Cryo-thermal Process For Fracturing Rock Formations

Abstract

A process for fracturing rock formations is disclosed using cryogenic fluids and exemplified in a process for the secondary recovery of oil wherein the major steps include: establishing a complex of elongated holes arranged in a predetermined geometric pattern which penetrate the oil production formation; injecting pressurized, superheated steam into the holes; fracturing the oil production formation using cryogenic techniques; and recovering the mobilized oil.

Parent Case Text

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of my application Ser. No. 823,306 filed May 9, 1969 for Cryo-Thermal Process For The Recovery of Oil which resulted in U.S. Pat. No. 3,581,821.

Description

FIELD OF INVENTION

This invention relates to the fracturing of rock formations.

BACKGROUND OF THE INVENTION

When oil production from a producing well ceases using primary recovery techniques, it is known that additional oil still exists in the stratigraphic trap.

Basic methods for the secondary recovery of oil include the following as applied to the oil-bearing field: (a) fire flooding, (b) water flooding, (c) steam flooding, (d) gas injection, and (e) excavation and retorting. Methods (a) through (d) employ procedures which will force the oil in place to migrate either directly or by reducing the viscosity of the oil. The latter is particularly important with regard to highly viscous oils.

Steam flooding has been used in conventional techniques to supply force and to reduce viscosity. The injected steam tends to raise the temperature of the petroleum reservoir. The resulting thermal expansion of the in-place petroleum in connection with gas pressure created by steam distillation of a portion of the petroleum forces the oil deposit to move or migrate to a relief zone. Usually this relief zone is the invasion hole drilled into the formation up through which the oil rises.

There are three zones created by conventional steam injection: first, a steam zone near the well bore; second, a hot water condensate zone where the steam condenses; and third, a water and condensate zone where the water and condensate reach the ambient temperature of the reservoir. The heat energy imparted to the deposit must be large enough to drive through two lower temperature zones to get to the deposit. In addition, the heat energy drive must be sufficient to penetrate the dirt and condensed water film covering the walls of the cavity which the injected steam creates.

In practice, the use of the conventional steam flooding technique starts with the invasion of a pay zone formation and is continued at a distinct level. The resultant cavity will eventually become so large that the steam is not able to retain its initial heat before reaching the cavity walls. This results in excessive condensation on the cavity walls and increasing difficulty in heating the cavity. The long periods required to stage a field using this technique also make it uneconomical as well as inefficient.

The present invention obviates many of the disadvantages of the conventional steam flooding operation and provides additional steps which make oil recovery techniques, particularly with regard to secondary oil recovery, highly efficient and economical.

An object of this invention, therefore, is to improve the recovery of oil using novel thermal techniques.

An additional object is to aid recovery of oil by the use of geometrically arranged and spaced hole formations.

Another object of this invention is to assist in the recovering of oil by the use of cryogenic methods.

A further object is to provide an oil recovery process which is efficient and economical using selected combinations of the above steps.

In accordance with the invention, a method for recovering oil from a formation containing oil in a non-flowable state comprises establishing a complex of elongated holes arranged in a predetermined geometric pattern which penetrate an oil production formation. The method further includes injecting pressurized, superheated steam into at least one of the holes, fracturing the oil production formation using cryogenic fluids and recovering the mobilized oil.

For a better understanding of the present invention together with other and further objects thereof, reference is made to the following description taken in connection with the accompanying drawings, and the scope of the invention will be pointed out in the appended claims.

In the drawings:

FIG. 1 illustrates a partly sectioned vertical view of an arrangement for providing pressurized, superheated steam to a bore hole;

FIG. 2 a-f shows an elevational view of embodiments of a steam jet nozzle for use with this invention;

FIG. 3 a-f has illustrated plan views of the preferred geometric hole arrangements for use with this invention;

FIG. 4 is a plan view of the five-hole pattern in more detail;

FIG. 5 is a vertical sectional view of the oil formation using the five-hole arrangement of FIG. 4; and

FIG. 6 illustrates a partially schematic and partial plan view of the steam stimulation-pressure well technique as implemented.

GENERAL DESCRIPTION OF THE INVENTION

Although the following cryo-thermal process of secondary oil recovery and its variations are to be described in general terms, it must be recognized that this technique will vary according to the characteristics of the in-place petroleum as well as the nature of the formation where the petroleum is found.

A major feature of the process is the injection of pressurized, superheated steam into a complex of elongated bore holes. Generally the number of holes will be ten or less and will be spaced between 75 feet minimum and 600 feet maximum of each other from center to center of the holes. The complex of holes is in specific geometric formation.

The use of superheated steam provides significant advantages over non-superheated steam. First, a moderate amount of superheat greatly increases the volume of the steam. The quantity of water required for a given amount of heat energy is therefore greatly reduced. Second, thermal conductivity of superheated steam is less than that of saturated steam, therefore, less heat is lost through pipe walls and more heat energy is delivered to the pay zone. Third, considerably more heat may be extracted from superheated steam for the same amount of fuel used.

The high temperature superheated steam is introduced into the hole complex under very high pressure to create thermal expansion in the oil, to lower the viscosity of the oil, to liberate the hydrocarbon gases from the in-place petroleum, and to flash much of the connate water present in the formation into additional steam. All of the above combine to generate a force to mobilize the in-place oil. In addition, pressure is rapidly built up in the hole which densifies the pay zone immediately adjacent the hole. The degree of densification of the stimulation well, in that region wherein the heat is being introduced, improves the local thermal conductivity, thus permitting more heat energy to bring about in-place petroleum migration.

Under certain conditions, the pressurized, superheated steaming alone will cause the oil adjacent the hole to move into the hole where it is carried up and out. The pressurized gases, which rise up the hole above the point at which the steam is energizing, create a pressure differential within the hole which also tends to pull out the oil in place.

A second feature of the process is the use of a cyrogenically produced oxidizing liquid such as liquid oxygen (LOX) in addition to the pressurized, superheated steam to provide additional heat energy to the pay load. Certain formations have faults or cracks which will tend to siphon off some of the injected steam, which detrimentally lower the pay zone temperature. If this occurs, liquid oxygen may be mechanically introduced into the pay zone. The effect of an oxidizing agent such as liquid oxygen being subjected to intense heat and pressure and contacting organic combustible material such as grease, oil, tar, asphalt and kerosene is to deflagrate or rapidly oxidize. The material so exposed will burn fiercely and will raise the temperature of the pay zone considerably, as well as the shaft of the elongated hole. The heat generated by the LOX oxidation will also flash the connate water in the interstices of the pay zone into additional steam. Secondarily, the deflagration will effectively contribute to fracturing the pay zone region of the formation. To reiterate, the purpose of the LOX is to accelerate the heat stimulation process. Fracturing may occur as a result of this high heat energy level by the mechanism of isotropic or anisotropic expansion. After its initial use the LOX will be used whenever the bore holes of the stimulation wells drop below the temperature of the injected superheated steam.

The hot bore hole produced by the superheated, pressurized steam and the LOX injection has further advantageous effects on oil recovery. First, the lighter hydrocarbons become gaseous and expand, which, on rising upward, expand further because of the reduction of ambient pressure. This tends to increase the pressure differential in the hole. Any bubbles of gas present in individual oil droplets tend to expand the oil, further destroying any cohesive property of the oil on sand grains and other detritus.

In addition, the existence of temperature differentials as well as pressure differentials at different levels in the bore hole effectively create a fractionating distillation column tending to break out additional chemical elements which may be present in the oil.

It should be emphasized that both pressurized, superheated steam and the LOX injection may be applied to all or selected holes of the hole complex mentioned earlier. This will largely be determined by the nature of the oil in place, the type of formation and the geographical features of the terrain.

A third feature of this process is the use of cryogenic freezing of moisture or fluid in the pay zone region. This may be accomplished by application of a cryogenic fluid to one or a number of the holes if there is sufficient moisture in the soil of the pay zone (e.g. more than 15 percent moisture content). If the moisture content is not this high, a fluid, preferably water, is injected in the desired hole or holes. Then the cryogenic fluid or solid is inserted which rapidly freezes the water. Liquid nitrogen is a preferred freezing agent although pressurized CO.sub.2 may also be effective.

The fluid in the hole is quick frozen. If it is water, it expands and swells and laterally, and to some extent vertically, displaces the pay zone in the region of the hole. The effect of this is to collapse the pores of the formation which squeezes the trapped oil and water in the interstices of the formation. Additionally, the connate water is frozen and expanded. A third effect is to improve, locally, the thermal conductivity of the pay zone both by freezing and by densification. The effect of the swelling in a central well, when coupled with the heat and pressure in adjacent or flanking well is to figuratively put the formation through a wringer and the petroleum literally wrung out.

The effect of improving local thermal conductivity is important if there is a desire to directionalize the heat flow in the pay zone. In addition to extracting the pay zone oil directly from the superheated hole, it may be desired to use certain of the extreme holes as stimulation holes (i.e. inject the high energy thermal sources there), and other extreme holes, preferably on the other side of the formation, as production holes (i.e. remove the oil there). The heat energy and pressure will cause migration of the oil from the stimulation holes to the production holes from which the oil will be removed. If the thermal conductivity of the formation were increased in the direction of migration of the oil, it would accelerate the migration process by directing the flow of heat from the stimulation holes to the production holes. To this effect, the rapid freezing of a central or pressure hole between stimulation and production holes will be advantageous.

It may be desirable to use pressurized nitrogen gas in addition to and in conjunction with the quick-freezing of the pressure well to create additional fracturing and better mobilization of oil.

The cryogenic technique may be used at any stage of the overall secondary oil recovery process as seems advantageous.

DETAILED DESCRIPTION OF The INVENTION

A more detailed discussion of specific embodiments of the invention will now be described.

Referring first to FIG. 1, a particular arrangement for injecting the pressurized steam is shown. A water pump 10 draws water through a suction line and supplies the water to a steam generator 11. The steam output is then supplied to a superheater 12 which provides the additional heat energy necessary to superheat the steam to the desired temperature. The pressure of the superheated steam will be controlled by the rate of conversion of water to superheated steam. The pump 10, steam generator 11 and superheater 12 may be provided on a mobile unit. The pressurized, superheated steam is directed through a flexible metal hose 13 which is guided by an overhead support 14. The metal superheated steam line 13, preferably created from stainless steel, is connected to a steam lance 15 which has a head 16 to which a plurality of nozzles 23 is attached. The steam lance 15 is inserted in a pre-drilled bore hole in tight formations or may be used to bore its own hole by virtue of the high pressure coming from the lance head 16. The diameter of the bore hole must be such as to accept a minimum 10 3/4 inch casing 19. The casing 19 must accommodate the steam lance 15 and the oil take-up pipe 20. Pipe 20 has attached to it pressure 21 and temperature 22 indicators which help to determine the nature of the bore hole and pay zone and the appropriate moment for oil removal. Pipe 20 is connected to storage tanks (not shown).

Although the superheated steam injection is similar whether oil is to be removed from the same hole or will be removed from a different hole, FIG. 1 is illustrative of oil removal from the steam-stimulated hole.

The heat 16 is shown in more detail in FIG. 2 a-f.

FIG. 2a shows a cross section of a jet nozzle 23 which is the heart of head 16. The nozzle 23 is shown to have an interior Venturi arrangement for optimizing the pressure of the steam. FIGS. 2b-2f show various arrangements and orientations of the nozzle as it is attached to head 16. In FIG. 2b, a single nozzle 23 is downwardly directed. FIG. 2c shows a single nozzle at 90.degree. to the vertical attached using a rounded elbow. FIG. 2d illustrates a straight tee arrangement for achieving a 90.degree. nozzle attachment which has the capability of adding an additional downward-directed nozzle. FIG. 2e indicates just such a connection by two 90.degree. oriented nozzles attached to steam head 16.

The preferred arrangement is 2f. This arrangement has five nozzles 23 connected to the steam head 16. One nozzle 23a is directed vertically downward and the others, 23b, 23c, 23d and 23e, are arranged concentrically and symmetrically in a horizontal plane. Other arrangements of nozzles 23 may also prove advantageous.

The steam slams out of jet nozzles 23 at a speed between a mach 2 and mach 4. The steam lance may thus be used to drill its own hole in loose formations.

As the steam pipe starts down the bore hole, it jets out a continuous flow of steam. The only time steaming is interruPted is when additional lengths of pipe are added. As the steam lance enters the pay zone, the descent of the lance is preferably interrupted and the region of the pay zone will be subjected to a period of steaming for approximately 15 minutes. Then the lance will continue its descent, stopping every 20 feet in soft formations and 10 feet in tight formations. The descent of the lance will continue until the bottom of the pay zone is reached. At this point, the lance will be raised some 3 feet and steaming will continue and will not be interrupted until the temperature of the bore hole drops below a certain level (e.g. 250.degree.F. less than the injected steam) at which point some other step such as LOX application will be used or the process discontinued as to this pay zone area.

Typically, in the initial steaming operation, if the temperature of the bore falls below this prescribed level, the injection of liquid oxygen will then commence. If the bore hole is large enough, the LOX will merely be dropped down the hole at intermittent intervals. In the event the bore hole is not large enough to contain both the steam line and the LOX container, steam injection will be interrupted and the steam line will be valved off, disconnected and removed. Then glass containers, about one liter in volume, filled with LOX and stoppered with a gas escape tube in place are intermittently dropped down the hole. The containers will burst open, striking the bottom of the hole and spattering the LOX which had not escaped through the gas relief tube around the region of the hole. The LOX, in coming into contact with the heated petroleum flashes into flame, developing intense heat.

In certain instances, this high localized heat may cause the silica portion of the sands in the pay zone to fuse. In this event, methyl alcohol, chilled to an appropriate temperature, to minus 60.degree. F. for example, and under a predetermined pressure, 300 psig. pressure for example, is released through a pipe down the bore hole to impact and splatter on the extremely hot fused silica. Thermal shock follows which shatters the silica, thereby creating a path for subsequently applied superheated steam.

Referring now to FIG. 3, shown are preferred arrangements of geometric hole formations for three to 10 holes. The number of holes will vary depending on the size and nature of the formation. The holes are to be used in applying the superheated steam and LOX steps of the invention and the appropriate holes for directionalizing the oil flow using the pressure well technique are indicated as well.

Regarding the pressure hole or well step, reference is now made to FIG. 4. In that figure a particular arrangement of five wells is shown to indicate the operation of the pressure well technique and the limits of the regions of influence of applied thermal energy.

The first two wells 25, 26, stimulation wells, are drilled in line. The third well 27 is spaced equidistant between the first two 25, 26 and at the same distance from the first two along a line normal to wells 25 and 26. Wells 28 and 29 are located similar to wells 25 and 26 and equidistant from well 27 along a line normal to wells 28 and 29. The center well is called the pressure well, while wells 28 and 29 are called the production wells. The approximate regions of influence of each well are shown by the larger circles surrounding each of the wells.

Well 27 is typically to be filled with water to a point some 10 feet above the pay zone of the formation. Tanks containing liquid nitrogen are lowered into the water. The liquid nitrogen tanks preferably have a quick-opening device to be controlled from the surface which rapidly expels the liquid nitrogen into the water. The water then quick-freezes and fractures the formation as previously described. An alternate approach is to circulate either liquid nitrogen or another suitable cryogenic fluid through a closed pipe in the bore hole. It may be desireable to use pressurized nitrogen gas in conjunction with the ice-creating pressures to provide additional pressures to the pay zone to further fracture the formation and mobilize the oil.

The pay zone in the vicinity of pressure well 27 is fractured which squeezes out some trapped oil and directionalizes the flow of heat from the stimulation wells 25 and 26 to the production wells. The in-place petroleum will follow the least resistant thermal path and the newly created fissures from the stimulation wells 26 and 27 to the production wells 28 and 29. The oil will then fill the bore holes of wells 28 and 29, flowing up to the surface and out. Certain highly viscous oils may require application of heat to the production wells' bore zone to increase the rapidity of flow.

After the pressure well-freezing has been applied, the superheated steam is further applied to the stimulation wells. When this is done, the well opening on the surface is closed by a collar. In the collar is welded a threaded pipe nipple.

The purpose of the nipple is to regulate the pressure build-up in the stimulation wells; in addition, it permits the attachment of a secondary heat line which may convey escaping heat energy into the bore holes of the production wells to facilitate oil flow as mentioned above.

Referring now to FIG. 5, the injection profile of the five-well approach using the center pressure well is shown, including the direction of heat flow. Also indicated are the limits of the local primary heat influence. Note that the depths of the wells are sloped toward the production wells to take advantage of natural drainage.

A preferred embodiment for implementing the pressure well technique is shown for a five-well arrangement in FIG. 6. A thermal energy generator 30 supplies the pressurized, superheated steam to a main steam line 31. The main steam line 31 is connected to the stimulation wells 32 and 33 which would typically be closed by a collar with an attached nipple as previously mentioned. Secondary heat lines 37 and 38 are attached to the nipples in the stimulation collars and connect to the production wells 34 and 35. The output oil lines from the production wells connect to line 44 which is brought to oil storage tanks 45. The gaseous portions of the production well output are forced out of the storage tanks 45 as the tanks fill up. The gas enters gas vent line 42 or auxiliary gas feed line 41. If the gas is transmitted through gas vent line 42, it is condensed in gas condenser 39 by the circulation of cryogenic fluids. The distilled gases are collected in distillate sotrage tank 40. The distillate may be drawn off by output line 43. The auxiliary gas feed line 41 disposes of the gas in some other predetermined manner. Thus, all portions of the oil well output are collected and may be used.

Extensions to a second well complex are shown by wells 47, 48 and 49. In this case, well 47 will be the pressure well and wells 48 and 49 will be the production wells. Similarly, a third and addition complexes may be added by adding three additional wells, e.g. 50, 51, 52, to the existing complex.

It should also be noted that if a fracture zone exists in the formation, it may not be necessary to use a center pressure well. Also, it may not prove necessary to directionalize the heat flow in an existing formation. What will determine if all or part of the above process is used are the following general criteria: (1) permeability of the formation pay zone; (2) porosity of the pay zone; (3) the percentage of oil, water or gas in place; (4) reservoir pressure (if any); (5) type of formation of the pay zone; and (6) the Terrastatic pressure confining the pay zone.

The following are examples of how the invention may be used in a particular oil-bearing formation. The examples were chosen to indicate multi-step application of the invention.

EXAMPLE 1

Conditions: Pay zone formation is located on the forward slope of an anticline. Depth is 1672 feet down. Permeability of the formation is 46 millidarcies. Percentage of porosity of the formation is 34.4. Of this 34.4%, 27% is oil and 56% is water, the rest being gas. A.P.A. gravity is 24.

Process: Construct the five-well complex as shown in FIG. 4. The wells are generally drilled to the depth shown in FIG. 5 taking advantage of natural drainage. Fill the center well with water from the bottom to a point 10 feet above the pay zone or 10 feet up into the rock mass capping the oil sands. Lower tanks filled with liquid nitrogen into the water and quickly release the liquid nitrogen to rapidly freeze the water, thereby trapping the nitrogen tanks in place.

Apply the superheated steam to the bore holes of both stimulation wells. Position the jet nozzles of the steam line approximately one inch from the face of the formation at the bottom of the pay zone. The nozzle is directed horizontally and tangent to the radius of influence of densification peripheral to the pressure well. Inject superheated steam at 950 psig and 950.degree.F. into the stimulation wells. The effective rate of steam flow per second is 1 1/2 lbs. (by weight).

Continue steaming for 24 hours, with a collar closure including pipe nipples around the bore holes.

After 24 hours, disconnect the steam line and remove it from the bore hole.

Drop liquid oxygen in liter glass containers down the bore hole at the rate of one every 15 minutes. Continue for 1 hour.

After LOX application, reconnect the steam lines and continue steaming for 7 days. Resume LOX treatment during the 7 days only if the bore hole temperature drops below the temperature of the steam.

After the seven days test the trapped tanks in the pressure well to determine whether they can be hauled up. (If they can, it means that sufficient heat has migrated to the center well and wholly or partially melted the ice.) After hauling up the tanks, check the water in the bore hole for gas infusion, for oil presence and for temperature. If the temperature is substantially above freezing, stop steaming, disconnect the steam lines and begin dropping LOX at 15-minute intervals for 2 hours in the stimulation wells. Reconnect the steam lines and resume steaming for another seven days. During this second 7-day interval continue checking the pressure well every 6 hours for temperature, oil show and gas infusion.

Once the entrapped tanks are removable and there are oil and/or gas infusions, it means the heat energy is migrating effectively. When the temperature of the center well reaches half that of the stimulation wells, the producing wells are quick-frozen to improve the thermal conductivity of that area by the techniques already described.

If the oil in the pay zone is more viscous between the pressure well and the producing well, than between stimulation well and pressure well, apply superheated steam to the production wells using steam lines and jet nozzles for a period of two hours at the same pressure and temperature as that existing in the stimulation wells.

If additionally necessary, remove steam lines, pump in water under high pressure of an amount in gallons equal to the pay zone depth and follow with liquid nitrogen to improve the directionalized heat flow toward the production wells.

Eventually the oil will begin to surge up the production wells.

EXAMPLE 2

Given the same conditions as Example 1 and following similar procedures, if at the end of the second 7-day period the middle well temperature has not risen substantially to at least half the temperature of the stimulation wells it means that the migrating heat energy has been stopped by a permeability barrier or fault.

Then steam or pump the water out of the center well. Connect a secondary steam line to the pipe nipple on the stimulation wells and insert in the center well. After three hours remoVe the steam line and pour some crude oil down the well so that it covers the pay zone but does not penetrate due to its viscosity. Drop LOX down the center bore hole at 15-minute intervals for an hour. Then an insulated line is used to apply methyl alcohol chilled to a temperature of approximately -60.degree.F. and forced under pressure (300 psig) down the center hole. The purpose, of course, is to apply thermal shock to fracture the formation and break through the fault. This procedure may be repeatd if necessary.

Eventually, oil or gas infusion will be visible in the center hole and the process described in Example 1 should be continued.

EXAMPLE 3

In a syncline geographic situation (trough), the general five-well implementation will use four stimulation wells and a center producing well without a central pressure well. The general procuesures of Example 1 should then be followed.

As indicated by the foregoing examples, the exact process depends on the nature of the terrain and the response of the pay zone to the applied steps.

While general reference has been made to the use of the present invention with regard to secondary recovery of oil, no limitation precludes use of any or all of the steps of this invention to primary oil recovery as well.

The utilization of this invention in the fracturing of a rock formation is characterized by flash freezing accomplished by the release of a cryogenic liquid in thermal association therewith. The employment of a cryogenic liquid is of critical importance. Heretofore refrigeration has been employed to freeze an earth or rock formation for such purposes as stabilization of the formation or preventing the percolation of a liquid therethrough. However, mere freezing has been found to be ineffective in accomplishing a useful amount of fracturing. This ineffectualness in the accomplishment of fracturing as compared with the fracturing that is obtainable in the practice of the invention by the employment of a cryogenic fluid is due to various causes. Thus, if the formation is merely subjected to the action of a refrigerant at a rate usually associated with freezing accomplished by refrigeration, the cooling off and freezing are more gradual. Under such conditions the ice formation tends to be oriented laterally to a lesser extent with resultant lessening of ting of directionalized expansive forces. There also is a tendency for water to migrate. Moreover, the expansive forces are applied more gradually and with less tendency to fracture. When, on the other hand, a cryogenic fluid is employed in intimate contact with water or with a rock formation containing water, the water that is present is frozen extremely rapidly, namely, is flash frozen as this term is known in the art. Under such conditions the crystal formation is such that the expansion forces are directed laterally to a much greater degree. Moreover, the expansion forces are applied with suddenness that is akin to a fracturing blow as distinguished from a gradually applied pressure. In addition, there is less tendency for the water to migrate and full advantage is taken of its presence at the points where fracturing is induced.

A cryogenic liquid, as this term is used herein, is one which remains liquid at a temperature of minus 298.degree.F. or less. The cryogenic liquid, as initially furnished, is under sufficient pressure to maintain it in the liquid state under conditions of normal temperature. When the pressure is released the expansion and evaporation produce extremely low temperatures virtually instantaneously, and water and rock in the adjacent area are reduced very quickly to a temperature which typically may be of the order of minus 60.degree.F. with concomitant conversion of the water to ice so suddently that fracturing of a rock formation is accomplished very effectively as the result of the flash expansion in volume upon the transformation of the water to ice.

As stated hereinabove, the fracturing may be caused solely by water contained in a rock formation so long as the water content is at least about 15 percent by volume. In such case when the amount of water is above 15 percent by volume, the flash freezing occurs in the rock formation adjacent the released cryogenic liquid, which flash freezing effectively fractures the adjacent formation. If the formation thereafter is permitted to thaw and additional water is taken up by the formation which has been rendered more receptive thereto by the initial fracturing, the amount and extent of the zone of fracture may be greatly augmented by a second release of a cryogenic liquid which causes flash freezing of the water in the formation.

As stated hereinabove, maximum fracturing effectiveness is obtained according to this invention when free water is caused to be present in the cavity in addition to any water naturally present in the formation. By the addition of free water, the amount of water in such interstices as there may be in the formation is increased to a maximum with resultant enhancement of the fracturing effect. Moreover, the body of free water, such as that in an 8-inch diameter oil well bore in the oil-bearing zone, itself applies great lateral forces to the walls of the bore with resulting fracturing effect. As stated above, ice tends to expand laterally under conditions of flash freezing. The necessity for flash freezing by the employment of a cryogenic liquid is especially well illustrated when free water is present in the bore of the oil well. When flash freezing is caused to occur, the expansion not only is sudden but most of it is lateral so as to fracture the walls of the bore. If, on the other hand, the water is gradually frozen as by the use of a refrigerant the ice as it is formed tends to accommodate itself to the confining walls of the bore with the result that most of the expansion is upwardly and there is very little fracturing effectiveness.

The enhancement of fracturing effect by repetition is particularly applicable when there is free water that is flash frozen in a cavity in the rock formation. After the initial fracture produced by flash freezing, a quantity of the free water may be remelted. For example, when water in the bore of an oil well has been flash frozen in the oil-producing zone, the ice in the upper portion of the oil-producing zone, e.g. to an extent of 5 to 10 feet, may be melted. The resulting water then is permitted to percolate into the rock formation which has been opened up by the initial fracturing. Upon again flash freezing the water in the bore, the fracturing becomes highly effective so as to extend laterally a greater distance as compared with the initial fracture.

By the employment of this invention, fracturing of rock formations such as those encountered in the oil-bearing zone of an oil well can be accomplished in greater amount both as to degree and lateral extent than that which is believed to have been possible by prior expedients.

As disclosed hereinabove, the release of the cryogenic liquid from the compressed state should be accomplished in as close proximity as possible to the zone in which it is desired to induce fracturing. This may be accomplished by the placing of a container for the pressurized cryogenic liquid in the desired proximate position and releasing it by opening a valve which quickly releases the pressurized liquid so as to rapidly become chilled and cooled to temperatures much below the freezing point of water. Alternatively, the pressurized cryogenic liquid can be directed from a container therefor to the desired location through a line having an outlet, ordinarily controlled by a valve, through which the cryogenic liquid may be released.

Fracturing induced by flash freezing according to this invention may be utilized wherever the fracturing of a rock formation is desired. Thus, it may be employed in mining which may be either the open pit type or underground. In such case penetration into the rock formation ordinarily would be of a much lesser order as regards depth of penetration and diameter of the drilled hole. However, the principle is the same as regards fracturing by flash freezing induced by release of a pressurized cryogenic liquid. In such an operation the flash freezing may be caused to occur at substantially the same time in a plurality of cavities in proximate spaced relation to each other as, for example, in connection with bench drilling. The invention has similar applicability in quarrying. It also has other applications as in tunnel driving, shaft drilling and boulder breaking.

By plugging a cavity containing free water the fracturing effect can be enhanced. Moreover, by anchoring a plug both below and above a zone where fracturing is desired the fracturing can be largely localized in the desired zone as well as rendered more effective in the desired zone. In applications such as these a container for the cryogenic liquid can be put in place at the desired location and a release valve therefor can be actuated by electrical circuit means passing through one of the plugs.

Another expedient for augmenting the effectiveness of the fracturing is that of undercutting the entrance to a cavity so that the inlet of the cavity has a smaller cross section than the cavity wherein the cryogenic liquid is released. For example, the bore of an oil well may be enlarged in the oil-producing zone of the rock formation by the employment of an expansion drill.

Liquid nitrogen is the preferred cryogenic liquid because of its inertness and likewise because of the extremely low temperatures obtainable upon its release. Nitrogen is liquid at temperatures from minus 321.degree.F. to minus 345.degree.F. It also is relatively economical to employ. Oxygen remains a liquid at temperatures of minus 298.degree.F. and lower and also is a cryogenic liquid that may be employed in the practice of this invention. However, care has to be exercised in its use in the presence of a combustible material. There are other cryogenic liquids but for reasons such as cost or hazard they are less practical to use in a commercial operation.

This invention has been illustrated in connection with an oil recovery operation wherein fracturing is accomplished by the release of liquid nitrogen in a well so as to cause water to be flash frozen to a level that is somewhat above the oil-producing zone. In preferred practice of this invention this is accomplished in increments. For example, enough liquid nitrogen is introduced into the bore of the well to flash freeze an amount of water in the bore that fills the bore to a distance of about 5 feet at the lowermost portion of the oil-bearing zone. The volume of water in this 5-foot zone of the bore in the case of a bore about 8 inches in diameter, for example, is approximately 13 gallons and flash freezing with lowering of the temperature to approximately minus 20.degree.F. may be accomplished by the release of 20 gallons of liquid nitrogen from a container that has been lowered into the bore to position immediately above the zone to be flash frozen and that is adapted to release the liquid nitrogen so as to come in intimate contact with the water in the zone and cause to be flash frozen. This operation may then be repeated in increments of approximately 5 feet each throughout the extent of the oil-bearing zone, for example approximately 40 feet, and preferably in an additional 5 feet above the oil-bearing zone. Shorter or longer increments also may be used. Moreover, the water in the region of the entire oil-bearing zone may be flash frozen in a single step.

While the preferred practice of secondary oil recovery has been described hereinabove wherein superheated steam is injected into a well that is adjoining the well in which fracturing occurs, it also is within this invention to effect cryothermal fracturing as aforesaid, permit the rock formation in the region of the oil-bearing zone to thaw out and thereafter inject superheated steam into the well to stimulate flow of oil into it so that it may be recovered.

The use of liquid nitrogen in the practice of this invention is to be distinguished from the use of liquid nitrogen or any other cryogenic fluid in the manner disclosed in my U.S. Pat. No. 3,152,651 according to which a cryogenic liquid is introduced into contact with rock which has been heated to a high temperature by means of superheated steam and which, therefore, is relatively dry, with the result that the rock is subjected to internal thermal stresses which embrittle it and cause it to fall apart rather than being fractured by the flat freezing of water. The present invention is directed to the surprising fracturing effectiveness of the flash freezing of water accomplished by a cryogenic liquid as compared with mere freezing as by exposure to sub-freezing weather or by being subjected to conventional refrigeration techniques.

While there have been described what are considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention and it is, therefore, aimed to cover all such changes and modifications as fall within the true spirit and scope of the invention.

Claims

I claim:

1. A method of fracturing a rock formation which comprises flash freezing of water by the release of a pressurized, cryogenic liquid so as to come in intimate contact therewith while said water is confined for imposing fracturing pressure on said formation concomitantly with its flash expansion in volume upon transformation into ice.

2. A method according to claim 1 wherein said cryogenic liquid is liquid nitrogen.

3. A method according to claim 1 wherein the water that is flash frozen comprises water which occurs in the interstices of the rock formation.

4. A method according to claim 1 wherein a hole is made into the rock formation and the pressurized cryogenic liquid is released in the hole.

5. A method according to claim 4 wherein water is introduced into the hole so as to occur as free water therein and wherein said free water is flash frozen by said release of a pressurized cryogenic liquid.

6. A method according to claim 4 wherein said hole is made in an oil-bearing zone of a rock formation and said cryogenic liquid is released in said zone with concomitant fracturing of said zone.

7. A method according to claim 6 wherein the fracturing of the rock formation in the oil-bearing zone is followed by the introduction of superheated steam which stimulates flow of oil into said hole.

8. A method according to claim 4 wherein a container having cryogenic liquid therein is introduced into the hole and the cryogenic liquid is quickly released therefrom.

9. A method according to claim 4 wherein a plug is anchored into the hole and the cryogenic liquid is released in the region of said hole confined by said plug.

10. A method according to claim 1 wherein the water in the rock formation is melted after the initial fracturing of the formation, additional water is added in the zone of initial fracture, and the fracturing step is repeated at the zone of initial fracture.