Abstract:
A downhole tool is provided that selectively opens and closes an axial/lateral bore of a tubular string positioned in a wellbore used to produce hydrocarbons or other fluids. When integrated into a tubular string, the downhole tool allows individual producing zones within a wellbore to be isolated between stimulation stages while simultaneously allowing a selected formation to be accessed. The downhole tools and methods can be used in vertical or directional wells, and additionally in cased or open-hole wellbores.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation-in-part of U.S. patent application Ser. No. 13/267,691, filed Oct. 6, 2011, which claims the benefit of U.S. Provisional Application No. 61/390,354, filed Oct. 6, 2010, the entire disclosures of which are hereby incorporated herein by reference in their entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    Embodiments of the present invention are generally related to selectively opening and closing one or more ports or access openings in a tubular string. More specifically, one embodiment allows selective access of a tubular annulus of a wellbore to provide a flow path between a tubular string positioned in the wellbore and a geologic formation that requires a treatment such as hydraulic fracturing. 
       BACKGROUND OF THE INVENTION 
       [0003]    A wellbore used in recovering oil/gas typically includes a production string placed within a casing string. In some wellbore designs, the entire length of the wellbore is lined with the casing string, which is cemented within the wellbore. Alternatively, in open-hole designs, the casing string is limited to an upper portion of the wellbore and lower portions of the wellbore are open. In both open-hole and cased-hole designs, the production string is typically placed into the lower portions of the wellbore and mechanical or hydraulic packers are used to radially secure the production string in a predetermined location. The outside diameter of the production tubing is less than the diameter of the internal wellbore or production casing, thereby defining a tubular annulus. 
         [0004]    To gain access to oil/gas deposits in the general area of the wellbore, selected portions of the production casing are perforated or, alternatively, sliding sleeves or other devices are used to provide a conduit to the oil and gas deposits. To enhance the flow of oil/gas into the tubular annulus, and to thus increase flow into the production tubing, hydraulic fracturing (i.e., “fracing”) of subterranean formations may be required, especially in low permeability formations. That is, in some instances subterranean formation that the wellbore penetrates does not possess sufficient permeability for the economic production of oil/gas so hydraulic fracturing and/or chemical stimulation of the subterranean formation is needed to increase flow performance. 
         [0005]    Hydraulic fracturing consists of selectively injecting fracturing fluids into a subterranean formation in openhole or via perforations or other openings in the production casing of the wellbore at high pressures and rates to form a fracture. In addition, granular proppant materials, such as sand, ceramic beads, or other materials are injected into the formation with the fracturing fluids to hold the fracture open after the hydraulic pressure has been released. The proppant material prevents the fracture from closing and thus provides a more permeable flow path within the subterranean formation, resulting in increased flow capacity. In chemical stimulation treatments, permeability and thus flow capacity is improved by dissolving materials in the formation or otherwise chemically changing formation properties. 
         [0006]    To gain access to multiple or layered reservoirs, or a very thick hydrocarbon-bearing formation by hydraulic fracturing, multiple fracturing zones are established and stimulated in stages. One technique currently being used with significant results utilizes the use of a directionally drilled well into a single reservoir. By drilling the well in a substantially horizontal orientation through the reservoir, the reservoir can be fractured in multiple locations to substantially improve the flow rate. To stimulate multiple fracturing zones, a target stimulation zone must be temporarily isolated from the already-stimulated zones to prevent injecting fluids into the already-stimulated zones. Various methods have been utilized to achieve zonal isolation, although numerous drawbacks to the current methods exist. 
         [0007]    A common method currently used to isolate a fracturing zone in multistage fracturing utilizes composite bridge plugs. According to this method, the deepest zone in the wellbore (or most distal in horizontal wellbores) is stimulated. Then, the stimulated zone is isolated by a bridge plug that is positioned above the perforations associated with the stimulated zone. The process is repeated in the next zone up the wellbore. At the end of the stimulation process, a wellbore clean-out operation removes the bridge plug. The major disadvantages of using one or more bridge plugs to isolate a fracture stimulated zone are the high cost and risk of complications associated with multiple trips into and out of the wellbore to position the plugs. For example, bridge plugs can become stuck in the wellbore and need to be drilled out at great expense. A further disadvantage is that the required wellbore cleanout operation may block or otherwise damage some of the successfully fractured zones. 
         [0008]    Another method used to isolate a fracturing zone utilizes frac baffles and balls. The first baffle, which contains the smallest inside diameter, is placed in the most distal portion of the wellbore. The succeeding baffles increase in diameter and are installed above the previous baffle. To achieve zonal isolation, a frac ball of a predetermined size is dropped that seats on the corresponding frac baffle at a specified depth or position to block a portion of the wellbore. The isolated zone is accessed by perforations or a sleeve is shifted then stimulated. After each stage, the process is repeated until all selected frac zones in the well are fracture stimulated. On the last day of operation, the frac balls typically are flowed back to the surface during the flow back of the fracturing fluids. The primary advantage of this method is that the frac baffles are installed within the casing and can be activated by dropping a ball from the surface, with little downtime between fracture stimulation stages. The disadvantages include the need to use progressively larger sized balls for subsequent fracturing stages, thus limiting the number of zones that can be treated for a given casing diameter. Additionally, the frac baffles and balls may need to be milled out of the casing string, which increases the number of wellbore operations and inherent risks and costs associated therewith. 
         [0009]    One method for successfully isolating one or more production zones utilizes a sliding sleeve that is associated with a tubular string, which may include casing, liners, tubing, etc. Opening the sleeve permits zonal isolation and stimulation of the formation via the tubular string through the selected sleeve. The sleeve can be operated by using a mechanical/hydraulic shifting tool attached to coiled or jointed tubular or by using a ball-drop system. In a ball-drop system, a ball pumped down the tubular string engages a sliding sleeve and shifts the sleeve from a closed position to an open position, thereby opening a passageway to the tubular annulus. The ball also isolates the already-stimulated zones located beneath the open sleeve. The advantages of this method are that the tubular annulus can be accessed without requiring various tools or costly trips into the wellbore to isolate the various formations. However, the method is limited by the need to use progressively larger sized balls for subsequent fracturing stages, thus limiting the number of zones that can be deployed for a given tubing string diameter. This system inherently restricts the production flow rate due to the necessity of using progressively smaller balls to open and close the sleeves. 
         [0010]    Accordingly, a need exists for an improved downhole tools and methods that efficiently isolates individual zones of a subterranean formation while (1) ensuring that stimulation fluids are directed to the desired location, (2) maintaining a desired inner diameter of the tubing string, (3) reducing the time between stimulations, and (4) is mechanically simplistic to operate and cost effective. 
         [0011]    The following disclosure describes improved downhole tools and methods for selectively isolating downstream portions of a tubular string while simultaneously allowing access to the tubular annulus of a wellbore such that a selected zone may be stimulated. The improved downhole tools and methods do not limit the number of fracture stimulation stages created in a vertical or directional wellbore. As used herein, ‘downstream’ and ‘lower’ refers to the distal portions of a tubular string disposed toward the toe of the wellbore. Further, as used herein, ‘treatment fluid’ may comprise acid, proppant material, gels, or other stimulation fluids generally used in the art. 
       SUMMARY OF THE INVENTION 
       [0012]    The downhole tools disclosed herein is designed for downhole well stimulation for oil and gas wells, but could be used for any downhole application where a shifting sleeve is used to selectively divert flow. Additionally, the downhole tools may be employed in either open or cased holes. Generally, a downhole tool is placed into a wellbore and provides for the opening of the tubular string to the geologic formation while simultaneously restricting the flow of fluid and proppant downstream of the downhole tool. Fluid with or without proppant is then pumped into the geologic formation through the openings to stimulate the rock through hydraulic fracturing (fracing) or other treatment processes. By progressing from the toe (bottom) of the well back toward the surface, it is possible to stimulate the subterranean formation in stages, thus improving the quality of the stimulation and/or minimizing fluid/proppant. The downhole tools disclosed herein improve upon existing shifting sleeve designs by 1) allowing for a very large number of stimulation stages (50-200), 2) minimizing the flow restrictions inherent in ball drop systems that rely on progressively smaller ball diameters, 3) providing a system that does not need to be drilled out in order to facilitate production, 4) using a single ball size for all stages, and 5) improving the speed and efficiency of the stimulation process. 
         [0013]    It is thus one aspect of embodiments of the present invention to provide a downhole tool that seals a selected portion of a wellbore between geologic formations while simultaneously allowing access to a tubular annulus defined between the interior of a casing string or open-hole wellbore and a production string positioned therein. According to at least one embodiment, the downhole tool is integrated by a threaded connection, or any similar connection commonly practiced in the art, into a tubular production string that is positioned within the wellbore. The downhole tool provides a path for fluids or tools to enter the tubular annulus and simultaneously isolates downstream portions of the tubular production string from the high pressures exerted by a stimulation procedure, e.g., hydraulic fracturing. Additionally, with the use of packers or cement to isolate the tubular annulus, the downhole tool isolates non-targeted stimulation zones from the high pressures exerted by a stimulation procedure. As used herein, packers may be swellable, hydraulic, mechanical, inflatable, or any other alternative known in the art. The downhole tool in some instances eliminates the need to perforate various strings of pipe or position other tools into the wellbore, thus saving time, costs, and the inherent risk of trapping a tool. The downhole tool may be constructed of metallic or non-metallic materials, such as the composite materials currently used in composite bridge plugs, and typically combinations of both. 
         [0014]    It is another aspect of embodiments of the present invention to provide a downhole tool that employs a flapper valve that is capable of moving between a first position and a second position to selectively open and close an axial bore and a lateral bore of the downhole tool. The axial bore of the downhole tool opens to and is in fluid communication with an internal bore of the tubular string. The lateral bore of the downhole tool opens to and creates a passageway to the tubular annulus. The flapper valve may be associated with a sealing element fabricated of an elastomeric, plastic, metallic, or any other sealing element known to one of ordinary skill in the art. In some embodiments, the flapper valve may be comprised of degradable materials. For example, after a predetermined period of time, the flapper valve may dissolve to allow production fluid to flow unrestricted through the axial and lateral bores of the downhole tool. A degradable flapper valve is disclosed in U.S. Pat. No. 7,287,596, which is incorporated herein by reference in its entirety. 
         [0015]    When in the first position, the flapper valve seals the lateral bore of the downhole tool such that fluid may be pumped through the axial bore of the downhole tool. The axial bore of the downhole tool may also allow passage of solid elements, such as wireline tools, tubing, coiled tubing conveyed tools, cementing plugs, balls, darts, and any other elements known in the art. The sealing area of the first position may be irregular in shape and comprised of several sealing surfaces. 
         [0016]    When in the second position, the flapper valve seals the axial bore of the downhole tool, thereby sealing the internal bore of the tubular string and allowing fluid to be pumped to the tubular annulus through the lateral bore of the downhole tool. The movement of the flapper from the first position to the second position effectively seals the downstream stimulation zone and opens a passageway to the tubular annulus, allowing the next stimulation zone to be immediately treated. 
         [0017]    It is another aspect of embodiments of the present invention to provide a restraining mechanism for maintaining the flapper in the first position. The restraining mechanism may be a ring, finger, a tubular member, such as a sleeve, or any other restraining device. The restraining mechanism exerts a force against the flapper valve to prevent external forces acting upon the outside of the flapper valve, such as the external pressures associated with circulating a fluid in the tubular annulus, from unseating the flapper valve from its first position. When the restraining device is disengaged, the flapper valve is free to move to the second position. According to at least one embodiment, the restraining mechanism is disengaged by an actuating mechanism deployed on electric wireline, a slickline, coiled tubing, jointed tubing, solid rods, or drop members. Examples of drop members include balls, plugs, darts, or any other members commonly used in the art. As used herein, ‘ball’ refers to any shaped device that is feasible of being pumped down a tubular string and is not limited to a circular-shaped device. For example, a ‘ball’ may be circular, oval, oblong, or any other shape known in the art. 
         [0018]    It is another aspect of embodiments of the present invention to provide a flapper valve that is biased toward the second position by a coiled spring, leaf spring band, or other similar energy storage system. The stored energy assists the movement of the flapper valve toward the second position. According to at least one embodiment, a spring is placed in the body of the downhole tool, and compressed, storing mechanical energy to aid in the movement of the flapper valve from the first position to the second position. Additionally, an explosive device may be used to assist the flapper valve movement. For example, cement located in the tubular annulus may interfere with flapper movement and the spring or explosive device would aid in breaking the flapper valve away from the cement. The activating tool used to move the flapper valve-restraining device also may assist in the movement of the flapper valve from the first position to the second position. 
         [0019]    It is another aspect of embodiments of the present invention to provide a downhole tool that is activated with drop members from the surface using a multi-pressure activation system. The multi-pressure activation system exposes the downhole tool to a predetermined pressure to selectively actuate a sliding sleeve that receives a drop member. For example, in one embodiment, a first higher pressure does not actuate the sliding sleeve. Instead, the higher pressure causes the drop member to pass through the axial bore of the downhole tool, by use of a spring operated catch mechanism, and travel through the internal bore of the tubular string to the next tool or to the distal end of the wellbore. The higher pressure may either deform the drop member to allow it to pass through the axial bore of the downhole tool or actuate a ball catch mechanism, such as a collet slidable device, collet deformable fingers, or any other ball catch mechanism currently employed in the art. Collet slidable devices are disclosed in U.S. Pat. Nos. 4,729,432, 4,823,882, 4,893,678, 5,244,044, and 7,373,974, which are incorporated herein by reference in their entireties. Collet deformable fingers are disclosed in U.S. Pat. Nos. 4,292,988 and 5,146,992, which are incorporated herein by reference in their entireties. 
         [0020]    In the above mentioned embodiment, a second lower pressure does not allow the drop member to pass through the axial bore of the downhole tool. Rather, the lower pressure keeps the drop member trapped, under pressure, in the axial bore of the downhole tool. The lower pressure is held for a period of time until the sliding sleeve moves, thereby allowing the flapper valve to move from the first position to the second position to block the axial bore of the tubular string and to open the lateral bore of the downhole tool. 
         [0021]    In operation, the drop member would be inserted into the tubular string. Once the drop member lands and engages the sleeve of a downhole tool, a higher pressure would be exerted at the surface of the wellbore. The higher pressure would cause the drop member to pass through that downhole tool without sleeve actuation, and continue to pass through each tool distally in the wellbore until the desired tool is reached. The sleeve of the desired downhole tool would then be activated by applying the lower pressure, which would move the sleeve and allow the flapper valve to actuate from the first position to the second position. Fracture stimulation materials may then be selectively pumped through the internal bore of the tubular string, through the lateral bore of the downhole tool, and into the tubular annulus. 
         [0022]    In another embodiment, utilizing hydraulics in the catch mechanism would allow a drop member to pass under a lower pressure; shifting would occur only under a higher pressure. 
         [0023]    Another aspect of embodiments of the present invention is to provide a sliding sleeve associated with a reservoir of hydraulic oil or other fluid that allows the sliding sleeve to shift, thereby freeing the flapper valve to move from the first position to the second position. The hydraulic oil or other fluid bleeds through an orifice to a second reservoir allowing the sliding sleeve to move over a period of time from an initial position to a position that allows the flapper to move. The sliding sleeve may be moved back to its first position by means of a spring or other stored energy device, which would in turn transfer the hydraulic fluid back through the orifice to the first reservoir. 
         [0024]    It is another aspect of embodiments of the present invention to provide a locking mechanism for securing a sliding sleeve in a shifted position. The locking mechanism prevents the sliding sleeve from shifting back to its initial position, thereby ensuring that the sliding sleeve does not disengage the flapper valve from its second position. 
         [0025]    It is another aspect of embodiments of the present invention to provide a downhole tool that is activated by coiled tubing or small diameter jointed tubing. In this embodiment, the treatment for a given wellbore stimulation would be pumped in an annulus formed between the coiled tubing, solid rods, and the inner surface of a tubular string, thereby allowing the coiled tubing to function as a dead string to monitor down hole treating pressures. A tool located at the end of the coiled tubing engages a shifting sleeve associated with the tubing string that is held in place by shear pins or any other similar device. The use of coiled tubing as the actuating tool allows an unlimited number of treatment stages to be performed in a well, thus providing an advantage over frac baffles, for example, which require smaller actuation balls to be used to engage frac baffles in more distal positions in the wellbore. Additionally, using coiled tubing as the activation member removes the need for pressurizing fluid pumped from the surface as described above, and the coiled tubing may be used to cleanout proppant between fracing stages. 
         [0026]    Another aspect of embodiments of the present invention is to provide a downhole tool utilizing a shifting sleeve that closes the tubular production string at a predetermined location and opens the annulus of the wellbore to allow fracing or other stimulation procedures in stages. In one embodiment a counter is embedded in the shifting sleeve and a uniform size ball is dropped into the well. Each shifting sleeve is preset with a unique counter number such that the counter locks in place after the proper number of balls have passed, catching and retaining the next ball. The ball then closes off the wellbore and shifts a sliding sleeve, opening the annulus and geologic formation to be treated at a predetermined depth or interval. The counter locking mechanism is designed to facilitate normal completion operations including flow back during screen out. As used herein, counting means refers to any form of counter that can increment and/or decrement. Sleeve activation means identifies any means that facilitates movement of an inner tubular member, such as a sleeve. For example, sleeve activation means include pressure activation, mechanical activation, and electronic activation techniques. Signal means identifies any form of electronic signal that is capable of conveying information. 
         [0027]    Another aspect of embodiments of the present invention is to provide a swellable ball that is dropped into the well and a downhole tool utilizing a sliding sleeve. The ball is configured to swell after a predetermined period of time in a fluid, such as fracing fluid. In operation, the swellable ball is pumped quickly to the correct location. The location can be verified by counting pressure spikes, which result from the ball passing through a seat disposed in a sliding sleeve. Once the swellable ball is located in the tubular string proximal to the sleeve to be shifted, pumping is discontinued. Thus, the swellable ball would be allowed to swell to a size that would prevent the ball from passing through the selected sleeve. The operator would then continue pumping. 
         [0028]    Another aspect of embodiments of the present invention is to provide a smart ball that is dropped into the well and a downhole tool utilizing a sliding sleeve. In one embodiment, the shifting sleeve has an embedded radio frequency identification (“RFID”) chip and the smart ball has an RFID reader built into it. When the ball passes the RFID chip, the RFID reader reads the number of the RFID chip. If the correct number is read, the ball releases a mechanism that expands the size of the ball. For example, the expansion could be a split in the middle of the ball that rotates part of the ball slightly. Alternatively, the top ⅓ of the ball may be hinged and would open upon the correct number being read. The larger ball would become stuck in the next seat. In another embodiment, the smart ball includes a timer that causes the ball to expand after a certain period of time. For example, in this embodiment, an operator would count the pressure spikes and stop pumping when the ball is in the right location and wait for the timer to go off. Pumping would then resume. 
         [0029]    Another aspect of embodiments of the present invention is to provide a ball that is dropped into the well and a downhole tool utilizing a smart sleeve. In one embodiment, each sleeve has an RFID reader and the ball has an RFID chip. When the correct ball passes, the device releases a mechanism to catch the ball, plugging the orifice and shifting the sleeve. In another embodiment, each sleeve has a pressure transducer and circuit board with logic to understand pressure signals. The sleeve receives hydraulic pressure signals from a signal generator on the surface. The proper signal triggers the sleeve to shift, thus opening the annulus and creating a seat for the ball to land on. Then, a ball is dropped to close off the axial bore of the tubular production string. 
         [0030]    It is another aspect of the present invention to provide a method for selectively treating multiple portions of a production wellbore, whether from the same geologic formation or different formations penetrated by the same wellbore. In one embodiment, a single sized ball is utilized multiple times to move a sleeve which isolates a lower portion of the wellbore, while providing communication to the annulus to treat the formation at a predetermined depth. After that zone is treated, subsequent balls of the same size are used to isolate and treat other zones at a shallower depth. After all the zones are treated, all of the balls may flow back to the surface, or disintegrate if manufactured from degradable materials. Dissolvable balls are disclosed in U.S. Patent Publication No. 2010/0294510, which is herein incorporated by reference in its entirety. 
         [0031]    It is still yet another aspect of embodiments of the present invention to provide a downhole tool that employs an external cover associated with the lateral bore of the downhole tool. The external cover prevents debris, such as cement, from interfering with the movement of the flapper from the first position to the second position. The external cover may be removed or deformed by fluid pumped through the internal bore of the tubular string and the axial bore of the downhole tool. Coiled tubing carrying fluids alone or fluids with abrasive particles may also be used to remove or deform the external cover, which will also form a tunnel through the cement to the formation. It is another aspect of embodiments of the present invention to provide a downhole tool that is used with external tubular packers positioned within the tubular annulus to isolate a stimulation zone and to prevent clogging of the lateral bore. External casing packers, conventional packers, swellable packers, or any other similar devices may be employed. External tubular packers isolate the frac zone and/or prevent cement from contacting the external portion of the downhole tool and blocking the lateral bore. 
         [0032]    Another aspect of embodiments of the present invention is to provide a downhole tool that facilitates tools exiting the tubular string through the lateral bore. According to at least one embodiment, the flapper valve may be longer in one axis such that when the flapper valve moves to the second position, it forms a whipstock slide that is angled with respect to a longitudinal axis of the tubular string. The whipstock slide guides drilling or workover tools to the lateral bore of the downhole tool. If the lateral bore is blocked by an external cover or by debris, the blockage may be removed by milling, drilling, acid, or other fluid, including abrasive particle laden fluids. Using the flapper valve as a whipstock slide may be particularly useful for short and ultra-short radius horizontal boreholes where the tubular string is the origin. The flapper valve may have an orienting mechanism, such as a crowsfoot&#39;s key that is commonly used to orient tools in a specified azimuth. When the flapper valve is in the second position, the orienting mechanism orients the tools to the lateral bore. 
         [0033]    According to another aspect of embodiments of the present invention, the downhole tool may include several longitudinally spaced flapper valves. Additionally, numerous smaller flapper valves could be arranged around the circumference of the downhole tool. The smaller flapper valves could be activated by an activating member as described above to open one or more additional bores to the tubular annulus. After being released by an activating member, the smaller flapper valves would move toward a second position, which may be disposed in a recess about the body of the downhole tool so as not to block the axial bore of the downhole tool. 
         [0034]    It is another aspect of embodiments of the present invention to provide a downhole tool that includes a flapper valve that does not open a lateral bore to the tubular annulus. In these embodiments, movement of an inner tubular member, such as a sleeve, opens ports to the annulus that allow fluid exchange between the axial bore of the tubular string and the subterranean formation. The movement of the inner tubular member allows the flapper valve to block the axial bore of the tubular string and thereby prevent fluid flow through the axial bore of the downhole tool to portions of the tubular string located downstream of the actuated flapper valve. 
         [0035]    It is another aspect of embodiments of the present invention to provide a downhole tool that may be used as a blowout preventer that prevents a large volume of fluid from passing upward through the internal bore of the tubular string. According to at least one embodiment, a downhole tool includes a flapper valve and an inner tubular member. The flapper has two stationary positions, a first position and a second position. When the flapper valve is in the first position, fluid may be freely pumped through the axial bore of the downhole tool. When the flapper is in the second position, the internal bore of the tubular string is sealed such that fluids downstream of the flapper valve cannot flow upward through the axial bore of the downhole tool. In this embodiment, the inner tubular member is pressure activated and comprises a ball, a ball seat, a ball cage, and flow restriction orifices. The inner tubular member is held in place by shear pins or any other similar means known in the art that are responsive to axial force. 
         [0036]    The inner tubular member allows fluid to be pumped from the surface in normal circulation and in reverse circulation. During normal circulation, fluid flows down the tubular string through the ball seat and the flow restriction orifices of the inner tubular member. The ball cage restricts the ball from moving distally in the tubular string. During reverse circulation, fluid flows up the tubular string causing the ball to seat in the ball seat, thus limiting the upward fluid flow by requiring the fluid to flow through flow restriction orifices. If a large volume of fluid attempted to pass upward through the downhole tool, such as in a blowout situation, the friction pressure through the orifices would overcome the shear pins, or any other similar means and shift the inner tubular member upwards. The upward shift of the inner tubular member allows the flapper valve to move from the first position to the second position. Once in the second position, the flapper valve seals the internal bore of the tubular member and fluid flow up the internal bore of the tubular string would be prevented. The flapper valve may have a sealing element fabricated of an elastomeric, plastic, metallic, or any other sealing elements customarily used in the art to prevent fluids from flowing up the inner bore of the tubular string. The sealing elements may be disposed on the flapper or on a flapper seat. Additionally, the downhole tool may include multiple flapper valves. 
         [0037]    According to at least one embodiment of the present invention, a downhole tool adapted for use in a tubular string to selectively treat one or more hydrocarbon production zones is provided, the downhole tool comprising: an upper end and a lower end adapted for interconnection to a tubular string; a catch mechanism positioned proximate to said lower end and adapted to selectively catch or release a ball traveling through said tubular string; a sleeve which travels in a longitudinal direction between a first position and a second position, and which is actuated based on an internal pressure in the tubular string, said sleeve preventing a flow of a treatment fluid in a lateral direction into an annulus of the wellbore while in said first position, and permitting the flow of the treatment fluid in the lateral direction through at least one port in said second position; and a locking mechanism positioned proximate to said catch mechanism, wherein when said catch mechanism is engaged with said locking mechanism, said sleeve is in said second position and said treatment fluid cannot be pumped downstream of said catch mechanism in the tubular string. 
         [0038]    According to at least another embodiment of the present invention, a method for treating a plurality of hydrocarbon production zones at different locations in one or more geologic formations is disclosed, the method comprising: providing a wellbore with an upper end, a lower end and a plurality of producing zones positioned therebetween; positioning a string of production tubing in the wellbore, said string of production tubing having an upper end and a lower end; providing a plurality of selective opening tools in said production string, each of said selectively opening tools having a catch mechanism adapted to selectively catch or release a ball traveling through said tubular string, a sleeve which travels in a longitudinal direction between a first position and a second position and which is actuated based on an internal pressure in the tubular string, said sleeve preventing a flow of a treatment fluid in a lateral direction into an annulus of the wellbore while in said first position, and permitting the flow of the treatment fluid in the lateral direction through at least one port in said second position, and a locking mechanism positioned proximate to said catch mechanism, wherein when said catch mechanism is engaged with said locking mechanism, said sleeve is in said second position and said treatment fluid cannot be pumped downstream of said catch mechanism in the tubular string; pumping a treatment fluid containing a ball through the production tubing at a predetermined first pressure until said ball engages the catch mechanism of a first selective opening tool positioned proximate to a first portion of the hydrocarbon production zone; maintaining said first pressure in said production tubing for a pre-determined period of time to displace said catch mechanism of said first tool and engage the locking mechanism of said first tool wherein a sleeve of said first tool is in a second position; pumping the treatment fluid into said first portion of said at least one geologic formation; reducing the pressure in said production tubing wherein said catch mechanism disengages from said locking mechanism and said sleeve returns to said first position; pumping said treatment fluid at a predetermined second pressure until said ball engages and passes through said catch mechanism of said first selective opening tool, said second pressure higher than said first pressure; reducing said treatment fluid pressure to said first pressure to position said ball in a catch mechanism of a second selective opening tool positioned proximate to a second zone of the hydrocarbon production zone, wherein said catch mechanism engages a locking mechanism of said second tool wherein a sleeve of said second tool is in second position; pumping the treatment fluid into said second portion of said at least one geologic formation. 
         [0039]    According to yet another embodiment of the present invention, system adapted for use in a tubular string for treating one or more hydrocarbon production zones, comprising: a plurality of downhole tools, each comprising: an upper end and a lower end adapted for interconnection to a tubular string; a catch mechanism positioned proximate to said lower end and adapted to selectively catch or release a ball traveling through said tubular string; a sleeve which travels in a longitudinal direction between a first position and a second position, and which is actuated based on an internal pressure in the tubular string, said sleeve preventing a flow of a treatment fluid in a lateral direction into an annulus of the wellbore while in said first position, and permitting the flow of the treatment fluid in the lateral direction through at least one port in said second position; and a locking mechanism positioned distal to said catch mechanism, wherein when said catch mechanism is engaged with said locking mechanism, said sleeve is in said second position and said treatment fluid cannot be pumped downstream of said catch mechanism in the tubular string; wherein when a treatment fluid containing a ball is pumped into said tubular string at a predetermined first pressure, said ball displaces a catch mechanism of a first downhole tool until engaging a locking mechanism of said first tool wherein a sleeve of said first tool is in a second position; wherein when a treatment fluid containing a ball is pumped into said tubular string at a predetermined second pressure greater than said first pressure, said ball passes through said catch mechanism of said first downhole tool until engaging a catch mechanism of a second downhole tool. 
         [0040]    The Summary of the Invention is neither intended nor should it be construed as being representative of the full extent and scope of the present invention. Moreover, references made herein to “the present invention” or aspects thereof should be understood to mean certain embodiments of the present invention and should not necessarily be construed as limiting all embodiments to a particular description. The present invention is set forth in various levels of detail in the Summary of the Invention as well as in the attached drawings and the Detailed Description of the Invention and no limitation as to the scope of the present invention is intended by either the inclusion or non-inclusion of elements, components, etc. in this Summary of the Invention. Additional aspects of the present invention will become more readily apparent from the Detail Description, particularly when taken together with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0041]    The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and together with the general description of the invention given above and the detailed description of the drawings given below, serve to explain the principles of these inventions. 
           [0042]      FIG. 1  is a cross-sectional view of a fracture stimulation system according to one embodiment of the present invention; 
           [0043]      FIG. 2  is a cross-sectional view of a well production system according to one embodiment of the present invention; 
           [0044]      FIG. 3  is a cross-sectional view of a downhole tool that is actuated by a shifting tool according to one embodiment of the present invention; 
           [0045]      FIG. 4  is another cross-sectional view of the embodiment of  FIG. 3 ; 
           [0046]      FIG. 5  is a cross sectional view of a horizontal well with multiple fracturing stages; 
           [0047]      FIG. 6  is a cross-sectional view of a downhole tool that is actuated by a pressure activation system according to one embodiment of the present invention; 
           [0048]      FIG. 7  is another cross-sectional view of the embodiment of  FIG. 6 ; 
           [0049]      FIG. 8  is yet another cross-sectional view of the embodiment of  FIG. 6 ; 
           [0050]      FIG. 9  is a cross-sectional view of a downhole tool that is actuated by a pressure activation system according to another embodiment of the present invention; 
           [0051]      FIG. 10  is a cross-sectional view of the downhole tool shown in  FIG. 9  in a non-shifted position; 
           [0052]      FIG. 11  is a cross-sectional view of the downhole tool shown in  FIG. 9  in a shifted position; 
           [0053]      FIG. 12  is a cross-sectional view of the downhole tool shown in  FIG. 11  during flow-back; 
           [0054]      FIG. 13  is a cross-sectional view of a downhole tool that is actuated by a counter system according to yet another embodiment of the present invention; 
           [0055]      FIG. 14  is a cross-sectional view of the downhole tool shown in  FIG. 13  in a shifted position; 
           [0056]      FIG. 15  is an end view of the downhole tool shown in  FIG. 13 ; 
           [0057]      FIG. 16  is a side view of the counter assembly shown in  FIG. 13 ; 
           [0058]      FIG. 17  is a top view of the counter assembly shown in  FIG. 16 ; 
           [0059]      FIG. 18  is a side view of a locking mechanism in a clockwise lock position; 
           [0060]      FIG. 19  is a side view of the locking mechanism of  FIG. 18  in a counterclockwise lock position; 
           [0061]      FIG. 20  is a side view of a counter assembly according to another embodiment of the present invention; 
           [0062]      FIG. 21  is another side view of the counter assembly shown in  FIG. 20 ; 
           [0063]      FIG. 22  is a cross-sectional view of a downhole tool that is employed as a whipstock slide according to one embodiment of the present invention; 
           [0064]      FIG. 23  is another cross-sectional view of the embodiment of  FIG. 22 ; 
           [0065]      FIG. 24  is a cross-sectional view of a downhole tool that is configured to prevent a well blowout according one embodiment of the present invention; 
           [0066]      FIG. 25  is another cross-sectional view of the embodiment of  FIG. 24 ; 
           [0067]      FIG. 26  is yet another cross-sectional view of the embodiment of  FIG. 24 ; 
           [0068]      FIG. 27  is a further cross-sectional view of the embodiment of  FIG. 24 ; 
           [0069]      FIG. 28  is yet a further cross-sectional view of the embodiment of  FIG. 24 ; 
           [0070]      FIG. 29A  is a cross-sectional view of a downhole tool in an unactuated state under a low axial bore pressure, the tool actuated by a drop member and catch/release mechanism according to another embodiment of the present invention; 
           [0071]      FIG. 29B  is a cross-sectional top view of section A-A of the catch/release mechanism of the embodiment of  FIG. 29A ; 
           [0072]      FIG. 29C  is a detailed cross-sectional side view of portion A of the catch/release mechanism of the embodiment of  FIG. 29A ; 
           [0073]      FIG. 30A  is a cross-sectional view of the downhole tool shown in  FIG. 29A  in an actuated state under a low axial bore pressure; 
           [0074]      FIG. 30B  is a detailed cross-sectional side view of portion A of the downhole tool shown in  FIG. 30A ; 
           [0075]      FIG. 31A  is a cross-sectional view of the downhole tool shown in  FIG. 29A  under a high axial bore pressure; 
           [0076]      FIG. 31B  is a cross-sectional top view of section A-A of the downhole tool shown in  FIG. 31A ; 
           [0077]      FIG. 31C  is a detailed cross-sectional side view of portion A of the downhole tool shown in  FIG. 31A ; 
           [0078]      FIG. 31D  is a cross-sectional view of the downhole tool shown in  FIG. 31A  under a high axial bore pressure after passage of the drop member; 
           [0079]      FIG. 32A  is a cross-sectional view of a downhole tool in an unactuated state under a high axial bore pressure during retrieval of the drop member; and 
           [0080]      FIG. 32B  is a detailed cross-sectional side view of portion A of the downhole tool shown in  FIG. 32A . 
           [0081]    In certain instances, details that are not necessary for an understanding of the invention or that render other details difficult to perceive may have been omitted. It should be understood, of course, that the invention is not necessarily limited to the particular embodiments illustrated herein. 
           [0082]    To assist in the understanding of one embodiment of the present invention the following list of components and associated numbering found in the drawings is provided. 
           [0000]    
         
           
                 
                 
                 
               
             
                 
                     
                     
                 
                 
                     
                   # 
                   Components 
                 
                 
                     
                     
                 
               
               
                 
                     
                    2 
                   Downhole tool 
                 
                 
                     
                    6 
                   Wellbore 
                 
                 
                     
                    10 
                   Subterranean formation 
                 
                 
                     
                    14 
                   Tubular string 
                 
                 
                     
                    16 
                   Packer 
                 
                 
                     
                    18 
                   Axial bore 
                 
                 
                     
                    22  
                   Lateral bore 
                 
                 
                     
                    26  
                   Fracture ports 
                 
                 
                     
                    30  
                   Flapper valve 
                 
                 
                     
                    34  
                   Sliding sleeve 
                 
                 
                     
                    38  
                   Stimulation fluid 
                 
                 
                     
                    42  
                   Shifting tool 
                 
                 
                     
                    46  
                   Production fluid 
                 
                 
                     
                    50  
                   Shear pins 
                 
                 
                     
                    54  
                   Hinge 
                 
                 
                     
                    58  
                   Torsion spring 
                 
                 
                     
                    62  
                   Compression spring 
                 
                 
                     
                    66  
                   Fracturing zones 
                 
                 
                     
                    70  
                   Sleeve 
                 
                 
                     
                    74  
                   High pressure 
                 
                 
                     
                    78  
                   Drop member 
                 
                 
                     
                    82  
                   Catch mechanism 
                 
                 
                     
                    86  
                   Lower pressure 
                 
                 
                     
                    88  
                   Flange 
                 
                 
                     
                    90  
                   Spring 
                 
                 
                     
                    92  
                   Spring force 
                 
                 
                     
                    94  
                   Upper reservoir 
                 
                 
                     
                    98  
                   Lower reservoir 
                 
                 
                     
                   102 
                   Orifice 
                 
                 
                     
                   106 
                   Radial port 
                 
                 
                     
                   110 
                   Seals 
                 
                 
                     
                   114 
                   Weep hole 
                 
                 
                     
                   118 
                   Sleeve locking mechanism 
                 
                 
                     
                   122 
                   Recess 
                 
                 
                     
                   126 
                   Downhole tool 
                 
                 
                     
                   130 
                   Shifting sleeve 
                 
                 
                     
                   132 
                   Counter assembly 
                 
                 
                     
                   134  
                   Counter mechanism 
                 
                 
                     
                   138  
                   Counter locking mechanism 
                 
                 
                     
                   142  
                   Rocker mechanism 
                 
                 
                     
                   146  
                   Counter spring 
                 
                 
                     
                   150  
                   Counter window 
                 
                 
                     
                   154  
                   Perforations 
                 
                 
                     
                   158  
                   Protrusion 
                 
                 
                     
                   162  
                   Chamber 
                 
                 
                     
                   166  
                   Pressure equalization device 
                 
                 
                     
                   170  
                   Manual setting mechanism 
                 
                 
                     
                   174  
                   Trip pin 
                 
                 
                     
                   178  
                   Gears 
                 
                 
                     
                   180  
                   Counter wheels 
                 
                 
                     
                   182  
                   Inner shaft 
                 
                 
                     
                   186  
                   Sliding lock 
                 
                 
                     
                   190  
                   Anchor 
                 
                 
                     
                   192  
                   Treatment fluid 
                 
                 
                     
                   194  
                   Radial button 
                 
                 
                     
                   196  
                   Rack 
                 
                 
                     
                   198  
                   Gear 
                 
                 
                     
                   206  
                   Fill material 
                 
                 
                     
                   210  
                   Inner tubular member 
                 
                 
                     
                   212  
                   Outer tubular member 
                 
                 
                     
                   214  
                   Sealing element 
                 
                 
                     
                   218  
                   Ball 
                 
                 
                     
                   218 A   
                   Ball position A 
                 
                 
                     
                   218 B   
                   Ball position B 
                 
                 
                     
                   222  
                   Ball seat 
                 
                 
                     
                   226 
                   Ball cage 
                 
                 
                     
                   230  
                   Flow restriction orifices 
                 
                 
                     
                   240  
                   Piston 
                 
                 
                     
                   240 A   
                   Piston position A 
                 
                 
                     
                   240 B   
                   Piston position B 
                 
                 
                     
                   242  
                   Fluid reservoir 
                 
                 
                     
                   250  
                   Catch/Release Mechanism 
                 
                 
                     
                   252  
                   Collet Finger 
                 
                 
                     
                   254  
                   Major inner diameter 
                 
                 
                     
                   256  
                   Minor inner diameter 
                 
                 
                     
                   258  
                   Deformed distal collet finger 
                 
                 
                     
                   260  
                   Locking mechanism 
                 
                 
                     
                   270  
                   Locking dog 
                 
                 
                     
                     
                 
               
            
           
         
       
       
    
    
     DETAILED DESCRIPTION 
       [0083]      FIGS. 1 and 2  show one embodiment of the present invention in which at least one downhole tool  2  and associated tubular string  14  is disposed in a wellbore  6 . According to this embodiment, the wellbore  6  is drilled through a subterranean formation. As shown in  FIGS. 1 and 2 , three tools  2  are connected to a tubular string  14 . Each tool  2  is vertically disposed within a formation  10 A,  10 B,  10 C that has been selected to be fracture stimulated and/or produced. One of skill in the art will appreciate that packers, cement, or other sealants may be located on either side of the formation  10 A,  10 B, and  10 C to provide annular hydraulic isolation. As shown in  FIG. 1 , packers  16  provide annular hydraulic isolation of formation  10 B. In this embodiment, each tool  2  has an axial bore  18 , a lateral bore  22 , fracture ports  26 , a flapper valve  30 , and a sliding sleeve  34 . 
         [0084]    Referring now to  FIG. 1 , a fracture stimulation of a multiple zone formation is shown. As illustrated, the lower formation  10 C has been fracture stimulated, the intermediate zone  10 B is currently being fracture stimulated, and the upper zone  10 A will be fracture stimulated in the future. Stimulation fluid  38  flows down the tubular string  14  (which includes downhole tools  2 A,  2 B and  2 C), through the downhole tool  2 A and into the downhole tool  2 B (identifying Tool  2  in formation B). As shown, the downhole tool  2 B has been actuated wherein the flapper valve  30  blocks the axial bore  18  of tool  2 B, thereby preventing fluid from entering a distal portion of the tubular string  14  below the flapper valve  30  of tool  2 B. The fluid  38  flows through the frac ports  26  and the lateral bore  22  of the downhole tool  2 B into the intermediate zone  10 B. Portions of the tubular string  14  not associated with the zone being stimulated may be isolated by cement, packers, etc. 
         [0085]    After the fracture stimulation of the intermediate zone  10 B is completed, a shifting tool  42  is conveyed down the tubular string  14  to the downhole tool  2 A. The shifting tool  42  activates the downhole tool  2 A by shifting the sleeve  34 , thereby releasing the flapper valve  30 . Once released, the flapper valve  30  moves toward its second position and blocks the axial bore  18  of the downhole tool  2 A to fracturing zone  10 A prevent fluid from flowing distally in the tubular string  14 . The second position may be held in place by a variety of locking means that are well known to one of ordinary skill in the art. The shifting tool  42  is removed from the tubular string  14  or repositioned within the tubular string  14  to the next stimulation zone. Stimulation fluid  38  is then pumped down the tubular string  14 , through the activated tool  2 A, and into the fracturing zone  10 A. As will be appreciated by one skilled in the art, this fracture sequence can be repeated without limit in a wellbore. Additionally, more than one downhole tool  2  may be deployed within each formation  10 . 
         [0086]    Referring now to  FIG. 2 , production of a multiple zone formation is shown. As illustrated in  FIG. 2 , three vertically displaced (or horizontally placed zones in a directional well) formations  10  are producing fluid and/or gas (hereinafter “fluid”). The three downhole tools  2  integrated into the tubular string  14  allow the production fluid  46  to enter and flow up the tubular string  14 . Flapper valves  30  open in response to fluid flow and pressure, allowing flow from both outside and below the downhole tool  2 . As shown, production fluid  46  is flowing from the stimulated zones  10  through the frac ports  26  and the lateral bore  22  of the vertically displaced tools  2  into the tubular string  14 . Once in the tubular string  14 , the production fluid  46  flows up the tubular string  14 . The flapper valve  30  in each respective tool  2  is moved between a first position, where the lateral bore  22  is blocked, and a second position, in which the flapper valve  30  blocks the axial bore  18 , in response to fluid flow and pressure from outside and below the respective tool  2 . 
         [0087]      FIGS. 3 and 4  show a downhole tool according to another embodiment of the present invention. According to this embodiment, a sleeve  34  restrains a flapper valve  30  in its first position, thus closing a lateral bore  22  of the downhole tool  2 . A shifting tool shifts the sleeve  34 , thereby releasing the flapper valve  30  and allowing the flapper valve  30  to move toward its second position. 
         [0088]      FIG. 3  shows the flapper valve  30  is restrained in its first position by the sleeve  34 . The sleeve  34  is held in place by shear pins  50 , which prevent the sleeve  34  from moving within the tubular string  14 . In this position, the axial bore  18  of the downhole tool  2  allows fluids and solid elements to pass through the downhole tool  2  into distal portions of the tubular string  14 , and the flapper valve  30  blocks access to a tubular annulus formed between the tubular string  14  and the wellbore. The sleeve  34  blocks the ports  26  and the flapper valve  30  blocks the lateral bore  22 . 
         [0089]    Referring now to  FIG. 4 , the sleeve  34  has been shifted in the downhole tool  2 , thereby releasing a flapper valve  30  from its first position. A hinge  54  connected to the bottom of the flapper valve  30  allows rotation. A torsion spring  58  connected to the bottom of the flapper valve  30  biases the flapper valve  30  towards its second position. A compressed spring  62  also may be included in the body of the downhole tool  2  to assist the movement of the flapper valve  30  from its first position toward its second position. As shown, the flapper valve  30  is in its second position to seal the axial bore  18  of the downhole tool  2 , thereby preventing fluid from flowing downward into distal portions of the tubular string  14 . Frac ports  26  and the lateral bore  22  of the downhole tool  2  create passageways to the annulus of the tubular string  14 . As will be appreciated by one of skill in the art, the lateral bore  22  is optional. Accordingly, in some embodiments, fluid exchange occurs solely through the frac ports  26 . 
         [0090]    Referring now to  FIG. 5 , a horizontal well with multiple producing zones is shown. As illustrated, a wellbore  6  is depicted which contains five fractured zones  66 . At least one downhole tool  2  but preferably five in this example may be disposed within the wellbore to isolate and allow production from the different zones in the geologic formation. Each of the downhole tools  2  may be activated by a sleeve  34  as discussed above or by a pressure activation system to allow the selective treatment of each zone and subsequent production simultaneously, thus optimizing economic performance of the producing formation. Although not shown, the fractured producing zones may be hydraulically isolated with packers or cement, for example, to isolate the annular space between the tubular string  14  and the wellbore or casing. 
         [0091]      FIGS. 6-8  illustrate a downhole tool  2  according to another embodiment wherein the downhole tool  2  is actuated by a pressure activation system. More specifically, the sleeve  70  is pressure activated such that the flapper valve  30  is released depending on the pressure exerted into the tubular string  14 . In operation, a high pressure  74  applied to the tubular string  14  does not actuate a downhole tool  2 . Instead, the high pressure  74  causes a drop member  78 , such as a ball, to pass through a downhole tool  2  and travel to the next tool  2  in the tubular string  14  or to the distal portion of the wellbore  6 . The drop member  78  passes through the downhole tool  2  by deforming or by actuating a catch mechanism  82 , as shown in  FIGS. 6-8 . 
         [0092]    A lower pressure  86  actuates the downhole tool  2  by shifting the sleeve  70 , thereby releasing a flapper valve  30  and allowing it to move from its first position to its second position. More specifically, the lower pressure  86  acts upon the drop member  78 , which is lodged in the catch mechanism  82 , to slide the sleeve  70  away from the flapper valve  30 . Using a flange  88 , the sleeve contacts and compresses a spring  90  as it moves. The sleeve  70  is associated with an upper reservoir  94 , a lower reservoir  98 , and an orifice  102  for fluid passage. The outer surface of the sleeve  70  forms a boundary between the reservoirs  94 ,  98  and the internal bore of the downhole tool  2 , and seals the reservoirs  94 ,  98  from pressure in the tubular string. Sealing elements may be provided to enhance the seal between the sleeve  70  and the reservoirs  94 ,  98 . Once the sleeve  70  is moved a predetermined distance, the flapper valve  30  is able to release. In one embodiment, a high pressure  74  of about 3000 psi causes the drop member  78  to pass through a downhole tool  2 , and a lower pressure  86  of about 1000 psi maintained in the tubular string  14  for roughly 15 seconds causes the drop member  78  to move the sleeve  70 . One of ordinary skill in the art would understand this embodiment uses a similar mechanism to that of a hydraulic fishing jar. As will be appreciated by one of skill in the art, the pressures may vary depending on design of the sleeve  70 , the drop member  78 , the catch mechanism  82 , and the spring  90 . Further design criteria include the depth of the wellbore, pressure from the producing formation, diameter of tubing string  14 , etc. 
         [0093]      FIG. 8  shows a shifted sleeve  70  and a released flapper valve  30  in its second position. Once the sleeve  70  no longer abuts the flapper valve  30 , a torsion spring  58  will rotate the flapper valve  30  from its first position toward its second position, thereby blocking the axial bore  18  of the downhole tool and opening the lateral bore  22  of the downhole tool. An additional spring  62  may be used to assist the movement of the flapper valve  30  from its first position towards the second position. 
         [0094]      FIGS. 9-12  illustrate a downhole tool  2  actuated by a pressure activation system according to another embodiment of the present invention. The downhole tool  2  shown in  FIGS. 9-12  operates in a similar fashion as that described above in connection with  FIGS. 6-8 . A flapper valve  30  is shown in  FIGS. 9-12 ; however, in some embodiments, the flapper valve  30  is not included in the downhole tool  2 . In these embodiments, the sleeve  70  blocks access to the tubular annulus while in a non-shifted position. A drop member  78  shifts the sleeve  70  to allow access to the subterranean formation through openings formed in the circumference of the downhole tool  2 . The drop member  78  remains seated in the catch mechanism  82  during stimulation of the selected stage to isolate downstream portions of the tubular string from the stimulation fluid and/or proppant. 
         [0095]    Referring to  FIG. 9 , a sleeve  70  is disposed in an initial, non-shifted position. As shown, the sleeve  70  blocks access to the tubular annulus through a radial port  106  and restrains the flapper valve  30  in its first position, thereby blocking lateral bore  22 . Seals  110  provide a fluid tight engagement between the sleeve  70  and the downhole tool  2 , thus preventing fluid exchange between the tubular production string and the tubular annulus. The sleeve  70  is interconnected to a flange  88 , which is associated with an upper reservoir  94  and a lower reservoir  98 . The flange  88  has a weep hole  114  that allows fluid exchange between the upper and lower reservoirs. In operation, the weep hole  114  acts like a dashpot and resists motion of the sleeve  70 . The rate of fluid exchange between the upper and lower reservoirs increases once the flange  88  enters the larger cross-sectional reservoir area. Accordingly, in at least one embodiment, the sleeve  70  shifts at two different rates. Initially, the sleeve  70  shifts at a slow rate because of the restricted fluid flow through the weep hole  114 . However, once the sleeve has shifted to the point that the flange  88  enters the larger cross-section reservoir area, the sleeve shifts at an increased rate because of the increased fluid flow path between the upper reservoir  94  and the lower reservoir  98 . 
         [0096]    As illustrated in  FIG. 9 , a drop member  78  is seated in a catch mechanism  82 . At higher pressures, the drop member  78  passes through the catch mechanism  82  and travels to the next downhole tool  2  in the tubular production string, as shown in  FIG. 10 . At lower pressures, the drop member  78  remains seated in the catch mechanism  82  and moves the sleeve  70  into a shifted position, as shown in  FIG. 11 . 
         [0097]    Referring to  FIG. 10 , the sleeve  70  remains in a non-shifted position and the drop member  78  has passed through the catch mechanism  82  and is travelling through the tubular string toward a downstream tool  2  disposed in the tubular production string. Referring to  FIG. 11 , the drop member  78  has shifted the sleeve  70 , thus allowing the flapper valve  30  to isolate the downstream portions of the tubular production string. A sleeve locking mechanism  118  prevents the sleeve  70  from shifting upward in the downhole tool  2  and unseating the flapper  30  from its second position. As shown, the sleeve locking mechanism  118  is spring loaded. Alternative actuation methods, as known in the art, may be used to activate the sleeve locking mechanism  118 . Additionally, the sleeve locking mechanism  118  may have the ability to reset to its original position, thereby allowing the sleeve  70  to reset to its initial non-shifted position. 
         [0098]      FIG. 11  also depicts a recess  122  in the downhole tool  2  configured to receive the catch mechanism  82 . In one embodiment, the catch mechanism  82  has an undeformed outer diameter that is larger than the inner diameter of the downhole tool  2 . Accordingly, in this embodiment, the inner diameter of the downhole tool  2  constrains the outer diameter of the catch mechanism  82 . By providing a selectively positioned recess  122  in the downhole tool  2 , the catch mechanism  82  is allowed to expand into the recess  122  when the sleeve  70  is in a shifted position. This expansion allows the full inner diameter of the sleeve to be utilized for ball return during flow back operations. In one configuration, the catch mechanism  82  is a spring loaded collet assembly. 
         [0099]    Referring to  FIG. 12 , the downhole tool  2  is shown during flow back. As shown, the flapper valve  30  has rotated toward its first position, thereby allowing the drop member  78  to flow up the tubular string from distal portions of the wellbore. Additionally, the catch mechanism  82  has retracted into a recess  122  formed in downhole tool  2 . This retraction allows the full bore of the tubular string to be utilized and prevents the catch mechanism  82  from interfering with the return of the drop members  78  to the surface during flow back. In some configurations, the flapper valve  30  may be locked in its first position during flow back by a latching mechanism. Locking the flapper  30  in its first position would increase the flow up the axial bore  18  of the tubular production string while allowing flow from the stimulated zones to continue through the ports  106 .  FIGS. 13-19  depict a downhole tool  126  that is actuated by a pressure activation system according to another embodiment of the present invention. Downhole tools  126  are selectively disposed within stimulation stages according to a predetermined stimulation process. Each downhole tool  126  utilizes a counter to actuate a sliding sleeve. Each counter is associated with a stimulation stage and is preset to a predetermined number. The counter indexes for every drop member  78  that passes through the downhole tool  126 . After the predetermined number is reached, the counter prevents subsequent drop members  78  from passing through the downhole tool  126  to downstream portions of the tubular production string. Accordingly, each drop member  78  that is dropped proceeds to a predetermined stage number. Once at the predetermined stage number, the drop member  78  seats in a catch mechanism and seals the axial bore of the tubular production string. Increased pressure in the tubular production string upstream of the predetermined stage number shifts the predetermined tool  126  and allows access to the subterranean formation through openings in the tubular production string. 
         [0100]    Referring to  FIG. 13 , a cross-sectional view of the downhole tool  126  in a pre-shifted position is illustrated. In the pre-shifted position, the downhole tool  126  allows fluid and/or proppant to pass through the downhole tool  126  to the stage being stimulated while restricting access to openings formed in the downhole tool  126 . The downhole tool  126  utilizes a shifting sleeve  130  that may be secured in a pre-shifted position by a shear pin  50 . The shifting sleeve  130  employs a counter assembly  132  to activate shifting of the sleeve  130 . The design of the counter assembly  132  may vary, as will be appreciated by one of skill in the art. As shown in  FIG. 13 , the counter assembly  132  includes a counter mechanism  134 , a locking mechanism  138 , a rocker mechanism  142 , a counter spring  146 , and a catch mechanism, such as a protrusion  158 . In at least one embodiment, the counter assembly includes a manual setting mechanism  170  that allows the counter mechanism  134  to be incremented or decremented manually through buttons or levers. In an alternative embodiment, an electronic setting mechanism may be provided that allows an operator to remotely set the counter to a predetermined number. The preset number for the counter mechanism  134  may be revealed in a window  150  constructed of suitable transparent materials, such as Lexan or other similar materials. The window  150  may be viewed either from the sidewall of the pipe or by looking down the tubular before installation. 
         [0101]      FIG. 14  depicts the downhole tool  126  in a shifted position, revealing perforations  154  in the tubular production string. In the shifted position, the downhole tool  126  allows fluid and/or proppant to pass through the perforations  154  while restricting access to downstream portions of the tubular production string. As illustrated in  FIG. 14 , the drop member  78  remains lodged in the shifting sleeve  130  and restricts flow that might otherwise pass on to stages that have already been stimulated. After stimulation, the drop member  78  is no longer needed to seal the inner bore of the downhole tool  126  and thus is allowed to flow back to the surface. As shown, a sleeve locking mechanism  118  prevents the shifting sleeve  130  from shifting back into its pre-shift position. 
         [0102]      FIG. 15  illustrates a simplified end view of the downhole tool  126  with a drop member  78  disposed therein. In  FIG. 15 , the counter mechanism  134 , the locking mechanism  138 , and the counter spring  146  are not shown for simplicity reasons. As illustrated, the drop member  78  is seated on the protrusion  158  and substantially seals the inner bore of the downhole tool  126 . To prevent sand or other proppants from interfering with the gears of the counter assembly  132  and to ensure adequate lubrication thereof, the counter assembly  132  may be housed in a chamber  162  that is filled with oil or other fluid. A pressure equalization device  166 , such as a pressure regulator, may be used to ensure that the pressure inside the chamber  162  does not drop substantially below the pressure in the tubular production string, thus minimizing the likelihood of contaminants reaching the counter assembly and ensuring proper operation of the counter assembly  132 . The pressure equalization device  166  is in fluidic communication with the chamber  162  and the inner bore of the tubular production string, and isolates the fluid in the chamber  162  from the fluid and proppants in the tubular production string. In at least one embodiment the pressure equalization device is a piston and cylinder. Additionally, a sealing element may be provided between the counter assembly and the inner bore of the tubular string to further isolate the counter assembly  132  from contaminants. 
         [0103]      FIGS. 16-19  illustrate in detail one embodiment of a counter assembly  132 . As shown in  FIGS. 16-19 , the counter assembly  132  includes a counter mechanism  134 , a locking mechanism  138 , a rocker mechanism  142 , a counter spring  146 , and a manual setting mechanism  170 . Referring to  FIGS. 16-17 , a catch mechanism, such as a protrusion  158 , interconnects with the rocker mechanism  142 . The rocker mechanism  142  interconnects to a counter mechanism  134 , a locking mechanism  138 , and a spring  146 . Upon contact with a drop member, the protrusion  158  rotates the rocker mechanism  142  and allows the drop member to pass through the internal bore of the downhole tool  126 . Upon rotation of the rocker mechanism  142 , the counter mechanism  134  indexes a running count number. Once the running count number reaches a predetermined number, the counter mechanism  134  moves a trip pin  174  which allows the locking mechanism  138  to shift, thereby preventing subsequent drop members from passing through the downhole tool  126  to downstream portions of the tubular string. In some embodiments, the counter mechanism generates an electronic signal to activate the locking mechanism. In these embodiments, once the predetermined number is reached, an electronic signal is sent to the locking mechanism, which shifts into a locked position upon receipt of the signal. In some embodiments, the counter mechanism also may generate an electronic signal to activate shifting of an inner tubular member, such as a sleeve. In these embodiments, the sleeve would not be activated by an internal pressure within the tubular string. 
         [0104]    A manual setting mechanism  170  allows the counter mechanism  134  to be incremented or decremented manually through buttons or levers, thereby allowing the counter mechanism  134  to be preset to a predetermined number. As discussed above, an electronic setting mechanism may be provided that allows an operator to remotely set the counter to a predetermined number. Accordingly, the counter mechanism  134  is settable such that each tool  126  in the tubular production string will have a unique number and will lock out only after the proper numbers of balls have passed by it. The counter assembly  132  also includes a counter spring  146  that interconnects with the rocker mechanism  142  and restrains rotation of the rocker mechanism  142 . The counter spring  146  is configured to prevent the counter assembly  132  from counting when fracing fluid with or without proppant is passed through the downhole tool under typical fracing conditions. Accordingly, the counter spring  146  ensures that the rocker mechanism  142  will rotate only under the force of a drop member  78  seated on the catch mechanism. The counter spring  146  is illustrated as a linear spring; however, in some embodiments the counter spring  146  may be a torsion spring disposed on the shaft of the rocker mechanism  142 . 
         [0105]    As depicted in  FIGS. 16-17 , the counter assembly  132  incorporates a plurality of gears  178  and a plurality of counter wheels  180  to enable counting to a predetermined number, which in turn facilitates engagement of the locking mechanism  138 . The counter mechanism  134  may incorporate geneva gears or other incrementing/decrementing gears to facilitate proper counting. For example, the device may have a gear for 1&#39;s, 10&#39;s and 100&#39;s places and may use geneva gears or other incrementing gears to facilitate proper counting between these places. 
         [0106]    As previously mentioned, the design of the counter assembly  132  may vary without departing from the scope of present disclosure. For example, in one embodiment, the counter is a disk that rotates to release the ball. In another embodiment, a button or section of the wall may move in the radial direction to allow the ball to pass and decrement the counter. As a further example, instead of utilizing a catch mechanism interconnected with a rocker mechanism  142 , the catch mechanism could translate in and out of the inner bore of the tubular production string to actuate a click counter. In this configuration, the motion of the protrusion  158  would be orthogonal to the central axis of the tubular production string. The orthogonal motion would actuate the counter mechanism  134  in a similar fashion as a hand-held clicker. Once the predetermined number is reached, the counter mechanism  134  would activate the locking mechanism  138  to prevent orthogonal movement of the protrusion. In this example, the protrusion  158  may have sloped surfaces to enable a drop member to force the protrusion  158  into the chamber  162  and to pass by the protrusion  158 . 
         [0107]      FIGS. 18-19  depict an embodiment of the locking mechanism  138 . In  FIGS. 18-19 , a trip pin  174  is disposed toward a lower, or downstream, end of the downhole tool  126 . Accordingly, during normal flow, the direction of fluid flow is from left to right in  FIGS. 18-19 . Referring to  FIG. 18 , the locking mechanism  138  is in a clockwise lock position. As illustrated, a sliding lock  186  prevents an inner shaft  182  of the rocker mechanism  142  from rotating clockwise, but allows the inner shaft  182  to rotate counterclockwise. A compression spring  62  biases the sliding lock  186  against a trip pin  174  and is disposed between the sliding lock  186  and an anchor  190  that is interconnected with the sleeve  130 . As shown in  FIG. 17 , the trip pin  174  is interconnected with the counter mechanism  134 . Once a predetermined number of drop members passes by the counter assembly  132 , the counter mechanism  134  pulls the pin  174 . Accordingly, in the clockwise lock position, the locking mechanism  138  allows drop members, such as balls, to pass by the counter assembly  132  to distal portions of the tubular production string. However, the locking mechanism  138  prevents drop members from passing by the counter assembly  132  in a reverse direction toward the surface. 
         [0108]    Referring to  FIG. 19 , the trip pin  174  has been pulled by the counter mechanism  134 . As shown, the compression spring  62  has shifted the sliding lock  186  into a counterclockwise lock position. In this position, the sliding lock  186  prevents the inner shaft  182  from rotating counterclockwise, but allows the inner shaft to rotate clockwise. The compression spring  62  maintains the sliding lock  186  in this counterclockwise lock position. By preventing counterclockwise rotation, the lock mechanism  138  prevents drop members from passing to downstream portions of the tubular production string. Thus, once the lock mechanism  138  is in this lock position, a subsequent drop member will seat on the protrusion  158  and substantially seal the inner bore of the tubular production string. Internal pressure will build in the inner bore of the tubular production string, thus shifting the sleeve  130  associated with the counterclockwise locked counter assembly  132  into a shifted position. Accordingly, in the counterclockwise lock position, the locking mechanism  138  allows drop members, such as balls, to pass by the counter assembly  132  toward the surface. However, the locking mechanism  138  prevents drop members from passing by the counter assembly  132  to distal portions of the tubular production string. 
         [0109]      FIGS. 20-21  depict a counter assembly according to another embodiment of the present invention wherein the counter assembly utilizes a button or section of the sleeve wall to allow a ball to pass and decrement the counter. In general,  FIGS. 20-21  illustrate a linear actuation method of incrementing/decrementing a counter. Referring to  FIGS. 20-21 , treatment fluid  192  is flowing toward distal portions of the tubular string. A button  194  has sloped surfaces and extends into an internal bore of a sleeve  130 . The button  194  is interconnected to a rack  196 , which is configured to intermesh with a gear  198  to increment/decrement a counter. The gear  198  may be, for example, a counter gear or a worm gear that is interconnected with a counter mechanism. A sliding lock  186  is interconnected with a spring  62 , an anchor  190 , and is in mechanical or electrical communication with a counter mechanism. Once a predetermined number of balls have passed by the button  194 , the counter mechanism will activate the sliding lock  186  to prevent subsequent balls from passing by the button  194 . As shown in  FIG. 20 , a drop member  78  has contacted the button  194 . The sliding lock  186  is not engaged, and thus the ball may depress the button in a direction orthogonal to the fluid flow  192  and continue flowing toward distal portions of the tubular string. Referring to  FIG. 21 , the drop member  78  has depressed the button  194  into the body of the sleeve  130 , and the rack  196  has engaged the gear  198 , thereby causing the gear  198  to rotate. The rotation of the gear  198  causes the counter mechanism to increment/decrement the running count number. 
         [0110]    According to at least one embodiment of the present invention, a method is provided that selectively stimulates stages using a single-sized ball. Following the stimulation of a stage, a ball is dropped into the well and pumped down the center of the tubular production string. The ball passes through each downhole tool  126  in the system under the force of the fluid pressure. Because of the diameter of the inner bore of the tubular production string, the ball may pass through a downhole tool  126  only if it decrements a counter. In one embodiment, the counter is a disk that rotates to release the ball. In another embodiment, a button or section of the wall may move in the radial direction to allow the ball to pass and decrement the counter. When the counter reaches zero, a lock is engaged and the counter will no longer allow the ball to pass through the downhole tool  126 . With the ball prevented from passing, the flow through the tubular is greatly restricted and a pressure differential will be created. This pressure differential will create sufficient force to move the sleeve from a non-shifted position to a shifted position. The downhole tool may or may not incorporate shear pins to ensure that the sleeve only shifts when a predetermined force is applied. In the shifted position, the ball remains held by the locked counter and provides sufficient flow restriction to divert the bulk of the flow to radial openings in the tubular production string and for the stage to be fraced. Alternatively, the shifting mechanism may activate a flapper device to seal the axial bore of the tubular production string. 
         [0111]    While in the non-shifted position, the downhole tool  126  will not allow balls to pass in the reverse direction. However, fluid will be allowed to pass by the ball relatively unimpeded because of the design of the tubular region. This feature allows the completions engineers to flow back in the event of a screen-out, but not accidently flow back beyond the next downhole tool. If this were to happen each ball would then decrement the counter as soon as fracing operations resumed and the sleeves would shift too soon. By preventing the ball from returning while in the downhole tool is in a non-shifted position, counting integrity is preserved. While in the shifted position, the reverse flow lock is removed and the downhole tool will allow relatively unrestricted flow of the balls through the downhole tool  126 . 
         [0112]    The axial bore of the downhole tool may also allow passage of solid elements, such as wireline tools, tubing, coiled tubing conveyed tools, cementing plugs, balls, darts, and any other elements known in the art. When all of the stages have been fraced, the pressure is reduced and the flow reverses direction. In this flow back mode, the balls will pass back by the counter with very little resistance. 
         [0113]      FIGS. 22-23  illustrate another embodiment wherein the flapper valve  30  is used as a whipstock slide. According to this embodiment, the flapper valve  30  is longer in one axis than in another, such that the flapper valve  30  forms a slide when in the second position. The angled flapper valve  30  assists the placement and extraction of tools through the lateral bore  22  of the downhole tool  2 . It is feasible that the lateral bore  22  of the downhole tool  2  may be filled with a fill material  206 , such as soft cast iron, cement, etc. that may need to be removed with a drilling apparatus or by chemical treatment. Additionally, an orienting key may be associated with the flapper valve  30  to orient and guide tools to the lateral bore  22  of the downhole tool  2 . In some embodiments, the orienting key is a separate member that is landed in a crowsfoot associated with the flapper valve  30 . The flapper valve  30  is restrained in its first position by a sleeve  34 , which is held in place by shear pins  50 . The flapper valve  30  may be held in place by other mechanisms described herein. 
         [0114]    Referring to  FIG. 23 , the sleeve  34  has been displaced vertically within the tubular string  14  by a shifting tool thereby allowing the flapper valve  30  to move from its first position to its second position. The shifting tool may be operated by wireline, slickline, coiled tubing, or jointed pipe as appreciated by one skilled in the art. A hinge  58  interconnects the lower end of the flapper valve  30  to the downhole tool and allows the flapper valve  30  to rotate. A torsion spring  58  biases the flapper valve  30  towards its second position. Another spring  62  may be provided to assist the movement of the flapper valve  30  from its first position to its second position. 
         [0115]      FIGS. 24-28  illustrate yet another embodiment wherein a downhole tool  2  is utilized to prevent a well blowout. According to this embodiment, an inner tubular member  210  is operably interconnected to the axial bore of the downhole tool  2  by shear pins  50  or other connecting means known in the art. Additionally, a sealing element  214  may be placed around the inner tubular member  210  to provide a seal between the inner tubular member  210  and the downhole tool  2 . The sealing element  214  may be elastomeric, plastic, metallic, or any other sealing elements known to one of ordinary skill in the art. The inner tubular member  210  restricts the movement of the flapper valve  30  and holds the flapper valve  30  in its first position. The upper portion of the inner tubular member  210  forms a chamber that houses a ball  218 . The chamber is also defined by a ball seat  222  and a ball cage  226 . 
         [0116]      FIG. 24  shows a condition where fluid is flowing down the tubular string  14 . As shown, the fluid flows into the inner bore of the downhole tool  2  and further into the inner tubular member  210  via a ball seat  222  and orifices  230 . The fluid flow and pressure forces the ball  218  to contact the ball cage  226 , which prevents the ball  218  from moving distally into the tubular string  14 . As illustrated, fluid flows around the ball  218  without unduly restricting the fluid flow. In this embodiment, the inner tubular member  210  is held in place within the downhole tool  2  by shear pins  50 . The annulus formed between the inner tubular member  210  and the downhole tool  2  is sealed by an o-ring  214  or other sealing elements commonly used in the art. As shown in  FIGS. 24-25 , three sets of vertically displaced shear pins  50  and o-rings  214  are utilized. As will be appreciated by one of skill in the art, the number of shear pins and sealing elements may vary. 
         [0117]    Referring to  FIG. 25 , as fluid flows up the internal bore of the tubular string  14 , it enters the downhole tool  2  and the inner bore of the inner tubular member  210 . The fluid flow and pressure causes the ball  218  to seat in the ball seat  222 , thus restricting the fluid flow through the inner tubular member  210  by redirecting the fluid flow through orifices  230  in the inner tubular member  210 . 
         [0118]      FIG. 26  shows an increased fluid flow associated by a well blowout that is represented by the dark arrows. The increased fluid flow flows through the orifices  230 , but in a restricted manner, which creates an upward force on the inner tubular member  210 . 
         [0119]    In  FIG. 27 , the increased fluid flow caused by the well blowout has sheared the shear pins  50  and thus the inner tubular member  210  has shifted upward in the downhole tool  2 . The upward movement frees the distal flapper valve  30 , which allows it to close the axial bore of the downhole tool  2 . The momentum of the fluid flow and the inner tubular member  210  causes the inner tubular member  210  to continue moving up the tubular string  14 , thus allowing a second proximal flapper valve  30  to close. The flapper valves  30  prevent fluid from flowing up the axial bore of the downhole tool  2 , thereby preventing the well blowout. As will be appreciated by one of skill in the art, more or less than two flapper valves  30  may be used without departing from the scope of the invention. 
         [0120]      FIGS. 29-32  illustrate a downhole tool  2  actuated by a drop member and catch/release mechanism  250  according to yet another embodiment of the invention. The downhole tool  2  allows access to the annulus of a tubular string placed in a wellbore via a unique valve with two stationary positions with separate bores. One bore of the valve is open to the interior of the tubular to which the valve is attached; the second bore may create a passageway to the annulus of the tubular string by use of a drop member. The inner tubular, when in the initial or first position, has sealing elements (e.g. elastomeric, plastic, metallic) that seal the space between the inner and outer tubular members. The seal allows fluids, such as drilling mud, cement, and fracturing fluids, to be effectively pumped through the bore of the tool with minimal or no leakage to the annulus. The sealing may be enhanced by use of elastomers, O-rings, softer metals or other techniques customary in downhole tools. The tool may also be constructed of metallic or non-metallic materials, such as the composite materials currently used in composite downhole tools. In one embodiment, the tool  2  is connected to the tubular string  14  by a threaded connection. 
         [0121]    The downhole tool  2  comprises an inner tubular member  210 , outer tubular member  212 , catch/release mechanism  250 , locking mechanism  260  and locking dog  270 . The downhole tool  2  is positioned such that the outer tubular member aligns with fracture ports  26 . Inner tubular member  210  is slidable relative to outer tubular member  212 . Stated another way, inner tubular member  210  may be actuated relative to outer tubular member  212 . The inner tubular member  210  engages with piston  240 . As the inner tubular member  210  moves downward, or distally, relative to outer tubular member  212 , the piston  240  compresses the spring  90  in communication with the upper reservoir  94  and the lower reservoir  98 . The catch/release mechanism  250  comprises collet fingers  252  and is dimensioned with major inner diameter  254  and minor inner diameter  256 . The ball  218  moves through axial bore  18  as a result of differential pressure on the upstream and downstream pressure on the back to engage the catch/release mechanism  250 . 
         [0122]    As will be discussed below, depending on the pressure applied within the axial bore  18 , the ball  218  may engage the catch/release mechanism  250  until the catch/release mechanism  250  moves distally, or downwards, within axial bore  18  so as to engage locking mechanism  260 , or alternatively, may momentarily engage catch/release mechanism  250  without catch/release mechanism  250  engaging locking mechanism  250 . Such alternatives allow the ball  218  to either draw the inner tubular member  210  distally or downward so as to create an opening  26  to axial bore  18 , or instead pass through catch/release mechanism  250  without creating such an opening. Thus the internal pressure within the axial bone can be used to selectively open the fracture ports  26  to allow fluid communication to the annulus of the wellbore. 
         [0123]    Referring to  FIGS. 29A-C , a downhole tool  2  interconnected to a tubular string  14  within a wellbore is depicted. The downhole tool  2  is shown in an unactuated state under a low axial bore pressure.  FIG. 29A  is a cross-sectional view of the downhole tool  2 ,  FIG. 29B  is a cross-sectional top view of section A-A of the catch/release mechanism  250  of the embodiment of  FIG. 29A , and  FIG. 29C  is a detailed cross-sectional side view of portion A of the catch/release mechanism  250  of the embodiment of  FIG. 29A . In the configuration of  FIGS. 29A-C , the downhole tool  2  has been positioned in a tubular string  14 , a ball  218  pumped into the axial bore  18 , with a generally pre-determined lower pressure  86  applied. The drop member ball  14  descends distally within axial bore  18  toward catch/release mechanism  250 . The inner tubular member  210  does not appreciably move relative to the outer tubular member  212  and remains in an unactuated state (deemed position one or a first position). As the ball  218  descends within the axial bore  18 , the ball  218  engages and lands in the catch/release mechanism  250  at collet fingers  252  and draws both catch/release mechanism  250  downward and inner tubular member  210  downward, until catch/release mechanism  250  engages locking mechanism  260  as depicted in  FIGS. 30A-B . The wellbore is thus sealed above the drop member ball  218  from the distal portions of the wellbore. It is important to note that under lower pressure  86 , ball  218  engages collet fingers  252  and pulls catch/release mechanism  250  downward, without axially-spreading collet fingers enough to effect passing through collet fingers  252 . 
         [0124]    When the lower pressure  86  is held in wellbore, it is below that necessary for the drop member ball  218  to disengage from and pass through the tool  2  to travel to any subsequent tool  2  distal from the first tool. The drop member ball  218 , when held in the tool at the lower pressure  86 , causes the inner tubular member  210  to move from a first position to a second position over a period of time, in a similar manner to the activation cylinder of a hydraulic jar. The activation cylinder, comprising an upper reservoir  94  and lower reservoir  98  of hydraulic oil or similar fluid, bleeds through a fluid communication means, such as a connecting aperture or around the activation cylinder, to allow the cylinder to move over a period of time from the first position to the second position, allowing the inner tubular member  210  to move from the initial (first, unactuated) position to the second (actuated) position. 
         [0125]      FIG. 30A  is a cross-sectional view of the downhole tool shown in  FIG. 29A  in an actuated state under a low axial bore pressure and  FIG. 30B  is a detailed cross-sectional side view of portion A of the downhole tool of  FIG. 30A . As depicted in  FIGS. 30A-B , the inner tubular member  210  has moved downward or distally so as to create an aperture in tubular string  14  at fracture ports  26 , thereby enabling stimulation fluid  38  to flow from axial bore  18  into a hydrocarbon formation adjacent fracture ports  26 . That is, the wellbore is open to the annulus of the tool  2 . Furthermore, inner tubular member  210  has moved downward or distally so as to activate locking dogs  270 , thereby preventing the inner tubular member  210  from moving upwards or proximally up tubular string  14  and closing fracture ports  26 . Locking dogs  270  have an unactuated profile within the inner tubular member  210 . One example of locking dogs known to those skilled in the art, is provided in U.S. Pat. No. 4,437,55 to Krause, Jr., which is hereby incorporated by reference in its entirety. In this configuration (i.e. when an aperture or flow channel has been created at fracture ports  26  so as to allow stimulation fluid  38  egress from axial bore  18 ), the inner tubular member  210  is in an actuated state deemed position two or a second position. 
         [0126]    Note that the further downward movement of the inner tubular member  210  from the second position, and passing of the drop member ball  218 , will be prevented given the change in profile of the stationary portion of the tool  2 . That is, the application of a higher pressure within axial bore  18  with the drop ball  218  in place will not cause the drop ball  218  to pass, since the change in profile as provided by the wedge shaped locking mechanism  260  will prevent the radial deformation of the collet fingers  252  and therefore prevent the passing of the drop member ball  218 . In fact, a higher pressure will cause the collet fingers  252  with the trapped drop member ball  218  to more tightly wedge into the change in profile. Note that inner diameter  256  of catch/release mechanism  250  is smaller than ball  218  diameter, thus prevents the drop ball  218  from traveling downhole from the wedge shaped locking mechanism  260 . 
         [0127]    The tool  2  has an internal bore that allows wellbore fluid to be pumped through the tool  2 , and also may allow physical passage of solid elements, such as wireline or slickline tools, tubing and coiled tubing conveyed tools, and drop elements, such as cementing plugs, balls and darts, which can pass through the tool when the tool is in the initial closed position. When the tool  2  is in the second (actuated) position, the bore of the tubing is effectively sealed, and fluid pumped into the wellbore is directed to the annulus of the tubular string, through the bore previously closed by the inner tubular member  210 . If the device  2  is used with external tubular packers, such as external casing packers, swellable packers or similar devices, it is not anticipated that cement will be on the external portion of the tool. If cement is contemplated to be placed around the tool  2  and hardened, it may be necessary to place an external cover, outside of the tool  2  in the initial position, to prevent the cement from interfering with the movement of the inner tubular member  210  to the second position. Such an external cover would be removed or deformed by the fluid pumped through the second bore. It also may be desired to pump acid or other fluid (including abrasive particle laden fluids) through the opening created by the movement of the inner tubular member  210  to the second position to remove debris and/or the cement from the annulus and improve a fracturing operation. 
         [0128]    If the tool  2  is activated with drop members from the surface as described above, it may be desirable to have a multi-pressure activation system. For example if the tool is to be deployed in a horizontal well that is to be fracture stimulated with multiple stages (See  FIG. 5 ), using multiple tools that are connected to and part of the tubular string in the wellbore, it would be desirable to have a tool that would be actuated by an applied first pressure exerted at the surface of the wellbore when the drop member lands and seals in the tool and have a second pressure at which the drop member passes through that tool without actuation, and continues to pass through each tool set distally in the wellbore until the desired tool is reached, and which would be selectively activated to allow a fracture stimulation to be pumped into the annulus of the tubular at a pre-determined location, as described below. 
         [0129]    When a sufficiently higher pressure  74  (relative to the lower pressure  86  described above) is applied to the downhole tool  2  and a ball  218  inserted into axial bore  18 , the downhole tool  2  operates in an alternative manner, as depicted in  FIGS. 31A-D .  FIG. 31A  is a cross-sectional view of the downhole tool shown in  FIG. 29A  yet under a high axial bore pressure.  FIG. 31B  is a cross-sectional top view of section A-A of the downhole tool of  FIG. 31A ,  FIG. 31C  is a detailed cross-sectional side view of portion A of the downhole tool of  FIG. 31A , and  FIG. 31D  is a cross-sectional view of the downhole tool of  FIG. 31A  under a high axial bore pressure after passage of the ball drop member. 
         [0130]    In the configuration depicted in  FIGS. 31A-C , the downhole tool  2  has been positioned in a tubular string  14 , a ball  218  inserted into the axial bore  18 , and a higher internal wellbore pressure  74  applied. A ball  218  has descended distally within the axial bore  18  toward the catch/release mechanism  250 . The inner tubular member  210  has slightly moved relative to the outer tubular member  212 , although not enough to open fracture port  26 . Under the higher internal wellbore pressure  74 , the ball  218  has engaged the catch/release mechanism  250  at collet fingers  252  and slightly drawn both the catch/release mechanism  250  and the inner tubular member  210  downward. However, in contrast to the operation of the downhole tool  2  under the lower pressure  86  operation as discussed above, under the higher internal wellbore pressure  74  the ball  218  engages collet fingers  252  and, while pulling catch/release mechanism  250  downward (and with it inner tubular member  210 ), the ball  218  axially-spreads collet fingers enough so as to pass through collet fingers  252 .  FIG. 31C  depicts the ball  218  under high pressure  74  as collet fingers  210  spread, such that the inner diameter  256  of the catch/release mechanism  250  is equal to the ball  218  diameter.  FIG. 31D  depicts downhole tool  2  after ball  218 , under high pressure  74 , has passed through catch/release mechanism  250 . In this state, the inner tubular member  210 , as engaged with piston  240  and actuation cylinder, is urged vertically upwards by spring force  92  so as to return to its first (unactuated) state. The actuation cylinder, and thus the inner tubular member  210 , is returned to the initial first position by any stored energy device including a spring  90 , stored hydraulic energy, etc. Note that a ball  218  which passes through tool  2  as described herein may subsequently travel through tubular string to another tool  2  or to the distal portion of the wellbore as necessary to complete the wellbore operation. 
         [0131]    The spring  90  and actuation cylinder also function to prevent premature deployment of the tool  2  resulting from the friction of fluid being pumped through the tool  2  and resulting higher wellbore pressure. Other embodiments employ alternative means to allow controlled passing of a drop ball  218 , to include collet slidable devices (e.g. U.S. Pat. Nos. 5,244,044, 4,729,432, 7,373,974, each incorporated by reference in their entirety), collet deformable fingers such as those described above (and also, e.g. U.S. Pat. Nos. 4,893,678, 4,823,882, 4,292,988, each incorporated by reference in their entirety) and other ball release mechanisms known to those skilled in the art. 
         [0132]    In another embodiment, the tool  2  could be configured to allow the return of drop members to the surface by placing an inclined surface on the distal portion of the inner tubular member  210 , allowing the drop members to move from tools deployed in the distal portions of the wellbore, back through the tools and returning to the surface. This would be accomplished in a similar manner to the drop members passing tools during stimulation operations, but in the opposite direction. The drop member would contact the inner tubular assembly from the distal end, and push the inner tubular assembly a small distance to engage the locking dogs. This small axial movement will allow the radial deformation of the collet fingers by a buildup of pressure on the drop member from the formations previously stimulated. The drop members could be composed fully or partially of a dissolvable material, such as described in U.S. Patent Appl. Publ. No. 2011/0132621, which is hereby incorporated by reference in its entirety, using nanotechnology, or other materials, such as a magnesium alloy, that will either result in the total dissolution of the drop member or cause a reduction in the ball size to allow the drop members to pass through the tools and back to the wellhead. 
         [0133]    Once a ball  218  has passed through downhole tool  2  via catch/release mechanism  250 , it may be returned as depicted in  FIGS. 32A-B .  FIG. 32A  is a cross-sectional view of a downhole tool in an unactuated state under a high axial bore pressure during retrieval of the drop member, and  FIG. 32B  is a detailed cross-sectional elevation view of portion A of the downhole tool shown in  FIG. 32A . To return ball  218 , a high internal wellbore pressure  74  is provided to axial bore  18  such that ball  218  engages the far or distal side of collet fingers  210 , as shown by ball position  218   A . With enough internal wellbore pressure the ball  218  will spread collet fingers  210  as depicted in  FIG. 32B  so as to allow passage vertically up the axial bore  18  of the tubing string, as shown by ball  218  at ball position  218   B . Note that although ball  218  is returned, inner tubular member  212  remains actuated, as depicted in  FIG. 32A , because of extended locking dogs  270 . Should the locking dogs  270  be configured for remote actuation or deactuation, the locking dogs  270  could be retracted, in which case inner tubular member  270  would ascend vertically or proximally so as to close fracture ports  26 . 
         [0134]    In one embodiment, the drop ball  218  is other than substantially round. For example, the drop ball  218  may be oblong spherical, bullet shaped, conical shaped, egg-shaped, or any shape that enables the functions herein described. 
         [0135]    Conventional drop members, such as non-metallic frac balls may also have a reduction in size due to the erosive nature of the wellbore fluids being produced through the tool. Even if the frac ball does not open the collet fingers fully to allow the full sized ball to pass and be recovered at the surface, it will cause some radial movement of the fingers, opening a small aperture that will pass wellbore fluid at high velocity. It is well known to one of ordinary skill in the art that small apertures leaking high velocity fluids may quickly become eroded and using a relatively soft non-metallic frac ball will enhance this phenomena to erode the outer diameter of the frac ball, to allow passage through the tool. 
         [0136]    Another method to handle the balls during flowback and production would be to extend several of the collet fingers, but not all, so that the balls would be prevented from plugging the tool during production, and that there would be significant flow area around the ball through the spaces of the collet fingers that were not extended, such that all production would bypass the ball and not cause a production shortfall due to plugging of the tools by the balls during flowback and production. Another means to return a ball include the use of a ball with a dissolvable outer layer which dissolves over time to create a smaller diameter ball which may pass through a catch/release mechanism. 
         [0137]    While various embodiments of the present invention have been described in detail, it is apparent that modifications and alterations of those embodiments will occur to those skilled in the art. Moreover, references made herein to “the present invention” or aspects thereof should be understood to mean certain embodiments of the present invention and should not necessarily be construed as limiting all embodiments to a particular description. However, it is to be expressly understood that modifications and alterations are within the scope and spirit of the present invention, as set forth in the following claims.