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FIELD OF INVENTION 
       [0001]    The present invention relates to devices for multi-stage, horizontal well isolation and fracturing. 
       BACKGROUND OF THE INVENTION 
       [0002]    An important challenge in oil and gas well production is accessing hydrocarbons that are locked in formation and not readily flowing. In such cases, treatment or stimulation of the formation is necessary to fracture the formation and provide passage of hydrocarbons to the wellbore, from where they can be brought to the surface and produced. 
         [0003]    Fracturing of formations via horizontal wellbores traditionally involves pumping a stimulant fluid through either a cased or open hole section of the wellbore and into the formation to fracture the formation and produce hydrocarbons therefrom. 
         [0004]    In many cases, multiple sections of the formation are desirably fractured either simultaneously or in stages. Tubular strings for the fracing of multiple stages of a formation typically include one or more fracing tools separated by one or more packers. 
         [0005]    In some circumstances frac systems are deployed in cased wellbores, in which case perforations are provided in the cemented in system to allow stimulation fluids to travel through the fracing tool and the perforated cemented casing to stimulate the formation beyond. In other cases, fracing is conducted in uncased, open holes. 
         [0006]    In the case of multistage fracing, multiple frac valve tools are used in a sequential order to frac sections of the formation, typically starting at a toe end of the wellbore and moving progressively towards a heel end of the wellbore. It is crucial that the frac valves be triggered in the desired order and that they do not open earlier than desired. Once open, it is also important that the frac valves do not become closed until it is desirable to close them. 
         [0007]    Many configurations have been developed in the field to frac multiple stages of a formation. For example fracing tools are known in which a ball is pumped into the tool and sits in a seat to block fluid flow through the central bore, thereby causing fluid pressure to build up and forcing fluid to flow through multiple jet nozzles located circumferentially around the liner. 
         [0008]    Other frac valve tools are known for use with coiled tubing, in which a ball is dropped to block flow down the liner and redirect flow through pressure firing heads in a fracking sleeve. Some downhole tools teach including a packer with a ball seat and a ball, in which fluid can be redirected to fracking ports on a fracing tool. Others teach the use of balls of different sizes to control downhole surge pressure. 
         [0009]    A need still however exists for frac valve tools that are simple in construction, small in size and effective at fracing formations in multiple stages 
       SUMMARY OF THE INVENTION 
       [0010]    In a first aspect, a frac valve tool is taught, said tool comprising one or more ports a sleeve movable between a closed position in which the sleeve prevents fluid flow through said one or more ports and an open position in which the sleeve allows fluid flow through said one or more ports and a ball receiving seat removably connected to the sleeve wherein receipt of a ball on the ball receiving seat moves said seat and said sleeve from closed to open positions. 
         [0011]    In a second aspect, a frac valve tool is taught, said tool comprising a ball receiving seat removably received within said tool, said seat comprising a seating profile for receiving a ball; wherein said seating profile matches a radius of said ball to nondeformably grip said ball. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0012]      FIG. 1  is a schematic diagram of a horizontal well fitted with the tools of the present invention; 
           [0013]      FIG. 2  is a cross sectional view of one example of the frac valve of the present invention in a closed position; 
           [0014]      FIG. 3  is a cross sectional view of one example of the frac valve of the present invention in an opened position; 
           [0015]      FIG. 4  is a cross sectional view of one example of the frac valve of the present invention in an open position with the seat drilled out; 
           [0016]      FIG. 5  is a cross sectional view of one example of the frac valve of the present invention in a closed position with the seat drilled out; and 
           [0017]      FIG. 6  is a cross sectional elevation view of a quality control inspection fixture for use with the frac valve of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0018]    A frac valve tool is provided that improve on existing ball drop, multi-stage, horizontal fracturing tools, by providing increased safety during installation, reduced rig time and greater dependability in fracing multiple stages in a horizontal section of the wellbore. 
         [0019]    By combining both a slim outside diameter and short length, the present frac valve tool eliminates the need for handling pup joints, thereby reducing the rigidity of the liner. These features permit the more flexible, reduced outside diameter tool string to be deployed into the wellbore with greater ease. 
         [0020]    The present frac valve tools can be lifted by hand and hand threaded onto the liner, which is typically gripped at the rig floor, and then a section of upper liner, typically gripped in an elevator or similar device, can lowered onto the frac valve tool and the one piece body of the frac valve tool allows torque to be applied from the upper liner section, through the frac valve tool and into the liner to make up the liner string. 
         [0021]    The present frac valve tool can be deployed with associated tools along a liner and deployed into the open hole section of the wellbore. The present frac valve tools provide a means of stimulating a section of the formation to induce fracturing of the formation and flow of formation fluids. The short length of the frac valve tool  400  eliminates the need for pup joints on either end. The small outside diameter and short length increases liner flexibility, further aiding deployment into the wellbore. In a preferred embodiment, the present frac valve tool  400  eliminates the typical threaded connection between the top of the tool and the mandrel. Instead, a box end connection and the mandrel are integral and an installation tool is utilized to insert the frac valve tool  400  inside the mandrel. The use of the special installation tool permits the elimination of a threaded connection thereby shortening the frac valve tool length. 
         [0022]    With reference to  FIG. 1 , in a preferred method of deployment, the present frac valve tools can be deployed on a tubing string further comprising a float shoe or guide  50  at the toe of the liner, an activation tool  100  at a pre-determined distance from the guide shoe  50 , a first stage frac valve tool  200 , and then an series comprising an open hole packer  300  alternated with the present frac valve tools  400  to a final cased hole packer  500 . It would be well understood by a person of skill in the art that  FIG. 1  merely represents one example of a tubular fracing string of tools and that additions, omissions and alterations to the illustrated string and its components can be made without departing from the scope of the present invention. 
         [0023]    The present frac valve  400  is located in the liner between two open hole packers  300  and is depicted in  FIGS. 2 ,  3 ,  4 ,  5  and  6 . The frac valve  400  comprises a mandrel  420  that is preferably full bore and has an inside diameter matching the inside diameter of the liner. One or more ports  410  are formed around the circumference of the mandrel, said ports  410  providing fluid communication between the inside of the liner and the open hole wellbore. The mandrel  420  contains within it a sleeve  408  connected to the mandrel by one or more shear screws  406 . In a closed position, the sleeve  408  blocks fluid passage through the one or more ports  410 . Within the sleeve  408  is a seat  404  that can receive a ball  402  that is deployed into the liner from the rig floor and pumped onto the seat  404  by fluid pressure. 
         [0024]    The present frac valve  400  is preferably pressure balanced due to sealing by o-rings that straddle the ports, such that the sleeve  408  is not shifted to the open position until the ball  402  lands on the seat  404 . After the ball  402  is pumped onto the seat  404 , liner pressure generates a force which shears shear screws  406  allowing the seat  404  and sleeve  408  to shift, opening communication through the one or more ports  410 . 
         [0025]    The seat  404  of the present frac valve  400  is preferably surface hardened to prevent erosion that can be caused by proppants pumped through them. The seats  404  are manufactured from a material and in a geometry that can withstand the stress generated by the ball  402  landing and seating under high differential pressure, while providing adequate support for the ball  402 . Suitable materials for the present seat  404  may be most cast irons, including Class 40 Gray Iron or Class 50 Gray Iron, although other suitable materials would be known to a person of skill in the art and are encompassed by the scope of the present invention. The seats are more preferably treated with liquid nitrogen to achieve a Rockwell hardness rating of HRC 50 to 55. 
         [0026]    The present seat preferably comprises a seating profile  416  that receives and in part grips the ball  402  in the seat to ensure the ball  402  is not inadvertently unseated until desired. The seat radius  416  is advantageously designed to allow gripping of the ball  402  without sheering the ball  402  or causing plastic deformation. The present seating profiles requires only low pump off pressure to lift the ball  402  off of the seat  404  by pressure from the formation, after fracturing is complete. This is due to the seating profile  416  being preferably matched to a corresponding ball  402  radius to prevent the ball  402  from deforming and becoming wedged into the seat  404 . 
         [0027]    The relationship between the geometry of the seating profile  416  and the matching ball  402  is preferably designed to permit a variety of ball  402  to seat  404  size ratios for a number of liner applications. The matching geometry of the seating profile  416  and the ball  402  permits a seat  404  of the present design to be adapted for use with many ball and seat sizes, thereby reducing the size increments of seats  404  that need to be manufactured. In a preferred embodiment, the size and geometry of the seating profile  416  can be adjusted relation to the size of ball  402  to be used, this reduces potential hoop stresses that can build up in the ball  402 , and ensure that an optimal relationship between proper seating and low pump off pressure. 
         [0028]    The ball  402  used with the present invention can be any ball well known and used in ball drop tools found in the state of the art. More preferably, the ball  402  is composed of a non-elastomeric material that shows strength, corrosion resistance against stimulant fluids and wellbore fluids and a degree of flexibility. Such materials can include but are not limited to phenolics, composites or aluminum. 
         [0029]    The seat  404  is preferably manufactured with a minimum amount of material to allow the seat  404  to be drilled out after use, thereby minimizing drill out times. In particular, the seat material is designed to be friable and crumble upon drilling, thereby reducing the chance of large drilled out fragments from blocking the liner. 
         [0030]    With reference to  FIG. 4 , the seat  404  of the present frac valve  400  is drilled out after fracturing is complete. The geometry of the seat  404  and the method used to fasten it to the sleeve  408  ensures the seat  404  will drill up into fine particles, eliminating the possibility of large pieces of debris falling onto the next seat  404  to be drilled out. Such debris adds to the time required for the subsequent seat  404  to be drilled out and tends to rotate and grind against the next seat  404 . 
         [0031]    Preferably one or more anti-rotation tabs  414  located inside the frac valve  400  assists seat drill out by holding the seat  404  stationary. More preferably the seat  404  is threaded into the sleeve  408  in such an orientation that drilling out the seat  404  urges the threads into tightening, thereby additionally serving to hold the seat  404  in place in the sleeve  408 . The threads  418  on the seat  404  and on the sleeve  408  are most preferably left hand threads that tend towards tightening when the seat  404  is drilled. These threads  418  also allow the seats  404  of any frac valve tool  400  to be changed as needed, for example should damage be detected in a seat  404 , or should on-site adjustments need to be made for different ball and seat sizes for one or more frac valve tools  400 . 
         [0032]    In a further preferred embodiment, a quality control inspection fixture  700 , illustrated in  FIG. 6  is used to check five dimensional characteristics of each frac valve  400 , to ensure correct placement of each valve in the liner. The quality control fixture  700  checks the bore hole size through the seat  404 , and the bore in which the ball  402  lands. It checks the concentricity of both bores to ensure proper sealing whenever the ball  402  lands on the seat  404 . The quality control inspection fixture  700  checks the geometry of the seat profile  416  and also the distance from the seat  404  to the top of the frac valve  400 , to ensure proper assembly of the frac valve tool  400 . The quality control inspection fixture  700  is preferably attached to a seat installation tool (not shown) to assist in ensuring the correct seat  404  is being installed into the frac valve tool  400 . 
         [0033]    In some cases, the frac valve seat  404  can be drilled out to the drift inside diameter of the liner Drift diameters are specified by the American Petroleum Institute (API) for each weight of casing. An object of a given drift diameter and given length as specified by API must fit through the inside diameter of the pipe. 
         [0034]    Although it is common to run one frac valve  400  per isolated section of the formation, it is also possible to place multiple frac valves  400  in any given isolated section. In a preferred embodiment, the frac valve  400  can be configured to have a closable feature. The closable frac valve  400  can be closed by a number of means. One embodiment permits the frac valve  400  to be closed before drilling out the seats, in this case a shifting tool run on tubing is used to close the frac valve  400 . A second embodiment, illustrated in  FIG. 5 , allows the frac valve  400  to be closed after the seat  404  is drilled out. Multiple frac valves  400  or a single frac valve  400  may be shifted from an open to a closed position with a further second style of shifting tool  412 . 
         [0035]    The total flow area through all of the fracture ports  410  of the frac valve  400  is preferably greater than the flow area through the liner. 
         [0036]    Sometimes a sand off occurs during the fracing operation when no more sand can be pumped into the formation and the sand remains indie the liner preventing the ability to pump the next ball down the well. In such cases, an opening tool (not shown) can be run through the tubing and landed on the seat  404 . In such cases, applied pressure in the annular area between the inside wall of the liner and the outside diameter of the tubing is used to pump the frac valve  400  into the open position. 
         [0037]    In one example of operation of the present frac valve tool  400 , a liner may be assembled with a float shoe  50 , an activation tool  100 , a liner, a first stage frac valve tool  200 , and then a series comprising a liner, an open hole packer  300 , a liner and the present frac valve tools  400 . Optionally, an open hole anchor may be used between the activation tool  100  and the first stage frac valve tool  200  to anchor the liner to the wellbore. Alternative to an open hole anchor centralizers, stabilizers or other suitable means known in the art may also be used for this purpose. 
         [0038]    Preferably up to 40 frac valves  400 , on a 4½″ liner for example, separated with open hole packer  300   s  can be used in a string. In operation, the seats  404  of the frac valve tools  400  sequentially increase in the size of ball  402  they can receive; with the smallest seat  404  being closest the toe end of the wellbore and the largest seat  404  being at the heel end. A cased hole packer  500  is attached to the upper end of the liner. A latch seal assembly or other known means can be used to attach the cased hole packer  500  to the work string. 
         [0039]    The liner is run into the conditioned bore hole by a work string or on a frac string. At a predetermined depth the activation tool  100  is activated to stop fluid flow. Pressure in the liner now increases from a triggering pressure at which both the cased hole packer  500  and the open hole packers  300  begin to set, to a final pack off pressure at which the cased hole packer  500  and open hole packers  300  are fully set. A pressure test may optionally be performed inside the casing to determine if the cased hole packer  500  has set properly. If the liner was run on a work string, the latch seal assembly or other connection means can next be removed from the cased hole packer  500  and the work string and latch seal assembly are removed from the well and a frac string and latch seal assembly are deployed. Otherwise, if the liner was run downhole on a frac string, no replacement has to be made. 
         [0040]    Further pressure is applied to the fracture string. At a pre-determined opening pressure that is higher than the pack off pressure, the first stage frac valve tool  200  shifts to the open position and stimulation fluid is pumped into the formation and causes it to fracture. Proppant is then pumped into the fracture. Next, a first ball  402  is pumped into the liner corresponding to the seat sizes of the frac valve tool  400  closest the toe of the wellbore. By this process the frac valve tool  400  is activated to thereby open ports  410  to allow communication between the inside of the liner and the isolated section of the formation between the two open hole packer  300  straddling the particular frac valve  400 . Subsequent frac valve tools  400  are similarly activated by pumping subsequent balls  402  into the liner in sequential size order. 
         [0041]    The stimulation fluid pumped through the ports of the frac valve  400  fractures the exposed formation between the open hole packers  300  used to isolate that stage. Whenever this stage has been fractured, a next frac valve  400  is activated and the process is repeated. The process can be repeated up to 40 times in total in a 4½″ liner, for example. Other sizes of liners have a different number of frac valve tools  400  and open hole packers  300 . When all the desired stages have been fractured, the well is allowed to flow and formation pressure from formation fluid flow acts to deactivate the frac valves  400  by pumping balls  402  off of the seats  404 , and allows formation fluid flow into the liner. Afterwards the frac string and connecting means can be removed from the well. 
         [0042]    If desired, the seats of the frac valves  400  can be drilled out at a later date. 
         [0043]    In the foregoing specification, the invention has been described with specific embodiments thereof; however, it will be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention.

Summary:
In a first aspect, a frac valve tool is taught, said tool comprising one or more ports a sleeve movable between a closed position in which the sleeve prevents fluid flow through said one or more ports and an open position in which the sleeve allows fluid flow through said one or more ports and a ball receiving seat removably connected to the sleeve wherein receipt of a ball on the ball receiving seat moves said seat and said sleeve from closed to open positions. In a second aspect, a frac valve tool is taught, said tool comprising a ball receiving seat removably received within said tool, said seat comprising a seating profile for receiving a ball; wherein said seating profile matches a radius of said ball to nondeformably grip said ball.