Patent Document

BACKGROUND 
     This disclosure relates to a method and apparatus for treating a subterranean well formation to stimulate the production of hydrocarbons and, more particularly, such a method and apparatus utilizing foam diversion in the well formation. 
     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. 
     Also, squeezing methods have been used which involve introducing stimulation fluids containing acids to 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. 
     In these methods, foam diversion is often used according to which foam is created and used to plug pores in the formation and thus promote the spreading of the fluids over a relatively large surface area of the formation. To this end, conventional 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. Moreover, using conventional procedures, a foam generated at the surface is sent through the same conduit as the other liquids. Therefore, if a foam is needed, it cannot be introduced into the formation until all the liquids used previously are cleared from the wellbore. The gas into the foam generator could be changed, but this change will not occur until all previously delivered foam clears the wellbore. This, of course, is very time-consuming. 
     SUMMARY 
     According to an embodiment of the present invention a method for acid treatment of a subterranean well formation is provided to stimulate the production of hydrocarbons which utilizes foam diversion which can be initiated substantially instantaneously in situ. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view of a fracturing system according to an embodiment of the present invention, shown in a vertical wellbore. 
     FIG. 2 is an exploded elevational view of two components of the system of FIG.  1 . 
     FIG. 3 is a cross-sectional view of the components of FIG.  2 . 
     FIG. 4 is a sectional view of a fracturing system according to an embodiment of the present invention, shown in a wellbore having a horizontal deviation. 
     FIG. 5 is a view similar to that of FIG. 1 but depicting an alternate embodiment of the fracturing system of the present invention shown in a vertical wellbore. 
     FIG. 6 is a view similar to that of FIG. 5, but depicting the fracturing system of the embodiment of FIG. 5 in a wellbore having a horizontal deviation. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, a stimulation system 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 system 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  could be placed just above the lower end of the casing  14  or could extend beyond, or below, the end of the casing  14  as viewed in FIG.  1 . 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. 
     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 gas, such as nitrogen or carbon dioxide. 
     The valve sub  26  is normally closed to cause flow of the gas 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 . Several different types of fluids are pumped into the annulus  28  from the ground, for reasons to be described. 
     The respective axes of the jet sub  20  and the valve sub  26  extend substantially vertically in the wellbore  10 . When the gas 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 . 
     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 gas from the work string  16  enters the housing  30 , passes through the passage  32  and is discharged from the openings  22 , with the discharge pattern being in the form of a disc extending around the housing  30 . 
     If the gas is introduced into the work string  16 , and discharges through the openings  22 , at a relatively high pressure, under conditions to be described, a jetting effect is achieved. This creates a relatively high differential discharge pressure, which accelerates the stimulation fluid in the annulus  28  to a relatively high velocity. Thus a relatively high shear occurs between the jetted gas and the fluid in the annulus  28 . This high shear causes the development of a high quality foam in situ for reasons to be explained. 
     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 . 
     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. 
     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. 
     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. 
     In operation, the ball  46  is dropped into the work string  16 , passes through the passage  42 , and seats on the shoulder between the passages  42  and  44 . A gas, such as nitrogen or carbon dioxide is pumped down the work string  16  and the fluid pressure thus builds up in the subs  20  and  26 . This pumping of the gas is continued until the system is fully charged at which time it is discontinued. 
     A preflush fluid is then pumped down the annulus  28  at pressures between the pressure of the pores of the formation and the fracture pressure. This preflush fluid removes carbonates and/or sweeps away harmful minerals from the wellbore  10  which would otherwise cause precipitates when contacting hydrofluoric acid at a later stage. The preflush fluid can be non-acidic, acidic, or both. 
     A stimulation fluid is then pumped down the annulus  28  at pressures at the reservoir  12  between the pore pressure and the fracture pressure. The stimulation fluid, can be in the form of a conventional acid that is used in squeezing or matrix acidizing, along with various additives that are well known in the art. Typical acids include mineral or organic acids, such as hydrochloric acid, hydroflouric acid, formic acid, or acetic acid, or a blend thereof. The stimulation fluid reacts with the formation to cause fracturing and squeezing, in a conventional manner. 
     An afterflush fluid is then pumped down the annulus  28  to sweep the hydrofluoric acid out of the wellbore. This afterflush fluid is generally non-acidic and can contain foaming agents for reasons to be described. It is noted that, during the above, some of the above gas may be present in the workstring  16  near or at its end, and some of the gas may have leaked into the annulus  28  as a result of the charging of the system, as described above. This gas is at a concentration, or pressure, to prevent the above fluids from rising up into the workstring  16 , but is not high enough in concentration to create a viscous foam when it mixes with the fluid at the openings  22  in the jet sub  20 . 
     After a predetermined pumping of the afterflush fluid, a diversion stage is initiated to insure that the fluid is spread over a relative large surface area of the formation. To this end, the pumping rate of the gas into the workstring  16  and through the openings  22  is initiated at an increased rate compared to the initial charging of the system, as discussed above. One of the following steps are taken to insure that foam is created in the annulus  28  at or below the jet sub  20  when the gas discharging from the openings  22  mixes with the afterflush fluid in the annulus  28 : 
     1) the differential pressure of the gas across the openings  22  will be high enough to create a homogeneous foam; 
     2) a foaming agent is added to the fluid; and/or 
     3) the gas-to-liquid ratio will be high enough to create a viscous foam. 
     The foam thus formed is directed to the formation and is forced into the pores thereof, creating a barrier so that the fluids of the next stage, or cycle, to be described are redirected to other untreated portions of the formation. 
     During this diversion stage, pressure increases or decreases occurring at the reservoir face  12  are monitored at the surface. Changes at the surface can be made with respect to either the fluid or gas rate to change the downhole foam&#39;s viscosity for fluid loss effects and stage sizes. 
     Once the desired diversion is accomplished, the above steps are repeated in another cycle and the above-mentioned barriers created by the foam caused by the diversion enables the fluid, and particularly, the stimulation fluid, to be spread over a relatively large surface area of the formation. Thus, in accordance with the foregoing, the foam is generated in situ on demand and substantially instantaneously. 
     The accelerated gas flow can be computed as follows: 
     Assuming Q is quality, V g  is the volumetric flow rate of gas at a certain pressure (in this example, pressure effects and gas expansion effects are ignored for clarity purposes; and it can be included in the future using common engineering know how) and V l  is the liquid rate; V g1  is the gas rate at Q 1 , and V g2  at Q 2 ; and dV is equal to (Vg2−Vg1), then, knowing that V 2 =(Q*V l )/(1−Q), the eventual gas flow can be computed at Q 2 ; which is V g2 =(Q 2 *V l )/(1−Q 2 ). In order to create the downhole step change and deliver the volume relatively quickly, this volume is V ADD =dV*V PIPE /V g2 ; where V PIPE  is the total volume of the conduit carrying gas. V ADD  must be delivered in addition to V g2  as quickly as possible. 
     After the above operations, if it is desired to clean out spent acid or 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 . 
     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.    
     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, positioned at the lower portion of casing  14 . As in the previous embodiment, gas, such as nitrogen, is introduced into the end of the work string  56  at the ground surface (not shown); while a stimulation fluid, described above, is pumped into the annulus of wellbore  50 . 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 gas 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 gas 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 gas 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 create a foam with the gas in the wellbore  50 . 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. 
     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 acid spotting and squeezing process discussed above can then be repeated numerous times throughout the horizontal wellbore section, such as every 100 to 200 feet. 
     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. The casing  60  must be either preperforated or perforated using conventional means; or it could be hydrajetted with sand using the jet sub  20 . Optionally, inside the casing  60  wire screens could be installed and packed with gravel in a manner well known in the art. 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 casing  60  to generate foam 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. 
     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. The casing section  62   b  must be either preperforated or perforated using conventional means; or it could be hydrajetted with sand using the jet sub  20 . Optionally, inside the casing section  62   b  wire screens could be installed and packed with gravel in a manner well known in the art. Then the stimulation operation described in connection with the embodiments of FIGS. 1-3, above, is initiated with the mixture of stimulation fluid and gas discharging, at a relatively high velocity, through the above openings in the casing  62 , and against the formation  12  to fracture squeeze 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 
     It is understood that variations may be made in the foregoing without departing from the scope of the invention. For example, although the above technique was described in connection with a process to matrix acidize sandstone reservoirs, it is understood that it is not exclusive to matrix sandstone acidizing with hydrofluoric acid, and can be used in carbonate matrix acidizing with other type acids which are compatible with carbonate reservoirs. 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. Further, the liquids may be injected through the workstring  16 , while the gas is pumped into the annulus  28 . Moreover, it may be decided that the dispensing of the reactive fluids, such as the acids, be spotted at different positions of the well. To do this, position of the jet sub  20  may be far below the casing  14  as shown in FIG.  1 . Still further, the above preflushes and afterflushes can be acidic or not acidic. 
     Also, the gas can be premixed with some liquids prior to entering the work string  16  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 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. 
     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.

Technology Category: 8