Patent Publication Number: US-9428988-B2

Title: Hydrocarbon well and technique for perforating casing toe

Description:
This invention relates to a hydrocarbon well and a technique of fracing the well without having to make an initial coiled tubing run. 
     BACKGROUND OF THE INVENTION 
     There has been a paradigm shift in the oil and gas industry with the advent of drilling wells having a horizontal leg and then conducting multiple frac jobs along the horizontal leg. This has allowed the development of oil and gas bearing formations which were heretofore uneconomic. For some years, the fastest growing segment of hydrocarbon production in the United States has been from shales or very silty zones that previously have not been economic. The current areas of increasing activity include the Barnett Shale, the Haynesville Shale, the Fayetteville Shale, the Marcellus Shale, the Eagle Ford Shale and other shale or shaley formations. It is no exaggeration to say that the future of oil and gas production in the United States involves drilling horizontal wells and then conducting multiple frac jobs in the horizontal leg of the well. Any development that improves the efficiency or lowers the cost of drilling and completing multi-fraced horizontal wells is important to the production of oil and gas. 
     One standard technique for completing a horizontal well is to run a string of casing into the horizontal leg of the well and then cement the casing in place by pumping a first cementing plug through the casing followed by cement followed by a second cementing plug. Drilling mud in the annulus between the well bore and the casing is displaced and flows toward the surface and ultimately discharges into surface equipment. The first cementing plug ultimately lands in float equipment near the end of the casing string whereupon cement flows out of the casing, through passages in the float equipment and then flows in the annulus toward the surface. When the second cementing plug reaches the float equipment, it latches in place and prevents back flow of cement into the casing. Thus, at the end of a successful cement job, a sheath of cement sets up around the casing string from the float shoe or toe of the casing string toward or beyond the heel where the well bore turns horizontal. 
     Although there are a variety of ways to conduct frac jobs on a well, one standard technique is to perforate the horizontal well near the float shoe by running a perforating gun on coiled tubing to a location adjacent the float shoe, perforate the casing, pull the coiled tubing out of the well and then release the coiled tubing unit. This is called perforating the toe because the end of the casing string is sometimes referred to as the toe. 
     The next step in the standard technique is to pump into the well a perforating gun having a rubber sealing against the inside of the casing and shoot a set of frac perforations near the end of the casing string. It will be seen that it is necessary to perforate the toe because, otherwise, the inside of the casing string is pressure tight and the perforating equipment cannot be pumped into the well because there is no place for the liquid to escape that is in front of the perforating equipment. Thus, a coiled tubing run is necessary to transport a perforating gun to the toe so a set of frac perforations can be shot. 
     A first zone is fraced by pumping a slurry of water, chemicals and proppant through the casing and through the perforations into the hydrocarbon bearing formation. To conduct multiple frac jobs, the casing is perforated in a second location nearer the surface, a bridge plug is placed to isolate the zone that has been fraced and a second frac job is done by pumping frac fluids through the second set of perforations. This process is repeated until the desired number of fracs have been done. 
     It has been proposed to provide a system where the second cementing plug moves a sleeve and opens a port through the float shoe in the process of cementing a horizontal well. This theoretically provides a passage through the casing toe which allows a wire line perforating gun to be pumped into the well so a coiled tubing run is not required. One problem with this system is the cement has not set up when the port is opened, meaning that cement is often found inside the casing above the float equipment. This is a problem because the only known means of removing set up cement inside the casing string is with a bit on the end of a work string, which is usually a coiled tubing unit, meaning that the intended purpose of avoiding a coiled tubing run has been thwarted. In addition, it is not a good idea to remove cement from outside the casing, where it is needed, and let it flow into the casing where it is a problem. 
     Disclosures of interest relative to this invention are found in U.S. Pat. Nos. 2,707,997; 4,673,039; 5,103,911; 5,425,424; 6,047,773; 6,397,950; 7,640,988 and 7,802,627, U.S. Printed Patent Applications 2006/0207763 and 2011/0017453, Canada Patent 2,692,377 and Great Britain Patent 2,452,858. 
     SUMMARY OF THE INVENTION 
     In the disclosed technique, it is recognized that a coiled tubing run to perforate the casing toe can be eliminated. This is important because, at mid-2011 prices, a run-of-the-mill coiled tubing unit in South Texas costs $55,000-75,000/day, there is a one day minimum and is not cheaper elsewhere. Avoiding a $55,000-75,000 cost, at present prices, on every horizontal well is significant. Although the disclosed method and apparatus is particularly suited for horizontal wells, it can be used on vertical wells and thereby avoid the time and expense of a perforating gun run in preparation for fracing operations. 
     To this end, one or more passages are provided in the casing string which are blocked by a soluble plug. These passages can be provided in a sub or short joint provided for this purpose which is installed in the casing string near the toe. In some embodiments, the soluble plug is a water or acid soluble ball that is well known in the art. In these embodiments, the passage can provide a first valve seat cooperating with the soluble ball to provide a check valve preventing flow into the casing string and a second valve seat cooperating with the soluble ball to provide a second check valve preventing flow out of the casing. 
     After the casing string is cemented in the horizontal leg of the well and the second plug is pumped down and latched in float equipment, the rig used to drill the well is normally released so it can be moved to its next drilling location. By the time a completion attempt is made on the well, the cement has set up in the annulus between the well bore and the casing and the soluble plug has dissolved in the liquid used to pump the second cementing plug into the casing. Completion operations can then begin without requiring a coiled tubing run in the case of a horizontal well or a perforating gun run in the case of a vertical well because there are already passages open at a suitable location between the casing and the cement sheath surrounding the casing. In some embodiments, a wire line perforating gun can be pumped into the well to create perforations through which frac fluid can be pumped. In other embodiments, a frac job can be conducted through the passages provided by the soluble plug or plugs. 
     It is accordingly an object of this invention to provide an improved technique for completing a horizontal or vertical well. 
     A more specific object of this invention is to provide the capability of commencing completion operations in a horizontal well without requiring a coiled tubing run. 
     A further object of this invention is to provide an improved well installation and method of completing the well which is simpler, less expensive and quicker than existing approaches. 
     These and other objects and advantages of this invention will become more apparent as this description proceeds, reference being made to the accompanying drawings and appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified cross-sectional view of a horizontal hydrocarbon well illustrating the condition of the well immediately after cementing casing in the well; 
         FIG. 2  is an enlarged longitudinal cross-sectional view of  FIG. 1  illustrating a sub providing passages through its tubular wall that are plugged by a soluble element; 
         FIG. 3  is a transverse cross-sectional view of the sub of  FIG. 2 , taken substantially along line  3 - 3  thereof, as viewed in the direction indicated by the arrows; and 
         FIG. 4  is an enlarged view of the end of a vertical well illustrating slightly different float equipment; 
         FIG. 5  is a view of a well illustrating an early step in conducting a series of fracing operations in the horizontal well of  FIG. 1 ; and 
         FIG. 6  is a view similar to  FIG. 5  illustrating another early step in conducting a series of fracing operations in a horizontal well, certain parts being broken away to disclose a plug. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIGS. 1-3 , a hydrocarbon well  10  includes a bore hole  12  drilled in the earth including a more-or-less vertical leg  14  transitioning through a heel  16  to a horizontal leg  18  which passes into an oil or gas bearing formation  20 . As the bore hole is being drilled, one or more strings of surface pipe (not shown) and/or protection pipe (not shown) may be run and cemented in the bore hole  12 . After the bore hole  12  is drilled, a casing string  22  is run into the well having float equipment at or adjacent an end or toe  26  of the casing string  22 . The float equipment can include a float shoe  24  at the end of the casing string to receive both cementing plugs as shown in  FIG. 1 , can comprise other types of float equipment shown in  FIG. 4  or can be of any other arrangement. 
     A first cementing plug (not shown) followed by cement is pumped down the casing string  22  while drilling fluid, usually known as drilling mud, in the bore hole  12  flows on the outside of the casing  22  toward the heel  16  and ultimately upwardly through the vertical leg  14  to the surface  28  where it is received and handled by suitable surface equipment (not shown). At the end of the cement batch, a second cementing plug (not shown) can be pumped into the casing string  22  to displace cement from inside the casing string  22  so it accumulates on the outside of the casing string  22  in an annulus  30  between the casing string  22  and the bore hole  12 . The cementing plugs can come to rest in the float equipment  24  at or near the end of the casing  22  as shown in  FIG. 1 . The well  10  may include centralizers (not shown), scratchers (not shown) or other cementing accessories in the annulus  30  but the annulus  30  is not blocked so drilling mud and cement are capable of readily flowing from the end of the bore hole  12  to the surface. 
     Horizontal wells are typically completed by hydraulically fracturing the formation  20  to produce a series of vertical fractures spaced along the horizontal leg  18  as suggested by the dashed lines  32 . The fractures  32  can be created in any suitable manner. Conventionally, the completion process begins by rigging up a coiled tubing unit on the well  10 , attaching a wire line perforating gun to the coiled tubing and running the coiled tubing into the casing string  22  until is approaches the casing toe  26  near the float equipment  24 . After perforating the casing string  22  adjacent the toe  26 , the coiled tubing is removed from the well and the coiled tubing unit is rigged down and released. 
     A wire line perforating gun (not shown) or water jet assembly (not shown) is pumped into the casing  22  on a wire line until it reaches a desired location near the end of the casing string  22  where it is used to create a first set of frac perforations through the casing string  22  and the cement sheath  34 . A bridge plug (not shown) may be set above the perforated casing toe  26  to isolate it from frac pressure. Pumping equipment (not shown) is rigged up to deliver a frac fluid through the casing string  22  to create a first fracture or a first set of fractures, depending on whether one interval has been perforated or whether several intervals have been perforated. By setting a bridge plug or other isolation tools between perforations, a series of fractures can be created in the horizontal leg  18 . Those skilled in the art will recognize that the well  10  and the completion technique, as heretofore described, is typical of current techniques for completing a horizontal well. 
     Instead of rigging up and using a coiled tubing unit to create a set of perforations adjacent the casing toe  26 , one or more passages are provided through the tubular wall of the casing string  22  which are sealed by a soluble plug. This may be accomplished by providing passages in a casing joint or collar but it can be done by providing a sub  36  inserted into the casing string  22  as shown in  FIGS. 1-2 . The sub  36  can accordingly comprise a body  38  having a passage  40  therethrough, a threaded pin  42  sized to be received in a collar  44  of a casing joint, float equipment or the like and a threaded box  46  sized to receive a pin end  48  of a casing joint. The sub  36  may be placed at any suitable location adjacent the casing toe  26  and is upstream or nearer the surface  28  than the most upstream float equipment  24  shown in  FIG. 1  or as shown in  FIG. 4 . Although the sub  36  may be at any suitable location, as a practical matter, the sub  36  can typically be less than 150′ from the end of the casing string  22  or less than 100′ from the float equipment  24 . 
     The sub  36  can comprise a series of passages  50  opening into the housing passage  40  and spaced axially and circumferentially around a central portion of the body  38 . The sub  36  may be of any suitable length to provide room for a large number of passages  50 , as desired. The passages  50  can be of any suitable shape to accommodate a soluble plug of any suitable size and shape. It may be preferred, however, that the soluble plug be a ball or sphere  52 . Soluble balls are desirable because they are widely used in oil and gas wells for a variety of purposes, have undergone a considerable amount of development, have become very reliable and potential customers have already become comfortable using them. Soluble balls are commercially available from many different suppliers including Magnum Oil Tools International, LLC, Corpus Christi, Tex. under the name BIOBALLS to which reference is made for a more complete description thereof. These soluble balls are available as water soluble balls and/or acid soluble balls in a variety of operating pressure and temperature ranges. 
     To accommodate the balls  52 , the passages  50  can include a first valve seat  54  acting to prevent flow from the annulus  30  into the housing passage  40 . A retainer plug or cap  56  provides threads  58  to mate with the threaded passage  50  to compress a seal  60  while a valve seat  62  acts with the ball  52  to prevent flow from the housing passage  40  to the annulus  30 . The plug  56  also includes an opening  64  completing a flow path through the plug  56  after the ball  52  dissolves. It will be seen that the soluble ball  52  acts as a valve preventing flow in either direction through the passage  50  while the well is being cemented and for a period that is long enough for the cement to set up. 
     Near the end of cementing the casing string  22  in the bore hole  12 , some technique is used to displace cement from inside the casing string  22  so it accumulates on the outside in the annulus  30 . Typically, a second cementing plug is pumped into the well followed by a completion fluid, which is typically water, 2% KCL water, field salt water, sea water or the like. In any event, the composition of the ball  52  and/or the composition of the completion fluid is selected so the ball  52  slowly dissolves in the completion fluid. The time interval to dissolve the ball  52  is subject to wide variation but is sufficiently long for the cement to set up to an extent where it is no longer flowable before the ball  52  is completely dissolved. This is typically not a problem because it takes at least several days to dismantle and move the drilling rig used to drill the bore hole  12 , assemble the equipment necessary to complete the well and start the completion operation. 
     For a variety of reasons, the completion fluid may include an acid and have a very low pH, typically lower than three. In such an event, the composition of the soluble ball  52  can be selected to be acid soluble. 
     After the casing string  22  has been cemented in the bore hole  12 , the drilling rig (not shown) used to drill the well  10  is released and moved from the location. After a period of time, in which the cement sets up, a completion attempt is made. During this interval, the ball  52  dissolves in the completion fluid thereby opening the passages  50 . When it is desired to commence fracing operations, one of many approaches can be adopted. As shown in  FIG. 5 , if care is taken to position the sub  36  in an area where it is desired to conduct the first frac operation of the formation  20 , the first frac operation can be conducted through the passages  50  simply by pumping a frac fluid down the casing string  22  and out through the passages  50 . 
     As shown in  FIG. 6 , if the sub  36  is located in an area where it is not desired to frac, as where the casing toe  26  is outside the hydrocarbon bearing formation  20 , the formation adjacent the sub  36  can be pumped into so a perforating gun (not shown) can be pumped into the casing string  22  to an location where the first fracture is to be created. In this circumstance, it will be seen that the passages  50  allow liquid in front of the perforating gun to exit the casing  22  thereby allowing the perforating gun to be pumped to the casing toe  26 . After shooting the first set of frac perforations  90  and retrieving the perforating gun with its wire line, a bridge plug or other suitable device  92  can be run into the casing  22  as shown in the broken away portion of  FIG. 6  to isolate the passages  50  thereby directing the frac fluid through the desired frac perforations  90 . In this situation, the first set of frac perforations  90  are nearer the surface  28  than the sub  36 . 
     Horizontal hydrocarbon wells are typically fraced a number of times in the horizontal leg. This is accomplished, in any suitable manner, as by pumping perforating equipment through the casing string  22  to the location where the second fracture is to be created, perforating a second set of frac perforations, isolating the first set of frac perforations with a bridge plug or the like and then pumping a frac fluid through the second set of perforation. Typically, bridge plugs are pumped into the casing string with liquid in front of the bridge plug exiting through the passages  50  or through perforations that will be isolated by the bridge plug. This process is repeated to create as many fractures as desired. Those skilled in the art will recognize this description of frac operations as being typical of current fracing techniques. 
     For example, to create a second set of perforation, after the first perforations are fraced, second wire line perforating equipment can be placed in the casing string and a liquid pumped into the casing string to propel the second wire line perforating equipment toward the casing toe to a location spaced from the first perforations where the well was fraced, liquid in front of the second perforating equipment exiting the well bore through the first perforations. A second set of perforations can be created in the casing string with the second perforating equipment. A second wire line isolation tool can be placed in the casing string and a liquid pumped into the casing string to propel the second isolation tool toward the casing toe to a location between the first and second perforations and setting the second isolation tool to isolate the first perforations from the second perforations. The well can then be fraced through the second perforations. 
     Although the disclosed technique has particular advantages in horizontal wells, it is also suitable for use in vertical wells as suggested in  FIG. 4  where a well  70  includes a more-or-less vertical well bore  72  extending through a hydrocarbon bearing formation  74  and a casing string  76  cemented in the well bore  72  by a cement sheath  78  in an annulus  80  between the casing string  76  and the well bore  72 .  FIG. 4  not only illustrates the disclosed technique in a vertical well, it also illustrates a slightly different type float equipment where a float shoe  82  is located on the bottom of the casing string  76  and a float collar  84  is located one or more casing joints  86  above the float shoe  82 . 
     When the casing string  76  is run into and cemented in the well bore  72 , a sub  88  of the same type shown in  FIG. 2  is located in the casing string  76  above the float collar  84  at a location opposite the formation  74  which is desired to be fraced. When running the casing string  76  into the well bore  72 , care is taken to position the sub  88  opposite the formation  74  to be fraced. After the cement sets up and the soluble plug or plugs in the sub  88  dissolve, a frac job can be conducted without requiring a perforating gun run thereby saving the time and cost to create a first set of frac perforations. 
     It will be seen that many modifications of the disclosed technique may be employed, as by placing the soluble plug in float equipment, such as a float shoe or float collar. 
     Although this invention has been disclosed and described in its preferred forms with a certain degree of particularity, it is understood that the present disclosure of the preferred forms is only by way of example and that numerous changes in the details of operation and in the combination and arrangement of parts may be resorted to without departing from the spirit and scope of the invention as hereinafter claimed.