CA2588297A1 - Method and apparatus for stimulating production from oil and gas wells by freeze-thaw cycling - Google Patents

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

METHOD AND APPARATUS FOR STIMULATING PRODUCTION
FROM OIL AND GAS WELLS BY FREEZE-THAW CYCLING
FIELD OF THE INVENTION

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

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).

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.

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's ability to hold proppants in a uniformly-dispersed suspension. Frac fluids commonly contain a variety of chemical additives to achieve desired characteristics.

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.

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 out extraneous or deleterious materials from the fissures in the formation, thus enhancing the formation'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.

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

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.

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.

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.

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.

In a second aspect, the present invention is an apparatus for practicing the method of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to the accompanying figures, in which numerical references denote like parts, and in which:
FIGURE 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.

FIGURE 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.

FIGURE 3 illustrates one embodiment of a nozzle and movable packer assembly in accordance with the present invention.

FIGURE 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.

FIGURE 4B is a side view of an expandable bladder for use in conjunction with the retainer assembly shown in Fig. 4A.

FIGURE 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.
FIGURE 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

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.
We1110 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 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 10 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.

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 30. 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.

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).

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.

Because the lower end 32 of return tubing 30 is plugged, the expanded refrigerant 70E 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.

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.

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.

Persons of ordinary skill in the art of the invention will appreciate that mechanical or electromagnetic means for generating LF 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.

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 we1110, 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.

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 2-7/8" 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., 1-1/4" diameter, for use in conjunction with 2-7/8"
coiled tubing) is inserted within return tubing 130 as shown, with a packer/nozzle assembly 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 connected to a suitable packer 170 such that the packer/nozzle assembly is sealingly movable within return tubing 130.

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.

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 70E 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).

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.

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 70E 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.

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 70E within sub-chamber 138.

In the particularly preferred embodiment shown in Figs. 4A, 4B, and 4C, 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).

Bladder 80 has a generally hemispherical first end 80A having a bolt hole 81 on the axial centreline of bladder 80, and an open second end 80B 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 element 84. Transition element 82 is formed with a flared perimeter lip 82A at its end adjacent to bladder 80.

The bladder retainer assembly comprises an end cap 90, a bladder transition housing 92, and an expandable tubular sleeve 96. End cap 90 has a generally hemispherical first end 90A with a concave inner surface 90B generally configured to accommodate first end 80A of bladder 80, and an open second end 90C with an annular interior recess 90D. 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 92A defining an axial bore 94 with an annular shoulder 94A;

= a concave inner surface 92B generally configured to accommodate a portion of bladder 80 adjacent to transition element 82; and = an open second end 92C with an annular interior recess 92D.

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 96A and second end 96B configured to be received, respectively, within annular recess 90D of end cap 90 and annular recess 92D of transition housing 92. A central section 96C between ends 96A and 96B is thus exposed such that it will be adjacent to the bore of return tubing 130 when packer 170 is inserted therein.

As illustrated in Fig. 4C, retainer tube 100 has a closed first end 100A and an open second end 100B, 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 100A. Second end 100B has a flared circumferential lip 104.

The assembly of this particular embodiment of packer 170 may now be readily understood with reference to Figs. 4A, 4B, and 4C. First, bladder 80 is positioned with its first end 80A disposed adjacent to concave inner surface 90B of end cap 90. First end 100A of retainer tube 100 is into inserted bladder 80 through open second end thereof, until bolt 102 extends through bolt hole 81 in first end 80A 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 96A of sleeve 96 within annular recess 90D of end cap 90. Transition housing 92 is then assembled by positioning split housings 93 around transition element 82 and second end 80B
of bladder 80, with second end 96B of sleeve 96 disposed within annular recess 92D of transition housing 92, with perimeter lip 82A of transition element 82 disposed against annular shoulder 94A, and with second end 80B of bladder 80 disposed adjacent to concave inner surface 92B 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.

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 wall 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 70E 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 96D 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.

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 a new location within return tubing 130.

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.

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.

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 Figure 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.
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).

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.

Well liner 12 and cement 11 are perforated in the vicinity of production zones 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.

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 wellbores (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.

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.

Beginning on page 16 of this specification, and incorporated in its entirety as a part hereof, is a 15-page "Freeze Fracturing Test Report" providing additional information regarding the present invention and experimental testing performed in connection therewith on behalf of the inventor.

FREEZE FRACTURING TEST REPORT
Design Concept f3ackqrounci nfor'mation The rfrrsigri concepl for freeze fracturing well is to rapiclly freeze water within the production casing. f=ront this fracturing in tlie well formation will result from two mechanisnis.
'i'he first fracturing mechanism is from the icefornied in the casing as it expand in volume relative to lhe liquid waterin the space thus exerting a conipressive force on IhE wall of Itie production casiny, 1'he ice should force away any debris in the casing perforations, but most iniporlantly exert a compressive force against the casting end thus the well formation to create new fractures in the freeze z.orie.

The secorid fracturing mechanism is froni theheat transfer and freezing lhat will coniinue -is long as refrigeration is conlinuously applied. Freezing of the water beyond the production casinU thal are contained within existing cracks in the formation that will force open the cracks as the ice expands. This freeze process is designed to be conipletecI
refalively.e{uickly so ttiat niulliple freeze thaw cycles can be employed on a well in a reasonable time span lo maximize Ihe fractuuring ability of the water. Below in Figrmre i is adiagrani of a typical oil well (irrrage sruJrcr:
(ronJ ,; t: tr. lNilc,~~ac~c'Ut-r:org}.

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~M ( 1'rodltcliotl ?~yt raFU1P
~'a ~ '~ =l .i, r~ = -. n~; ~~ " t ~~ '.
i,l . f~~,(' ~ ~~'' L}t~~ . ' ~',al ~il ~ ..=,~. K r '4 "t ,{~w~ t~
wtV~p c~ypl~i~+a')i!1 ~~ht~; CC 1 ~.! , ,, i ~=xf= . ~ Yy ): .a~l'~
, ,r',~r.' ,~=,~fi,~t~ ~'~r, -~~~, ~~'ti ~r~i' '<<'' .~~+~~,, r~,~: 1~~r~
~~~',~~t,s~,.
t~ry~~.'!~1J~ J~}=~~,'1iq(T'lt,~irj+'l~'~!=".yt{~'y1 . 'y~ r-= ,.i:' - ~,t' j~)~~~~~.: ~,+; ~ ~~' j1i-~~'~;k,Ii' 7 }t~Vt=t( = FjdtR f. ~J1.r.' ir:71 ~~' 5X7~'.J=4~~:r ~)tl~1h=
~~7 r~i' n+. 'la' L~wYrl rJt,l" , 1 ~t'~ ~~ ~~k.; Oil enl~rs ~, v, tFti ' 1{ t .~,~r~ !'cTrcrrali~us ~
r=Trb,~1 ,~~ 4i'k u17U11 j~tl ~'1~"' ~.hf '~~~ ~:~ ~" '"~ ~' t %%(r ~1~ ~I~
, 4i cr ti~~r Sl; F L>, t 't1~ 4 rP~biY~ f' ~S ti S'}Ji n~,~i ~yr .c,~y1.~7 , ,'~= , , ,~ i1~~ ~ "i. ~l Z r ~ 1~,:+u $ "
r.vti = i ~Y', ~~..7ti ,~ ~n.KIL,a~,-j/.fa~t .r ,u:,a"t:.,.,rr=.":~r. :rh I~.1a.rr r.13s~ :~. (r ~. /~ = ~t..}q.C.~P_;. .l Figure 1: Typical Oil tVefl 1('0 f:)esion Criteria 'I'he general design concept (or the freeze fracttlring system was to design a portable self contauned closed circuil refrigeratiori plant that could be used on lypical slialtow oil and gas wells.
The syslem design critc-ria is a 4 to 6 ho.ur freeze, however we expect this to vary with well water ancl formation compositions. The riiain design issues that had to be addressed were:
= Refrigeranl circulation rate 4 Refrigeration capacity = Refrigerant handling and sysleni design pressure = Refrigerant temperature Refriqerant Circulalion Rate:
For a cornmercially viable freeze fracturing system the refrigeration system employed on v:ell sires must be compatible with niachinery and pipe systerns used in typical wc:lls. It was rccodnized thal Ihe freeze fracluring refrigeration system must be contained wilhin Ihe production tLlt)rnrl. in v:hich Ihe inner diameter is approximatety 2'1/7 for shallow wells and coal bed melltane w:!Ils. Tiiis t=roulcl require thal for the refrigerant to re-circulate to lhe freeze zone and back lo thLa refriqraration plant the refrigerant be supptied in a pipe within the tubing, and returriing in lhe annulus space between Ihe supply pipe and tubing. The size of ll'te supply ancl relLrrn pipe vjill diclat6' tlhe maximurtti mass flow of refrigeranl in lhe syslem, which will intern dictate the maximum rofrigeration capacity since this is proportional to lhe refrigeranf niass flow rate. Re,:rtistically nvr::inlurn flow rates thail can be acltleve trsing a 1" supply pipe ancl the resulting anrnrlus in Ihe :"-'=" in tire produc(ian tubing is 10-13 gallons per minute @ appro>:imately 'lOft of heErcl loss per 1f;Oft trf pipe Rr:frigerant l.apacily:
Initially it was proposed lo utili--e a refrigeratecl brine solution to be re-circulaled to ancl frc,m Ilrc: freeze zone. l'he refrigeration brine would be cooled in the refrigeralion plzrnl on the suri~,ce and Men pumped ciown lhe supply tubing artd relurn irr the anntdus space. Due lo the rel,,livOly smVIll supply I,rpe ancl relurn annulus pipe size available lor flow, the flow r~-Jtes woufcl be too lov:, lo achieve refrigeration capacity requirer.l for large scale cooling of tlie frec:z e r_one. 7o provicie ;.11'.1equate refrigeration capacily a ciirect expansion refrigeration system was the best option. I\nolher important benefit of this systeni is that less pumping work will be neecled as cornpFrrecf to a re-circulated liquid system as the liquid in lhe freeze area will be vapourized and rirav:inr) back to the surface as vapour, which requires much less puniping work. In this type of sySler Iuqh pressure liquid refrigerant will be supplied lo the freeze zone and allowed to "boil off"

r Ihe .1nnrrlus space. The phase change of the refrigerant froni liquid to vapor will draw heat from flre water arid well formalion outsicle of tlie produclion tubing. In the diagram below iri 1=irjorr. 2 is a cut away cliagram showing the pipe arid lubing in the freeze fracturing refrigeration . .....=.,__.__. _ _. , ~\
I- orma tion~- ~ '.:'=:: , / j"

:::,::.= :
Production Casing Fornirrl ion Freeze Zorie'- --Ice I'rocluclion Tubing f:e(rigerant Ttrbing Refrigerant ;f f Su ) )!
Y ~~'~'= ,.' ~\ ~~~ .~
I:efrigeMnl f;clurn õ . '~. .
\\'\ ..~;'' r; .. '~~~~ = l% /

~ ~ :' ,;=~~ ,s~~~~ ~~~ , i~~~ r= ~=. , ', _ir ~~~fjh' ,~ , Frc1rrr'e 2.= Diaf7ram of Freeze Fr'rx: Rc(rigeraliorn Piixr VariOus refrigeranis such as ammonia, nilrogen, and carbon dioxide tvere investigated for ilieir suitataility in this freeze fracturing refrigeration applicalion.
Carborr dioxide provided the besl cornbinalion for higli latent hcat, liquid density, vapour pressures, tr.mperatures, ease of hanclling, _rnrl ~:vailabilily, 1=a provide rapid water freezing in the freeze zone carbon dioxide will (heorolically provide len-rperalures of -45.5C at '105 psig. Nitrogen ;vas also considc:rr-,d as a sultablc: rc.friqeranl Eis even lower ternperatures were possible. but due to Ihe considerably lower i,rlc:nt heat vaporizalion relative to carbon dioxide, the maximuni theoretical refrigeration capacity ul tlle sysleni trsing nilrogen would nol be great enough to freeze a 7one of reasonable size.
R'elriner)nt hancfling anci syslern design pressure:
'I'Iie working pressures for carbon clioxide woulcl range froni approximately 90 psig, lo a rna>:inium of 1069 psig if ltie carbon clioxide reaclies 30.5C. Ammonia was also consicferecl but lo a'-:hinve Ihe low lemperalures required for this systern lhe system worrld neecl to operate al vacuum pressurras. ilnother praclical problem witli the use of antnionia woulcl be lhe Ir,xicily and IlLInIr Erbilily issues (or operalors unfamiliar witli the substartce. In Table 1 shown orl Ihe= followina t agc: are Ilie properlies of ihe refrigerants consiclered for Ihe systerrr.

{g ' Table 1: Properties of Refrigerants Carbon Dioxide Ammonia fJitrogen ...__.__._.__ Laient f=leat of Vaporization kJlk ) 322 1389 _ 48.88"
Lic)uid Density ~ 40C (m3/kg) 115.8 Oi~9=9 _ 424=7"_ _ Saturated Vapour Pressure (@ -40C (psia) 145.8 10.4 3:2 ' 5.rturatecl Vapour Pressure n 20C sia 830.7 N.A
= -This pressure is in vacuum, " These values are @-148C
Re(ric~eranl 7emperature:
In orcler to achieve Iiigh heat transfer rates froni the water and well formation to the r,_:frigerint a large temperature difference is required. The high heat transfer rate will help rc:move heat froni the water quickly so the freeze cycle tinie is short, and so that a greater cliameter of the surrouncling formation also cools to sub freezing tc:mperatures. Since there is a Izrrcte thermal mass and Iiigh temperature below ground, the lower refrigerant temperzrlures are recluired to cool as much area around the well as possible to maximize the freeze frrcturing eifect. I-)reliniinary calculations have indicated that teniperatures less than -40C will be desirable lo create su(ficienl temperature differential between lhe refrigerant and well formation to achieve the clesirecl speed of the freeze and frozen area.

C:;oeriment Desiqn Sinc:e the calculations irivolved in determining the heat transfer from the waler to the refrigerant are extremely complicated due to the flow dynamics in the water, and in fhe refrigerant as well as Ihe phase changes testing with a sniall scale refrigeration system aricl tesl becl viill provide valuMble data irl which results can be exterided to esfirnating performance in the actual ciov, n hole application. In the clesign of the test bed the temperatures and materials useci to sinlulate clovvn-hole formafion condifions niust accurately replicate the worst case scenario. This will c.lefermine if fhe proposed design can provicie sufficient refrigeration capacity, and thal arouqh ice is forr ed so that nieaningful work can be extracted from the ice.
Patterns in which tlr. ic:e. forrns ancl how the ice nielts will also be important pieces ef qualitative data frorn the experiment. Other important data froni Ihe tests will be manipufation of clesign variGrlfles in tht:
contrcrllecl tesl environment to see what impact Ihey have on systerii performance. Flelow the identified experimental variables are fisted.

Important rnanipulafecl variables in the experinient are:
= l_iquicl refrigerarit supply temperature = I_ictuicl refrigerant supply niass flow rate (Continue(l on Ilte following page) o System evaporation pressure = Dawn hole lcnlperalures = ( reeze walcr salinity In1l1nrlanl qur,rlildlive experimental data froiii tlle experiment are:
= Shape and pattern of the ice forrnalion o Type of ice fornled froiii the process = Inteclrih/ of the production tubing freml ice compression In11p0rlanl rlW1ntila6ve experimental clata froiii lhe experinlenl are:
= Duralion lo fornl ice of practical size = Duration lo clefrost ice for'nlalion = l'otal size of ice fornlation = '1'enlperalure of return refrigerant = Compressive force fronl ice expansion < Refrigeration capacily required per length of pipe 1'c l hefricleration ('lant Desicn 1=ree:_e Fracturiilg Refrigeration Plant Design: 13asecf on ihe clirect expansio(l ~c.lricl; r 11ion ccncepl, the freeze fracluring refrigeration plant will Gc requirecl to provicle low Ic:rnperalrrre carhon diOxicle liquicl to freeze ::one pipe. The carbon dioxicle refrigerant t=vill be c:uilainc:cl in the planl and production lubinc7 as a closecl loop refrigercrtion systenl. The low L.rnprralure carbon dioxide vvill be pumped froiii Ihe plant via the refrigerant lubinq down to the frt~C3ze :.one ~)nd discharged into Ifle annulus space in the produclion tubinq. The lictuicl carbon ciio::iilc: in lhr2 annulus space will absorb heat froiii the production tubirlg and "boil ofi," <Ind return rh,:: I:rlant ;: : vnpour vrith the help of lhe suction fronl rcfrigerint compressors. Compressors ill.;x nlptess Ille catbon cliosicle vapour lo a hiqh pressure, anci then lhe 11igh pressurc~ vapour ill r:onr.lnnsr; in a c.onclenser unit as I1ic)h presstrre liquid. 'f'l1e hicdh pressure liquicl '::rll hc:
llo.v ;rl tr> flasli to a low lemperature presSure liquid, lo be delivered back lo Ilie freeze zone to :onil~l lr~ Ihe closed loop refrigeration cycle. On lhe following page in l=irltrt't., 3 is a cliaclr;:mi of a t; l1ir.; ~t closerl loop refriqcralion cycle.

f=cu lhe experiments the tesl refrigeration plant must be able 10 cleliver liquicl carbon :iir,>:icf,! ,:11 I:;nillcraluri?s expeclecl for Ihe actual down-h0le refri9 eration. 'rhe desigri, fabrication, :111cl r:;apr,nse of a small scile closecl loop refrigeration plant complete will all lhe conlporienls ';:o!11SJ noi I)i' ecoflC111llcz-11ly felslbie and COLIICI IIoI be G(1lpleleCl 1V1Ihin Ih(-'. lJc)IlI III11~:I1nL' for IOSIIfI(I It SvOs I)r0OOSCCI IO tlse a refrlgerafiofl sI(ICl l'JhICh LIUI1ZP.s tc711hs of high pressl.lft liC)lllcl carbon dioxide as lhe refrigeranl, which vrould negale Ihe requirement of equipmenl such as r,omhressors as conclensers. The high pressure carbon clioxicle liquid frorn lhe supply tanl(s V1oulcl be equivalenl to refrigeranl that has been conlpressecl, ancJ then condensed by compressors ar1cJ condensers in a closed loop refrigerafion systenl, In lhe experiment syslenl the rciri3eranl V:oulci nnl be n closed loop, and the refrigerant woulcl not be usecl in a closed cycle.
I:rrlt inste;ad venlecl to atmosphere after il has provicled refrigeralion vrorl(. The liigh pressure liqui;l carbon dioxicle would be supplied lo lhe test refrigeration plant as phase ntm1ber 2 in lhe reirigeralion cycle cliagranl, ancl its pressure is sharply reducecl in the refrigeralion plant Io lower Ilic lic!uicJ Iemporallrres al 3 and punlpecJ info lhe tesl pipe which acts as an evaporolor. TFIc, iiquicl carbon clioxicle absorbs heat ancl boils off in pari =1 vihere ii is discharged to Ihc:
:1Irnospllarc. Since we are able to manipulate evapor=ator Ixessures via pressure rcclulalors on llro tc:'st r~frirat;ration skid, we can rcplicate lhe same refrigeralion corlclitions at points 3,:lnd 4 of !I. rcrrirl;.+ralion cycle, and Ihus providing r:quivalerlt refrigeralion perfornlarlce of a closed cycle r(:(riclr_r~llion lllant :=ritlloul compressors or condensers, The scclle of the refrigeraliorl lest is r,=_I,)liv,!I, snl~,lll, 010 lol.al discharge quanlily of carbon clioxide qas is relatively snlall= and so Ihe environnlenlal inlpacl should be nonlinal.

I_irv. 'I'crnperalurO =J'enlperalurc Ln:: I'ressure I_iquicl Expansion Device 1-ligh Pressure Liquicl I Hr:al In (=vaporalor Conclr ns r'.~, ~ I_le:al C>I11 I liclh Teniperattlrr: Conlpressor 1-ligll 'I'enihr~ralure !_c;v: f ressurr Vapour 1---- - ~ I 1-ligh f'ressure Vapotlr ~~----Fiqurc 3: 7 y~)ir 111%e/iirjr;v-~j(ion Cycle A cfiacJranl of tlle= lesl refrigeration skicl is provided in Appendix A at the back of Ihc raport Th,1=_,sl re1ri9eMlion planl ::=ill conlain the following maj)r components on the following paqc n Carbon rJioxide flash vessel wilh pressure regulators lo bring lhn.
tenlperalure of the high pressure refrigerant down.
= Refrigerant relurn pressure regulator to manipulale evaporalor 1'rressures.
= Liquid refrigerant punlp with flow meter lo nlonitor and adjusl flow rales.
= Autonlatic level conlrols for feedincl supply refrigerant to the plant.
1'r.~s( I?ed Desirill (=he lest bed invotves a heated and insulalerJ PVC water tank large enough to lnlerse unl of lest pipc. Water temperatures can be adjusled anci regulaled Io sin-rulate Ihe hc.al Iransfer from lhe wr=_II fornlatiorl to tlle refrigerated pipe. The physical dimensions of the test 1-11nc VVill br: full scale, bul the refriger'ation capacity, nlass flow of refrigerant of the re:friqeration I,l_rnl ~rrlll l1P, scaled clol:on to llle length of the lest bed, th115 SIr11UlatlllCJ IUIC: r:lctual rr:frigi;ration per Ieilqtil c.if pipe.

Trslinra Procecluro Teslinq v,oill be I:erforntecl in three separate stalJes each with a clifferent c:nlhlr,rsis ccn lhe r;rirr:;ralicm system in order to efficiently gather reliable ciala and thus optirni; e tlle s+,+stenl clesirln Data on heal transfer ancl ice formation will be of signific~rnt irnporlance, jrnci Iic lesl c;onclitiOn:, nlust sinlutale con(litions expected down-hole since estimations frorn calculations c~n he unrchai)lC- since lhr-= heat transfer dynamics involved in Ille system are very complicated. A
Ie~.1 run v:ill be completc;cl when the refrigeralion systenl has reached equilibriunl t=,rilh the n:irc nin ~nl. when no further ice wlll forlll.

I'Ir rsc I l h"2 phLIse 1 tOsls will be run in orcier lo eslablish Ihe effects of refrigerant supply t.:n~l~ : ralcrrc , evaporalion hressr.rre, ancl liqr.ricl flow rate on heal transfer. l'he lest pil-,e will be suI>nic.rsr:cl in lhe water reclulLIteci at a constant tenlperalure vrithoul any production pckeling.
i'hrs %Vrll DIlrnv visual inspection and documentation of where and how quickly the ice lornls orl lhc prnduC-lion fuhinrJ, and rlivc: an indication of Ihe effective heat transfer.
Nolr Ihal sinc~:: the pipe is nol j;lck,.=Ic!il by produclion lubing in Ihe test bed water, natural convection will draw Iltal to the Ice si rfnce fornlecl on Ihe pipe nlalcing this test nluch more difficull lhan actual conclilions for tlle refriyeralii,n systenl to Ilcrilcl ice. Ice thickness will be the goal in lhe second phase of Icstinc7.
C)nr.c: Ihc infcrmcition Ilas been galhered on Ihe variabh?s which most greatly affect lie.rt Iransfer.
tllcrn fl}c., systenl can be oplinlizecl which is the seconcl phase of testing.

Phase *1 Procecitrre 1. llcljusl and record initial tank lemperatures.
2. /lcljusl anci record liquid carboti clioxide supply teniperature.
3. Adjusf and record liquid carbori dioxide supply flow rale.
4. AdJust and record carbon dioxide return pressure.
f). Record lhe lime when refrigerant delivery begari.
6. Observe and record how and ilie tirne ilie ice forrns on lhe production ttrbing.
7. Observe: and recorcl any ternperalure changes in Ihe water tanic.
8, Slop refricleranl supply and conclude lest when ice growth lias slopped.
9. Record lhe arnounl of ice formed and timc-: required, 10. Recorcf ihe time when refrigeranl supply has sloppecf_ t'I. Obserw~- arncl record ilie lime it lalces to fully c.Iefrosl Ihe f7roclucli0n fuhing.
I?. fulalte changes lo test cortclitions and repeat test.

Plras":: In lhe seconcl phasc of testing onct: lhe mechanics lhat dictate heat transfi2r fronl the ~lcr tc7 Ihi: refrigeralecl pipe i, unclersloocl, llie syslern will lie lested on how quickly ;:rrtcl hot;"
Urcl; Iti~. ice can be fornied on the production tttbing, The refrigerated production pipe will be jar.keted tvith lypical prcxluction-casing in llie test bed. This will relain chilled waler :;urrotrnding Ihl. reiricleiatecl pipe ancl reduceleliminate any natural coriveclion c:ttrrents with exterior test becl ~.vcrler that would siov;, ice formafion. f=rom fhis lesl the syslem will be limed for Ihe cluration r0q1rirr!cl lo build ice cotnnletely in the procluctiori casing, and checlted if lhe production casing r.,In b,: coolecl to a poinl where ice can be rormed on lhe outside of the casinc) pipe Ac(dilionzU
forrnatiorl clirtlr "vill be recordrrd. Nalural convection can be redtteed oulsicfe of lhe= casirig pipe I?j tlic u;:,D of ~ carrar polycarbonale ltibing to isolate the waler surrounding lhe casiny. 'f I~is :=:oulcf :imulale Ihe e>;lent of Ihe fonmalion. Effects of compressive stresses on the c~i:rng pipe arrcl on Ihc refricterant pipe v:ill be carefully inspected between tesl runs.

I'liarsr; 2 larocc.ciure i Adjust and recor'cf initial lank temperalures.
Aclju si ancl n:cord fic ricl carbon dioxide supply lemperature.
3. Acljust Cuxi rec.ord liquid carbon clioxide supply flow rate.
,I. Adjust and rec.orcl carbon clioxicle returrr pressure.
:i. f;ecorcl the time when refrigeranl clelive=ry began.
G. Observe and recorci how Lind lhe tirne ilie ice forms on llte proclucti0n lubing.
(Continued on the followirig page) ;. Observa @ncl recorcf any lemperature changes in the ~a~ater Izlnk.
B. Stop refrigerant supply and conclucle test when ice grotivlh has stopped.
9. Record Illc anlount of ice fornled and lime requireCi.
'10. Record the tinle when refrigerant supply has stopped.

11. Observe r:tncl record the tinlc it takes to fully defrost the production tubing.

12. Make changes to test conditions and repeat test.

Phase 5: In the final ancl third phase of Ihe lesting is tcr deterrninc: Ilie compressive loadiny lhat can b:, achic:ved by ttlC: VoIIIllletrlc expansion qualily of water when it chatiges state to ice. The clear polycarbonale tulzing will be replaced by 8" seanlless pipe, vMcll will be capped at both ends and welded to the produclion casting for form a pressure vessel. Once filfed completely t=:illl v;ater Ille jacket will be pressured lo simulate clown-hole formation pressures, and Ihcn rr:friqi:rrrted to form ice. 7he ice Ihal will form in the vessel will expand prr=.ssur'izing ille jacket vi:ssel. I he yield slreiiglli of ice ranges from 600 to 1600psi depenciing on the ariiouril of air Irappr.cl in the ice. A pressure rdauge on Ilie vessel will recorcf pressures reached from the torn rlion of ice insicfe Ihe jackel. Once Ilie pressure tests have been complete lhe prodtrction lubinq v;ill b-0 inspectr:rl far any effects from the cornpression fronl the ice lormecf.

Phase 3 Procedure I Adjust and rccorcl initial tank temperalures.
2. Acijusl and record liquict carbon dioxide supply lemperature.
3.
Adjlrsl and recorcl liquid carbon dioxide supply flow rate.
4. Adjtrtit and recorct carbon dioxide return pressure.
!i. /ldjusl and recorc! the jacket pressure. (Cnsure there is no air in 1hc :;yslem) G. Record Ilie linie when refrigerant clelivery began.
+'. Observa ancf record pressure changes in the vessel.

8. Stop rc'frir)r?rOn1 5upply and concltlde lest wllen I)I'eSSUre I)Ulld 11c1S
Sl,lf)iliLL='Cl Or Ilas climbed lo unsafe levels.
9. Observe if lllero~ is any visible damage to Ihe prodlrelion lubing.
IU. 11bke changes to lest condilions and repeat test.

Irrr~~94s 1 and 2 r-c:speclively in Appendix 0 shows the lest refrideration plant liquicl r,arbon dioxide c.irnlainer,;, r. su;nrl~;~ry of t-r;e test equipnienl is provided on the following page Test f:cluipment:
f?efrigeralion Skici clvr bulk C02 supply system I Ic:at Iracecl t'. insulrrlecl 2'x3'x20' PVC lanlc, vol. 897.6 U.S, gals, 2-7/8" concentric steel lubing .235 wall fliickness c/w'/" S.S. inner fube d -1i2" slollecl sleel casing .205 wall thickness prepared vrith %"" slols @
'120 clegree l:,iiasing covering 3'A of total area (Continrrecl) 'I-1M" steel spiral wound inner floaling jaclcel = mulli-parj cliffuser nozzle equipped with 15/G4" primary orifice = Scheclirle 10 Allison ASTM A 53 G/ 5L B 8">:322x21' HTbVO
Test c:;onclilions:
til~r:rlr r balh lernpcarature mainlain-r.cl by 5.51(w heater and in sulalecl cover vol ~184.38 LI.S. gals.
lenrl)0r0lure +'I0 lo +?~t clegrees C
municipal supply PI-1 7.,} adjusted by acldition of salt to PI-I 8. 3 C02 refriguranl temprrature - 4+0 clegrees C
Indoor shop lemperatUre '15 cle grees C average sirnulalecl clo+vn 11ole condilions 1000psig, 24 C and saline waler a pl-I 8.3 fReSLIItS
1 Teslini=I Sunlnlar;~
Tlle ainl of lhe first phase of lesling was lo delenllirle and optimize the heal Ironsfer of Ihc: refrigerant in llle proctuction lubing. Inilial test resulls from phase 'I of Ille leslinrl were very posilive. It ':vas found Ihat lhe heat transfer of (lle water to the production pipe is n,11 urally very cloocl, which translatecl to ctuick ice fornlcition. During the various teSts to Optinllze tlc?:~I transfer it ,ivC1s clclerniinecl ltlal Ihe systelll is cIL11Ie sensltlve to the flow IllecllanlCs in the productlon ILIhIII(J.
Sincr, Ihc: flow rale of Ihe plant is scaled dowrl fronl the actual plant, Ihe heat Iransfer in Ihe test bCcl shoutcl be less Ihan aclual conclitions, so lhe resulls fronl this test should be a conservative indicrrtion of the heat transfer capability between refrigerant and water/
ice. This is clue to tlle fact illilt since II1er4 is less flow relative to thc pipe area, tJle ftow in lhe tesl case will be far less turbuh;nl Dncl ttius provide less hcat Iransfer. Icleally the flow in produclion tubinq is liiyll enougtr lo b:; in ltte hrrhulent reqion Vlllere heat transfer is greatest. Various refriyeranl clelivr:ry rlo7zles and Il esI transfer enhancement devices were lesled until a cfesign allowecl for optirnize heal Uransfer bc:hvecin refrigerant ancl ice V.1hile being practical for dowm-Ilole use.

The ice fornlation linles and thickness were recorded, but the data was usecl rnE+inly as qu,lnlilalive: clala lo compare the differences between irldividual phase 1 tesls. This clala cannot he reliat>Iy extrapolatecl lo ice forniation in clown-llole conclitions since tlle phase 1 lest conclilions do not r,.flect the heal transfcr dynamics expected down-hole. At tI1C
cOnClusion of I)hase 1 lessting heat transfor rates achi&:ved were iri excess of our preliminary calculations, ancl ice forn;ation was olso very uniforni around the pipe, F'tiasc:! I eslincl Sunlnmn, In tlie sacancl plrrise of testincl Ilie refrigerated production lubing is encasecl by Ille prucluc,liulr r;~sinq, rrncl Ihen boih are encased by a clear polycarbonale pipe which has fan inner di,irnelc-'r similar to dmvn-I1ole fornlation dianlelers. This tesl vrill sinlulale the physic;~rl confiqurotion of waler thnl %:'oulcl be expected in a clown-hole system, and total ice girth and ice fornl;ilion 1111l,;s shoulcl Ue representative to expected results down-hole.
Since lhe Ileal trarlsfer ::r3s optirnr::ecl in plrase I uf testing, ltle best refriqeration settincds, ancl heat transfer clevice;;
::ere used lo see their actual ice procluction capacity. The first couple Iesl runs werr: v,,illr nluriir.ipol laf~ :roler wilhoul any Z-lcljustnlent la Itle water pt-I.

21(~-Complete freezing of llte tesl pipe was achieved at tank water lemperafures varying from + 15 lo =+=24C, in approximately 2 lo 2.51xs. It was fotrnd Ihal ltie initial ice formalion was slowecl ~lt the beginning relalive to wlien lhe production lubing was not encased clue to Ihe I)roduction casino's E-Jbility lo conclucl heal to Itie productiott tubing. This difference effectively ar.lded adailional thermal mass lo be cooled to subfreezing temperatures before any ice coul(l be formed on II1: prociuction lubing. Once Ihe production casing was sufficittntly COGI@CI iC(: t)r0r1uCIl011 rapicfly commencecl within [lie production casing. The ice formation continuecl with ict: forming r:>;fcric+r lo lhe produclion casing and compfete freezirig of all ilie water witliin the polycarbonale Itrbt."

In thc ne>:t lest the water in Ihe lanle was dissolved wilh sodium hydroxide tci raisc the pt-I
froni 7.3 la 8.3 to test the effect of salt concentralion with freezing lime.
This concGnlration of salt v,otrlcl be Itidltest expeclecl iri ciown-hole corlditions, so [lie test was rnm with the salt water S+iilltlOn 0nc- ilutial tjnlc ternperatures of =+=24G. It was (ound tlial freeze times wcre not greally c:xlc:rlclecl hy tltc: salt in the water. Another important factor idcntifiecl dtlring Ihese lesls is thal any ci+nvective current in the waler has a large effect on [lie speecl ancJ
amounl of ir.,e Ihat can be io1-1110rJ. '1lrhen Ihe polycarbonale pipe was open=ended, the resulling natural conveclion current wilh wa(or tanlc v:ater t=xterior lo [lie pipe greatly reduced the amount of icc~ formecl aI tlie enc)s.

In ~rctual dowrl-hole condition we exper.t thal (hc waler :irxrlcl L)e n;lalivt~--Iy r.unslriclecl tivilh in the well forrnalion, and thu;: should restrict ttle fornlation of cc)rrvecaian currenls. This concluded (he second phase of Ihe lesling indicatecl tl)al enoucJfI ice in sufficient rhiC-10ness cirn be built ty) in and ciround the production casing, and in a linlc-: period thal is praraical F?ftia_;,i Te stincl Slrnlrlia Irl Ihe Ihircl and last plic7se of lesting focused an tesling the effective pressun.;, Ihal tlle r>;i)an;;ian af ice can prcicluce, and if (here was Liny issue wilh compressive datnage io Ilie procluclion lubing from lhe compressive lorces. In order for freeze frrcturing to bU irultrslrially vi,able suffici~-~n1 r.ompressive forces nlust be able lo be produced by [lie expanding ic'? wilhout anly resulling damage to Ihe produclion casing or tubing. In Fi_ryurc; 4 founcl on 111E.1 following page is a cut=rr%v,ty cliaclram showing the compressive forces ori Ihe formation ancl on Ihe refrigerant tubinq Ja~~y' r q ~ M
s 4;
Compressive Force on f ormalion Compressive 1=orce ori hefrigeranl 'f'ubing r ~' ~ , ~ Iij I=+i~i~~ , C= ~ ~ ~.~~'~~~11~~2' a ~/
Ice ~f l .\( I U j rt r~ ~~
Rc:frig_ elan! Tubing ProclLIC6011 CasInCJ

t 'V
Formalir~n +, Frurrre 4: Dirrgr'ann of Compressive Forces In this lesl lhe test pipe ancl jacket pipe was prc:-pressurized lo 1000 psict to simulate rlcrvjlt-hal,~ formation pressures. As the refrigeration commenced ancl ice formecl inside lhe pressun;, vessel, a pressure c0auge would measure any change in pressure.

In the first lest the pressure rose to approximately 2,100 psig as shown in ini~rclh 'I B. bsefore falling Luicl stabilizing at 2100 psig. The loss in pressure was possibly ciue to a leak In one ot llic, filtinOs. In the second run of lhe lest the pressure build-up was far greater runnin,i to .] peak of fl000psiq before a ruplure occurred al onc cnd of llie :asia, r-.rnil llre lesl '::as c:nclccl zII lhat poinl.
-rlie prWsslu'rDI vessel Was disassembled and lhe reffigeranl lubing inside was checked for any visible damage. L~~en after lwo cycles of testing Ilie refrigerant tubing dicl riol show any visible signs of frol"r Ihi, compressive slresses from lhe ice expansion.

2g .

Analysis ;;umrll,.~ of Results Froni lhe lhree phases of testing all the niajortlesign issues such as refrigerant tubing clarnaqc:, sufficienl ice forniation girth, and ice productiori rate were all resolved. Wllh methodical testinq proceclures "to were able to test the performance of lhe refrigeration system in controlled conditions to gather valuable data that can be reasonably applied lo actual dovrn-hole systenis.
From the testing, ice formation girth up to 6.5" was achieved in approximately 2.5hrs. The compressive force from the ice cxpansion is coriservatively arounc12,100-4000psi. Vc-Iluable operational clata was also gathered fronl the testing such as refrigeration capacity require/length of pipe, rnaxinlum refrigeration capacity, expected defrost tinies for differenl envirorlmental Ir nrl~i.r.atwes, as wetl as specialized start-up procedures for the clown-hole carbon dioxide refriqr,r,:rtion system, suc:h as preventing clry ice forniation in the refrigerant system.
Conclusion l7espite the compressed design, fabrication, and testing schedule the result:-, frorn this projec:t have been very encouraging and suggest that a viable industrial application for freeze fr;;rcturinq is a roal possibility. The next step woulcl be fi.rll scale freeze fracturing tests in an actual ":,21l site, In ils currenl specification the experimental refrideration slcid has a refrigerant (Jelivery cap,icily of approximately 3TR (refrigeralion tons.) The calculated refrigcration capacily iri u;p; rinr~~nt~~l conditions is approximately 'I lo '1.2TR, If the experimental skicl was utilized in real :vcrrlcl lests a conservative estimated freeze zone wotlld be approximately 12-15rn lonq.
Compressiori Forces ~ %' S~e~so~, I egldp ~ . ~ r ,= , I
Refngorant Dellvery õ, , Refn erant f"low~{
g ,2P;~41C1 <~~'_~4t5 ' Sz ~.i',ri~1..Lr',.Ik Te o0l,r?' 0 L ~'+ r ~ n Back to;Surface \..
. . ~, ~~..\ ~ . = ! j N." Stressed;Regiqn.
1=i9rire 5: Uiagrnnr o(Frc;r:zc: Frrclurilui !?c:qioll VVilh Ihe abilily to apply compressive forces in specific regions aricl since lhe frceze proc:e:: s, is rel::rtively quick, niultiple freeze thaw cycles should be accomplished in a worlc day to provicle aclclilional options; in formation fracturing.

Claims (31)

1. 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.

2. The method of Claim 1 comprising the further step of introducing LF wave energy into the formation in association with the freezing cycle.

3. The method of Claim 2 wherein the LF wave energy is provided in a form selected from the group consisting of electromagnetically-generated waves and mechanically-generated waves.

4. The method of Claim 2 wherein the LF wave energy is introduced into the formation by LF wave-generating means associated with the expander means.

5. The method of Claim 2 wherein the frequency of the LF waves is between approximately 15 cycles per second and 300 cycles per second.

6. The method of Claim 2 wherein the LF wave energy is pulsed.

7. The method of Claim 1 wherein the expander means comprises a section of open-bottomed tubing incorporating a streamlined flow restriction configured to induce a pressure drop in a refrigerant fluid passing the flow restriction.

8. The method of Claim 1 wherein the expander means comprises a nozzle having an interior chamber in fluid communication with a plurality of outlet jets, said nozzle being connected to the lower end of the supply tubing such that a refrigerant fluid can flow from the supply tubing into said interior chamber and out of the nozzle through the outlet jets, said nozzle having associated means for inducing a pressure drop in a refrigerant fluid passing through the nozzle.

9. The method of Claim 8 wherein one or more of the outlet jets are rearwardly angled.

10. The method of Claim 9 wherein one or more of the rearwardly-angled outlet jets are oriented to induce helical flow of a refrigerant fluid exiting therethrough.

11. The method of Claim 8 wherein the means for inducing a pressure drop comprises an adjustable orifice means for restricting the flow of refrigerant from the supply tubing into the nozzle.

12. The method of Claim 1 wherein the aqueous fluid comprises fresh water.

13. The method of Claim 1 wherein the aqueous fluid comprises a brine solution.

14. The method of Claim 1 wherein the step of ensuring that an aqueous fluid is present within the well annulus to a selected level comprises the additional step of introducing an appropriate volume of aqueous fluid into the well annulus.

15. The method of Claim 1 wherein the liquid refrigerant is selected from the group consisting of liquid nitrogen, liquid carbon dioxide, liquid propane, and calcium chloride brine.

16. The method of Claim 1 wherein the thaw cycle comprises the additional step, subsequent to discontinuation of the flow of refrigerant, of circulating a warm fluid down the supply tubing and back through the tubing annulus.

17. The method of Claim 16 wherein the warm fluid comprises a fluid selected from the group consisting of warm water, steam, warm air, and warm oil.

18. The method of Claim 1 wherein the thaw cycle comprises the additional step, subsequent to discontinuation of the flow of refrigerant, of circulating a gas down the supply tubing and back through the tubing annulus, said gas being a gas known to give off heat in response to a reduction in the pressure of the gas.

19. The method of Claim 1 wherein the thaw cycle comprises the additional step of providing heat to the first region of the formation using induction heating methods.

20. The method of Claim 1 wherein vaporized refrigerant flowing upward through the well annulus is recovered and compressed for re-introduction into the supply tubing, in a closed-loop refrigeration cycle.

21. The method of Claim 1 wherein an annular flow restrictor baffle is disposed around the supply tubing at a selected location within the tubing annulus, thereby defining an annular sub-chamber within the tubing annulus extending between the flow restrictor baffle and the closed-off end of the return tubing, said flow restrictor baffle having means for restricting the flow of refrigerant fluid from said sub-chamber into the portion of the tubing annulus above the flow restrictor baffle.

22. The method of Claim 21 wherein the means for restricting the flow of refrigerant fluids comprises an adjustable orifice.

23. The method of Claim 1 wherein the plug means is a permanent closure associated with the return tubing.

24. The method of Claim 21 wherein the plug means is a packer associated with the expander means and is sealingly movable within the return tubing.

25. The method of Claim 24 comprising the further steps of:

(a) repositioning the supply tubing, expander means, movable packer, and flow restrictor baffle within the return tubing so as to reposition the annular sub-chamber adjacent a second region of the formation;

(b) initiating a freezing cycle substantially as described in Claim 1 so as to freeze said second region of the formation; and (c) initiating a thaw cycle substantially as described in Claim 1 so as to thaw said second region of the formation.

26. The method of Claim 24 wherein:

(a) the expander means comprises a nozzle having a lead end, a coupling end, and an interior chamber in fluid communication with a plurality of outlet jets, said nozzle being connected at its coupling end to the lower end of the supply tubing such that liquid refrigerant can flow from the supply tubing into said interior chamber and out of the nozzle through the outlet jets, said nozzle having associated means for inducing a pressure drop in a refrigerant passing through the nozzle;

(b) the nozzle is connected to the packer, such that movement of the supply tubing within the return tubing will case corresponding movement of the packer.

27. The method of Claim 21 wherein a portion of the supply tubing within the annular sub-chamber is enclosed within a cylindrical jacket.

28. The method of Claim 27 wherein helical fluting is disposed around the cylindrical jacket to induce swirling flow of refrigerant within the tubing annulus.

29. The method of Claim 26 wherein the nozzle has a forward jet extending through the nozzle's lead end, and wherein the packer comprises:

(a) an expandable generally cylindrical bladder in fluid communication with the nozzle's interior chamber via the forward jet; and (b) an elastic tubular sleeve disposed around the bladder, the diameter of said sleeve in its relaxed state being slightly less than the inside diameter of the supply tubing;

such that the introduction of refrigerant into the supply tubing will cause vaporized refrigerant to enter the bladder, thereby causing inflation of the bladder and consequent radial expansion of the tubular sleeve so as to urge the sleeve into sealing contact with the cylindrical inner wall of the return tubing.

30. The method of Claim 1 wherein the wellbore is substantially vertical.

31. The method of Claim 1 wherein the wellbore includes a substantially horizontal section.