Method for evaluating shale fracturability based on well logging data of rock physics experiment

Abstract

The present application relates to the field of well logging evaluation technology of oil and gas exploration and development, and particularly relates to a method for evaluating shale fracturing property based on rock physical experiment, comprising the following steps: S1, obtaining rock fracturing property experimental parameters by using laboratory core fracturing triaxial experiment; S2, analyzing the obtained experimental parameters to obtain analysis results; S3, combining the analysis results with logging parameters to establish a fracturing strength model, a fracture complexity model and a fracture extension model; and S4, using the analysis results as a standard scale to establish a fracturing property index model based on rock physical experiment by using the related models. The present application is based on triaxial compression rock physical experiment, and by analyzing three processes of applying pressure, fracture opening and fracture extension in fracturing, the correlation between each process and logging parameters is established, and finally a fracturing property index is formed to evaluate the shale fracturing property.

Description

Technical Field

[0001] This invention relates to the field of well logging evaluation technology for oil and gas exploration and development, specifically a method for evaluating the fracturing capability of shale based on well logging data from rock physics experiments. Background Technology

[0002] Currently, the brittleness index is an important parameter for evaluating the compressibility of shale oil (especially tight oil). The most widely used methods are the Poisson-Young method and the method for calculating brittle mineral content.

[0003] However, the Poisson-Young method lacks experimental evidence and has strong regional applicability; the methods for calculating brittle mineral content are inconsistent in their identification of brittle minerals. Among indoor experimental evaluation methods, the brittleness index calculation method based on stress-strain curve analysis is the most widely used, but its descriptive perspective differs from the earlier proposed brittleness index's ability to form complex fracture networks. Furthermore, the brittleness index does not consider rock strength and cannot fully characterize shale fracturing capability. In evaluating the fracturing capability of shale reservoirs, its variation is small, its stratification ability is weak, and it has significant inapplicability.

[0004] In the current exploration and development of the Daqing Oilfield, shale oil and gas has gradually become an important area for oil and gas replacement. The shale reservoirs unique to Daqing are characterized by relatively high clay content, low porosity, and low permeability. Due to the tight lithology and low natural productivity of these reservoirs, the engineering fracturing approaches or methods used for conventional oil and tight oil reservoirs cannot achieve maximum development and exploitation. To achieve economical development, fracturing enhancement operations are necessary. Therefore, fracturability has become an important parameter in the exploration and development of this type of reservoir. Establishing an effective method for evaluating shale fracturability and providing a quantitative fracturability index is of great significance for evaluating sweet spots and designing fracturing operations. Summary of the Invention

[0005] To address the problems existing in the prior art, this invention provides a method for evaluating the fracturing capability of shale based on well logging data from rock physics experiments. Specifically, considering the characteristics of the Daqing Qingshankou Formation shale, which requires low external force for fracturing, has high post-fracturing fracture complexity, but poor fracture extension, this invention proposes to use a comprehensive fracturing capability index to quantitatively evaluate the fracturing capability of shale.

[0006] This invention is achieved through the following technical solution:

[0007] A method for evaluating the fracturing capability of shale based on well logging data from rock physics experiments includes the following steps:

[0008] S1, Experimental parameters of rock fracturing at different stages were obtained using triaxial core fracturing experiments;

[0009] S2, the obtained experimental parameters were analyzed for core fracturing strength, post-fracturing fracture complexity, and post-fracturing fracture extension, and the analysis results were obtained.

[0010] S3 combines the analysis results with logging parameters and establishes fracturing strength model, fracture complexity model and fracture propagation model;

[0011] S4. Using the analysis results as the standard scale, a fracturability index model based on rock physics experiments is established using the fracturing strength model, fracture complexity model, and fracture propagation model.

[0012] Preferably, in S3, the logging parameters are obtained based on logging curves, and the logging curves are obtained based on logging data.

[0013] Preferably, the logging curves include curves obtained from conventional logging and curves obtained from array sonic logging.

[0014] Preferably, in S4, the fracturability index model based on rock physics experiments is:

[0015]

[0016] Among them, F i The fracturability index for triaxial core fracturing experiments;

[0017] A, B, C, and D are the correlation coefficients calibrated by rock physics experiments;

[0018] S S The standard strength for core fracturing triaxial tests;

[0019] F C The complexity of fractures in a triaxial core fracturing experiment;

[0020] FE i The fracture extension is measured in a triaxial core fracturing experiment.

[0021] Preferably, in S2, the analysis results include core fracturing strength, core fracture complexity, and post-fracturing fracture extension; the core fracturing strength is obtained from core fracturing strength analysis; the core fracture complexity is obtained from post-fracturing fracture complexity analysis; and the post-fracturing fracture extension is obtained from core fracture extension analysis.

[0022] Preferably, the standard strength S of the core fracturing triaxial test S The expression is obtained by utilizing the correlation between core fracturing strength and logging parameters:

[0023] S S =F(YMOD)

[0024] In the formula, S SThe standard strength is the triaxial test strength of the core fracturing test; YMOD is the Young's modulus calculated from the array acoustic logging curve.

[0025] Preferably, the fracture complexity F in the core fracturing triaxial experiment is... C The relationship between core fracture complexity and logging parameters is obtained, expressed as:

[0026] F C =f(K / TH)

[0027] In the formula, F C denoted as fracture complexity in a triaxial core fracturing experiment; K represents potassium content; TH represents thorium content.

[0028] Preferably, the fracture propagation in the triaxial core fracturing experiment is obtained using the correlation between the fracture propagation and logging parameters, expressed as:

[0029] FE i =f(VSH,YMOD)

[0030] In the formula, FE i The value represents the fracture propagation in a triaxial core fracturing experiment; VSH represents the clay content. Compared with existing technologies, this invention has the following advantages:

[0031] The method for evaluating the fracturability of shale based on well logging data from rock physics experiments in this invention is based on laboratory triaxial compressive rock physics experiments. By analyzing the three processes of pressure application, fracture initiation and fracture propagation during fracturing, correlations are established with well logging curves, and finally a fracturability index is compiled to evaluate the fracturability of shale.

[0032] Based on the three processes of laboratory rock fracturing, the external force is gradually increased from the start of fracturing until it reaches the rock's strength. Hydraulic fracturing reaches the rock's fracture pressure, while uniaxial or triaxial fracturing reaches its uniaxial or triaxial compressive strength. Next, once the external force reaches the rock's strength, cracks begin to form. Different rocks produce different crack morphologies; for example, sandstone produces only one shear fracture during axial fracturing, while shale produces multiple cracks of different shapes. Finally, if pressure is continued after the cracks open, the cracks begin to extend. Rocks with high brittleness produce cracks that extend quickly and far, while rocks with low brittleness produce cracks that extend and expand more slowly. The brittleness index, analyzed based on the stress-strain curve, describes this capability of the rock.

[0033] Corresponding to the three stages mentioned above, a complete evaluation of rock fracturing capability should describe all three stages: the magnitude of the force required for fracturing, the rock's ability to form a fracture network, and the propagation capacity of the generated fractures. By establishing correlations between the three experimental rock analysis parameters and well logging data, models for fracturing intensity, fracture complexity, and fracture propagation are established using well logging data. Then, using laboratory analysis data as the standard scale, a comprehensive fracturing capability index model is established using well logging data.

[0034] This invention uses well logging data to quantitatively calculate the fracturing intensity, post-fracturing fracture complexity, and fracture extension of shale reservoirs. The fracturability index formed by fitting these three parameters quantifies the comprehensive fracturability required to evaluate engineering sweet spots, achieving a qualitative leap in the evaluation of shale fracturability. It can greatly meet the engineering needs for evaluating rock fracturability, which is reflected in the design of fracturing construction parameters and the prediction and evaluation of engineering sweet spots.

[0035] This invention has unparalleled advantages in quantitatively evaluating the fracturing capability of shale, and therefore has great potential for widespread application.

[0036] Furthermore, based on quantitative analysis in the laboratory, by establishing correlations with conventional logging and array sonic logging data, the three key parameters involved in calculating shale fracturing capability from logging data were determined, and finally, a calculation model for the comprehensive fracturing capability index was determined. Attached Figure Description

[0037] Figure 1 This diagram illustrates the calculation steps of a method for evaluating the fracturability of shale based on well logging data from rock physics experiments, as described in this invention.

[0038] Figure 2 This is a schematic diagram showing the comparison between the fracturability index and the fracturability of core analysis in this invention;

[0039] Figure 3 This is a schematic diagram illustrating the actual well data processing effect of the present invention. Detailed Implementation

[0040] The present invention will be further described in detail below with reference to specific embodiments. These descriptions are for explanation purposes only and are not intended to limit the scope of the invention.

[0041] This invention discloses a method for evaluating the fracturing capability of shale based on well logging data from rock physics experiments, referring to... Figure 1 This includes the following steps:

[0042] S1, Experimental parameters of rock fracturing at different stages were obtained using triaxial core fracturing experiments;

[0043] S2, the obtained experimental parameters were analyzed for core fracturing strength, post-fracturing fracture complexity, and post-fracturing fracture extension, and the analysis results were obtained.

[0044] The analysis results include core fracturing strength, core fracture complexity, and post-fracturing fracture extension. Core fracturing strength was obtained from core fracturing strength analysis; core fracture complexity was obtained from post-fracturing fracture complexity analysis; and post-fracturing fracture extension was obtained from core fracture extension analysis.

[0045] S3 combines the analysis results with logging parameters and establishes fracturing strength model, fracture complexity model and fracture propagation model;

[0046] Among them, logging parameters are obtained from logging curves, and logging curves are obtained from logging data, including conventional curves obtained from conventional logging and curves obtained from array sonic logging.

[0047] (1) Utilizing the correlation between core fracturing strength and logging parameters, a reservoir standard strength model is established using Young's modulus:

[0048] S S = f(YMOD);

[0049] In the formula, S S The standard strength of the core fracture triaxial test is given in MPa; YMOD is the Young's modulus calculated from the array acoustic waveform, in GPa.

[0050] (2) Utilizing the correlation between core fracture complexity and logging curves, a model for calculating fracture complexity was established using the potassium-thorium ratio:

[0051] F C =f(K / TH)

[0052] In the formula, F C The dimensionless value represents the fracture complexity in the triaxial core fracturing experiment; K represents the potassium content in %; and TH represents the thorium content in ppm.

[0053] (3) Establish a fracture extension model by utilizing the correlation between core fracture extension and logging curves:

[0054] FE i =f(VSH,YMOD)

[0055] In the formula, FE i VSH represents the fracture extension in triaxial core fracturing experiments, in degrees; VSH represents the clay content, in percent.

[0056] S4. Using the analysis results as the standard scale, a fracturability index model based on rock physics experiments is established using the fracturing strength model, fracture complexity model, and fracture propagation model.

[0057] Ideally, a well-fracturable rock core should possess the following characteristics: first, low strength, requiring minimal external force for fracturing; second, high post-fracturing fracture complexity, generating numerous fractures with a high degree of network structure, resulting in good fracturing effect; and third, high fracture extension, leading to rapid fracture opening and long-distance extension after fracturing, further enhancing the fracturing effect. Analysis results show that shale exhibits low fracturing strength, high post-fracturing fracture complexity, but poor fracture extension.

[0058] By analyzing the relationship between the three fracturability characterization parameters, they can be unified into a single parameter to characterize the fracturability of shale reservoirs. A comprehensive fracturability index model is established using standard strength, fracture complexity, and fracture extension. The physical meaning of the fracturability index is defined as the fracture complexity that can be generated by applying a unit of energy to a unit volume of rock.

[0059] The fracturability index model is as follows:

[0060]

[0061] Among them, F i The fracturing index for triaxial core fracturing experiments is expressed in cm. 3 / J;

[0062] A, B, C, and D are the correlation coefficients calibrated by rock physics experiments;

[0063] S S The standard strength for triaxial core fracturing tests is given in MPa.

[0064] F C The fracture complexity in a triaxial core fracturing experiment is dimensionless.

[0065] FE i The value represents the fracture extension in a triaxial core fracturing experiment, expressed in degrees.

[0066] When analyzing specific engineering problems, it is necessary to consider three fracturing parameters. For example, to estimate the complexity of the fracture network formed after fracturing, the complexity of the fractures needs to be considered. When designing the pressure during fracturing operations, the standard strength should be referenced. When designing fracturing stimulation operations such as sand addition and well shut-in, the post-fracturing fracture extension capacity should be considered.

[0067] Figure 2 This diagram illustrates the comparison between the fracturability index calculated from well logging data and the fracturability index calculated from core analysis, in accordance with the present invention's method for evaluating shale fracturability based on rock physics experimental data. This is to demonstrate the accuracy of the calculation results using well logging data.

[0068] refer to Figure 3The first channel is the formation channel; the second channel is the logging curves for well diameter and natural gamma ray; the third channel is the depth channel; the fourth channel is the logging curves for deep and shallow lateral and microsphere focusing; the fifth channel is the logging curves for compensated neutron, sonic transit time, and compensated density; the sixth channel is the clay content curve and core analysis data for clay content; the seventh channel is the fracture pressure curve; the eighth channel is the Young's modulus and Poisson's ratio curves; the ninth channel is the calculated compressibility index curve and core analysis data for compressibility index; the tenth channel is the calculated fracture propagation curve and core analysis data for propagation; the eleventh channel is the calculated fracture complexity curve and core analysis data for complexity; the twelfth channel is the calculated standard strength curve and core analysis data for standard strength; and the thirteenth channel is the logging interpretation layering conclusions.

Claims

1. A method for evaluating the fracturing capability of shale based on well logging data from rock physics experiments, characterized in that, Includes the following steps: S1, Experimental parameters of rock fracturing at different stages were obtained using triaxial core fracturing experiments; S2, the obtained experimental parameters were analyzed for core fracturing strength, post-fracturing fracture complexity, and post-fracturing fracture extension, and the analysis results were obtained. S3 combines the analysis results with logging parameters and establishes fracturing strength model, fracture complexity model and fracture propagation model; S4. Using the analysis results as the standard scale, a fracturability index model based on rock physics experiments is established using the fracturing strength model, fracture complexity model, and fracture propagation model. The analysis results include core fracturing strength, core fracture complexity, and post-fracturing fracture extension; the core fracturing strength is obtained from core fracturing strength analysis; the core fracture complexity is obtained from post-fracturing fracture complexity analysis; and the post-fracturing fracture extension is obtained from post-fracturing fracture extension analysis. Standard strength of core fracturing triaxial test The expression for the fracturing strength model is obtained by utilizing the correlation between core fracturing strength and logging parameters: In the formula, ; YMOD Calculate Young's modulus for array acoustic logging curves; The expression for the fracture complexity model is obtained by utilizing the correlation between core fracture complexity and logging parameters: In the formula, ; K Potassium content; TH Thorium content; The fracture propagation in triaxial core fracturing experiments is obtained using the correlation between core fracture propagation and logging parameters. The expression for the fracture propagation model is as follows: In the formula, ; VSH This refers to the mud content.

2. The method for evaluating the fracturing capability of shale based on well logging data from rock physics experiments according to claim 1, characterized in that, In S3, the logging parameters are obtained based on the logging curves, and the logging curves are obtained based on the logging data.

3. The method for evaluating the fracturing capability of shale based on well logging data from rock physics experiments according to claim 2, characterized in that, The logging curves include those obtained from conventional logging and those obtained from array sonic logging.

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