Abstract:
The invention relates to the dividing up and testing of bulk samples from cores of rock. The method involves the extraction of several plugs for triaxial testing which are derived from exactly the same level in the core, and also Brazilian test samples and samples for compositional and textural analysis from the same level. Triaxial tests with different confining pressures may be performed to destruction on separate triaxial plugs, allowing a reliable full Mohr-Coulomb analysis to be performed. Mechanical properties may be related more reliably to composition and texture of rock. The technique is especially useful for non-conventional rock such as shale.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application 61/836,929, filed on Jun. 19, 2013, the entire contents of which are hereby incorporated by reference. This application is related to, and incorporates by reference in its entirety, U.S. patent application Ser. No. 14/309,390 entitled “Mechanical characterization of core samples”, filed concurrently herewith. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    None. 
       FIELD OF THE INVENTION 
       [0003]    This invention relates to the processing of rock core samples. 
       BACKGROUND OF THE INVENTION 
       [0004]    For many years, scientists and engineers involved in hydrocarbon exploration and extraction fields have prepared and analyzed core samples of geological formations, which are often taken during or after drilling operations. This analysis is useful for studying the formation in-depth and possibly directing oilfield decisions in order to maximize recovery of hydrocarbons. It is customary to extract a core (often cylindrical in shape) of a certain diameter and cut a certain conventional length from that core, known as a bulk sample, take a plug sample from that bulk sample and subject it to mechanical testing. Core analysis can include evaluation of rock properties, anisotropy, organic matter content, fluid content, geomechanical properties and the like. 
         [0005]    Conventionally, a bulk core sample is a length (usually approximately 6 inches long) cut from a standard 4 inch diameter core (there are other standard core diameters). One third of the core, cut longitudinally, is typically removed and submitted to relevant authorities or agencies for archiving. A suitable place is then selected for cutting a plug for mechanical testing. 
         [0006]    A current standard method for sampling and testing cores involves taking a single cylindrical plug for triaxial testing from an end of a 6 inch (15 cm) long bulk sample. Additional plugs for Brazilian tensile strength testing, X-ray diffraction analysis and geochemical analysis are taken from different places, further along axis of the sample. If a Mohr-Coulomb analysis is to be performed, the same triaxial test plug may be subjected to sequential tests with different confining pressures, each test taking the plug to a point just before failure. 
         [0007]    Despite its widespread usage, current core testing techniques can suffer from inaccuracy and poor consistency. As a result, massive oilfield operations can be guided by incorrect data. These issues are particularly problematic when characterizing non-conventional rocks such as shale. Part of the problem also lies in the inadequacy of currently accepted models of rock properties and behaviors. For example, in non-conventional organic source rock reservoirs, elastic parameterization of the mechanical response (of the matrix) does not apply because organic source rocks tend to have higher volumes of clay and kerogen which contribute to elastic-plastic constitutive behavior. 
         [0008]    The inventors have sought to establish methodology for processing core samples that includes improved ways of dividing up and testing the core and of correlating mechanical test results with composition and texture of the sample. The resulting core analysis data should be reliable and correlate more closely with real-world rock properties compared to some conventional methods. 
       BRIEF SUMMARY OF THE DISCLOSURE 
       [0009]    In one embodiment of the invention, a method of processing a bulk sample of a cylindrical rock core is provided, the said bulk sample being created by making two transverse cuts through the core such that the bulk sample is of cylindrical shape with the same cross section as the core, having two end faces and a curved face. The method comprises making a cut through the bulk sample, parallel to and spaced from the axis, to remove a slab from the bulk sample, thereby creating a plane face on the bulk sample extending parallel to the axis, the removed slab having a corresponding plane face; then removing from one of said end faces of the bulk sample three or more cylindrical plugs having substantially the same dimensions; and performing triaxial compressive testing to failure on each of said plugs. Triaxial testing is well known and routinely used e.g. for testing samples of rock in the oil and gas industry and other industries. It involves enclosing a cylindrical rock sample with a constraining sleeve around its curved surface, whilst applying a compressive load to the end faces of the sample. Axial stress and strain are recorded as well as constraining stress. 
         [0010]    Because a fresh plug is used for each compressive test, the second and subsequent tests are not influenced by whatever effect the previous test of test had on the structure of the plug. Furthermore, instead of taking the plug to a point near failure and deriving a value for compressive strength based on the readings of stress and/or strain at that near-failure point, the plug may be taken up to and beyond its failure point because it does not have to be re-used. Further testing is conducted on plugs from exactly the same level in the core, thereby ensuring that the same type of rock is tested each time. Small scale stratigraphic variation in the sample, which may be commonly found especially in shale and other unconventional rock, therefore potentially has no effect or at least much less effect on the overall result. 
         [0011]    The method may further comprise removing one or more disc shaped plugs either from said plane face of the bulk sample or from said corresponding plane face of the slab, at a location with respect to the length of the bulk sample which corresponds to that of the cylindrical plugs, and subjecting said disc shaped plugs to Brazilian testing. Brazilian testing is another well-known and commonly used test like the triaxial test. It involves applying a compressive load to the curved surface of the sample disc, and provides a measure of tensile strength of rock. In this way, tensile strength results can be obtained from rock at the same level in the core as the triaxial plugs. At least two disc shaped plugs may be subjected to Brazilian testing in respective different directions, thereby obtaining a measure of anisotropy. It may be possible to take two or more Brazilian disc samples, arranged along the axis of the core, with both (or all) encompassed within the same length of core as the triaxial plugs. 
         [0012]    It is preferable to analyse the cylindrical (triaxial) plugs and/or the Brazilian disc or discs prior to mechanical testing, e.g. by computer tomography (CT) scanning One reason for doing this is to find fractures/cracks within the samples which may affect the results of the mechanical testing. Analysis, e.g. by CT scanning, may also be performed on the plugs or disc samples after the triaxial or Brazilian testing; this can help determine the type of mechanical failure. 
         [0013]    In one embodiment, two or more of said cylindrical plugs are subjected to respective different confining pressures during triaxial testing. This may allow a full Mohr-Coulomb analysis to be performed using the results from the triaxial testing and, optionally, the Brazilian testing. 
         [0014]    The axes of said cylindrical plugs may be substantially parallel to the axis of the bulk sample. However, in another embodiment, the axes of two or more of the cylindrical plugs are at a non-zero angle to each other. In this way, bedding dip strength anisotropy can be measured. Optionally, the axis of a first one of said cylindrical plugs is substantially parallel to the axis of the bulk sample and the axis of a second one of said cylindrical plugs is substantially perpendicular to the axis of the bulk sample; the axis of a third one of said cylindrical plugs may be inclined with respect to the axes of the first and second plugs. 
         [0015]    A bulk sample carcass remains after the cylindrical and disc shaped plugs have been removed, and this carcass may be subjected to analysis to determine one or more of: 
         [0016]    (a) matrix mineral composition; 
         [0017]    (b) porosity 
         [0018]    (c) pore space constituent; and 
         [0019]    (d) total organic content. 
         [0020]    Preferably, the analysis is performed on samples from the carcass at the same location with respect to the axis/length of the core as the triaxial plugs 
         [0021]    Analysis to determine permeability may also be conducted, which may be by one or more of: 
         [0022]    (a) Bulk density analysis; 
         [0023]    (b) Grain density analysis; 
         [0024]    (c) Gas filled porosity determination; 
         [0025]    (d) Fluid saturation determination; and 
         [0026]    (e) Effective total interconnected porosity determination. 
         [0027]    Thin section analysis may also be performed to determine one or more of: 
         [0028]    (a) rock texture; 
         [0029]    (b) grain size distribution; and 
         [0030]    (c) degree of cementation. 
         [0031]    Petrographic analysis may also be performed. 
         [0032]    The analysis may further include identification of one or more of: 
         [0033]    (a) shale matrix composition; 
         [0034]    (b) secondary cements; 
         [0035]    (c) clays; 
         [0036]    (d) pore types; and 
         [0037]    (e) natural fractures. 
         [0038]    Finally, results from all the various rock composition and texture analysis may be correlated to the results of said triaxial and Brazilian testing. This correlation may have particular importance in building up a database of consistent data linking composition and texture characteristics to mechanical characteristics, e.g. to aid hydrocarbon exploration and production elsewhere. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0039]    A more complete understanding of the present invention and benefits thereof may be acquired by referring to the follow description taken in conjunction with the accompanying drawings in which: 
           [0040]      FIG. 1  is an end view and a section through the length of a bulk sample from a 4 inch core, showing where various plugs and samples may be taken from the bulk sample; 
           [0041]      FIG. 2  comprises views similar to  FIG. 1  of a 3.5 inch core; 
           [0042]      FIG. 3  comprises views similar to  FIG. 2  of a 3.5 inch core as shown in  FIG. 2  but showing an alternative arrangement of plugs and samples; 
           [0043]      FIG. 4  comprises views similar to the previous figures of a 2⅝ inch core showing where plugs and samples may be taken if the entire core is available; and 
           [0044]      FIG. 5  comprises views similar to the previous figures illustrating how individual core plugs could be sampled to evaluate the rock strength as a function of angle to the plane of bedding. 
           [0045]      FIGS. 6 and 7  shows plots generated from triaxial testing as described in Example 2. 
       
    
    
     DETAILED DESCRIPTION 
       [0046]    Turning now to the detailed description of the preferred arrangement or arrangements of the present invention, it should be understood that the inventive features and concepts may be manifested in other arrangements and that the scope of the invention is not limited to the embodiments described or illustrated. The scope of the invention is intended only to be limited by the scope of the claims that follow. 
         [0047]    The following examples of certain embodiments of the invention are given. Each example is provided by way of explanation of the invention, one of many embodiments of the invention, and the following examples should not be read to limit, or define, the scope of the invention. 
         [0048]    The present invention provides tools and methods for characterizing core samples taken from a subterranean formation. More specifically, the present invention provides core analysis protocol that links standard mechanical test data (triaxial and Brazilian) to rock texture and composition measurements. 
         [0049]    One of the goals of the present invention is to improve the accuracy and consistency of results from testing core samples, especially unconventional rock such as shale. Once an accurate and consistent protocol is established, the protocol can be employed to generate a meaningful database of knowledge correlating mechanical properties of rocks with their composition and texture. 
         [0050]    As alluded to earlier, conventional core analysis techniques may be hampered by multiple issues. The inventors have discovered that during conventional core analysis, after a first triaxial test to an assumed point close to failure, the characteristics of a plug may change. This is again especially true of shale and other unconventional rock which, unlike conventional rock, is prone to plastic deformation. Consequently, combining results from successive triaxial tests on the same sample may be suspect. 
         [0051]    Conventional methods also tend to favor multistage triaxial testing on a single plug. Multistage triaxial testing loads a single plug in several incremental cycles (“multiple stages”) to simulate different levels of in-situ stress. This testing method requires that each loading cycle is performed systematically and with precision so that each stage is independent from previous loading history. Unfortunately, the method is hampered by at least two substantial issues: pre-failure damage (e.g., microcracking) and hysteresis. Damage can accumulate, for example, around 50% of the failure stress (the maximum differential stress at failure) and permanently change the rock by degrading its elastic properties and introducing mechanical flaws that weaken internal strength parameters. 
         [0052]    Moreover, the inventors have discovered that, especially in unconventional rock such as shale, there may be variations in physical properties over the length of a standard 6 inch bulk sample. Taking, for example, a Brazilian testing sample further along the axis than the triaxial sample can amount to testing different rock, making combination of results from the two tests potentially suspect. 
         [0053]    The protocol of the present invention performs a facies based mechanical rock classification by characterizing various core samples made from the same facies sample interval. The triaxial plugs, Brazilian plugs, and thin sections are all cut from the same 2 inch interval of preserved core material. This lessens the likelihood that the tested plugs are significantly different in their physical properties. The remaining carcass is used in further petrophysical measurements to determine porosity, permeability, mineralogy and organic content. The triaxial tests performed according to the present invention are single stage and performed to failure and beyond in order to study post failure deformation. An embodiment of the present invention can provide one or more of the following advantages:
       1. Mohr-Coloumb characterization performed on individual plugs to complete failure instead of multistage triaxial test on a single plug;   2. All triaxial test plugs are sampled at same depth reference which ensures the rock facies is equivalent for all trixial test for each Mohr-Coloumb characterization;   3. At least one Brazilian test is performed to determine tensile strength on same rock carcass as the extracted triaxial plugs (completing the mechanical characterization of the rock sample);   4. Lithology determination is made to determine mineral composition of the same core carcass as the extracted triaxial plugs, which allows for direct correlation of the mechanical characterization to mineral composition; and   5. A thin section taken along same axis on the same core carcass as the extracted triaxial plugs, which provides a means to determine how the mechanical characterization is related to rock texture.
 
Other advantages are apparent from the disclosure herein.
       
 
       Example 1 
       [0059]    This example describes a protocol for preparing and analyzing core sample according to an embodiment of the present invention. A schematic of the sampling protocol along with the various measurements is shown in  FIG. 1 . A standard procedure is for a bulk sample having length  103  of 6 inches (15 cm) and diameter  104  of 4 inches (10 cm) to be cut along its axis so that a quarter  105  of the core is removed and submitted for archiving, whilst the remaining three quarter core  100  is available for testing. 
         [0060]    The core analysis process begins when the preserved core samples arrive at the lab for testing. The preserved ¾whole core sample  100  is first CT scanned to validate the preserved rock material is suitable for core plugging and mechanical testing. This step is required to identify fractured, broken or desiccated core that might not be suitable for mechanical testing. 
         [0061]    After the CT scan, the sample is prepared for plugging. A total of 4—1″×2″ (2.5 cm by 5 cm) plugs  101  are cut for triaxial testing and an additional 2—½″×1″ (1.25 cm by 2.5 cm) plugs  102  are sampled for Brazilian strength tests. The triaxial plugs and Brazilian plugs are then sent to the Rock Mechanics Lab for testing to determine the Mohr-Coulomb and tensile mechanical characterization. 
         [0062]    The composition and texture are determined with the remaining carcass (left behind after the initial plugging). The carcass is sent to the Core Analysis Lab to perform the petrophysical measurements for composition including: porosity, permeability, mineralogy and organic content. Thin sections are taken from the carcass and prepared for petrographic classification and to highlight textural features that might influence the mechanical behavior of the rock. 
         [0063]    It is the integration of the texture and composition that will lead to the definition of the mechanically characterized facies. 
         [0064]    The plugged samples arrive at the Rock Mechanics Lab for triaxial and Brazilian testing. Triaxial testing is performed on 4 samples  101  at different confining pressures for Mohr-Coulomb analysis. The confining stresses used are: 0 psi, 1000 psi, 2000 psi and 3000 psi. Before the plugs are subject to the triaxial test, they are first machined to perfect right angle cylinders. It takes approximately 2 weeks to machine all the plugs. After the plugs have been prepared for mechanical testing they are CT scanned just prior to the triaxial test. The CT scan is performed to identify any hairline cracks and record the micro-heterogeneity. A CT scan is also performed immediately after testing to observe and record the exact type of mechanical failure. A similar protocol is followed with the Brazilian test. The Brazilian samples are machined into cylinders. CT scans are performed before and after the tests. The Brazilian test is performed twice, once at a bedding parallel orientation and once at bedding perpendicular. 
         [0065]    After both the triaxial and Brazilian tests are complete, a Mohr-Coulomb plot is created and a rock mechanical characterization is performed. The mechanical characterization from this series of measurements will provide the following mechanical properties: Young&#39;s Modulus, Poisson&#39;s Ratio, Unconfined Compressive Strength, Cohesion, Angle of Internal Friction and Tensile Strength. 
         [0066]    The carcass from the plugged sample is sent to the Core Analysis Lab for determination of Composition and Texture. The composition is determined through a number of petrophysical measurements to determine the matrix mineral composition, porosity, pore space constituent and total organic content. The permeability is also determined with the TRA method. TRA method includes sample preparation, bulk density (including volume fraction), grain density, gas-filled porosity determination, fluid saturation (oil, water and clay bound water saturation) and effective total interconnected porosity. Mineral composition is measured with XRD analysis, however, thin section mineral reconstruction may also be carried out to ensure the mineral composition determination is consistent across the entire sample. A bulk density measurement is made on the carcass by weighing the sample and determining its volume through fluid displacement. The mineralogy, organic content, pore volume and pore constituent are then used to reconstruct the bulk density and match it to the measured value. The bulk density re-construction is an important QC step to ensure the composition elements sum up to the measured whole. 
         [0067]    The rock texture; grain size distribution, degree of cementation and other relevant petrographic features are determined with thin section analysis. The thin sections are cut from the carcass along the axial direction matching that from the triaxial plug. The thin section analysis includes a detailed petrographic analysis. The carcass is first impregnated with red dye epoxy so when the thin section is prepared porosity appears red. The analysis includes identification of shale matrix composition, secondary cements, clays, pore types and natural fractures. Photomicrographs are included as necessary to characterize the various components. 
         [0068]    In addition to the 4″ core diameter sampling methodology, we have also created additional protocols for sampling smaller diameter cores. The next common core has a diameter  204  of 3 inches (7.5 cm). This is shown in  FIG. 2 . In some areas (for example, Canada) a slab  205  comprising one third of the whole core must be surrendered to the government for permanent archive. This only leaves a ⅔ portion  200  of the whole core to work with.  FIG. 2  is an illustration for applying our protocol to the 3.5 inch core with ⅓ withheld for permanent archive. Note: in this particular configuration, we are forced to sacrifice one of our triaxial test plugs  201  because we can not physically plug a fourth 1″ diameter plug with the available remaining material. As with the 4 inch core, two Brazilian test plugs/discs  202  can be taken from the core within the 2 inch length of the triaxial plugs  201 . 
         [0069]    In many instances, coring with a 3.5″ core bit will provide a full whole core to work with. This is shown in  FIG. 3 : as in  FIG. 2 , the core has a diameter  304  of 3.5 inches (7.5 cm), but the slab  305  and the remaining portion  300  are available for use. This allows us the opportunity to test  4  individual plugs  301  to failure for the Mohr-Coulomb analysis; the Brazilian plugs  302  may be taken from the slab  305 . In addition to the standard 1″×2″ triaxial test size, smaller plug sizes may be used to provide greater application for smaller core diameters. 
         [0070]    The smallest whole core that we have tested is the 2⅝ inch whole core, shown in  FIG. 4 . Smaller 0.625″×1.25″ triaxial plugs  401  are used (cut from the main part  400  of the bulk sample), but the same size Brazilian plug. The Brazilian plug is cut from the slab  405 . A thin section  406  for texture and petrophysical measurements for composition would also be performed at the same facies interval. Smaller diameter whole cores such as this are cut in wells that have limited bit size options. 
         [0071]    Additional mechanical parameterization includes the bedding dip strength anisotropy measurement. This type of mechanical characterization can be used to help understand the weak bedding rock failure that is commonly encountered while drilling the build section in horizontal wells.  FIG. 5  illustrates how the individual core plugs are sampled to evaluate the rock strength as a function of angle to the plane of bedding. Four individual triaxal test plugs  501  (at 0°, 30°, 60° &amp; 90° to bedding) are removed from the same facies sampling interval. A thin section  506  for texture and petrophysical measurements for composition would also be performed at the same facies interval. The composition and texture measurements would be used to categorize the facies and linked back to a mechanical stratigraphic classification that could be used to predict layering response in undrilled wells. 
       Example 2 
       [0072]    This example illustrates some of the differences between single stage triaxial testing of an embodiment and multistage triaxial testing.  FIG. 6  shows a sample graph plotting deviatory stress versus strain of multistage triaxial testing where confining pressure increased at each imminent failure point. There are at least three imminent failure points represented by the plateaus in the axial strain. Success of multistage triaxial testing is largely dependent on the experimenter&#39;s ability to determine the imminent failure point for each confining stress. It is not uncommon to misplace the appropriate termination point (imminent failure) for each loading cycle thereby introducing significant error in determining Mohr-Coulomb yield parameters. By contrast,  FIG. 7  shows a sample graph plotting deviatory stress versus strain of single stage triaxial testing where confining pressure is constant throughout the entire test. No human error is introduced in the form of judging where the termination point is. 
         [0073]    A series of single stage triaxial tests can be performed at progressively higher confining pressures to generate a sample Mohr-Coulomb plot ( FIG. 9 ). In  FIG. 8 , the confining stress increases from left to right. 
         [0074]    In closing, it should be noted that the discussion of any reference is not an admission that it is prior art to the present invention, especially any reference that may have a publication date after the priority date of this application. At the same time, each and every claim below is hereby incorporated into this detailed description or specification as additional embodiments of the present invention. 
         [0075]    Although the systems and processes described herein have been described in detail, it should be understood that various changes, substitutions, and alterations can be made without departing from the spirit and scope of the invention as defined by the following claims. Those skilled in the art may be able to study the preferred embodiments and identify other ways to practice the invention that are not exactly as described herein. It is the intent of the inventors that variations and equivalents of the invention are within the scope of the claims while the description, abstract and drawings are not to be used to limit the scope of the invention. The invention is specifically intended to be as broad as the claims below and their equivalents.