Abstract:
In a well stimulation method, a subsurface formation is fractured by freezing a water-containing zone within the formation in the vicinity of a well, thereby generating expansive pressures which expand or created cracks and fissures in the formation. The frozen zone is then allowed to thaw. This freeze-thaw process causes rock particles in existing cracks and fissures to become dislodged and reoriented therewithin, and also causes new or additional rock particles to become disposed within both existing and newly-formed cracks and fissures. The particles present in the cracks and fissures act as natural proppants to help keep the cracks and fissures open, thereby facilitating the flow of fluids from the formation into the well after the formation has thawed. Preferably, the freeze-thaw steps are carried out on a cyclic basis. Optionally, propagation of the freezing front into the formation may be enhanced by the introduction of low-frequency wave energy into the formation.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit, pursuant to 35 U.S.C. 119(e), of U.S. Provisional Application No. 60/746,937, filed on May 10, 2006, and said provisional application is incorporated herein by reference in its entirety. 
     
     FIELD OF THE INVENTION 
       [0002]    The present invention relates in general to methods for enhancing the efficiency of recovery of liquid and gaseous hydrocarbons from oil and gas wells. In particular, the invention relates to methods for fracturing a subsurface formation to facilitate or improve the flow of hydrocarbon fluids from the formation into a well. 
       BACKGROUND OF THE INVENTION 
       [0003]    A well drilled into a hydrocarbon-bearing subsurface formation, during an initial post-completion stage, commonly produces crude oil and/or natural gas without artificial stimulation, because pre-existing formation pressure is effective to force the crude oil and/or natural gas out of the formation into the well bore, and up the production tubing of the well. However, the formation pressure will gradually dissipate as more hydrocarbons are produced, and will eventually become too low to force further hydrocarbons up the well. At this stage, the well must be stimulated by artificial means to induce additional production, or else the well must be capped off and abandoned. This is a particular problem in gas wells drilled into “tight” formations—i.e., where natural gas is present in subsurface materials having inherently low porosities, such as sandstone, limestone, shale, and coal seams (e.g., coal bed methane wells). 
         [0004]    Despite the fact that very large quantities of hydrocarbons may still be present in the formation, it has in the past been common practice to abandon wells that will no longer produce hydrocarbons under natural pressure, where the value of stimulated production would not justify the cost of stimulation. In other cases, where stimulation was at least initially a viable option, wells have been stimulated for a period of time and later abandoned when continued stimulation became uneconomical, even though considerable hydrocarbon reserves remained in the formation. With recent dramatic increases in market prices for crude oil and natural gas, well stimulation has become viable in many situations where it would previously have been economically unsustainable. 
         [0005]    There are numerous known techniques and processes for stimulating production in low-production wells or in “dead” wells that have ceased flowing naturally. One widely-used method is hydraulic fracturing (or “fraccing”). In this method, a fracturing fluid (or “frac fluid”) is injected under pressure into the subsurface formation. Frac fluids are specially-engineered fluids containing substantial quantities of proppants, which are very small, very hard, and preferably spherical particles. The proppants may be naturally formed (e.g., graded sand particles) or manufactured (e.g., ceramic materials; sintered bauxite). The frac fluid may be in a liquid form (often with a hydrocarbon base, such as diesel fuel), but may also be in gel form to enhance the fluid&#39;s ability to hold proppants in a uniformly-dispersed suspension. Frac fluids commonly contain a variety of chemical additives to achieve desired characteristics. 
         [0006]    The frac fluid is forced under pressure into cracks and fissures in the hydrocarbon-bearing formation, and the resulting hydraulic pressure induced within the formation materials widens existing cracks and fissures and also creates new ones. When the frac fluid pressure is relieved, the liquid or gel phase of the frac fluid flows out of the formation, but the proppants remain in the widened or newly-formed cracks and fissures, forming a filler material of comparatively high permeability that is strong enough to withstand geologic pressures so as to prop the cracks and fissures open. Once the frac fluid has drained away, liquid and/or gaseous hydrocarbons can migrate through the spaces between the proppant particles and into the well bore, from which they may be recovered using known techniques. 
         [0007]    Another known well stimulation method is acidizing (also known as “acid fracturing”). In this method, an acid or acid blend is pumped into a subsurface formation as a means for cleaning but extraneous or deleterious materials from the fissures in the formation, thus enhancing the formation&#39;s permeability. Hydrochloric acid is perhaps most commonly as the base acid, although other acids including acetic, formic, or hydrofluoric acid may be used depending on the circumstances. 
         [0008]    Although fraccing and acidizing have proven beneficial capabilities, there remains a need for new and more effective methods for stimulating production in oil and gas wells. In particular, there is a need for stimulation methods that are more economical than known methods, and which can enable recovery of higher percentages of non-naturally-flowing hydrocarbons from low-permeability formations than has been possible using known stimulation methods. Even more particularly, there is a need for such methods that do not entail the injection of acids or other chemicals into subsurface formations, and that do not require the introduction of proppants into the formation. The present invention is direction to these needs. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    In general terms, the present invention is a well stimulation method whereby a subsurface formation is fractured by injecting an aqueous solution (e.g., fresh water) into the formation and then inducing freezing such that the aqueous solution expands, thereby generating expansive pressures which widen existing formation cracks and fissures in the formation and/or cause new ones to form. This process causes rock particles in existing cracks and fissures to be dislodged and reoriented therewithin, and also causes new or additional rock particles to become disposed within both existing and newly-formed cracks and fissures. Thawing is induced in the frozen formation, such that the aqueous solution drains from the formation. The particles present in the cracks and fissures act as natural proppants to help keep the cracks and fissures open in substantially the same configuration as created during the freezing step. 
         [0010]    Accordingly, in a first aspect the present invention is a method for stimulating flow of petroleum fluids from a subsurface formation into a wellbore drilled into and exposed to the formation, said method comprising the steps of:
       (a) providing a string of return tubing having an upper end and a lower end;   (b) providing a string of supply tubing having an upper end and a lower end, said lower end being open, and said supply tubing having expander means associated with said lower end;   (c) disposing the return tubing string within the wellbore so as to position the lower end of the return tubing at a selected depth, and so as to form a well annulus between the return tubing and the wellbore;   (d) disposing the supply tubing string within the return tubing string so as to position the expander means at a selected depth, and so as to form a tubing annulus between the supply tubing and the return tubing, with the return tubing string having associated plug means sealing off the tubing annulus at a selected location below the expander means;   (e) ensuring that an aqueous fluid is present in the well annulus to a selected level above the depth of the expander means;   (f) initiating a freezing cycle by introducing a flow of liquid refrigerant into the supply tubing, such that the refrigerant passes through the expander means and resultantly vaporizes and flows into the tubing annulus, and continuing the flow of refrigerant to freeze the aqueous fluid in a zone adjacent the expander means and to freeze an adjacent first region of the formation; and   (g) initiating a thaw cycle by discontinuing the flow of refrigerant and allowing said first region of the formation to thaw.       
 
         [0018]    Preferably, the freeze-thaw steps are carried out on a cyclic basis. Each additional freeze-thaw cycle will cause additional formation fracturing, plus the creation of additional natural proppant particles. The appropriate or most effective number of freeze-thaw cycles in a given application will depend on a variety of factors including the physical properties of the formation materials. 
         [0019]    In preferred embodiments of the method of the present invention, means are provided for subjecting the subsurface formation to LF wave energy during the freezing cycle of the method. This will have the effect of reducing the time required for each freezing cycle, for a given extent of penetration of the freezing front into the formation, thereby reducing the total time required for the well stimulation operation, thus enabling the well to be returned to production sooner. 
         [0020]    In a second aspect, the present invention is an apparatus for practicing the method of the invention. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    Embodiments of the invention will now be described with reference to the accompanying Figures, in which numerical references denote like parts, and in which: 
           [0022]      FIG. 1  is a cross-section through a vertical well extending into a subsurface formation, with refrigeration apparatus in accordance with one embodiment of the invention. 
           [0023]      FIG. 2  is a cross-section through a horizontal well extending into a subsurface formation, with refrigeration apparatus in accordance with another embodiment of the invention. 
           [0024]      FIG. 3  illustrates one embodiment of a nozzle and movable packer assembly in accordance with the present invention. 
           [0025]      FIG. 4A  is a cross-section through the retainer assembly and tubular sleeve of an alternative embodiment of a movable packer in accordance with the invention. 
           [0026]      FIG. 4B  is a side view of an expandable bladder for use in conjunction with the retainer assembly shown in  FIG. 4A . 
           [0027]      FIG. 4C  is a side view of a retainer tube for use in conjunction with the retainer assembly shown in  FIG. 4A  and the bladder shown in  FIG. 4B . 
           [0028]      FIG. 5  is a cross-section through a vertical well, illustrating how multiple subsurface zones at different depths can be simultaneously freeze-fractured in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0029]    The method of the invention is schematically illustrated in  FIG. 1 , which shows a vertical well  10  drilled into a hydrocarbon-bearing subsurface formation  20 . Well  10  will typically have a well liner  12 , with perforations  14  in the production zone (i.e., the portion of well  10  that penetrates formation  20 ) to allow hydrocarbons H to flow from formation  20  into well  10 . In some geologic formations it may be feasible to for well  10  to be unlined, such that hydrocarbons can flow directly into well  10 . In either case, well  10  can be said to be exposed to formation  20 , for purposes of this patent specification. When well  10  is producing, formation fluids comprising liquid and/or gaseous hydrocarbons are conveyed to the surface through a string of production tubing (not shown) which is disposed within well  10  down to the production zone. 
         [0030]    To use the well stimulation method of the present invention, the production tubing (if still present) is withdrawn from well  10 , and then a string of refrigerant return tubing  30  is inserted into well  10 , creating a generally annular well annulus  16  surrounding return tubing  38 . The lower end  32  of return tubing  30  is sealed off by suitable plug means  34 ; by way of non-limiting example, plug means  34  may be in the form of a conventional packer disposed within the bore of return tubing  30  in accordance with known methods, or in the form of a permanent welded end closure. A string of refrigerant supply tubing  40  extends within return tubing  30 , creating a generally annular tubing annulus  36  surrounding return tubing  30 . The lower end  42  of supply tubing  40  incorporates or is connected to a flow restrictor or other type of expander means (conceptually indicated by reference numeral  50 ) for creating a pressure drop so as to induce vaporization of a liquid refrigerant, in accordance with well-known refrigeration principles and technology. 
         [0031]    In many cases where formation pressure has been depleted to the point that hydrocarbons will no longer flow naturally, water  60  will have accumulated within well  10 , and will permeate formation  20 . However, to use the present method in depleted wells that are not already water-laden, water  60  is introduced to a desired height within well annulus  16 , from which it may flow into cracks and fissures in formation  20  (either directly or through perforations  14 ). 
         [0032]    A suitable liquid refrigerant  70  (e.g., liquid nitrogen, liquid carbon dioxide, calcium chloride brine, or, preferably, liquid propane) is pumped downward through bore  44  of supply tubing  40 . Liquid refrigerant  70  is forced past expander means  50 , causing the liquid refrigerant  70  to expand. Expander means  50  may take any of various forms in accordance with known refrigeration technology. In the embodiment illustrated in  FIG. 1 , expander means  50  is a streamlined flow obstruction that will cause an increase in flow velocity of liquid refrigerant  70 , thus causing a pressure drop in accordance with known principles of fluid dynamics, resulting in expansion and evaporation (i.e., phase change) of liquid refrigerant  70 . 
         [0033]    Because the lower end  32  of return tubing  30  is plugged, the expanded refrigerant  70 E is forced upward through tubing annulus  36  to the surface, where it passes through a condenser (not shown) for recirculation into supply tubing  40 . In accordance with well-known refrigeration principles, the circulation of refrigerant  70  through supply tubing  40  and return tubing  30 , as described above, results in the absorption and removal of heat from water  60  by refrigerant  70 , to the point that water  60  freezes. A freezing front propagates radially outward from well  10  into formation  20  as refrigerant  70  continues to circulate and remove more heat, with the result that water within cracks and fissures in formation  20  freezes and expands, causing fracturing of formation  20  as previously described. 
         [0034]    It has been found that the propagation of a freezing front through a geological formation can be enhanced or expedited by introducing low-frequency wave energy into the formation. In this context, low-frequency (or LF) waves should be understood as being waves in the approximate range of 15 to 300 cycles per second; i.e., 15-300 Hertz (Hz). The LF waves may be generated either electromagnetically or mechanically. Accordingly, in preferred embodiments of the invention, means for generating LF waves will be provided in association with lower end  32  of return tubing  30  or lower end  42  of supply tubing  40 . 
         [0035]    In a particularly preferred embodiment, the LF wave-generating means will be incorporated into expander means  50 . Where expander means  50  is in the form of a flow obstruction, it may be adapted to generate LF waves mechanically, as shock waves caused by the movement of liquid refrigerant  70  past the flow restriction. In alternative embodiments, an electromagnetic wave transmitter is provided in association with lower end  32  of return tubing  30  or lower end  42  of supply tubing  40 . In such embodiments, the amplitude and frequency of LF waves can be regulated by control means (not shown) located at the surface. Preferably, the LF waves are generated in pulsed fashion, which is believed to enhance the effectiveness of the wave energy in advancing the freezing front within formation  20 . 
         [0036]    Persons of ordinary skill in the art of the invention will appreciate that mechanical or electromagnetic means for generating LP waves can be provided in a variety of forms using known technology; accordingly, embodiments of the invention involving the use of LF waves are not to be limited to the use of any specific type of LF wave generation means. 
         [0037]    After being frozen as described above, preferably in conjunction with exposure to LF waves, the affected region of formation  20  is allowed to warm up so that water that has frozen therewithin will melt and drain into well  10 . Most preferably, formation  20  will be exposed to multiple freeze-thaw cycles, enhanced with the introduction LF waves into formation  20 . When formation  20  has been exposed to a desired number of freeze-thaw cycles, return tubing  30  and supply tubing  40 , are removed from well  10 , along with expander means  50  (and the LF wave-generating means, if being used). Well  10  is then ready to be returned to production in accordance with conventional methods. 
         [0038]    The method of the present invention may also be advantageously used in a horizontal wellbore  110 , as conceptually illustrated in  FIG. 2 . It should be noted that  FIG. 2  is not to scale; horizontal wellbore  110  will typically be hundreds of feet long. A string of return tubing  130  (e.g., in the form of 2⅞″ diameter coiled tubing, by way of preferred but non-limiting example) is inserted into wellbore  110  as shown, forming a well annulus  116  between return tubing  130  and wellbore  110 . A string of refrigerant supply tubing  140  (e.g., 1¼″ diameter, for use in conjunction with 2⅞″ coiled tubing) is inserted within return tubing  130  as shown, with a packer/nozzle assembly  150  connected to the lower end  142  of supply tubing  140 . The insertion of supply tubing  140  into return tubing  130  results in the formation of a tubing annulus  136  between supply tubing  140  into return tubing  130 . Supply tubing  140  passes through a flow restrictor baffle  134  located at a selected distance from packer/nozzle assembly  150 . Flow restrictor baffle  134  has one or more orifices (preferably adjustable) or other suitable means for permitting restricted flow of gaseous or liquid fluids across or through baffle  134 . As best seen in  FIG. 3 , supply tubing  140  terminates in a diffuser nozzle  160  connected to a suitable packer  170  such that the packer/nozzle assembly is sealingly movable within return tubing  130 . 
         [0039]    A portion of tubing annulus  136  thus forms an annular sub-chamber  138  extending longitudinally between packer  170  and flow restrictor baffle  134  as shown in  FIG. 2 . The portion of supply tubing  140  that is disposed within annular sub-chamber  138  will be referred to herein as the “stinger” section  180 , having a length L corresponding to the distance between packer  170  and flow restrictor baffle  134 . On the other side of flow restrictor baffle  134 , the remaining portion of tubing annulus  136  extends toward and up the vertical portion of wellbore  110 . Flow restrictor baffle  134  may be considered part of stinger  180  and is longitudinally movable, with stinger  180 , inside return tubing  130 . 
         [0040]    Using apparatus generally as described above, the subsurface formation  20  adjacent to horizontal wellbore  110  can be freeze-fractured by the following procedure. First, well annulus  116  is flooded with an aqueous fluid (e.g., fresh water or a brine solution), resulting in permeation of the aqueous fluid into cracks and fissures in the surrounding formation  20 . A suitable refrigerant  70  (e.g., liquid carbon dioxide, liquid nitrogen, or liquid propane) is pumped into supply tubing  140 , and exits the nozzle in vaporized form into annular sub-chamber  138 . As the refrigerant travels toward flow restrictor baffle  134 , it absorbs heat from the water in well annulus  116  (and the surrounding formation  20 ), resulting in expansion and vaporization of refrigerant  70 . The vaporized refrigerant  70 E passes through flow restrictor baffle  134  (in either liquid or gaseous phase, or in mixed-phase form) into tubing annulus  136 , and up to the surface where it will preferably be recovered, recompressed, and re-used (i.e., in a closed-loop refrigeration cycle). 
         [0041]    In accordance with well-known refrigeration principles, the foregoing process results in cooling and eventual freezing of formation  20  adjacent to annular sub-chamber  138 , producing desired freeze-fracturing effects as previously discussed. The frozen formation can then be thawed, either naturally by the effects of latent geothermal heat, or by circulating a warm fluid (e.g., water, steam, oil, or air) through the refrigerant tubing. As used in this context, the term “warm fluid” denotes a fluid having a temperature greater than zero degrees Celsius; persons skilled in the art will appreciate that the efficacy of the thawing process will be enhanced by using fluids having a temperature considerably higher than zero degrees Celsius. Alternative thawing methods may involve circulation of hydrogen, helium, argon or other gases known to give off heat in response to a reduction in pressure. As well, known induction heating methods may be used during the thaw cycle, alone or possibly in combination with other heating methods. The effectiveness of induction heating may be enhanced by implementing “skin effect” techniques in accordance with known methods. 
         [0042]      FIG. 3  illustrates one embodiment of the packer/nozzle assembly  150 , located at the end of the stinger section  180 . A refrigerant diffuser nozzle  160 , which is connected to refrigerant supply tubing  140 , has an interior chamber  162  and a nozzle wall  164 , plus a number of outlet jets  166  extending through nozzle wall  164 . Refrigerant  70  flowing through supply tubing  140  enters interior chamber  162  and exits as expanded or vaporized refrigerant  70 E through outlet jets  166  into sub-chamber  138 . Nozzle  160  is connected to a flexible packer  170  (either directly or by means of a nozzle receiver  172  or other suitable transition element) such that packer  170  will move longitudinally with stinger  180  when stinger  180  is inserted in or retracted from return tubing  130 , while at the same time providing an effective seal against the inner wall  132  of return tubing  130 . Packer  170  may be fabricated from rubber or other suitable flexible material. Preferably, an adjustable orifice means  142  is provided in association with nozzle  160  (e.g., incorporated into nozzle  160 , or within supply tubing  140  as shown), for varying the rate and velocity of refrigerant injection into sub-chamber  138 . 
         [0043]    The effectiveness of the refrigeration cycle may be enhanced by encasing stinger  180  within a cylindrical “floating” jacket  144 , which has the effect of reducing the cross-sectional area of sub-chamber  138  and in turn increasing the velocity of refrigerant flow within sub-Chamber  138 . Refrigeration efficiency may be further enhanced by providing helical fluting  146  around at least a portion of the supply tubing  140  within the stinger section  180  (or around floating jacket  144 , as shown in  FIG. 3 ), to promote uniform diffusion of the vaporized refrigerant  70 E within sub-chamber  138 . 
         [0044]    In the particularly preferred embodiment shown in  FIGS. 4A ,  4 B, and  4 C, packer  170  comprises:
       an expandable and generally tubular bladder  80  ( FIG. 4B );   a bladder retainer assembly ( FIG. 4A ) for receiving bladder  80 ;   a flexible, expandable tubular sleeve  96  ( FIG. 4A ); and   a hollow retainer tube  100  assembly ( FIG. 4C ).       
 
         [0049]    Bladder  80  has a generally hemispherical first end  80 A having a bolt hole  81  on the axial centreline of bladder  80 , and an open second end  80 B which is securely connected to a tubular connection element  84  by means of a crimped ferrule or other suitable transition element  82  such that the interior of bladder  80  is in fluid communication with the bore of tubular connection clement  84 . Transition element  82  is formed with a flared perimeter lip  82 A at its end adjacent to bladder  80 . 
         [0050]    The bladder retainer assembly comprises an end cap  90 , a bladder transition housing  92 , and an expandable tabular sleeve  96 . End cap  90  has a generally hemispherical first end  90 A with a concave inner surface  90 B generally configured to accommodate first end  80 A of bladder  80 , and an open second end  90 C with an annular interior recess  90 D. A bolt hole  91  extends through end cap  90  on the axial centreline of end cap  90 . Bladder transition housing  92  comprises a pair of split housings  93  which, when assembled (using suitable bolts, machine screws, or the like), form a generally hemispherical assembly having:
       a first end  92 A defining an axial bore  94  with an annular shoulder  94 A;   a concave inner surface  92 B generally configured to accommodate a portion of bladder  80  adjacent to transition element  82 ; and   an open second end  92 C with an annular interior recess  92 D.       
 
         [0054]    Tubular sleeve  96  may be made of rubber or any suitable elastic material. Sleeve  96  has a relaxed (i.e., unstressed) diameter approximately equal to or slightly less than the inside diameter of return tubing  130  so that it can be easily moved within return tubing  130  when in its relaxed state, and preferably has an inner diameter approximately equal to or slightly small than the outer diameter of bladder  80 . Sleeve  96  has first end  96 A and second end  96 B configured to be received, respectively, within annular recess  90 D of end cap  90  and annular recess  92 D of transition housing  92 . A central section  96 C between ends  96 A and  96 B is thus exposed such that it will be adjacent to the bore of return tubing  130  when packer  170  is inserted therein. 
         [0055]    As illustrated in  FIG. 4C , retainer tube  100  has a closed first end  100 A and an open second end  100 B, and also has one or more spaced refrigerant openings  101  extending through its cylindrical sidewall. A bolt  102  or threaded rod extends coaxially from first end  100 A. Second end  100 B has a flared circumferential lip  104 . 
         [0056]    The assembly of this particular embodiment of packer  170  may now be readily understood with reference to  FIGS. 4A ,  4 B, and  4 C. First, bladder  80  is positioned with its first end  80 A disposed adjacent to concave inner surface  90 B of end cap  90 . First end  100 A of retainer tube  100  is into inserted bladder  80  through open second end  80 B thereof, until bolt  102  extends through bolt hole  81  in first end  80 A of bladder  80 , with flared lip  104  seated within and against tubular connection element  84 . End cap  90  is then placed over the bladder/tube subassembly such that bolt  102  extends through bolt hole  91  of end cap  90 , and a nut (not shown) is spun onto bolt  102 . Tubular sleeve  96  may then be slid over bladder  80  so as to dispose first end  96 A of sleeve  96  within annular recess  90 D of end cap  90 . Transition housing  92  is then assembled by positioning split housings  93  around transition element  82  and second end  80 B of bladder  80 , with second end  96 B of sleeve  96  disposed within annular recess  92 D of transition housing  92 , with perimeter lip  82 A of transition element  82  disposed against annular shoulder  94 A, and with second end  80 B of bladder  80  disposed adjacent to concave inner surface  92 B of transition housing  92 , thereby effectively clamping bladder  80  within transition housing  92 . With split housings  93  being securely connected to each other, the nut may be tightened on bolt  102  to complete the assembly of packer  170 . 
         [0057]    To use packer  170 , tubular connection element  84  is connected (using suitable adapter means, not shown) to a diffuser nozzle  160  having a forward jet (not shown) extending through nozzle wail  164  at or near the axial centreline of nozzle  160  (in addition to the rearwardly-oriented outlet jets  166 ). The interior of bladder  80  is thus in fluid communication with interior chamber  162  of nozzle  160  via the forward jet. Packer  170 , along with its associated supply tubing  140  is then inserted into return tubing  130 . When refrigerant  70  is introduced into supply tubing  140  and flows into interior chamber  162  of nozzle  160 , it expands and vaporizes and exits interior chamber  162  through the forward jet as well as through outlet jets  166 , such that expanded refrigerant  70 E enters retainer tube  100  and exits through refrigerant openings  101  into bladder  80 . This causes bladder  80  to inflate and expand radially outward, which results in the exertion of radially outward pressure against inner surface  96 D of tubular sleeve  96 , thus causing radial expansion of sleeve  96  such that its outer surface is urged into sealing contact with the inner cylindrical wall of return tubing  130 , whereupon the method of the invention can be put into operation to freeze-fracture an adjacent zone within the subsurface formation. 
         [0058]    To carry out freeze-fracturing operations in a different location within wellbore  110 , the flow of refrigerant is stopped, thus relieving pressure within bladder  80  such that tubular sleeve  96  returns to its relaxed state, such that packer  170  can be easily moved to anew location within return tubing  130 . 
         [0059]    Optionally, sleeve  96  may have annular grooves  97  so as to form annular ribs  98 , to enhance the effectiveness of the seal between sleeve  96  and return tubing  130  when sleeve  96  is in a radially expanded state. For the same purpose, hollow annular chambers  99  may be formed within ribs  98 . 
         [0060]    It is to be noted that the nozzle and packer assemblies shown in  FIGS. 3 and 4  are exemplary only. Persons skilled in the field of the invention will understand that nozzle/packer assemblies of various different designs and configurations could be used to beneficial effect with the method of the present invention. 
         [0061]    In a particularly preferred embodiment of the method, formation  20  is frozen in intermittent sections along the length of horizontal wellbore  110 . Stinger  180  is positioned inside return tubing  130  until it reaches an initial position in the vicinity of the toe  115  of wellbore  110 , as schematically depicted in  FIG. 2 . The refrigeration (or freezing) cycle is then initiated, resulting in formation freezing in a first zone surrounding stinger  180 , over a horizontal distance roughly corresponding to stinger length L. Stinger  180  is then partially retracted to a selected second, position within return tubing  130  so as to leave a space between the first frozen zone and stinger  180  in its second position. The freezing cycle is then commenced once again so as to create a second frozen zone, which will be separated from the first frozen zone by a substantially unfrozen zone. Stinger  180  can then be moved to a third position to create a third frozen zone laterally spaced from the second frozen zone, and so on as desired along the length of horizontal wellbore  110 . 
         [0062]    A particular benefit of this intermittent freezing method is that the presence of an unfrozen zone between freezing zones facilitates the generation of fracturing forces in three directions, not just radial forces. In alternative versions of the method, stinger  180  can be repositioned to freeze formation  20  in the unfrozen areas between the frozen zones; this secondary procedure can be carried out after the initially frozen zones have been thawed, or the thaw cycle can be delayed until formation  20  has been frozen along the full length of the wellbore. Of course, formation  20  can also be frozen in continuous linear stages, without leaving spaces between freezing zones (e.g., by simply retracting stinger  180  a distance approximately equal to L after each freezing stage). 
         [0063]      FIG. 5  illustrates how the method of the invention can be used to simultaneously freeze-fracture multiple production zones  22  at different levels within a subsurface formation  20 . As shown in  FIG. 5 , vertical wellbore  10  is cased with a well liner  12 , with cement  11  having been injected into the space between liner  12  and the surrounding formation  20 . A refrigeration apparatus in accordance with the present invention—comprising a refrigerant supply tubing string  40  disposed within a return tubing string  30 , with the lower end of supply tubing string  40  being fitted with a stinger section  170  (not shown in FIG.  5 )—is centrally positioned within wellbore  10 , creating a well annulus  16  as previously described. Suitable packers  17  (of conventional type or, optionally, ice packers) are disposed within well annulus  16  and around return tubing string  30  at selected elevations so as to block off a sub-chamber  18  within well annulus  16 . 
         [0064]    Well liner  12  and cement  11  are perforated in the vicinity of production zones  22  in accordance with known methods, thus effectively exposing sub-chamber  18  to production zones  22 . Sub-chamber  18  is then flooded with water  60 , which seeps into flooded zones  24  of production zones  22  and fills cracks and cavities  24  therein. A flow of refrigerant  70  is introduced into supply tubing  40  in accordance with the method of the invention, freezing water  60  to form ice  61  within sub-chamber  18  while freezing water within flooded zones  24 , thus inducing expansion forces to fracture production zones  22 . Optionally, well annulus  16  above sub-chamber  18  can also be filled with water to produce an “overbalanced condition” helping to direct the expansion forces from the formation of ice  61  within sub-chamber  18  radially outward from wellbore  10 . 
         [0065]    It will be readily appreciated by those skilled in the art that various modifications of the present invention may be devised without departing from the essential concept of the invention, and all such modifications are intended to come within the scope of the present invention and the claims appended hereto. It is to be especially understood that the invention is not intended to be limited to illustrated embodiments, and that the substitution of a variant of a claimed element or feature, without any substantial resultant change in the working of the invention, will not constitute a departure from the scope of the invention. By way of non-limiting example, various features and techniques described in association with freeze-fracturing formations surrounding vertical well bores (e.g., as in  FIG. 1 ) may be applied with freeze-fracturing methods associated with horizontal wellbores (e.g., as in  FIG. 2 ), and vice versa. 
         [0066]    In this patent document, the word “comprising” is used in its non-limiting sense to mean that items following that word are included, but items not specifically mentioned are not excluded. A reference to an element by the indefinite article “a” does not exclude the possibility that more than one of the element is present, unless the context clearly requires that there be one and only one such element.