WO2016065478A1

WO2016065478A1 - Dynamic loading and thermal fracturing of hydrocarbon formations

Info

Publication number: WO2016065478A1
Application number: PCT/CA2015/051106
Authority: WO
Inventors: Fred Schneider
Original assignee: Resource Innovations Inc.
Priority date: 2014-10-30
Filing date: 2015-10-29
Publication date: 2016-05-06

Abstract

A method for creating fractures in a subterranean formation adjacent an uncased wellbore or other cavity near the formation, which comprises injecting a gaseous oxidant into the cavity to mix with a hydrocarbon fuel that either pre-exists in the cavity or which is introduced into the cavity, followed by initiating an auto-ignition reaction of a pyrophoric gas which in turn initiates combustion of the oxidant and fuel, causing shock waves to travel along the cavity and form fractures in the subterranean formation. A pressure control means such as a vent well, may be used to control the fracture pressure. The method enables the fracturing of an entire formation in one event, reduces water usage and provides thermal energy for increasing production, among other advantages.

Description

DYNAMIC LOADING AND THERMAL FRACTURING OF HYDROCARBON

FORMATIONS

BACKGROUND

[0001 ] Unconventional hydrocarbons are hydrocarbons that come from subterranean rock formations, or reservoirs, that were previously deemed unproductive and uneconomic. Due to recent technological innovations and an abundant in-place supply, unconventional hydrocarbons have emerged as the potential energy resource of the future. Shale rock and/or tight rock are examples of unconventional hydrocarbon sources which are currently being exploited for the recovery of hydrocarbons as a reliable, affordable, energy source. Relatively large reserves of hydrocarbon resources trapped in tight formations or between non-permeable laminations have become more accessible over the past decade based on the combining of two established technologies: multistage hydraulic fracturing and horizontal drilling. Historical processes to fracture rock include using dynamite, freezing, perforating explosives, pressurized water and other fluids that can hydraulically fracture.

[0002] Hydraulic fracturing is a process used in most unconventional hydrocarbon wells. Large amounts of fracturing fluids including water, sand or proppants, and chemicals, are pumped underground through a wellbore and delivered to a hydrocarbon- bearing subterranean rock formation, to hydraulically break apart the rock for release of the hydrocarbons contained inside and provide fluid pathways to promote gravity drainage. In shale formations the natural structure both encapsulates and restricts vertical drainage. Fracking techniques are therefore targeted at promoting vertical communication.

[0003] Typically, large hydraulic fracturing operations (also known as hydro- fracking or "fracking") break subterranean rock formations by using pressurized fluids to create pathways for hydrocarbon flow to the wellbore. Post-treatment, the hydrocarbons are conducted to surface through the wellbore. Hydraulic fracturing therefore "stimulates" the reservoir by simply creating fractures within a hydrocarbon formation to increase conductivity, by creating flow pathways for hydrocarbons to travel along from the reservoir to the wellbore. [0004] In unconventional hydrocarbon recovery, horizontal wells are drilled and completed with multistage fracturing in order to effectively yield more stimulated subterranean rock. Each well can utilize multi-stage hydraulic fracturing spaced along the entire length of the horizontal well, each stage requiring large volumes of water. Generally, multi-stage fracturing technology has brought enormous resource into the economically recoverable categories. However the processes are expensive and have raised environmental concerns, not the least of which is the consumption of and subsequent sequestration of large volumes of surface water.

[0005] Current hydraulic fracturing technologies use large quantities of pressurized fluids, typically water, in order to effectively fracture rock formations and stimulate the reservoir. Proponents of hydraulic fracturing point to the economic benefits of the vast amounts of formerly inaccessible hydrocarbon energy which the process can extract. Opponents point to potential environmental impacts, including consumption of large volumes of water (both fresh and brackish), risk of breakthrough to and contamination of ground water, and the hydraulic fracturing chemicals causing contamination. For these reasons typical hydraulic fracturing has come under scrutiny internationally, with some countries suspending or banning it. [0006] In response to the increasing environmental concerns and regulations surrounding hydraulic fracturing and the use of water in fracturing, the industry has proposed alternate methods of fracturing, including the use of nitrogen, also known as cryogenic fracturing.

[0007] Cryogenic fracturing involves replacing some or all of the water component of typical fracturing fluids with nitrogen. While nitrogen can be stored and transported as a liquid at ambient temperatures, the vaporization of nitrogen during the fracking process creates an auto-refrigeration affect that lowers reservoir temperatures. This has a number of negative effects on post fracking production including:

• Cooling the rock and reservoir fluids reduces gas disassociation from the liquid phase, reducing the expansion gas drive needed to mobilize the oil (the primary production mechanism). Adding incremental heat on the other hand, enhances this drive mechanism.

• In hydrocarbons containing paraffins in a liquid state, lowering of the reservoir pressure or cooling of the fluids will promote paraffin precipitation. The paraffin molecules have a tendency to flocculate and adhere to solid surfaces.

[0008] Utilizing nitrogen to frack hydrocarbon formations containing waxy oil deposits runs the risk of causing paraffin precipitation within the fractured rock that could plug the newly created fractures and reduce hydrocarbon recovery factors.

[0009] Unconventional hydrocarbons are emerging as game changing energy resource for the future, however further technological breakthroughs are required to remediate environmental concerns and increase recovery factors of these non-renewable resources. Accordingly, a need remains for a fracturing process method that overcomes the above-noted shortcomings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 is a representative drawing of prior art multi-stage hydraulic fracturing methods;

[001 1 ] Figures 2 to 7 illustrate steps in an embodiment of the method described herein. Figure 2 is a representative drawing of an embodiment of a first step of the method, illustrating the drilling of a horizontal wellbore and a vertical wellbore in fluid communication with the horizontal wellbore; [0012] Figure 3 is a representative drawing of an embodiment of a next step of the method, illustrating a surface liquid storage tank associated with the vertical wellbore, and the low pressure injection and propagation of a fuel, such as CH₄, and an oxidant, such as O2, into the horizontal wellbore and into the formation;

[0013] Figure 4 is a representative drawing of an embodiment of a next step of the method, illustrating high pressure injection and propagation of CH₄ and O2 along the wellbore and into the formation; [0014] Figure 5 is a representative drawing of an embodiment of a next step of the method, illustrating the injection of a pyrophoric gas (accelerant) for ignition of the CH₄ and O2 mixture;

[0015] Figure 6 is a representative drawing of an embodiment of the next step of the method, illustrating ignition of the CH₄ and O2 mixture, and thermal and gaseous expansion;

[0016] Figure 7 is a representative drawing of the result of practicing the embodiment of the method described in Figures 2 to 6, illustrating the fracturing of the entire hydrocarbon formation; [0017] Figure 8 is a representative drawing of an alternate embodiment of the method illustrating a sump reamed or created about a vertical wellbore for fluidly communicating with the horizontal wellbore;

[0018] Figure 9 is a representative drawing of an alternate embodiment, illustrating a vertical combustion cavity created below the wellbore for delivering fuel and an oxidant to the formation for ignition therein;

[0019] Figure 10 is a representative drawing of the embodiment of Fig. 9, illustrating an optional sacrificial liner or bladder for oxygen or fuel isolation with an associated vent valve or rupture mechanism;

[0020] Figures 1 1 A and 1 1 B are a representative drawing of the steps involved in an embodiment of the method described herein; [0021 ] Figure 12 is a representative drawing of an embodiment illustrating a sump for fluidly connecting a vertical wellbore and a horizontal wellbore, and injection of the fuel gas, oxidant and pyrophoric gas through the horizontal wellbore;

[0022] Figure 13 is a representative drawing of an embodiment of the method for dynamic loading and thermal fracturing about a slanted wellbore.

[0023] Figure 14 is a graph showing the pressure and temperature near a subterranean cavity at the bottom of a vertical wellbore substantially similar to the embodiment illustrated in Figure 9, during a small-scale test of the initiation of rapid combustion using a pyrophoric gas as illustrated in Figure 5. SUMMARY

[0024] Described is a method for fracturing a subterranean formation by initiating a combustion reaction in a subterranean cavity with a pyrophoric gas that auto-ignites on contact with oxygen. The combustion reaction generates heat and gas, which may cause a pressure/shock wave in the cavity, and consequent fracturing of the subterranean formation. The method can fracture an entire formation in one event, reduces water consumption as compared to hydraulic fracturing, and can generate heat that increases production.

[0025] In one aspect, described is a method for creating fractures in a subterranean formation adjacent a cavity, said method comprising: a. injecting a gaseous oxidant into the cavity to mix with a hydrocarbon fuel in the cavity, and b. introducing a pyrophoric gas into the cavity to initiate an auto-ignition reaction that initiates combustion of the oxidant and fuel, said combustion causing shock waves to travel along the cavity and form fractures in the subterranean formation adjacent to the cavity. [0026] The cavity may be an uncased section of a horizontal wellbore, an uncased section of a vertical wellbore, or a combustion cavity created by mechanically reaming a wellbore to increase its volume. In some embodiments the hydrocarbon fuel or portion thereof is injected into the cavity. In other embodiments the hydrocarbon fuel pre-exists in the cavity. [0027] In one embodiment, one of the gaseous oxidant or the hydrocarbon fuel is injected into a bladder disposed in the cavity, and the other of the gaseous oxidant or the hydrocarbon fuel is disposed in the cavity about the bladder. In a preferred embodiment the gaseous oxidant is disposed inside the bladder and the hydrocarbon fuel is on the outside of the bladder. In embodiments using a bladder the hydrocarbon fuel may be a liquid and in some embodiments the liquid may further comprise a proppant.

[0028] In some embodiments a first end of the cavity extends from a first cased wellbore, and method further comprises: a. providing a vent at a second end of the cavity; b. before initiating the auto-ignition reaction, temporarily isolating the vent from the cavity; and c. opening fluid communication between the vent and the cavity. [0029] The temporary isolating of the vent may be by applying a hydraulic head to the cavity. In some embodiments a moveable plug is positioned between the cavity and the vent and the hydraulic head is applied in the vent well uphole of the movable plug.

[0030] In some embodiments the pyrophoric gas is silane. [0031 ] In another aspect, described herein is a method for creating fractures in a subterranean formation adjacent a cavity, comprising the steps of: a. injecting a gaseous oxidant into the cavity to mix with a hydrocarbon fuel gas in the cavity, to form a combustible mixture; and b. introducing a pyrophoric gas into the cavity to initiate an auto-ignition reaction with the oxidant, which further causes combustion of the combustible mixture, said combustion causing Shock waves to travel through the formation and form fractures in the formation adjacent to the cavity.

[0032] The cavity may be an uncased section of a horizontal wellbore, an uncased section of a vertical wellbore, or a combustion cavity created by mechanically reaming a wellbore to increase its volume. In some embodiments the hydrocarbon fuel or portion thereof is injected into the cavity. In other embodiments the hydrocarbon fuel pre-exists in the cavity.

[0033] In some embodiments the cavity is isolated from a cased wellbore in fluid communication with the cavity. This isolation may be, for example, with a packer assembly. [0034] In some embodiments the method further comprises a. providing a vent to the cavity; and b. isolating the vent from the cavity before initiating the auto-ignition reaction.

[0035] The method of claim 16 wherein the isolating of the vent comprises applying a hydraulic head to the cavity. The method may further comprise placing a moveable plug between the cavity and the vent and applying the hydraulic head in the vent uphole of the movable plug.

[0036] In some embodiments the pyrophoric gas is silane.

[0037] In another aspect, described herein is a method for creating fractures in a subterranean formation adjacent a cavity, comprising the steps of: a. installing a bladder into the cavity; b. injecting a gaseous oxidant into the bladder and providing a hydrocarbon fuel in the cavity outside of the bladder, or c. injecting a hydrocarbon fuel into the bladder and injecting a gaseous oxidant into the cavity outside of the bladder; and d. injecting pyrophoric gas into the cavity or into the bladder, to initiate an auto- ignition reaction of the pyrophoric gas, causing rupture of the bladder and combustion of the hydrocarbon fuel and the oxidant, said combustion causing shock waves to travel through the formation and forming fractures in the formation adjacent to the cavity. [0038] The cavity may be an uncased section of a horizontal wellbore, an uncased section of a vertical wellbore, or a combustion cavity created by mechanically reaming a wellbore to increase its volume.

[0039] In some embodiments the hydrocarbon fuel is a liquid. In some embodiments the method further comprises adding a proppant to the cavity.

[0040] In some embodiments the bladder has a frangible section that releases the gaseous contents of the bladder into the cavity when the pressure differential between the inside of the bladder and the surrounding cavity exceeds a predetermined value.

[0041 ] In some embodiments the cavity is isolated from a cased wellbore in fluid communication with the cavity. This isolation may be accomplished, for example by using a packer assembly.

[0042] In some embodiments the method may further comprise: a. providing a vent to the cavity and b. isolating the vent from the cavity before initiating the auto-ignition reaction. [0043] The step of isolating of the vent may be by applying a hydraulic head to the cavity. In some embodiments a moveable plug is positioned between the cavity and the vent and the hydraulic head is applied in the vent uphole of the movable plug.

[0044] In some embodiments the pyrophoric gas is silane. DETAILED DESCRIPTION

[0045] With reference to Fig. 1 , prior art fracturing techniques involved multi-staged fracturing along a horizontal wellbore 10. The multi-staged fracturing would typically begin at a toe of the horizontal wellbore, wherein a first section of the wellbore would be isolated and fractured by the injection of high pressured fracturing fluid, mainly comprising of water and other components such as proppants, chemicals and the like.

[0046] Upon completion of fracturing of the first section, operators would reposition the fracturing tool or equipment in a subsequent section of the wellbore uphole of the first section, isolate this subsequent section and fracture the formation adjacent this subsequent section by injecting fracturing fluid at high pressure. This process of repositioning the downhole fracturing tool, isolating the section and fracturing the formation adjacent the section is repeated until all sections of the horizontal wellbore are fractured.

[0047] Described herein is a method of fracturing a hydrocarbon-containing formation that is adjacent a cavity - that is, the cavity is entirely within the formation, partially within the formation, or outside of the formation but close enough to it so that initiating a combustion reaction in the cavity will cause fracturing of the hydrocarbon- containing formation. In some embodiments the cavity is an uncased (openhole) portion of a wellbore, such as the uncased portion of a vertical wellbore or a deviated wellbore such as a horizontal or slanted wellbore. In some embodiments the cavity is a combustion cavity reamed or otherwise created within the formation about a downhole end of a wellbore. In some embodiments the cavity is a naturally-occurring cavity. While preferred methods contemplate that the cavity is entirely within the hydrocarbon-containing formation to be fractured, in some embodiments the cavity is partially within or close to that formation.

[0048] The method uses a combustion reaction to generate heat and gas in the cavity, thereby increasing both temperature and pressure in the cavity. As a result of the combustion reaction, the pressure is increased to a pressure that is in excess of the fracture pressure of the formation. More specifically, the dynamic loading fracturing method described herein comprises the injection and propagation of a hydrocarbon fuel and a gaseous oxidant into a cavity disposed in a hydrocarbon formation, and then causing the fuel and oxidant to react or combust within the cavity and optionally within the formation itself, to create a thermal expansion gas event with sufficient energy to fracture the targeted geological structure. The rapid thermal gaseous expansion of the reactants creates a pressure or shock wave that impacts the geologic structure to create fractures within the affected area. The pressure wave can either be totally absorbed within the formation or through surface venting via an associated wellbore, or by other means. As an enhancement to the technology, and in the embodiment shown in Fig. 2, a vertical well installed at the toe of a horizontal wellbore can be incorporated into the design, wherein the downhole reservoir pressure would be limited to the pressure needed to overcome annulus water head pressure. [0049] After auto-ignition, the resulting hot gases propagate into the surrounding formation. Applicant believes that horizontal wells will likely promote fracturing along a predominantly vertical plane (creating vertical reservoir permeability), while vertical wells will likely promote fractures along a predominantly horizontal plane (creating horizontal reservoir permeability).

[0050] The combustion reaction contemplated herein is the combustion of a hydrocarbon fuel and an oxidizing agent. The hydrocarbon fuel may be a liquid or a gas. Different hydrocarbon fuels can be used, according to their chemical properties and depending on the properties of the formation. As examples, CH₄ (methane), C2H8 (ethane), natural gas and diesel may be used. C2H8 (ethane) may be preferred over CH₄ (methane) when there is a need for more thermal energy to enhance the fracking process. In some embodiments, the hydrocarbon fuel gas can be injected from the surface. In alternative embodiments, or in addition, the hydrocarbon fuel gas may not be injected from the surface, as hydrocarbons from the formation can be sourced directly therefrom, most effectively, natural gas. The oxidizing agent may be a compound that contains oxygen, and includes O2 gas, air, oxygen enriched air, or other oxygen containing compounds (e.g., N2O). [0051 ] For the combustion reaction, the hydrocarbon fuel and oxidizing agent are preferably present in the cavity at an approximately stoichiometric ratio, however depending on the reservoir conditions, in some embodiments a rich or lean mixture may be used. Because some embodiments of the methods contemplate using a hydrocarbon fuel that already exists in the formation, the amount of hydrocarbon fuel and oxidizing agent that is/are injected into the cavity will be determined by taking into account the amount and type of pre-existing hydrocarbon fuel. If the combustion reaction is between CH₄ (methane) and O2 (oxygen), for example, the mass ratio of methane:oxygen in the cavity may be approximately 4: 1 assuming the reaction proceeds as follows:

[0052] This mass ratio may be achieved by injection of these gases into the cavity in a 4: 1 ratio, or by reducing the amount of methane/increasing the amount of O2 injected, after taking into consideration any pre-existing methane that is in the cavity.

[0053] In the methods contemplated herein the combustion reaction between the hydrocarbon fuel gas and the oxidant may be initiated by use of a pyrophoric gas. A pyrophoric gas ignites spontaneously in air at or below 54 °C (130 °F). Exemplary pyrophoric gases potentially useful for the methods described herein are silane, diborane, arsine and phosphine. Silane is a preferred pyrophoric gas contemplated for use herein, and it may auto-ignite via the following reaction:

[0054] Applicant contemplates at least two general embodiments for initiating the combustion reaction by auto-ignition of the pyrophoric gas. In a first embodiment, the cavity is filled with a mixture of a gaseous hydrocarbon fuel and the gaseous oxidant. The pyrophoric gas is then added to the mixture in the cavity, alone or in combination with a slug of oxygen, wherein it will auto-ignite. Injection of silane concomitantly with a slug of oxygen leads to a more reliable auto-ignition, however this is not always necessary. In this embodiment of the method, for safety and efficacy reasons, care is taken to prevent auto-ignition of the mixture of the gaseous hydrocarbon fuel and the gaseous oxidant in the cavity, before the pyrophoric gas is added. This embodiment of the method has the advantage that it can be repeated one or more times in the same cavity. [0055] In a second embodiment, a bladder is inserted into the cavity, and the bladder may be filled with either the hydrocarbon fuel (gaseous or liquid) or with the gaseous oxidant; the cavity surrounding the bladder is filled with either the gaseous oxidant, or the hydrocarbon fuel (gaseous or liquid), respectively. The pyrophoric gas is then added to the cavity alone or in combination with a slug of oxygen, wherein it will auto-ignite causing the bladder to rupture and release its contents for the combustion reaction to proceed. In this embodiment there is less risk that the combustion reaction between the hydrocarbon fuel and the gaseous oxidant will initiate before the pyrophoric gas is added, leading to better control of the fracturing process. However re-treatment of the same cavity is difficult using this method, as a second bladder would need to be installed in the cavity.

[0056] Some embodiments of the method further contemplate the use of a pressure control mechanism to limit the pressure in the cavity and formation resulting from the combustion reaction. The pressure control means assists in ensuring that the pressure in the cavity does not exceed a predetermined level which would damage the cavity (e.g., wellbore) or the formation, which would be unsafe, or which would lead to otherwise unpredictable results. The pressure control means also assists in ensuring that a selected predetermined pressure, optimal for fracturing, is attained in the cavity. The pressure control means may be, for example, a column of dampening fluid, a mechanical pressure release valve, a bridge plug, a wiper plug, a packer, or some combination of the above, that is fluidly connected to the cavity, and which can absorb some of the pressure generated therein by the combustion reaction. [0057] Having thus described the basic method, specific embodiments will now be described, as shown in the accompanying Figures. While the method will be described in conjunction with these embodiments and the Example, it should be understood that this is by illustration only. The scope of the claims should not be limited to the preferred embodiments but should be given the broadest interpretation consistent with the description as a whole.

[0058] An embodiment of the dynamic loading and thermal fracturing method is now described with reference to Figs. 2 to 7. As shown, this embodiment comprises drilling a first wellbore having a wellhead 75, an upper section 70 comprising casing 25 cemented into the wellbore, and a lower uncased horizontal section 10, such as an openhole wellbore. A second vertical cased wellbore 20 is drilled near a toe 30 of the horizontal section 10, and fluidly connected thereto; or in an embodiment the vertical wellbore 20 can be drilled to intersect the toe 30. The vertical wellbore 20 creates a directional path for any fluids that are injected into the horizontal section 10, and further serves as a vent.

[0059] With reference to Fig. 3, prior to charging the horizontal section 10 with the hydrocarbon fuel and oxidant, a surface liquid storage container or tank 40 can be fluidly connected to the vertical wellbore 20, and this section of the wellbore can be isolated from the horizontal section 10 by, for example a packer or a sacrificial plug 50, which can be positioned in the vertical wellbore 20 adjacent toe 30 of the horizontal section 10.

[0060] Two conveyance strings 60 and 62, such as coiled tubing strings, can be run into the vertical section 70 of the first wellbore to about the heel 80 of the horizontal section 10. Packers 85 are set in the annulus between the casing and the conveyance strings, thus isolating the vertical section 70 from the horizontal section 10 of the wellbore.

[0061 ] The uncased (openhole) horizontal wellbore section can then be charged by injecting a fuel, such as a hydrocarbon fuel gas, for example natural gas or CH₄, and an oxidant, such as O2 gas into the wellbore. In an embodiment, and as shown, the oxidant can be injected into the horizontal section 10 through the bore of the conveyance string 60, while the fuel can be injected into the horizontal section 10 through the bore of the conveyance string 62.

[0062] In an embodiment, and if necessary the hydrocarbon fuel can be spiked to a concentration well below its lower explosive limit (LEL) with air or enriched air to enhance the exothermic reaction and promote auto-ignition.

[0063] The injected oxidant (O2) and fuel (CH₄) can mix at about the heel 80 of the horizontal section 10 and then flow along the horizontal section to towards the toe. As shown by the arrows entering the formation in Figs. 3 and 4, the fuel (CH₄) and oxidant (O2) can flow along the horizontal section 10 of the wellbore, and may enter into the surrounding formation and propagate or permeate through the formation using natural fissures and cracks, and/or pre-existing pores.

[0064] The hydrocarbon fuel gas and oxidant may be injected at approximately a stoichiometric ratio. The use of an approximately stoichiometric ratio of the injected fuel and oxidant maximizes consumption or reaction of the injected O2 during subsequent ignition. That is, there is a limited amount of O2 injected into the formation, allowing for more precise control of ignition. [0065] Fig. 4 illustrates the thermal gaseous expansion of propagation of the gaseous fuel and oxidant into the surrounding reservoir within the formation adjacent the horizontal section 10. The propagation may move from adjacent the heel 80 towards the toe 30. The embodiments shown in Fig. 3 and 4, depict a low pressure injection followed by a higher pressure injection of the gaseous fuel and oxidant. In other embodiments of the method only a higher pressure injection is used. The pressure in the formation before the pyrophoric gas is injected (i.e., before initiation of the combustion reaction) is sufficiently high so that, on combustion, the pressure in the well will exceed fracture pressure, thereby fracturing the formation. [0066] High pressure injection increases the process yield, ensuring predetermined energy release. Subsequent, secondary fracking events could be initiated within the thus created high permeability region, or alternately within depleted regions of a target reservoir by simply injecting oxygen into the fractured structure and auto-igniting.

[0067] The high pressure hydrocarbon fuel (CH₄) and oxidant (O2) injection increases the overall reservoir pressure which may cause the sacrificial bridge plug 50 in the second vertical wellbore 20 to dislodge. Accordingly, the increasing reservoir pressure acting on the bridge plug 50 can be counteracted with a head of dampening fluid 90, such as brackish water, above the bridge plug 50. The head of dampening fluid 90 can further act to control any Shockwaves or pressure increases caused by the subsequent ignition of the CH₄ and O2. [0068] In an embodiment, a combustion cavity 100 can be reamed or otherwise created about the heel of the horizontal wellbore wherein the auto-ignition of the silane and subsequent ignition of the hydrocarbon fuel and the oxidant can be initiated.

[0069] With reference to Figs. 5 and 6, after a period of time sufficient to maximize propagation of the injected fuel (CH₄) and oxidant (O2) into the formation (if this occurs at all), the fuel and oxidant are ignited. In a preferred embodiment and as shown in Fig. 5, a slug of pyrophoric gas (accelerant), such as silane, can be injected downhole to mix with the fuel (CH₄) and oxidant (O2) mixture. The pyrophoric gas can be introduced alone into the mixture via conduit 62, or it can be administered with additional oxygen (which is introduced via conduit 60). The introduction of the pyrophoric gas to the CH₄ and O2 mixture, under downhole conditions, causes spontaneous combustion or auto-ignition of the pyrophoric gas, and subsequent ignition of the CH₄ and O2 mixture, to result in an exothermic explosive reaction (see Fig. 6).

[0070] In an embodiment, manipulation of oxidant or hydrocarbon fuel concentrations through the use of a containment bladder can ensure maximum consumption of injected oxidant during combustion events. That is, there is a limited, targeted volume of oxidant injected into the formation to promote auto-ignition and sustain the combustion event, allowing for more precise control of ignition (see Fig. 10).

[0071 ] The injection of the pyrophoric gas and/or oxidant can be chased with a nitrogen gas purge, the inert nitrogen being injected through the bore of the conveyance string used to deliver these compounds to the wellbore. [0072] As shown in Fig. 7, the exothermic reaction creates a gas expansion front that propagates throughout the formation through existing fractures (where possible). This increases the existing network of channels within the geological structure, creating additional pathways which may release trapped hydrocarbons therein and permit them to move therethrough. The reaction also cascades from about the heel 80 towards the toe 30 of the horizontal section 10, and from the horizontal section 10 radially into the surrounding formation on a vertical plane, creating further fractures. The thermal and gaseous expansion cooperatively causes multiple fractures within the formation surrounding the entire length of the horizontal section 10. [0073] The reservoir access created by the fractures not only improves reservoir permeability and hydrocarbon mobility, it also allows thermal propagation into the formation that enhances the expansion gas drive, reduces viscosities and promotes liquid expansion.

[0074] As the expansion gas reaction increases reservoir pressures, the sacrificial plug 50 could force the head of dampening fluid, such as water, in the second vertical wellbore in a manner to create a hydraulic pressure relief mechanism. The liquid head pressure can act as a dampener and pressure control mechanism as the reservoir pressure displaces the dampening fluid to the surface for collection and reuse.

[0075] Figs. 3 to 7 illustrate the fuel and oxidant being injected through the first wellbore 70 and the dampening fluid in the second vertical wellbore 20. In alternate embodiments the fuel and oxidant can be injected through the second wellbore while the dampening fluid can be injected through the first wellbore (see e.g., Fig. 1 1 ). In such instances, the surface storage tank 40 should be fluidly connected to the first wellbore.

[0076] In an alternate embodiment of the method, and as shown in Fig. 8, a blow case sump 100, can be reamed or otherwise created within the formation about a downhole end of the vertical wellbore 20. The sump 100 is created sufficiently large to fluidly connect the horizontal section 10 with the vertical wellbore 20.

[0077] After low pressure propagation of fuel (CH₄) and oxidant (O2) into the horizontal wellbore, the vertical wellbore 20 and the sump 100 can be filled with a dampening fluid 90, such as brackish water, to create a head of fluid in the vertical wellbore 20. The height of this liquid column within the vertical well bore is directly linked to reservoir pressure (i.e., height of the head of fluid) As the reservoir pressure increases as fuel gas is injected, the liquid level increases to accurately reflect the near wellbore pressure downhole.

[0078] As shown, the sump 100 can be filled with the dampening fluid by a conveyance string 1 10 fluidly connected to the surface liquid storage tank 40. The annulus between the conveyance string 1 10 and the vertical wellbore 20 can be plugged using a bridge plug, cement 1 15, or by other means.

[0079] Once the sump 100 is filled with the dampening fluid to create a liquid seal between the reservoir vapor phase and the wellbore, high pressure hydrocarbon fuel (CH₄) and oxidant (O2) can be injected into the horizontal section 10 while the operator monitors and supplements the volume of dampening fluid available to maintain the liquid seal. Subsequently, auto-ignition can be initiated as previously described above. [0080] As previously described, upon auto-ignition of the hydrocarbon fuel source and the oxidant, the explosive reaction causes both thermal and gaseous expansion from about the heel 80 of the horizontal section 10 towards the toe 30, and from the horizontal section 30 outwardly into the formation, causing fracturing of the formation. [0081 ] Residual energy from the combustion reaction can be limited and dampened by the second vertical wellbore 20 and sump 100. The residual energy can be absorbed by the head of dampening fluid within the second vertical wellbore 20, causing the dampening fluid to rise. In the case of excessive pressure, the dampening fluid can flow to the surface and into the surface storage tank through the conveyance string 1 10 in the vertical wellbore 20. Any fluids forced to the surface would be captured for storage and eventual reuse from surface storage tank 40.

[0082] In a further embodiment of this method, shown in Fig. 12, the second vertical well 20, in addition to the first horizontal well 10, is used as a production well. The top panel of Fig. 12 shows the explosive oxidation reaction causing both thermal and gaseous expansion and fracturing of the formation (arrows). The bottom panel of Fig. 12 shows mobilized hydrocarbons from the formation draining by gravity into the horizontal wellbore and into the sump 100 (vertical arrows) and being produced from the second vertical well and the horizontal well, with a thermally-enhanced gas drive.

[0083] With reference to Fig. 9, an embodiment of this dynamic loading and thermal fracturing method can be adapted to be used with a single vertical wellbore in a hydrocarbon formation, where the geological structure is prone to channelling because of the inherent permeability. Accordingly, Applicant believes that fracturing along a horizontal plane within such formations using dynamic loading and thermal fracturing can create incremental horizontal permeability in conjunction with enhanced fluid drive mechanisms; reduced tendency for early breakthrough on gravity drainage applications, solution gas drive, over riding gas drive, decreased viscosity, etc. [0084] In an embodiment, and as shown, a vertical wellbore 20 can be drilled into a hydrocarbon reservoir having geological structures that promote vertical channeling and inhibit horizontal communication necessary to support enhanced hydrocarbon exploitation. A combustion cavity 105 can be reamed out or otherwise created about the downhole end of the wellbore 20. In an embodiment, the combustion cavity 105 can be drilled into the bottom rock structure below the hydrocarbon rich pay zone, typical of most carbonate structures.

[0085] Conveyance strings 120 and 122, such as coiled tubing strings, can be run down the vertical wellbore 10, such that a downhole end of the conveyance strings 120 and 122 are positioned within the combustion cavity 105. A packer 50 may be used to isolate the combustion cavity 105 from the cased wellbore 20 above. In an embodiment, and as shown in Fig. 10, an isolation bladder 130 can be used to provide segregation of the two reactants, fuel and oxygen.

[0086] As previously disclosed, and as shown in Fig. 9, a gaseous hydrocarbon fuel, such as CH₄, can be injected into cavity 105 via conveyance string 122, and a gaseous oxidant, such as enriched air or O2, can be injected into cavity 105 via conveyance string 120. [0087] Once the fuel and oxidant reach the desired pressures in cavity 105, a slug of pyrophoric gas, such as silane, can be injected into the cavity via conveyance string 122. A slug of gaseous oxidant may also be injected into the cavity via conveyance string 120, to facilitate the auto-ignition reaction. The injection of the pyrophoric gas can be chased with a nitrogen gas purge, the inert nitrogen being injected into the bore of the conveyance string 122 and optionally also into the bore of conveyance string 120.

[0088] The ignition of the fuel and oxidant causes the combustion reaction to proceed, leading to an increase in temperature and pressure within the cavity and outwardly into the formation, causing fracturing of the formation. [0089] In an alternate embodiment of the method, and as shown in Fig. 10, a bladder 130, which may be flexible, inflatable or rigid, and a valve mechanism 140 can be incorporated into the conveyance string 120 to act as a trigger mechanism and a finer control for auto-ignition of silane and ignition of the fuel (CH₄) and oxidant (O2). The bladder may be, for example, a plastic liner. The bladder may be filled with hydrocarbon fuel or oxidant, and the cavity surrounding the bladder with oxidant or hydrocarbon fuel, respectively, thus separating the fuel and oxidant until ignition by the pyrophoric gas. In some embodiments a mixture of oxidant and hydrocarbon fuel may be disposed outside of the bladder within the cavity. In embodiments using a bladder, the hydrocarbon fuel within or outside of the bladder may be liquid or gaseous. [0090] If there is oxygen in the bladder, silane may be injected into the cavity surrounding the bladder via a conveyance string, which may be the same conveyance string through which the methane was injected into the cavity. The cavity contains methane and in some embodiments also oxygen. Oxygen may be injected at the same time that the silane is injected, but down a separate conveyance string. This may be the conveyance string that is connected to the bladder, and at the interface between the conveyance string and the bladder a valve assembly may be operated to switch the outlet of the conveyance string from the bladder to the surrounding cavity. The oxygen would now flow into the surrounding cavity to contact the silane, causing the silane to auto- ignite.

[0091 ] If there is methane in the bladder, silane may be injected down the same tubular as the methane but out into the cavity via a valve that can direct a gas from the conduit into the bladder or into the cavity. The cavity contains oxygen and in some embodiments also methane. Oxygen may be injected at the same time as the silane is injected, down the other tubular.

[0092] In some embodiments of the method the bladder 130 is disposed along all or a part of the horizontal section of a horizontal well. This may be accomplished, for example, as described in US 4,064,935. One preferred embodiment of the method uses a liquid hydrocarbon fuel, such as diesel, in the cavity, and a gaseous oxidant in the bladder. The liquid fuel may optionally contain a proppant, so that when the combustion reaction occurs the proppant is disposed in the fractures formed.

[0093] In one embodiment, once the cavity is filled with the desired amount of fuel (CH₄) and/or oxidant (O2), a rupture mechanism or valve mechanism 140 can cause the bladder 130 to discharge the oxidant or fuel into the combustion cavity 105. Then, when the pyrophoric catalyst comes into contact with the oxidant (or added oxidant), spontaneous combustion or auto-ignition causes a thermal and gaseous propagation. The injection of the pyrophoric gas causes a threshold combustion event sufficient to melt the plastic liner and permit the oxidant and fuel to mix in the cavity. The two combusting reactants create a gas expansion front moving radially away from the wellbore, subsequently migrating into pre-existing fractures with enough energy to further fracture the associated geological structure.

[0094] In an alternate embodiment, the fuel in the combustion cavity 100 can be enriched with a concentration of the oxidant, although care must be taken to ensure that the fuel to oxygen ratio stays below its lower explosive limit (LEL), in order to prevent premature auto-ignition.

[0095] Further still, in another embodiment, other means for breaching the plastic liner 130 can be used, such as a frangible section on the plastic liner 130. This frangible section can initiate rupture of the bladder when the bladder is pressurized beyond a selected pressure by the gas contained therein, for example. [0096] Applicant believes embodiments of vertical dynamic loading and thermal fracturing can be applied to formations that are prone to vertical channeling, such as carbonate formations and/or formations that have undergone primary production, for example CHOPS (cold heavy oil production with sand) depleted formations. In this embodiment, existing channels in the formation form the directional flow path, thus obviating the need for a second vertical wellbore beyond the existing CHOPS wellbore.

[0097] Figs. 1 1A and 1 1 B illustrate various steps in another embodiment of the dynamic loading and thermal fracturing method described herein. As shown, in this embodiment, a directional path can be created by fluidly connecting the uncased (openhole) horizontal wellbore 10 to a second vertical wellbore 20 by forming a combustion cavity 100, such as by reaming or other known methods, or by directly intersecting the vertical wellbore with the horizontal wellbore (top panel in Fig 1 1 A). Dampening fluid 90 can be injected into the vertical portion of the horizontal wellbore (and possibly also into the vertical wellbore) to serve to dampen or absorb the pressure wave that will result from dynamic loading and thermal fracturing (middle panel in Fig 1 1 A). A wiper plug 50 may be inserted to isolate the uncased (openhole) horizontal wellbore from the column of dampening fluid. A surface liquid storage tank 40 can be fluidly connected to either or both of the horizontal wellbore or the vertical wellbore for receiving and storing any dampening fluid that may exit the wellbores during fracturing.

[0098] Hydrocarbon fuel gas and gaseous oxidant can be injected into the cavity 100 using concentric tubing via conduits 62 and 60, respectively, and permitted to propagate throughout the wellbore and potentially also into the formation, to saturate the formation as much as possible with a mixture of fuel gas and oxidant. The gases may be injected first at a low pressure (just above formation pressure) and second at a higher pressure (a pressure that will result in a pressure, on combustion, that exceeds fracking pressure) (middle and lower panel in Fig 1 1 A). Conduits 60 and 62 may further comprise check valves as a back pressure control means. [0099] After the fuel and oxidant are injected, a pyrophoric gas, such as silane, can be injected into cavity 100 via conduit 62 (top panel in Fig 1 1 B). Oxygen can be concurrently injected into cavity 100 via conduit 60, to enhance the auto-ignition of the pyrophoric gas. The injection of the pyrophoric gas and oxygen can be followed by a purge of inert gas, such as nitrogen, for safety or other reasons.

[0100] As the pyrophoric gas reaches the combustion cavity, it will auto-ignite, leading to initiation of the combustion of the fuel and oxidant therein (middle panel in Fig. 1 1 B). Wiper plug 50 and the column of dampening fluid may be forced upwards in wellbore. The thermal energy and gaseous expansion caused by the propagating combustion reaction forms fractures throughout the formation (bottom panel in Fig. 1 1 B).

[0101 ] Fig. 13 illustrates an embodiment after the fracturing method described herein was employed on an uncased (openhole) slanted wellbore 10, which is in production phase. As shown, a second vertical wellbore 20 was drilled and fluidly connected to the slanted wellbore 10 by a sump 100 located near toe 30 of the slanted wellbore. During fracturing the sump was a water sump used to control pressure during the combustion stage, as described previously. In production it is an oil sump, the oil being delivered uphole via pump 150. [0102] Applicant believes that embodiments of the dynamic loading and thermal fracturing method offer at least the following advantages: 1 ) the ability to fracture an entire formation in one event; 2) the reduction of water consumption and sequestration; 3) the reduction of the use of fracturing fluids; 4) the addition of thermal energy for increasing production; 5) and increase and maintenance of reservoir pressures for increasing production; 6) a reduction of risk of paraffinic precipitation in waxy deposits; and 7) a reduction of surface footprint. EXAMPLE

[0103] The following is a representative example of the fracturing method described herein.

[0104] The method of initiating rapid combustion in a subterranean cavity to initiate a shock wave for the purpose of creating fractures in a formation to stimulate the production of oil or gas has been tested in a small-scale pilot arrangement. Applicant simulated the ignition process described for example in Fig. 5, using a vertical well arrangement substantially similar to the arrangement shown in Fig. 9.

[0105] More particularly, a combustion cavity at the bottom of a vertical well was reamed such that the target formation was exposed at the downhole end of the wellbore. Two coiled tubing conveyance strings were run down the vertical wellbore such that the downhole end of the conveyance strings were positioned within the cavity.

[0106] Low volumes of CH₄ were injected through one tubing string while similarly low volumes O2 was injected through the second tubing string. The flow of oxygen was stopped and a flow of silane was injected through the CH₄ tubing string. The flow of O2 was resumed once the silane exited the tubing string within the cavity. Temperature in the reservoir was about 20°C, and pressure about 500 kPa.

[0107] The graph in Fig. 14 shows the temperature and pressure near the cavity, as measured by instrumentation installed in a water column in the wellbore above the cavity and in fluid communication with the cavity. The graph shows a rapid rise in pressure to about 900 kPa, and temperature to about 100°C coinciding with the auto- ignition of the silane and O2 mixture, which in turn initiated rapid combustion of the O2 and CH₄ mixture. After ignition was observed through the pressure and temperature indicators, the flow of oxygen was stopped and the pressure and temperature returned to pre-ignition levels. This small-scale, low-pressure test demonstrates the feasibility of a silane accelerant to achieve auto-ignition and initiation of rapid combustion, and that higher-volume, higher-pressure injection of O2, CH₄, and an accelerant will prove effective when utilized in the embodiments described herein.

Claims

1 . A method for creating fractures in a subterranean formation adjacent a cavity, said method comprising: a. injecting a gaseous oxidant into the cavity to mix with a hydrocarbon fuel in the cavity, and b. introducing a pyrophoric gas into the cavity to initiate an auto-ignition reaction that initiates combustion of the oxidant and fuel, said combustion causing shock waves to travel along the cavity and form fractures in the subterranean formation adjacent to the cavity.

2. The method of claim 1 wherein the cavity is an uncased section of a horizontal wellbore, an uncased section of a vertical wellbore, or a combustion cavity created by mechanically reaming a wellbore to increase its volume.

3. The method of claim 1 or 2 wherein the hydrocarbon fuel, or a portion thereof, is injected into the cavity.

4. The method of any one of claims 1 to 3, wherein one of the gaseous oxidant or the hydrocarbon fuel is injected into a bladder disposed in the cavity, and the other of the gaseous oxidant or the hydrocarbon fuel is disposed in the cavity about the bladder.

5. The method of claim 4 wherein the gaseous oxidant is injected into the bladder and the hydrocarbon fuel is a liquid.

6. The method of claim 5 further comprising adding a proppant to the cavity.

7. The method of any one of claims 1 to 6, wherein a first end of the cavity extends from a first cased wellbore, the method further comprising: a. providing a vent at a second end of the cavity; b. before initiating the auto-ignition reaction, temporarily isolating the vent from the cavity; and c. opening fluid communication between the vent and the cavity.

8. The method of claim 7 wherein the temporary isolating of the vent comprises applying a hydraulic head to the cavity.

9. The method of claim 7 further comprising placing a moveable plug between the cavity and the vent and applying the hydraulic head in the vent well uphole of the movable plug.

10. The method of any one of claims 1 to 9, wherein the pyrophoric gas is silane.

1 1 . A method for creating fractures in a subterranean formation adjacent a cavity, comprising the steps of: a. injecting a gaseous oxidant into the cavity to mix with a hydrocarbon fuel gas in the cavity, to form a combustible mixture; and b. introducing a pyrophoric gas into the cavity to initiate an auto-ignition reaction with the oxidant, which further causes combustion of the combustible mixture, said combustion causing Shock waves to travel through the formation and form fractures in the formation adjacent to the cavity.

12. The method of claim 1 1 wherein the cavity is an uncased section of a horizontal wellbore, an uncased section of a vertical wellbore, or a combustion cavity created by mechanically reaming a wellbore to increase its volume.

13. The method of claim 1 1 wherein the hydrocarbon fuel, or a portion thereof, is injected into the cavity.

14. The method of any one of claims 1 1 to 13, wherein the cavity is isolated from a cased wellbore in fluid communication with the cavity.

15. The method of claim 14 wherein the cavity is isolated from the cased wellbore by a packer assembly.

16. The method of claim any one of claims 1 1 to 15, further comprising a. providing a vent to the cavity; and b. isolating the vent from the cavity before initiating the auto-ignition reaction.

17. The method of claim 16 wherein the isolating of the vent comprises applying a hydraulic head to the cavity.

18. The method of claim 17 further comprising placing a moveable plug between the cavity and the vent and applying the hydraulic head in the vent uphole of the movable plug.

19. The method of any one of claims 1 1 to 18 wherein the pyrophoric gas is silane.

20. A method for creating fractures in a subterranean formation adjacent a cavity, comprising the steps of: a. installing a bladder into the cavity; b. injecting a gaseous oxidant into the bladder and providing a hydrocarbon fuel in the cavity outside of the bladder, or c. injecting a hydrocarbon fuel into the bladder and injecting a gaseous oxidant into the cavity outside of the bladder; and d. injecting pyrophoric gas into the cavity or into the bladder, to initiate an auto- ignition reaction of the pyrophoric gas, causing rupture of the bladder and combustion of the hydrocarbon fuel and the oxidant, said combustion causing shock waves to travel through the formation and forming fractures in the formation adjacent to the cavity.

21 . The method of claim 20 wherein the cavity is an uncased section of a horizontal wellbore, an uncased section of a vertical wellbore, or a combustion cavity created by mechanically reaming a wellbore to increase its volume.

22. The method of claim 20 or 21 wherein the hydrocarbon fuel is a liquid.

23. The method of claim 22 further comprising adding a proppant to the cavity.

24. The method of any one of claims 20 to 23 wherein the bladder has a frangible section that releases the gaseous contents of the bladder into the cavity when the pressure differential between the inside of the bladder and the surrounding cavity exceeds a predetermined value.

25. The method of any one of claims 20 to 24 wherein the cavity is isolated from a cased wellbore in fluid communication with the cavity.

26. The method of any one of claims 20 to 25 wherein the cavity is isolated from the cased wellbore by a packer assembly.

27. The method of any one of claims 20 to 26, further comprising a. providing a vent to the cavity and b. isolating the vent from the cavity before initiating the auto-ignition reaction.

28. The method of claim 27 wherein the isolating of the vent comprises applying a hydraulic head to the cavity.

29. The method of claim 27 further comprising placing a moveable plug between the cavity and the vent and applying the hydraulic head in the vent uphole of the movable plug.

30. The method of any one of claims 20 to 29, wherein the pyrophoric gas is silane.