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BACKGROUND  
         [0001]    This disclosure relates to a method for treating a subterranean well formation to stimulate the production of hydrocarbons and, more particularly, such an apparatus and method for fracturing and squeezing the well formation.  
           [0002]    Several techniques have evolved for treating a subterranean well formation to stimulate hydrocarbon production. For example, hydraulic fracture acidizing methods have often been used according to which a portion of a formation to be stimulated is isolated using conventional packers, or the like, and a stimulation fluid containing gels, acids, sand slurry, and the like, is pumped through the well bore into the isolated portion of the formation. The pressurized stimulation fluid pushes against the formation at a very high force to establish and extend cracks on the formation. However, the requirement for isolating the formation with packers is time consuming and considerably adds to the cost of the system.  
           [0003]    Also, squeezing methods have been used which involve introducing stimulation fluids containing acids to carbonate type formations at a pressure that is higher than the formation pressure (but not as high as the fluid pressure in the fracturing methods), causing the fluid to infiltrate the pores in the formation and react with the formation to enlarge the pores. However, this is a relatively slow process and produces relatively small openings in the formation. Also, since these stimulation fluids are usually very reactive, especially at elevated temperatures, the fluid is often prematurely spent close to the wellbore in the formation. Thus, no extended reach is achieved and the fluidentry point is often greatly enlarged. As a result, it is impossible to form multiple, relatively long, and effective acid fingering throughout the wellbore face, especially in low-permeability reservoirs that require deep penetration.  
           [0004]    Chemical reactivity of the acid can be reduced using many ways, and one of them is the use of foams. Since foams are also good leak off prevention material, they help in creating large fractures. Conventionally, foaming equipment is provided on the ground surface that creates a foam, which is then pumped downhole. Foams, however, have much larger friction coefficients and reduced hydrostatic effects, both of which severely increase the required pressures to treat the well.  
           [0005]    Therefore, what is needed is a stimulation treatment that combines most or all features of the above types according to which the need for isolation packers is eliminated, the foam generation is performed in-situ downhole, the depth of penetration improved, and the reaction of the fracturing acid with the formation is controlled so that premature reaction of the acid with the formation is prevented.  
         SUMMARY  
         [0006]    According to an embodiment of the present invention, the techniques of acid fracturing and squeezing are combined to produce an improved stimulation of the formation. To this end, a stimulation fluid is discharged through a workstring and into a formation at a relatively high impact pressure and velocity without the need for isolation packers to fracture the formation. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    [0007]FIG. 1 is a sectional view of a fracturing apparatus according to an embodiment of the present invention, shown in a vertical wellbore.  
         [0008]    [0008]FIG. 2 is an exploded elevational view of two components of the apparatus of FIGS. 1 and 2.  
         [0009]    [0009]FIG. 3 is a cross-sectional view of the components of FIG. 2.  
         [0010]    [0010]FIG. 4 is a sectional view of a fracturing apparatus according to an embodiment of the present invention, shown in a wellbore having a horizontal deviation.  
         [0011]    [0011]FIG. 5 is a view similar to that of FIG. 1 but depicting an alternate embodiment of the fracturing apparatus of the present invention shown in a vertical wellbore.  
         [0012]    [0012]FIG. 6 is a view similar to that of FIG. 5, but depicting the fracturing apparatus of the embodiment of FIG. 5 in a wellbore having a horizontal deviation.  
     
    
     DETAILED DESCRIPTION  
       [0013]    Referring to FIG. 1, a stimulation apparatus according to an embodiment of the present invention is shown installed in an underground, substantially vertically-extending, wellbore  10  that penetrates a hydrocarbon producing subterranean formation  12 . A casing  14  extends from the ground surface (not shown) into the wellbore  10  and terminates above the formation. The stimulation apparatus includes a work string  16 , in the form of piping or coiled tubing, that also extends from the ground surface and through the casing  14 . The work string  16  extends beyond, or below, the end of the casing  14  as viewed in FIG. 1, and one end of the work string  16  is connected to one end of a tubular jet sub  20  in a manner to be described. The jet sub has a plurality of through openings  22  machined through its wall that form discharge jets which will be described in detail later.  
         [0014]    A valve sub  26  is connected to the other end of the jet sub  20 , also in a manner to be described. The end of the work string  16  at the ground surface is adapted to receive a stimulation fluid, to be described in detail, and the valve sub  26  is normally closed to cause flow of the stimulation fluid to discharge from the jet sub  22 . The valve sub  26  is optional and is generally required for allowing emergency reverse circulation processes, such as during screenouts, equipment failures, etc. An annulus  28  is formed between the inner surface of the wellbore  10  and the outer surfaces of the workstring  16  and the subs  20  and  26 .  
         [0015]    The stimulation fluid includes a conventional acid that is used in squeezing or fracture acidizing, discussed above, along with various additives that are well known in the art. Typical fluids include mineral or organic acids, such as hydrochloric acid, formic acid, acetic acid, or a blend thereof. A more specific, but non-limiting, example of the type of fluid is a 28% hydrochloric acid containing gelling agents, corrosion inhibitors, iron-control chemicals, and chemicals for controlling sulfide cracking. Also, some sand and a foaming agent may be added to the fluid for reasons to be described. This mixture will hereinafter be referred to as “stimulation fluid.” 
         [0016]    The respective axes of the jet sub  20  and the valve sub  26  extend substantially vertically in the wellbore  10 . When the stimulation fluid is pumped through the work string  16 , it enters the interior of the jet sub  20  and discharges through the openings  22 , into the wellbore  10 , and against the formation  12 .  
         [0017]    Details of the jet sub  20  and the ball valve sub  26  are shown in FIGS. 2 and 3. The jet sub  20  is formed by a tubular housing  30  that includes a longitudinal flow passage  32  extending through the length of the housing. The openings  22  extend through the wall of the casing in one plane and can extend perpendicular to the axis of the casing as shown in FIG. 2, and/or at an acute angle to the axis of the casing as shown in FIG. 3, and/or aligned with the axis (not shown). Thus, the stimulation fluid from the work string  16  enters the housing  30 , passes through the passage  32  and is discharged from the openings  22 . The stimulation fluid discharge pattern is in the form of a disc extending around the housing  30 .  
         [0018]    As a result of the high pressure stimulation fluid from the interior of the housing  30  being forced out the relatively small openings  22 , a jetting effect is achieved. This is caused by the stimulation fluid being discharged at a relatively high differential pressure, such as 3000-6000 psi, which accelerates the stimulation fluid to a relatively high velocity, such as 650 ft./sec. This high velocity stimulation fluid jetting into the wellbore  10  causes drastic reduction of the pressure surrounding the stimulation fluid stream (based upon the well known Bernoulli principle), which eliminates the need for the isolation packers discussed above.  
         [0019]    Two tubular nipples  34  and  36  are formed at the respective ends of the housing  30  and preferably are formed integrally with the housing. The nipples  34  and  36  have a smaller diameter than that of the housing  30  and are externally threaded, and the corresponding end portion of the work string  16  (FIG. 1) is internally threaded to secure the work string to the housing  30  via the nipple  34 .  
         [0020]    The valve sub  26  is formed by a tubular housing  40  that includes a first longitudinal flow passage  42  extending from one end of the housing and a second longitudinal flow passage  44  extending from the passage  42  to the other end of the housing. The diameter of the passage  42  is greater than that of the passage  44  to form a shoulder between the passages, and a ball  46  extends in the passage  42  and normally seats against the shoulder.  
         [0021]    An externally threaded nipple  48  extends from one end of the casing  40  for connection to other components (not shown) that may be used in the stimulation process, such as sensors, recorders, centralizers and the like. The other end of the housing  40  is internally threaded to receive the externally threaded nipple  36  of the jet sub  20  to connect the housing  40  of the valve sub  26  to the housing  30  of the jet sub.  
         [0022]    It is understood that other conventional components, such as centering devices, BOPs, strippers, tubing valves, anchors, seals etc. can be associated with the system of FIG. 1. Since these components are conventional and do not form any part of the present invention, they have been omitted from FIG. 1 in the interest of clarity.  
         [0023]    In operation, the ball  46  is dropped into the work string  16  while the stimulation fluid is continuously pumped from the ground surface through the work string  16  and the jet sub  20  and to the valve sub  26 . In the valve sub  26 , the ball  46  passes through the passage  42  and seats on the shoulder between the passages  42  and  44 . The fluid pressure thus builds up in the subs  20  and  26 , causing the stimulation fluid to discharge through the openings  22 .  
         [0024]    The pumping rate of the stimulation fluid is then increased to a level whereby the pressure of the stimulation fluid jetted through the openings  22  reaches a relatively high differential pressure and high discharge velocity such as those set forth above.  
         [0025]    During the above operation, a gas, consisting essentially of carbon dioxide or nitrogen, is pumped from the ground surface and into the annulus  28  (FIG. 1). The gas flows through the annulus  28  and the stimulation fluid mixes with and carries the gas from the annulus towards the formation causing a high energy mixing to generate foam with the resulting mixture hereinafter being referred to as a “mixture.” 
         [0026]    The mixture is jetted towards the formation and impacts the wall of the formation forming the wellbore  12 . The confined mixture will pressurize the cavities in the formation and, as each of the cavities becomes sufficiently deep, the formation will fracture when the pressure is sufficiently high. Paths for the mixture are created in the bottoms of the above cavities in the formation which serve as output ports into the formation, with the annulus  28  serving as an input port to the system. Thus a virtual jet pump is created which is connected directly to the fracture. Moreover, each cavity becomes a small mixing chamber which significantly improves the homogeneity and quality of the foam. This high quality foam is then either pushed into the fracture or returned into the wellbore area.  
         [0027]    If the jet pressure and the pressure in the annulus  28  is not high enough to cause fracturing, and if this combined pressure is higher than the pore pressure in the formation, then “squeezing” will occur. Alternatively, if, after the fracturing discussed above, it is desired to squeeze, the pressure of the mixture in the annulus  28  is reduced to a squeeze level pressure which is higher than the pressure in the pores in the formation.  
         [0028]    In either of the above cases, according to the squeezing process, a greater quantity of the mixture will go through the larger pores in the formation than through the smaller pores, and the larger pores will be substantially increased in size to form channels or “wormholes” for the mixture to flow through. The presence of the foam in the mixture retards the reaction of the acid in the mixture with the formation so that the reach of the mixture into the formation is substantially extended when compared to techniques in which foam is not employed. Furthermore, the foam is of a high quality which increases the selectivity and effectiveness of the treatment. As the mixture in the wellbore  10  is pressurized against the wellbore walls and fracture faces in the manner discussed above, the foam bubbles tend to plug the smaller pores while entering the larger pores so that the acid portion of the mixture reacts with the formation material, thus further enlarging the larger pores. Thus, significant squeezing is achieved to create channels, also termed “fingering” or “wormholing,” in the fracture faces and the wellbore wall, with the reaction of the mixture with the formation being relatively slow so that the mixture can penetrate deep into the formation matrix. At the end of the squeeze, as the annulus  28  pressure is reduced, the fracture closes, and the flow back of the mixture to the wellbore creates channeling or wormholes along the fracture face.  
         [0029]    If it is desired to create a relatively large fracture, the pressure of the mixture in the annulus  28  around the sub  20  is controlled so that it is greater that the squeeze pressure, and slightly less than the hydraulic fracturing pressure, discussed above. The impact or stagnation pressure will bring the net pressure substantially above the required fracturing pressure; and therefore a substantially large fracture (such as 25 ft to 500 ft or more in length) can be created. In this process, the foam reduces mixture loss into the fracture face and/or the natural fractures. With the reduced loss of the mixture, most of the mixture volume can be used as a means for extending the fracture to produce the relatively large fracture. Since the fracture pressures are higher than the squeeze pressure discussed above, fingering of the mixture into the fracture face can occur simultaneously as discussed in the squeezing operation discussed above.  
         [0030]    After the above operations, if it is desired to clean out foreign material such as debris, pipe dope, etc. from the wellbore  10 , the work string  16 , and the subs  20  and  26 , the pressure of the stimulation fluid in the work string  16  is reduced and a cleaning fluid, such as water, at a relatively high pressure, is introduced into the annulus  28 . After reaching a depth in the wellbore  10  below the subs  20  and  26 , this high pressure cleaning fluid flows in an opposite direction to the direction of the stimulation fluid discussed above and enters the discharge end of the flow passage  44  of the valve sub  26 . The pressure of the cleaning fluid forces the ball valve  46  out of engagement with the shoulders between the passages  42  and  44  of the sub  26 . The ball valve  46  and the cleaning fluid pass through the passage  42 , the jet sub  20 , and the work string  16  to the ground surface. This circulation of the cleaning fluid cleans out the foreign material inside the work string  16 , the subs  20  and  26 , and the well bore  10 .  
         [0031]    After the above-described cleaning operation, if it is desired to initiate the discharge of the stimulation fluid against the formation wall in the manner discussed above, the ball valve  46  is dropped into the work string  16  from the ground surface in the manner described above, and the stimulation fluid is introduced into the work string  14  as discussed above.  
         [0032]    [0032]FIG. 4 depicts a stimulation system, including some of the components of the system of FIGS.  1 - 3  which are given the same reference numerals. The system of FIG. 4 is installed in an underground wellbore  50  having a substantially vertical section  50   a  extending from the ground surface and a deviated, substantially horizontal section  50   b  that extends from the section  50   a  into a hydrocarbon producing subterranean formation  52 . As in the previous embodiment, the casing  14  extends from the ground surface into the wellbore section  50   a.    
         [0033]    The stimulation system of FIG. 4 includes a work string  56 , in the form of piping or coiled tubing, that extends from the ground surface, through the casing  14  and the wellbore section  50   a,  and into the wellbore section  50   b.  As in the previous embodiment, stimulation fluid is introduced into the end of the work string  56  at the ground surface (not shown). One end of the tubular jet sub  20  is connected to the other end of the work string  56  in the manner described above for receiving and discharging the stimulation fluid into the wellbore section  50   b  and into the formation  52  in the manner described above. The valve sub  26  is connected to the other end of the jet sub  20  and controls the flow of the stimulation fluid through the jet sub in the manner described above. The respective axes of the jet sub  20  and the valve sub  26  extend substantially horizontally in the wellbore section  50   b  so that when the stimulation fluid is pumped through the work string  56 , it enters the interior of the jet sub  20  and is discharged, in a substantially radial or angular direction, through the wellbore section  50   b  and against the formation  52  to fracture and squeeze it in the manner discussed above. The horizontal or deviated section of the wellbore is completed openhole and the operation of this embodiment is identical to that of FIG. 1. It is understood that, although the wellbore section  50   b  is shown extending substantially horizontally in FIG. 4, the above embodiment is equally applicable to wellbores that extend at an angle to the horizontal.  
         [0034]    In connection with formations in which the wellbores extend for relatively long distances, either vertically, horizontally, or angularly, the jet sub  20 , the valve sub  26  and workstring  56  can be initially placed at the toe section (i.e., the farthest section from the ground surface) of the well. The fracture acidizing and squeezing process discussed above can then be repeated numerous times throughout the horizontal wellbore section, such as every 100 to 200 feet. Alternatively, this process can be performed in a continuous manner by moving the workstring  56 , and therefore the sub  20 , relatively slowly and continuously towards the ground surface causing the sub to be dragged through hills and valleys of the wellbore. When the jet sub  20  is at the top of the “hill,” i.e., when the jets are almost touching the inner surface of the wellbore, a fracture occurs; when it is in a valley, fractures can not start. Also, squeezing will occur as discussed earlier.  
         [0035]    The embodiment of FIG. 5 is similar to that of FIG. 1 and utilizes many of the same components of the latter embodiments, which components are given the same reference numerals. In the embodiment of FIG. 5, a casing  60  is provided which extends from the ground surface (not shown) into the wellbore  10  formed in the formation  12 . The casing  60  extends for the entire length of that portion of the wellbore in which the workstring  16  and the subs  20  and  26  extend. Thus, the casing  60 , as well as the axes of the subs  20  and  26  extend substantially vertically.  
         [0036]    Prior to the introduction of the stimulation fluid into the jet sub  20 , a liquid mixed with sand is introduced into the jet sub  20  and discharges from the openings  22  in the jet sub and against the inner wall of the casing  60  at a very high velocity, causing tiny openings to be formed through the latter wall. Then the operation described in connection with the embodiments of FIGS.  1 - 3  above is initiated, and the mixture of stimulation fluid and foamed gas discharge, at a relatively high velocity, through the openings  22 , through the above openings in the casing  60 , and against the formation  12  to fracture and squeeze it in the manner discussed above. Otherwise the operation of the embodiment of FIG. 5 is identical to those of FIGS.  1 - 4 .  
         [0037]    The embodiment of FIG. 6 is similar to that of FIG. 4 and utilizes many of the same components of the latter embodiments, which components are given the same reference numerals. In the embodiment of FIG. 6, a casing  62  is provided which extends from the ground surface (not shown) into the wellbore  50  formed in the formation  52 . The casing  62  extends for the entire length of that portion of the wellbore in which the workstring  56  and the subs  20  and  22  are located. Thus, the casing  62  has a substantially vertical section  62   a  and a substantially horizontal section  60   b  that extend in the wellbore sections  50   a  and  50   b,  respectively. The subs  20  and  26  are located in the casing section  62   b  and their respective axes extend substantially horizontally.  
         [0038]    Prior to the introduction of the stimulation fluid into the jet sub  20 , a liquid mixed with sand is introduced into the work string  16  with the ball valve  46  (FIG. 3) in place. The liquid/sand mixture discharges from the openings  22  (FIG. 2) in the jet sub  20  and against the inner wall of the casing  62  at a very high velocity, causing tiny openings to be formed through the latter wall. Then the stimulation operation described in connection with the embodiments of FIGS.  1 - 3 , above, is initiated with the mixture of stimulation fluid and foamed gas discharging, at a relatively high velocity, through the openings  22 , through the above openings in the casing  62 , and against the wall of the formation  52  to impact it in the manner discussed above. Otherwise the operation of the embodiment of FIG. 6 is identical to those of FIGS.  1 - 3 .  
       Equivalents and Alternatives  
       [0039]    It is understood that variations may be made in the foregoing without departing from the scope of the invention. For example, gas flowing in the annulus  28  can be premixed with some liquids prior to entering the casing  14  for many reasons such as cost reduction and increasing hydrostatic pressure. Moreover the makeup of the stimulation fluid can be varied within the scope of the invention. Further, the particular orientation of the wellbores can vary from completely vertical to completely horizontal. Still further, the particular angle that the discharge openings extend relative to the axis of the jet sub can vary. Moreover, the openings  22  in the sub  20  could be replaced by separately installed jet nozzles that are made of exotic materials such as carbide mixtures for increased durability. Also, a variety of other fluids can be used in the annulus  28 , including clean stimulation fluids, liquids that chemically control clay stability, and plain, low-cost fluids.  
         [0040]    Although only a few exemplary embodiments of this invention have been described in detail above, those skilled in the art will readily appreciate that many other modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the following claims. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents, but also equivalent structures.

Summary:
A method of stimulating a downhole formation according to which a plurality of jet nozzles are located in a spaced relation to the wall of the formation to form an annulus between the nozzles and the formation. An acid-containing, stimulation fluid is pumped at a predetermined pressure through the nozzles, into the annulus and against the wall of the formation. A gas is pumped into the annulus so that the stimulation fluid mixes with the gas to generate foam before the mixture is jetted towards the formation to impact the wall of the formation. It is emphasized that this abstract is provided to comply with the rules requiring an abstract that will allow a searcher or other reader to quickly ascertain the subject matter of the technical disclosure, and is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims under 37CFR 1.72