WO2014008514A2 - Shaped-charge well stimulation for increasing access to liquid in an underground reservoir - Google Patents

Abstract

The present invention provides for a method of enhancing access in an underground liquid reservoir by using a shaped charge in a well to create a rubble zone.

Description

Shaped-Charge Well Stimulation for Increasing Access to Liquid in an Underground

Reservoir

Inventor: George J. Moridis

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Patent Application Ser. No.

61/668,935, filed on July 6, 2012, which is hereby incorporated by reference.

STATEMENT OF GOVERNMENTAL SUPPORT

[0002] The invention was made with government support under Contract Nos. DE-AC02- 05CH11231 awarded by the U.S. Department of Energy. The government has certain rights in the invention.

FIELD OF THE INVENTION

[0003] The present invention is in the field of the extraction of liquid from an underground reservoir.

BACKGROUND OF THE INVENTION

[0004] Low- and Ultra-Low Permeability Reservoirs (LULPR) are unconventional reservoirs that are best described as oil and/or gas accumulations that are difficult to characterize and commercially produce by "conventional" exploration and production technologies. These resources are typically located in very "tight," heterogeneous, extremely complex, and often poorly understood geologic systems, often easy to find but difficult to produce. The universal feature of all tight and ultra-tight reservoirs is the unavoidable need for well and reservoir stimulation: the matrix permeability is extremely low (often at the nano-Darcy level) and, even with the presence of a system of natural fractures, it cannot support flow at anything approaching commercially viable rates without permeability enhancement. Such

enhancement/stimulation is provided by a number of methods, all of which are designed to develop a new system of artificial fractures that increase the permeability of the system, in addition to increasing the surface area (over which reservoir fluids flow from the matrix to the permeable fractures) and providing access to larger volume of the reservoir. Thus, stimulation techniques are the only means of rendering resource-rich but unproductive natural reservoirs into commercially viable entities. Fundamentally, it is stimulation technology that made gas production from shales possible, and it is this same technology that has effected production increases of orders of magnitude over the last few years (Sutton, et al; Cipolla, et al).

[0005] While it is not difficult to find extensive LULPRs in many basins, their behavior is unpredictable, and hinges to a very large extent on successful stimulation processes (which often dwarf the cost of well drilling and completion). Thus, establishing petroleum fluid flow at commercial rates can be the overriding cost concern in these operations. These types of considerations (low permeability, complex geology and unpredictable stimulation effects) are responsible for the high risk factors and unpredictable results often associated with LULPR exploration and development projects, and it is the high uncertainty coupled with high capital costs that inhibit industry investment in these resources despite their potentially vast magnitude. While it is not possible to alter geology and matrix permeability, we feel certain that we can effectively address the issue of stimulation, developing a new technique that can make such LULPR resources accessible to production both by lowering the stimulation costs as well as by decreasing the associated uncertainty.

[0006] Conventional stimulation techniques involve the creation of a system of individual fractures emanating from particular points along the wellbore. Although it may be possible to develop additional fractures (e.g., stress release fractures) that are possible to develop, the fracture system resulting from conventional stimulation is dominated by the "main" fractures (planar or dentritic) that occur at the locations of stimulation treatment.

[0007] There are significant problems with conventional stimulation techniques. If these are variants of "hydro-fracturing" (in which the near-incompressibility of water is exploited to deliver a shock that induces rock fracturing stemming from the target point), water injection into the fractures and into the matrix poses serious permeability problems because only a small fraction of the injected water is recovered, and the we believe that the water which remains has been "imbibed" into the shale matrix, which creates a relative permeability barrier to oil and gas flow from the matrix into the fractures. This effect is magnified in geologic media where the intrinsic permeability is already at the nano-Darcy level, and while high irreducible water saturations are expected, the process of imbibitions is irreversible, and is (unfortunately) a reality in the hydro-fracturing treatment. Various treating agents

(surfactants, acids, etc.) are used to mitigate these effects, but the fact remains that the total recovered water from a shale gas stimulation treatment varies from 10 to 30 percent.

[0008] Other problems of the artificial fractures resulting from stimulation are their effectiveness and their long-term performance. The effectiveness is a function of the reservoir volume these artificial fractures intercept and drain, and is often described by the surface area over which reservoir fluids move from the matrix to the fractures. The few distinct fracture systems that are created using conventional stimulation methods provide access to a very limited portion of the reservoir, with the rest being locked into a low permeability matrix (which, at nano-Darcy levels, cannot recharge the fractures and maintain production at commercial rates for a long period). The long-term performance of the fracture system is a major concern, as it tends to close over time as fluid production continuously transfers stresses from the fluids to the geologic media. The effectiveness of proppants to keep the fractures open has been limited by the fluids used to transport such proppants deep into the fractures, and by the "embedment" issue whereby angular grains actually embed into the rock matrix (a particular problem in gas shales at high temperature/high pressure (HT/HP) conditions).

[0009] The situation becomes even more complex when gels are used as the transport agent for proppants, as these gels can fail to "break" (i.e., dissociate) which has a profound effect on fracture conductivity (effectively serving as a plug, rather than a conductive fluid path). This is failure to "break" is rare, but it happens, effectively making that particular fracture stage unproductive. Another phenomenon is that of the "fracture screen out" whereby proppant particles "bridge" to form a barrier to injection. On a small scale we believe that proppant barriers can cause fractures to change paths, and may even lead to "erosion" of the rock matrix— but this is dependent on the geology and the stimulation process, and is not something that can be "designed" (other than using experience). In a full "screen out," the proppant forms a "bridge" that cannot be broken except by excessive hydraulic pressure which would also exceed the pressure capacity of the tubulars, hence, the treatment is stopped, and the stimulation may be completely inadequate for production.

[0010] Use of explosives as stimulation technologies (which are based on the fast release of compressible gases) alleviates the relative permeability issue, but offers no mechanism to maintain the fractures' aperture, since proppants cannot be transported by the released gases. A universal problem in all conventional stimulation is that, after an initial burst, oil and gas production often declines precipitously as the near-fracture media are exhausted, the stimulated volume and the surface area are limited, and the low-permeability in the matrix does not allow a sufficiently fast replenishment of the flow to the fractures.

[0011] To address all of these issues, very large-scale stimulation efforts (using staged "hydro fracturing") are often undertaken at each well (sometimes in parallel— so-called "zipper- fracs") [King]. This process significantly increases the cost of the well (well stimulation treatments regularly constitute 2/3 the total well costs); while still incurring the same limitations, risks and uncertainties. Additionally, all conventional techniques pose the risk of unintended fracturing of additional geologic strata if improperly designed, which can lead to stimulation of "non-reservoir" rock— as well as posing risks to groundwater resources and causing at least environmental concerns for relatively shallow formations (1000-1500 m).

SUMMARY OF THE INVENTION

[0012] The present invention provides for a shaped charge stimulation (ShaCS) technology for the enhancing of production in low and ultra-low permeability reservoirs.

[0013] The present invention provides for a method of enhancing access in an underground liquid reservoir, comprising: (a) deploying a shaped charge in a well, (b) exploding the shaped charge, such that access to the liquid reservoir is enhanced or increased, and (c) optionally extracting at least part of a liquid from the liquid reservoir. The reservoir is surrounded by one or more geological materials. The shaped charge explodes in manner as determined by the shape and design of the shaped charged. A practitioner of the invention can select the specific shaped charge or shaped charges to use, and the timing of placement of the shaped charge(s) so that the resulting explosion produces a specific desired rubbled or rubble zone. The desired rubble zone in turn enhances or increases access to the liquid reservoir so that the rate and/or total amount of the liquid can be recovered from the reservoir is increased.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The foregoing aspects and others will be readily appreciated by the skilled artisan from the following description of illustrative embodiments when read in conjunction with the accompanying drawings. [0015] Figure 1. A laboratory setup for seismic measurements on a sheared, fluid-saturated fracture in a granite sample. A pair of seismic transducers is attached to the cell. A cylindrical core of granite containing a fracture (at the center of the core across the core axis) is inserted in this cell. The pore pressure within the fracture can be controlled via a pair of fluid injection/extraction ports attached to the cell. The two halves of the cell chamber containing a fractured rock core are hydraulically sealed by a pair of thick O-rings mediating a center slip plate. During an experiment, this setup is loaded in a bi-axial loading frame shown in Figure 2.

[0016] Figure 2. Biaxial loading frame. The cylindrical rock core (granite) in the above picture is replaced by the test cell shown in Figure 1.

[0017] Figure 3. (A) Pressure isosurface and (B) damage map from a 3D Baneberry simulation. The pressure isosurface plot illustrates the lack of spherical symmetry in the cavity region due to medium layering and nearby faults. The damage map shows three damaged regions (a weak clayey region in the vicinity of the working point, the faults, and a cone-shaped spalled region near surface ground zero) connected to one another, thereby forming a continuous damaged region that connects the WP to the surface fissure where a prompt radioactive release was first detected just after the test.

DETAILED DESCRIPTION OF THE INVENTION

[0018] Before the invention is described in detail, it is to be understood that, unless otherwise indicated, this invention is not limited to particular sequences, expression vectors, enzymes, host microorganisms, or processes, as such may vary. It is also to be understood that the terminology used herein is for purposes of describing particular embodiments only, and is not intended to be limiting.

[0019] As used in the specification and the appended claims, the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to a "shaped charge" includes a single shaped charge as well as a plurality of shaped charge, either the same shape or of different shapes.

[0020] In this specification and in the claims that follow, reference will be made to a number of terms that shall be defined to have the following meanings: [0021] The terms "optional" or "optionally" as used herein mean that the subsequently described feature or structure may or may not be present, or that the subsequently described event or circumstance may or may not occur, and that the description includes instances where a particular feature or structure is present and instances where the feature or structure is absent, or instances where the event or circumstance occurs and instances where it does not.

[0022] The present invention provide for a method of enhancing access in an underground liquid reservoir surrounded by one or more geological materials, comprising: (a) deploying a shaped charge in a well in liquid communication with the underground liquid reservoir, (b) exploding the shaped charge, such that access to the liquid reservoir is enhanced or increased, (c) optionally drilling through the rubble zone using the stem of the well to at least the liquid reservoir, and (d) optionally extracting at least part of the liquid from the liquid reservoir. In some embodiments, step (a) comprises deploying a plurality of shaped charges in the well. In some embodiments, step (b) results in the creation of a rubbled or rubble zone at or near the location the shaped charge was deployed in the well, wherein the rubbled or rubble zone comprises the loosening and/or breaking up of the geological material. Any suitable means for extracting liquid from an underground liquid reservoir, such as oil from an oil reservoir, can be used, and they are well known to those skilled in the art.

[0023] In some embodiments, the underground liquid reservoir has an at least low

permeability. In some embodiments, the underground liquid reservoir has an ultra-low permeability. In some embodiments, the low permeability reservoir has a surface routine average air absolute permeability of equal to or less than about 10 mD, 5 mD, 1 mD, 0.5 mD, 0.1 mD, 0.05 mD, or 0.01 mD. In some embodiments, the ultra-low permeability reservoir has a surface routine average air absolute permeability of equal to or less than about 0.1 mD, 0.05 mD, or 0.01 mD. In some embodiments, the low permeability reservoir is a reservoir with a permeability of about equal to or less than 1 mD for oil and 0.1 mD for gas. There are three categories of low permeability reservoir: (1) fractured reservoirs, (2) tight reservoirs, and (3) tight reservoirs.

[0024] "Liquid" refers to both liquid and gas. In some embodiments, the liquid is crude oil, tight gas, or tight oil. In some embodiments, the gas is natural gas or methane.

[0025] In some embodiments, the geological material is shale, siltstone, sandstone, or tight sand.

[0026] The shaped charges suitable for use in the present invention are known to those skilled in the art. The shaped charges can open up the geological material around drilled wells. The shaped charges can be used in the oil and gas industry to pierce metal, concrete, and/or other solid materials, such as geological material. In an oil or gas well, a metallic casing is cemented to the borehole walls to maintain the borehole integrity. The shaped charge(s) are incorporated in a hollow carrier gun or a strip positioned in the casing. The shaped charge(s) are activated to pierce the well casing and the geologic formation at the hydrocarbon producing zone. The hydrocarbons enter the casing through such perforations and are transmitted to the well surface.

[0027] The shaped charges sever targets by jetcutting. The shaped charges utilize special housings that are designed to create a cavity or void between the explosive material and target wall. Employing a phenomenon known as the Monroe Effect, the shock wave produced at detonation accelerates and deforms the shaped housing into a high-velocity (such as 24,000-27,000 fps) plasma jet within the void space. The formed jet is able to cut through steel targets of various thicknesses based upon the void shape and the "stand-off distance to the target wall. Because the "cutting" efficiency of shaped charges is several times greater than that of bulk charges, they can often greatly reduce the net explosive weight needed to sever similar-sized targets.

[0028] In some embodiments, the shaped charge is a linear-shaped charges (LSC) comprising a void shaped into a chevron or inverted "V" along its entire length, and it is designed to and capable of cutting linearly through its target. One skilled in the art is capable of using LSC's on a wide range of decommissioning targets in many different configurations depending on the cutting requirements. Suitable LSC are commercially available from Accurate Arms Company, Inc. (McEwen, TN). In some embodiments, the shaped charge is a conical-shaped charge (CSC) which is capable of producing a cavity created in the shape of a cone designed to cut round holes and to penetrate deep into targets. In some embodiments, there is a plurality of shaped charges comprising CSC, LSC, or both thereof.

[0029] Examples of suitable shaped charges include those described in U.S. Patent

Application Pub. Nos. 2005/0056459 and 2011/0139505 (both hereby incorporated by reference). [0030] In the present invention, the shaped charge(s) are exploded to create a "rubble zone" comprising network of non-closing, high-density fractures in the geological material to a controlled distance from the well. Each shaped charge creates one or more non-closing, high- density fractures in the geological material to a controlled distance from the well.

[0031] In some embodiments, deploying the shaped charge comprises deploying a plurality of shaped charges in the well, and exploding the shaped charge comprises a particular firing sequence of the plurality of shaped charges. In some embodiments, the exploding step results in the creation of a "rubble zone" comprising a network of non-closing, high-density fractures in the geological material to a controlled distance from the well. The controlled distance is a distance or range of distance that the practitioner of the present invention intends by the intended use of the shaped charge(s). In some embodiments, the exploding step does not result or essentially or substantially does not result in any discrete fracture. The "rubble zone" created results in an increased access to the reservoir, such that more liquid in the reservoir can be extracted to above ground. In some embodiments, the reservoir is a tight reservoir. In some embodiments, the geological material is shale and the liquid comprises tight gas and/or tight oil.

[0032] Stimulation technology refers the means to increase access to a reservoir using the present invention. To develop and implement the stimulation technology one skilled in the art can use the most advanced shaped charges, firing sequence and/or controlled explosions, and involve the application of military grade advanced or conventional explosives to produce a "rubble zone". One skilled in the art can design the stimulation technology to produce the desired magnitude and orientation of all three principal stresses and the hydromechanical characteristics of the native fracture network in the original undisturbed system in order to produce the desired "rubble zone".

[0033] One skilled in the art can predict and/or measure the effectiveness of the shaped charge in producing the increase in access to the reservoir by means of theoretical analysis, numerical simulation, laboratory studies and/or field experiments known to those skilled in the art. One skilled in the art can develop one or more models to predict or produce the desired coupled flow, geomechanical, geophysical, and geochemical behavior of a "rubble zone". The models can be of the earliest stages of well stimulation to long-term production, and include the feasibility of using geophysical means to monitor long-term production. [0034] In some embodiments, the use of the present invention substantially increases production of oil and/or gas from a tight oil and/or tight gas system while minimizing environmental risks. One skilled in the art can determine the short- and long-term behavior of the "rubbled zone", such as massively-fractured ("rubbled") system and its effect on production, and the underlying interrelationship between flow, geophysics and geomechanics in such reservoirs. One skilled in the art can also determine possible geophysical markers that track the evolution of the flow and fracture characteristics of the reservoir under production to allow system monitoring and prediction of long-term behavior. One skilled in the art can further determine a set of pressure/production curves describing rubbled systems. The "rubble zone" can be characterized by fracture mapping.

[0035] One skilled in the art can determine models (numerical, analytical, and/or hybrid) for the prediction of long-term production performance (pressure and/or production behavior) for a rubbled reservoir system. A "rubbled reservoir system" is a system for extracting the liquid from a well with a "rubble zone" created by the present invention.

[0036] The technologies and methodologies described herein are known to one skilled in the art. The proposed technology is designed to alleviate the main problems of conventional stimulation techniques. By relying on explosives, it alleviates the adverse effects of hydro- fracturing liquids on the matrix relative permeability. Using shaped charges and relying on differential timing of the firing sequence, the dislodging of the fractured blocks can cause the fractures to dilate and create sustained aperture resultant from surface contact, thus alleviating the need for proppants. By appropriate design of the explosion events, a "rubble" zone of intensely fractured geologic materials is created around the well, increasing the surface area (and, consequently, potential production) by orders of magnitude as well as providing the side benefit of better coupling to in situ fracture networks. Finally, by employing the most recent advances in explosion technologies (such as targeting a particular structure without harming the adjacent structures), the extent of the rubble zone can be controlled, while ensuring that it never crosses into an adjacent strata where it may endanger the groundwater resources.

[0037] The present invention aims to maximize gas and/or oil production from LULPRs (including shale reservoirs). It achieves the object of increasing the supply of domestic natural gas and other petroleum resources through reducing the cost and increasing the efficiency of exploration for and production of such resources, while improving safety and minimizing environmental impact.

[0038] In some embodiments, the well is drilled using air. In some embodiments, the well is a completed open hole, without production casing. In some embodiments, the well is a shale gas well. In some embodiments, the well is prepared using traditional horizontal well and hydro-fracturing. These techniques are known to those skilled in the art. In some

embodiments, the reservoir is at a depth equal to or more than about 500 m deep. All "depths" herein are measured from the ground level). In some embodiments, the reservoir is at a depth equal to or more than about 1000 m deep. In some embodiments, the reservoir is at a depth equal to or more than about 2000 m deep. In some embodiments, the reservoir is at a depth equal to or more than about 2500 m deep. In some embodiments, the reservoir is at a depth from about 2500 m to about 3500 m deep (as measured from the ground level).

[0039] In some embodiments, the method comprises steps, means, modifications and/or alternative forms described in International PCT Patent Application No. PCT/US 13/45476, filed June 12, 2013, titled "HIGH STRAIN RATE METHOD OF PRODUCING

OPTIMIZED FRACTURE NETWORKS IN RESERVOIRS" (hereby incorporated by reference).

[0040] The implementation of the present invention has the potential to fundamentally affect the exploitation of LULPRs (tight oil, tight gas and shale gas reservoir systems) in the U.S., and by extension, to the world.

[0041] The most demanding (and uncertain) task today in such reservoir systems is the design of the stimulation system, and its effect on estimating reserves and production performance. The present invention provides a quantum leap in one's ability to stimulate LULPRs, which has the potential to obviate the vexing problems of conventional stimulation. By such a stimulation technology that increases the surface flow area by orders of magnitude, industry gains access to a much larger portion of the reservoirs under their control (and ones to be discovered), with commensurate increases in production and estimate of reserves.

Additionally, the method significantly prolongs the peak production capacity of reservoirs, and significantly reduces the uncertainty, unreliability and risk involved in hydro-fracturing applications.) This work is a combination of fundamental and applied science, and its unique combination of tasks has potential to redefine the practice of estimating reserves, in addition to designing stimulation methods. [0042] The present stimulation method utilizes practically no water in the stimulation process, and thus eliminates the risk of water-borne contamination groundwater and the heavy environmental impact that hydraulic fracturing places on water resources. The avoidance of fracturing of adjacent formations (and of the corresponding risks to groundwater contamination) is a critical design factor and a key advantage of the proposed technology. This has always been an indispensable component and an acceptance criterion in the design of subsurface tests of explosive technologies for defense purposes, and the specialty of Lawrence Livermore National Laboratory (LLNL).

[0043] The present method can be characterized by one or more of the following: flow- geomechanics-geophysics-geochemical analyses. The system can be also analyzed by LLNL using one or more of the following hybrid multi-physics codes capable of representing the response of geologic media to dynamic loading: GEODYN, GEODYN-L and LDEC, that are all massively parallel 3D codes that run on practically all of the laboratory's high

performance computing platforms. Such LLNL high performance computing platforms can be utilized to perform modeling and simulations for the analysis and design of the ShaCS explosive fracturing operations.

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[0045] It is to be understood that, while the invention has been described in conjunction with the preferred specific embodiments thereof, the foregoing description is intended to illustrate and not limit the scope of the invention. Other aspects, advantages, and modifications within the scope of the invention will be apparent to those skilled in the art to which the invention pertains.

[0046] All patents, patent applications, and publications mentioned herein are hereby incorporated by reference in their entireties.

[0047] The invention having been described, the following examples are offered to illustrate the subject invention by way of illustration, not by way of limitation.

EXAMPLE 1

SHAPED-CHARGE STIMULATION (ShaCS) TECHNOLOGY FOR PRODUCTION ENHANCEMENT IN LOW- AND ULTRA-LOW PERMEABILITY RESERVOIRS

[0048] This example involves developing and implementing a new stimulation technology that is based on modern advances in shaped charges, firing sequence and controlled explosions, and involves the adaptation of military applications of advanced or conventional explosives to develop a "rubble zone" of an extensive network of non-closing high-density fractures in the geological materials (as opposed to discrete fractures) to a controlled distance from the well, thus substantially increasing production from tight reservoirs (shale and tight gas, tight oil). The design of the stimulation technology accounts for the magnitude and orientation of all three principal stresses and the hydromechanical characteristics of the native fracture network in the original undisturbed system.

[0049] This involves evaluating the stimulation technology by means of theoretical analysis, numerical simulation, laboratory studies and field experiments, and developing models of the coupled flow, geomechanical geophysical and geochemical behavior of these "rubbled" systems from the earliest stages of well stimulation to long-term production, including the feasibility of using geophysical means to monitor long-term production.

[0050] Conventional hydro-fracturing is replaced with a technology that is based on the most advanced developments in shaped charges, firing sequence and controlled explosions, and involves adaptation of military applications of conventional or advanced explosives.

Generally, the ShaCS technology involves the following steps:

[0051] Drill an open-hole well (vertical or horizontal, depending on the reservoir thickness), and isolate the lowest part of the vertical stem

[0052] Using the most advanced simulation capabilities (developed for military applications), design a stimulation involving the use of shaped charges with differential explosion timings and sequence, and resulting in a cylindrical "rubble" zone around the wellbore. The radius of the rubble zone can be quite large, and is a design parameter. The shaped-charge explosions are designed to effect significant fracturing and dislocation in the rubble zone, obviating the use of proppants to keep the fractures open. This method is expected to increase the fracture surface area by several orders of magnitude. Protection of the adjacent (and possibly groundwater-bearing) strata is a critical component of the design.

[0053] Following the creation of the rubble zone, the rubble zone is drilled again through the rubble zone, using the same stem, to complete the well.

[0054] The following is proposed: (1) To first define by means of numerical simulation the geological envelope of potential applicability of the proposed ShaCS method in terms of geomechanical properties and local stresses, and a preliminary matching and evaluation of general types of geologic media (tight sands, shales, etc) with correspondingly appropriate variants of the ShaCs technology (in terms of the properties of the explosives to be used, and the characteristics of the firing sequence and shaped charges).

[0055] (2) To conduct a field test using an available well. All known properties and characteristics of the reservoir in question from all possible sources (well tests, wellbore geophysics, geological surveys, lithological and core analyses, etc.) are assembled and evaluated. Furthermore, a laboratory study is conducted to analyze the geologic media of the targeted reservoir, and seeks to determine the lithological, flow, geophysical, geomechanical and storage (e.g., sorption) properties of the matrix and of the natural fractures of the formation. Cores of representative reservoir sections are used for the purpose. Results from the laboratory study provide important data that may be currently unavailable (such as the geomechanical properties), and help refine general estimates already available from other sources (earlier core studies, geophysical surveys, etc.)

[0056] (3) This field test is preceded by an extensive study and evaluation of the properties and characteristics of the specific geologic formation where the reservoir is located at the site of the well. A critical component of the design is the protection of adjacent (and possibly groundwater-bearing) formations from fracturing to eliminate environmental concerns about polluting water resources and providing pathways for upward release of petroleum reservoir fluids. These analyses are conducted by means of numerical simulations using codes and techniques developed for the analysis of similar tests conducted for defense applications, and use data from previous studies and from the laboratory investigations. In particular, the GEODYN code developed at Lawrence Livermore National Laboratory (LLNL) (Livermore, CA), has been used extensively to study the response of a wide variety of underground formations to complex explosive loading and to develop appropriate constitutive models that describe the response over many orders of magnitude of scale and strain rates. The study culminates with the development of a complete design package for the field application of the ShaCS technology at the site, including the type and amount of the explosives to be used, location of the explosives in the wellbore, firing sequence and differential timing, and predicted/expected explosives performance and reservoir response in terms of fracture development and extent of the intensely-fractured rubble zone.

[0057] (4) Concurrent with the ShaCS explosive component deign, a parallel analysis (using all data from previous studies and from the laboratory investigations) is conducted by means of numerical simulation to (a) establish and describe the baseline (pre-ShaCS) system production behavior, reservoir properties and natural fracture characteristics, and (b) to provide estimates of improved production performance and pattern based on the expected results of application of the ShaCS technology and the predicted characteristics, properties and extent of the rubble zone. Note that the production patterns from such rubble zones are expected to be significantly different from typical results from LULPRs. A parallel geophysical study seeks to determine the geophysical properties and signature of the rubble zone, and to identify geophysical markers for field evaluation of its extent.

[0058] (5) Following the completion of the preparatory studies, a field test is conducted at the well of the industrial partner. During the test, the field and well are instrumented for monitoring the ShaCS process and the evolution and extent of the rubble zone.

[0059] (6) The stimulation is followed by an evaluation process aiming to determine the effectiveness of the ShaCS method in terms of (a) creating the desired massively-fractured rubble zone, and (b) comparing the rubble zone characteristics (extent, fracture density and aperture, etc.) to the simulation predictions. A field geophysical analysis using the signature and the markers identified in the earlier study is employed.

[0060] (7) Following the ShaCS application, the well is completed and brought to production. Production data are collected and analyzed for a period of at least one year. The production performance is compared to that from the Pre-ShaCS application.

[0061] From the preceding, the following are determined: the long-term behavior of the rubble zone and its effect on production, identifying possible geophysical markers that can track the extent and characteristics of the rubble zone of the reservoir under production, and allow system monitoring and prediction of long-term behavior, and improved

pressure/production curves for the description of long-term production from rubbled reservoirs.

EXAMPLE 2

STUDIES OF THE SHAPED-CHARGE STIMULATION (ShaCS) TECHNOLOGY

Definition of the Envelope of the ShaCS Technology Applicability

[0062] This example describes an investigation by means of numerical simulation of the applicability of the ShaCS technology in various broad types of formations (e.g., tight sands, shales, and the like) and under general categories of conditions (including initial/discovery pressures and stress distributions) and geomechanical properties. The emphasis is in the determination of conditions under which ShaCS is not applicable or recommended, thus eliminating early such systems from further consideration. Additionally, the effort in this task includes the development of a preliminary "User's Guide" of the technology, providing broad guidelines as to the ShaCS applicability vis-a-vis the properties and characteristics of the targeted formations, the desired properties of the intended rubble zone, the

correspondingly appropriate variants of the ShaCs technology to be used (in terms of the general types of explosives to be used, and the specifics of the firing sequence and shaped charges), and the expected extent, properties and characteristics of the rubble zone.

Development of knowledge in this area is expected to lead to: (1) identification of, if any, the limitations of the ShaCS technology, (2) definition of broad categories of geologic media and conditions requiring substantially different ShaCS designs and applications, and

corresponding design methodologies, and (3) a guide to the application of the technology, including expectations of extent and characteristics of the rubble zone.

[0063] The simulation codes useful for this example, such as GEODYN, LDEC and ALE3D are available from LLNL. They have been extensively used, tested and verified in numerous defense application of subsurface testing of explosives. Data Collection and Analysis of Targeted Reservoir

[0064] This task includes the collection and analysis of all data that have ever been produced and are available on the reservoir of the industrial partner that is to be used for the field test. These data can include well and production tests, drilling logs, well log geophysics, geological surveys, lithological and core analyses, sorption properties and performance, geomechanical properties, previous stimulation history, etc., and is used as inputs in the simulation analyses that are described in subsequent tests.

Laboratory Studies

[0065] In tight gas systems, mechanical, hydrological, and geophysical (seismic) properties are dominated by both natural and induced fractures. In this task one seeks to determine the coupled geomechanical-hydrological-geophysical properties of fractures in the targeted reservoir via laboratory experiments, and focus on subjects that have not been previously explored or whose earlier analysis is deemed by the project team to be insufficient. These experiments use cores containing natural or laboratory-induced fractures. The primary objectives of the laboratory experiments are the measurements of baseline intact rock properties; measurements of baseline fracture properties, and the measurements of fracture properties with stimulation.

Measurements of baseline intact rock properties

[0066] Representative reservoir cores are tested for static mechanical properties (elastic moduli), physical properties (φ and k), and seismic properties (P and S wave velocities at ultrasonic frequencies) to establish baseline intact rock properties. Seismic properties are measured using a low frequency (~1 kHz) resonant bar apparatus for short rock cores. This is because earlier studies indicate that shale samples (outcrop cores of Mancos and Pierre shales, possibly containing a small amount of gas) containing natural and induced fractures exhibit extremely large attenuation, with a quality factor Q as low as 5 (Suarez-Rivera et al, 2001). This large attenuation can potentially be an indicator of the presence of dense, open fractures containing reservoir fluids.

Measurements of baseline fracture properties

[0067] Cores containing a single natural fracture (or an induced fracture if such cores are unavailable) are tested for mechanical properties (normal and shear fracture compliances), hydraulic properties (fracture k), and seismic properties (P and S wave transmission coefficients), as a function of confining stress and pore pressure. The three-dimensional surface topography of the fracture is determined via surface profilometry to provide an understanding of the surface roughness and fracture angle.

Measurements of fracture properties after stimulation

[0068] Changes in the fracture properties (mechanical, hydrological, and geophysical properties), resulting from several different stimulation methods described below are examined via the laboratory experiments in the following activities:

[0069] Fluid-injection-induced changes. The pore fluid pressure within a fracture is elevated to increase the fracture aperture. Although the effect on the increases in the fracture properties should be the same as reducing the confining stress, a slow loss of pore fluid into the matrix from the fracture may result in time-dependent changes in the fracture properties.

[0070] Shear-induced changes. Shearing of a rough fracture resulting in fracture surface slips can result in increased local aperture. This leads to increased mechanical compliance and hydraulic conductivity. The slip can be induced by either mechanical force or by increasing pore pressure under pre-applied shear stresses, or by a combination of the two. In the laboratory, a specially designed shear loading cell is used to conduct concurrent mechanical, hydrological, and seismic measurements. An example of such a setup is shown in Figures 1 and 2.

[0071] A critical component of the design is the protection of adjacent (and possibly groundwater-bearing) formations from fracturing to eliminate environmental concerns about polluting water resources and providing pathways for upward release of petroleum reservoir fluids.

[0072] The proposed computational study encompasses 2D parametric simulations as well as 3D high fidelity simulations based on the most accurate geologic and geophysical data available. The simulations are performed using GEODYN (Lomov and Rubin, 2003), a 3D massively parallel Eulerian Godunov code with adaptive mesh refinement capabilities. A state of the art constitutive model for the deformation and failure of geologic material (Antoun et ah, 2006) is implemented in GEODYN and used to simulate a wide range of problems involving wave propagation in geologic materials, including underground nuclear explosions (Lomov et al., 2003), granular shaped charges (Lomov et al., 2003), penetration (Antoun et al., 2003), and hypervelocity impact (Antoun et al., 2006). The model is thermomechanically consistent, and it accounts for the effects of scaling, bulking, yielding, strain rate, material damage, and porous compaction on the material response (Antoun et al., 2001).

[0073] Under explosive loading, waves propagate from the source region out into the geologic formation. Heterogeneities and impedance mismatches at interfaces between dissimilar layers cause wave reflections, and the reflected waves interact with the outward propagating waves leading to complex stress states and equally complicated damage distributions throughout the medium. An example that illustrates these effects is depicted in Figure 3, which shows the results of a 3D GEODYN simulation of the BANEBER Y underground nuclear test.

[0074] One important attribute of the dynamic response of geologic materials is their weakness under tension, with the tensile strength typically weaker than the compressive strength by as much as an order of magnitude. This property can be exploited to optimize explosively generated fracture networks. One of the main objectives of this task is to perform parametric studies to explore the design space by varying explosive charge characteristics and timing sequences so as to maximize fracturing of the rock within the gas-bearing formation without compromising the integrity of surrounding geologic layers.

[0075] The study includes the development of a complete design package for the field application of the ShaCS technology at the site, including the type and amount of the explosives to be used, location of the explosives in the wellbore, firing sequence and differential timing. It also includes a discussion of predicted/expected explosives

performance, and the corresponding near- well and reservoir response in terms of fracture development and extent of the intensely-fractured rubble zone. The design package identifies the monitoring equipment needed during the field test. Needless to say that the design radius of the rubble zone is the maximum possible (thus maximizing the stimulated volume and the surface of the flow area) that can be safely attained.

Analysis of Baseline Reservoir Performance

[0076] This task involves analyses by means of conventional reservoir, flow and production analysis, as well as of numerical simulation, and seeks to (a) establish and describe the baseline (pre-ShaCS) system production behavior, reservoir properties and natural fracture characteristics, and (b) to provide estimates of improved production performance and pattern following the application of ShaCS. Well and production data is used to determine the current (pre-stimulation) reservoir behavior and well performance using history matching (inverse modeling). Standard analyses techniques— pressure transient analysis (PTA), model-based production analysis (PA), and modern decline curve analysis (DCA), is used to evaluate well/reservoir performance, as well as numerical simulators. While "pre- stimulation" flow is often problematic in shale gas reservoirs, it is possible to use legacy data — including data from typical vertical and horizontal wells (with hydro-fracturing) to establish a "baseline" of performance prior to stimulation. One can model and show relevant differentiation of the traditional well stimulation treatments and the new "rubbled" reservoir stimulation technique.

[0077] Codes describing coupled flow-geomechanics (TOUGH+ with the commercial geomechanical package FLAC3D, see Rutqvist and Moridis, 2008) and coupled flow- geomechanical-geophysical codes (Kowalsky et al, 2008a) are available to the project. The coupled flow-geomechanics model has been successfully applied to simulate multi-phase flow and geomechanical interaction associated with high-temperature nuclear waste disposal at Yucca Mountain (Rutqvist and Tsang, 2003, Rutqvist et al, 2008), C0₂ storage in brine formations (Rutqvist et al, 2007, 2008), geothermal energy extraction and cold water injection into steam reservoirs (Rutqvist and Oldenburg, 2008) as well as during production of methane gas from hydrate bearing sediments (Rutqvist and Moridis, 2008). The FLAC3D code (Itasca, 2006) is widely used in soil and rock mechanics engineering, and for scientific research in academia. FLAC3D has built-in constitutive mechanical models suitable for soil and rocks, including various elastoplastic models for quasistatic yield and failure analysis, and viscoplastic models for time dependent (creep) analysis. This simulator has been applied to study the relative contribution to cause and mechanisms of production and injection- induced microseismicity in geothermal steam fields (Rutqvist and Oldenburg, 2008).

[0078] Predictions on the effects of the ShaCS treatment on production patterns and well performance can be based on data and information that are made available from the experiments described herein, including the predicted characteristics, properties and extent of the rubble zone. It is expected that the production curves from rubble zones following a ShaCS treatment are expected significantly different from the ones that are typical in production from tight systems.

[0079] Within this task, a parallel geophysical study seeks to determine the geophysical properties and signature of the rubble zone. This question addressed involves the possible existence of geophysical markers that can relate changes in flow, geomechanical properties and the fracture characteristics to the geophysical signature of the system, and how they can be used to evaluate its extent and properties. Within a framework of coupled hydrological- geomechanical-geophysical simulation, one can effectively investigate the sensitivity of various geophysical markers to geomechanical changes in the reservoir using available codes. One can examine the feasibility of using time-lapse geophysical methods (with a focus on seismic exploration approaches) for tracking the evolution of flow properties and fracture characteristics of the rubble zone both at rest and under production.

Field Test Execution and Operations

[0080] This task involves all the activities related to the field application of the ShaCS technology, including instrumentation of the well, possibly nearby monitoring wells, and of the ground to monitor the progress of fracturing and the extent of the fracture zone. Active sensors are deployed to monitor the explosive fracturing operation in real time, to assess the detonation process and explosives performance, and to provide important data for comparison to model predictions of the development and propagation of the fracture network.

Analysis and Evaluation the Properties and Characteristics of the Rubble zone from the ShaCS Treatment

[0081] This analysis proceeds immediately after the conclusion of the ShaCS treatment (possibly prior to the completion of the well), and determines the effectiveness of the ShaCS method in terms of (a) creating the desired massively-fractured rubble zone, and (b) comparing the rubble zone characteristics (extent, fracture density and aperture, etc.) to the simulation predictions. A field geophysical analysis using the signature and the markers identified herein is employed, in addition to tiltmeters to monitor the long-term pressure behavior of the system over the extent of the rubble zone by tracking the surface deformation. The effort may also include analysis and evaluation of very early production data

immediately after the beginning of production. Comparison of field data to the simulation results is used to assess the effectiveness of the explosive fracturing design methodology at predicting the stimulation parameters in terms of the extent and permeability of the fracture network as well as actual reservoir performance. This helps refine the modeling and simulation tools for use in future applications.

Analysis of Long-Term Data of Well Production and Rubble Zone Behavior

[0082] Data on production, well performance, rubble zone behavior, and reservoir geomechanical response data is collected and analyzed. The production performance is analyzed and compared to that from the Pre-ShaCS application. These data are used to (a) evaluate the effectiveness of ShaCS as a method for long-term enhancement of production from LULPRs, (b) assess the quality of production predictions, (c) estimate important field parameters, and (d) determine the reasons for possible discrepancies between observations and predictions. Along with the results of the previous analysis, these results form the ultimate basis/criterion for the evaluation of the success and potential of the overall ShaCS technology.

[0083] While the present invention has been described with reference to the specific embodiments thereof, it should be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the true spirit and scope of the invention. In addition, many modifications may be made to adapt a particular situation, material, composition of matter, process, process step or steps, to the objective, spirit and scope of the present invention. All such modifications are intended to be within the scope of the claims appended hereto.

Claims

What is claimed is:

1. A method of enhancing access in an underground liquid reservoir surrounded by one or more geological materials, comprising: (a) deploying a shaped charge in a well in liquid communication with the underground liquid reservoir, (b) exploding the shaped charge, such that access to the liquid reservoir is enhanced or increased, and (c) optionally extracting at least part of a liquid from the liquid reservoir.

2. The method of claim 1, wherein step (a) comprises deploying a plurality of shaped charges in the well.

3. The method of claim 1 or 2, wherein step (b) results in the creation of a rubble zone at or near the location the shaped charge was deployed in the well, wherein the rubble zone comprises the loosening and/or breaking up of the geological material.

4. The method of claim 3, the rubble zone comprises a network of non-closing, high- density fractures in the geological material to a controlled distance from the well.

5. The method of claim 3 or 4, wherein the rubble zone is a specific desired rubble zone.

6. The method of one of claims 1-5, wherein the underground liquid reservoir has an at least low permeability.

7. The method of claim 6, wherein the underground liquid reservoir has a surface routine average air absolute permeability of equal to or less than about 10 mD.

8. The method of claim 7, wherein the underground liquid reservoir has a surface routine average air absolute permeability of equal to or less than about 5 mD.

9. The method of claim 8, wherein the underground liquid reservoir has a surface routine average air absolute permeability of equal to or less than about 1 mD.

10. The method of claim 9, wherein the underground liquid reservoir has a surface routine average air absolute permeability of equal to or less than about 0.5 mD.

11. The method of one of claims 1-6, wherein the underground liquid reservoir has an ultra-low permeability.

12. The method of claim 11, wherein the underground liquid reservoir has a surface routine average air absolute permeability of equal to or less than about 0.1 mD.

13. The method of claim 12, wherein the underground liquid reservoir has a surface

routine average air absolute permeability of equal to or less than about 0.05 mD.

14. The method of claim 13, wherein the underground liquid reservoir has a surface

routine average air absolute permeability of equal to or less than about 0.01 mD.

15. The method of one of claims 1-14, wherein the geological material is shale, siltstone, sandstone, tight sand, or a mixture thereof.

16. The method of one of claims 1-15, wherein the shape charge is a linear- shaped charge (LSC) or a conical-shaped charge (CSC).

17. The method of claim 16, wherein the shape charge is a LSC.

18. The method of claim 16, wherein the shape charge is a CSC.

19. The method of one of claims 1-18, wherein the liquid is oil or gas.

20. The method of claim 19, wherein the liquid is oil.

21. The method of claim 19, wherein the liquid is gas.