Evaluation method of compressibility of conglomerate reservoir

Abstract

The present invention provides a method for evaluating the compressibility of a conglomerate reservoir. The method for evaluating the compressibility of a conglomerate reservoir comprises: obtaining a brittleness index of the conglomerate reservoir; obtaining a fracture index of the conglomerate reservoir; obtaining a reformed zone width index of the conglomerate reservoir; calculating the compressibility index of the conglomerate reservoir according to the brittleness index, the fracture index and the reformed zone width index; and evaluating the compressibility of the conglomerate reservoir according to the compressibility index. The present invention solves the problem that the evaluation of the compressibility of conglomerate reservoirs in the prior art is difficult to implement.

Description

Technical Field

[0001] The invention relates to the technical field of oil and gas development, and in particular to a method for evaluating the compressibility of a conglomerate reservoir. Background Art

[0002] At present, as a new type of reservoir, conglomerate reservoirs have promoted the development of petroleum geology. However, conglomerate reservoirs have the characteristics of low porosity and low permeability, and hydraulic fracturing is still the main measure to increase production. Before hydraulic fracturing construction, it is necessary to first evaluate the compressibility of the conglomerate reservoir, evaluate the difficulty of forming a hydraulic fracture network, and demonstrate whether "volume fracturing" can be effectively carried out.

[0003] Existing studies believe that the key factors affecting the ability of reservoirs to form fracture networks are horizontal stress difference, rock brittleness, and the development state of natural fracture systems. At present, the most widely used method is to use the brittleness index calculated based on mineral composition or elastic parameters as an evaluation index for reservoir fracture formation ability, and on this basis, combine stress difference, natural fractures, fracture toughness and other factors to calculate the compressibility method. However, conglomerate is different from rocks in fine-grained sedimentary systems such as shale. Conglomerate is well developed, with a particle size of tens of centimeters and extremely strong heterogeneity. At the same time, the heterogeneity of conglomerate reservoirs is not used for reservoirs with developed fractures or bedding. The size, content and cementation strength of gravel edges are the key to its heterogeneity, which directly affects the formation of hydraulic fractures in conglomerate. Therefore, it is extremely important to establish a compressibility evaluation method for rock mechanics special diagnosis of conglomerate reservoirs for pre-hydraulic fracturing evaluation of conglomerate reservoirs. The existing compressibility evaluation methods require many parameters, the calculation method is complex, and it is difficult to apply them on site. The entire set of calculation methods is difficult to implement in a complex on-site construction environment.

[0004] As can be seen from the above, there is a problem in the prior art that it is difficult to implement the compressibility evaluation of conglomerate reservoirs. Summary of the invention

[0005] The main purpose of the present invention is to provide a method for evaluating the compressibility of a conglomerate reservoir, so as to solve the problem that the evaluation of the compressibility of a conglomerate reservoir in the prior art is difficult to implement.

[0006] In order to achieve the above-mentioned purpose, the present invention provides a method for evaluating the compressibility of a conglomerate reservoir, comprising: obtaining a brittleness index of the conglomerate reservoir; obtaining a fracture index of the conglomerate reservoir; obtaining a reformed zone width index of the conglomerate reservoir; calculating the compressibility index of the conglomerate reservoir according to the brittleness index, the fracture index and the reformed zone width index; and evaluating the compressibility of the conglomerate reservoir according to the compressibility index.

[0007] Furthermore, obtaining the brittleness index of the conglomerate reservoir includes: drilling a first core sample of the conglomerate reservoir, performing a uniaxial compression test on the first core sample, obtaining an axial strain-stress test curve, selecting an expansion point and a peak failure point on the axial strain-stress test curve, respectively reading the corresponding axial stresses and calculating the brittleness index.

[0008] Furthermore, the calculation formula of the brittleness index is:

[0009]

[0010] Where B is the brittleness index; σ ci is the expansion point stress, in MPa; σ f is the peak stress, in MPa.

[0011] Furthermore, obtaining the fracture index of the conglomerate reservoir includes: preparing a second core sample of the conglomerate reservoir, performing a three-point bending test on the second core sample, obtaining a displacement-load test curve, obtaining a peak load according to the displacement-load test curve, and calculating the mode I fracture toughness of the second core sample.

[0012] Furthermore, the calculation formula of mode I fracture toughness is:

[0013]

[0014]

[0015]

[0016] Among them, K IC is the mode I fracture toughness, unit is MPa·m 1 / 2 ; R is the radius of the second core sample, in m; B is the thickness of the second core sample, in m; a is the length of the crack notch of the second core sample, in m; s is the span of the second core sample, in m; P max is the peak load, in N.

[0017] Furthermore, obtaining the fracture index of the conglomerate reservoir also includes: normalizing the mode I fracture toughness to obtain the fracture index.

[0018] Furthermore, the calculation formula of the fracture index is:

[0019]

[0020] Where K is the fracture index; is the minimum value of mode I fracture toughness of conglomerate reservoir, in MPa·m 1 / 2 ; K' ICis the value to be tested of the mode I fracture toughness of the conglomerate reservoir, in MPa·m 1 / 2 .

[0021] Furthermore, obtaining the reformed zone width index of the conglomerate reservoir includes: fitting the initial section after the peak load on the displacement-load test curve and obtaining the slope of the initial section after the peak load, and calculating the reformed zone width index according to the slope.

[0022] Furthermore, the calculation formula of the transformation belt width index is:

[0023]

[0024] Where W is the width index of the transformation zone; W' is the absolute value of the slope; W min is the minimum absolute value of the slope.

[0025] Furthermore, the calculation formula of the compressibility index is:

[0026]

[0027] Among them, FI is the compressibility index; B is the brittleness index; K is the fracture index; and W is the transformation zone width index.

[0028] Furthermore, the compressibility of the conglomerate reservoir is evaluated according to the compressibility index, including: when the compressibility index is greater than or equal to 50, the compressibility of the conglomerate reservoir is good; when the compressibility index is less than 50, the compressibility of the conglomerate reservoir is poor.

[0029] By applying the technical solution of the present invention, by obtaining the brittleness index of the conglomerate reservoir, the fracture index of the conglomerate reservoir, and the reformed zone width index of the conglomerate reservoir, the compressibility index of the conglomerate reservoir is calculated according to the brittleness index, the fracture index, and the reformed zone width index, and the compressibility of the conglomerate reservoir is evaluated according to the compressibility index. The parameters required for the compressibility evaluation are easy to obtain and the data are accurate. The entire evaluation process does not involve complicated calculations and is simple and easy to use and easy to implement on site, thereby solving the problem of difficulty in implementing the compressibility evaluation of conglomerate reservoirs in the prior art. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The drawings constituting a part of the present application are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings:

[0031] Figure 1 A flow chart showing a method for evaluating the compressibility of a conglomerate reservoir in a specific embodiment of the present invention; and

[0032] Figure 2shows an axial strain-stress test curve diagram of a first core sample in a specific embodiment of the present invention;

[0033] Figure 3 A schematic structural diagram of a second core sample in a specific embodiment of the present invention is shown;

[0034] Figure 4 Shows Figure 3 Cross-sectional view at CC in FIG.

[0035] Figure 5 A displacement-load test curve diagram of a second core sample in a specific embodiment of the present invention is shown. DETAILED DESCRIPTION

[0036] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other. The present invention will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0037] It should be noted that, unless otherwise specified, all technical and scientific terms used in this application have the same meanings as commonly understood by ordinary technicians in the technical field to which this application belongs.

[0038] In the present invention, unless otherwise specified, the directional words used, such as "up, down, top, bottom", usually refer to the directions shown in the drawings, or to the components themselves in the vertical, perpendicular or gravity directions; similarly, for ease of understanding and description, "inside and outside" refer to the inside and outside relative to the outline of each component itself, but the above-mentioned directional words are not used to limit the present invention.

[0039] Obviously, the above-described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0040] In order to solve the problem that it is difficult to evaluate the compressibility of conglomerate reservoirs in the prior art, the present invention provides a method for evaluating the compressibility of conglomerate reservoirs.

[0041] like Figure 1 As shown, the method for evaluating the compressibility of the conglomerate reservoir includes: obtaining a brittleness index of the conglomerate reservoir; obtaining a fracture index of the conglomerate reservoir; obtaining a reformed zone width index of the conglomerate reservoir; calculating the compressibility index of the conglomerate reservoir according to the brittleness index, the fracture index and the reformed zone width index; and evaluating the compressibility of the conglomerate reservoir according to the compressibility index.

[0042] By obtaining the brittleness index of the conglomerate reservoir, the fracture index of the conglomerate reservoir, and the reformed zone width index of the conglomerate reservoir, the compressibility index of the conglomerate reservoir is calculated according to the brittleness index, the fracture index and the reformed zone width index, and the compressibility of the conglomerate reservoir is evaluated according to the compressibility index. The parameters required for the compressibility evaluation are easy to obtain and the data are accurate. The entire evaluation process does not require complicated calculations and is simple and easy to use and easy to implement on site, which greatly reduces the difficulty of implementing the compressibility evaluation of the conglomerate reservoir.

[0043] like Figure 2 As shown, in this embodiment, obtaining the brittleness index of the conglomerate reservoir includes: drilling a first core sample of the conglomerate reservoir, performing a uniaxial compression test on the first core sample, obtaining an axial strain-stress test curve, selecting an expansion point and a peak failure point on the axial strain-stress test curve, reading the corresponding axial stresses respectively, and calculating the brittleness index. Specifically, the maximum value of the volumetric strain is the expansion point, and the corresponding stress is the expansion point stress.

[0044] Specifically, the calculation formula of the brittleness index is:

[0045]

[0046] Where B is the brittleness index; σ ci is the expansion point stress, in MPa; σ f is the peak stress, in MPa.

[0047] like Figures 3 to 5 As shown, in this embodiment, obtaining the fracture index of the conglomerate reservoir includes: preparing a second core sample of the conglomerate reservoir, performing a three-point bending test on the second core sample, obtaining a displacement-load test curve, obtaining the peak load according to the displacement-load test curve, and calculating the mode I fracture toughness of the second core sample. Specifically, the second core sample is a semi-circular three-point bending test sample. The diameter D (=2R) of the second core sample is 0.076m, the thickness B is 0.03m, the crack notch length a is 0.018m, and the span s during loading is 0.05m. During loading, the pressure head above the second core sample continues to move downward until the second core sample breaks. To avoid the influence of dynamic effects, the loading rate is set to 0.1mm / min. The experimental data is collected from the time the pressure head contacts the second core sample to the time the second core sample breaks. Read the peak load P obtained from the experimental test max .

[0048] Specifically, the calculation formula for mode I fracture toughness is:

[0049]

[0050]

[0051]

[0052] Among them, K IC is the mode I fracture toughness, unit is MPa·m 1 / 2 ; R is the radius of the second core sample, in m; B is the thickness of the second core sample, in m; a is the length of the crack notch of the second core sample, in m; s is the span of the second core sample, in m; P max is the peak load, in N.

[0053] In this embodiment, in order to further determine the fracture index, obtaining the fracture index of the conglomerate reservoir further includes: normalizing the type I fracture toughness to obtain the fracture index. The fracture index is used to characterize the difficulty of the initiation and extension of the main fracture tip. The larger the value, the easier it is for the main fracture to initiate and extend. The higher the fracture index, the easier it is for the hydraulic fracturing main fracture to extend, and vice versa, the more difficult it is to open the main fracture.

[0054] Specifically, the calculation formula of the fracture index is:

[0055]

[0056] Where K is the fracture index; is the minimum value of mode I fracture toughness of conglomerate reservoir, in MPa·m 1 / 2 ; K' IC is the value to be tested of the mode I fracture toughness of the conglomerate reservoir, in MPa·m 1 / 2 .

[0057] like Figure 5 As shown, in this embodiment, obtaining the reformed zone width index of the conglomerate reservoir includes: fitting the initial section after the peak load on the displacement-load test curve and obtaining the slope of the initial section after the peak load, and calculating the reformed zone width index according to the slope. Specifically, the slope of the initial section after the load peak represents the bearing capacity of the water fracture reformed zone after the fracture, which is directly related to the reformed zone width. The wider the reformed zone width, the greater the bearing capacity of the reformed zone, and the flatter the post-peak curve of the three-point bending test.

[0058] Specifically, the calculation formula of the transformation belt width index is:

[0059]

[0060] Where W is the width index of the transformation zone; W' is the absolute value of the slope; W min is the minimum absolute value of the slope.

[0061] In this embodiment, the calculation formula of the compressibility index is:

[0062]

[0063] Among them, FI is the compressibility index; B is the brittleness index; K is the fracture index; and W is the transformation zone width index.

[0064] In this embodiment, the larger the brittleness index, the longer the main crack. The larger the fracture index, the easier it is for the main crack to start, extend and expand. The larger the reconstruction zone width index, the more complex the microcracks around the main crack, and the wider the reconstruction zone.

[0065] In this embodiment, the compressibility of the conglomerate reservoir is evaluated according to the compressibility index, including: when the compressibility index is greater than or equal to 50, the compressibility of the conglomerate reservoir is good; when the compressibility index is less than 50, the compressibility of the conglomerate reservoir is poor.

[0066] In a specific embodiment, the compressibility of several conglomerate reservoirs was evaluated according to the method for evaluating the compressibility of conglomerate reservoirs of the present application. Six first core samples were drilled in each conglomerate reservoir and numbered A to F. Specifically, the first core sample was a cylindrical sample of Φ2.5cm×5.0cm. Six second core samples homologous to the first core sample were prepared, i.e., semi-disc three-point bending test samples, numbered corresponding to the first core sample. Uniaxial compression tests and three-point bending tests were performed on the first core sample and the second core sample, respectively, and the experimental data were analyzed and calculated to obtain the compressibility index evaluation results of each sample.

[0067] Table 1 Evaluation results of conglomerate reservoir compressibility index

[0068] Sample No. A B C D E F Brittleness Index B 94.38 94.17 87.82 80.23 65.52 60.06 <![CDATA[Fracture toughness K IC > 251.57 228.66 158.13 141.79 135.84 108.10 Fracture index K 42.97 47.28 68.36 76.24 79.58 100.00 Absolute value of post-peak slope W' 25530 28818 2901 2432 2726 1616 Transformation belt width index W 6.33 5.61 55.70 66.45 59.28 100.00 Compressibility Index FI 47.89 49.02 70.63 74.31 68.13 86.69

[0069] As shown in Table 1, the calculated compressibility index of the first core sample and the second core sample numbered A and B is less than 50, indicating that the corresponding conglomerate reservoir has poor compressibility. The calculated compressibility index of the first core sample and the second core sample numbered C to F is greater than 50, indicating that the corresponding conglomerate reservoir has good compressibility.

[0070] From the above description, it can be seen that the above embodiments of the present invention achieve the following technical effects: by obtaining the brittleness index of the conglomerate reservoir, obtaining the fracture index of the conglomerate reservoir, and obtaining the reformed zone width index of the conglomerate reservoir, the compressibility index of the conglomerate reservoir is calculated according to the brittleness index, the fracture index and the reformed zone width index, and the compressibility of the conglomerate reservoir is evaluated according to the compressibility index. The parameters required for the compressibility evaluation are easy to obtain and the data are accurate. The entire evaluation process does not have complicated calculations, is simple and easy to use, and is easy to implement on site, which greatly reduces the difficulty of implementing the compressibility evaluation of the conglomerate reservoir.

[0071] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present application. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprise" and / or "include" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or combinations thereof.

[0072] It should be noted that the terms "first", "second", etc. in the specification and claims of the present application and the above-mentioned drawings are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence. It should be understood that the numbers used in this way can be interchanged where appropriate, so that the embodiments of the present application described herein can be implemented in an order other than those illustrated or described herein.

[0073] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for evaluating the compressibility of a conglomerate reservoir, characterized in that: include: Obtain the brittleness index of conglomerate reservoir; Obtaining a fracture index of the conglomerate reservoir; Obtaining a reformation zone width index of the conglomerate reservoir; Calculating the compressibility index of the conglomerate reservoir according to the brittleness index, the fracture index and the reformed zone width index; evaluating the compressibility of the conglomerate reservoir according to the compressibility index; The obtaining of the brittleness index of the conglomerate reservoir comprises: Drilling a first core sample of the conglomerate reservoir, performing a uniaxial compression test on the first core sample to obtain an axial strain-stress test curve, selecting a capacity expansion point and a peak failure point on the axial strain-stress test curve, reading the corresponding axial stresses respectively, and calculating the brittleness index; The calculation formula of the brittleness index is: Wherein, B is the brittleness index; σ ci is the expansion point stress, in MPa; σ f is the peak stress, in MPa; The obtaining of the fracture index of the conglomerate reservoir comprises: preparing a second core sample of the conglomerate reservoir, performing a three-point bending test on the second core sample to obtain a displacement-load test curve, obtaining a peak load according to the displacement-load test curve, and calculating the mode I fracture toughness of the second core sample; The calculation formula of the mode I fracture toughness is: Among them, K IC is the mode I fracture toughness, in MPa·m 1 / 2 ; R is the radius of the second core sample, in m; B is the thickness of the second core sample, in m; a is the length of the crack gap of the second core sample, in m; s is the span of the second core sample, in m; P max is the peak load, in N; The step of obtaining the reformed zone width index of the conglomerate reservoir comprises: Fitting the initial section after the peak load on the displacement-load test curve and obtaining the slope of the initial section after the peak load, and calculating the transformation belt width index according to the slope; The calculation formula of the transformation belt width index is: Wherein, W is the width index of the transformation zone; W' is the absolute value of the slope; W min is the minimum value of the absolute value of the slope; The calculation formula of the compressibility index is: Wherein, FI is the compressibility index; B is the brittleness index; K is the fracture index; and W is the transformation zone width index.

2. The method for evaluating the compressibility of a conglomerate reservoir according to claim 1, characterized in that: The obtaining of the fracture index of the conglomerate reservoir further comprises: The mode I fracture toughness is normalized to obtain the fracture index.

3. The method for evaluating the compressibility of a conglomerate reservoir according to claim 2, characterized in that: The calculation formula of the fracture index is: Wherein, K is the fracture index; K ICmin is the minimum value of the mode I fracture toughness of the conglomerate reservoir, in MPa·m 1 / 2 ; K' IC is the value to be tested of the mode I fracture toughness of the conglomerate reservoir, in MPa·m 1 / 2 .

4. The method for evaluating the compressibility of a conglomerate reservoir according to any one of claims 1 to 3, characterized in that: The evaluating the compressibility of the conglomerate reservoir according to the compressibility index comprises: When the compressibility index is greater than or equal to 50, the compressibility of the conglomerate reservoir is good; When the compressibility index is less than 50, the compressibility of the conglomerate reservoir is poor.

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