Method for judging compressibility of rock stratum

Abstract

The present invention relates to the field of mining technology, and particularly to a method for judging the compressibility of rock formations, including: obtaining the percentage of brittle mineral components, elastic modulus, fracture pressure and tensile strength of each rock formation based on in-situ borehole tests; obtaining the standardized parameters of the percentage of brittle mineral components, elastic modulus, fracture pressure and tensile strength of each rock formation; obtaining the weight of each index based on the analytic hierarchy process; and obtaining the compressibility coefficient of each rock formation based on the standardized parameters of the percentage of brittle mineral components, elastic modulus, fracture pressure, tensile strength of each rock formation and the weight of each index. By testing multiple in-situ data in the borehole and using a comprehensive evaluation method to judge the compressibility of rock formations at different horizons, the testing and evaluation efficiency is greatly improved. At the same time, the testing results of the borehole are more accurate in describing the rock formations and their distribution characteristics, and can more reasonably select the fracturing horizons, carry out parameter design, process optimization and equipment selection to achieve a better fracturing effect.

Description

Technical Field

[0001] The present invention relates to the technical field of mining, and particularly relates to a method for judging the compressibility of rock strata. Background Art

[0002] The application of hydraulic fracturing is becoming increasingly widespread. In particular, the fracturing of hard rock strata has become an important means for the prevention and control of disasters induced by strong mine pressure, rock burst, and strong mining-induced stress caused by hard roof, and for the advanced treatment of threats. However, the selection of fracturing targets is still relatively blind at present. Most of them directly select thick sandstone strata or limestone strata, etc., which are hard roofs in the traditional sense, through the stratigraphic column. And most of the stratigraphic columns are comprehensive stratigraphic columns. The distribution of underground rock strata changes rapidly and greatly, and the guiding significance of the comprehensive stratigraphic column is not great. Even for the sandstone strata, limestone strata, conglomerate strata, etc. marked in the stratigraphic column, the actual strength, structure and other characteristics may lead to poor compressibility. This will lead to unreasonable selection of fracturing horizons and poor fracturing effects, and the engineering effects cannot be achieved. Therefore, the compressibility of rock strata should be evaluated before fracturing.

[0003] In the traditional oil and gas extraction industry, the evaluation of rock mass compressibility mainly adopts a multi-index evaluation method, considering the geological parameters, physical properties parameters, physical and mechanical parameters and reservoir stress environment of the fracturing target layer. Most of them need to carry out core tests to obtain various indexes. For the fracturing of hard roofs in coal mines, physical properties do not need to be considered, and it is basically not necessary to form a complex fracture network structure. Only the main factors such as whether cracks are formed, whether the cracks are easy to initiate and easy to expand need to be considered.

[0004] Generally, the core test method is adopted to obtain the physical and mechanical parameters of coal and rock mass in coal mines. Although relatively accurate data can be obtained by taking cores for laboratory tests, due to the maximum fracturing height can reach up to 100m, the difficulty, cost and efficiency of core taking and testing are high. Summary of the Invention

[0005] The present invention provides a method for judging the compressibility of rock strata, which is used to solve one of the defects in the prior art. By judging the grouting feasibility of surrounding rock, the quantification effect of the compressibility of rock strata is realized, which can intuitively reflect the feasibility of surrounding rock grouting reinforcement and increase the scientificity of grouting engineering decision-making.

[0006] The present invention provides a method for judging the compressibility of rock strata, including:

[0007] Based on in-situ borehole tests, obtain the percentage of brittle mineral components, elastic modulus, fracture pressure and tensile strength of each rock stratum;

[0008] Based on the percentage of brittle mineral components, elastic modulus, fracture pressure and tensile strength of each rock stratum, obtain the standardized parameters of the percentage of brittle mineral components, elastic modulus, fracture pressure and tensile strength of each rock stratum;

[0009] Based on the analytic hierarchy process, the weights of each index are obtained;

[0010] Based on the standardized parameters of the brittle mineral composition percentage, elastic modulus, fracture pressure, and tensile strength of each rock stratum and the weights of each index, the compressibility coefficients of each rock stratum are obtained.

[0011] According to a method for judging the compressibility of a rock stratum provided by the present invention, the obtaining of the brittle mineral composition percentage of each rock stratum based on in-situ borehole testing includes:

[0012] Based on natural gamma testing, the gamma intensity values of each rock stratum are obtained;

[0013] Based on the test results of laboratory rock mineral composition, the gamma intensity values of each rock stratum are calibrated to obtain the shale content percentage of each rock stratum;

[0014] Based on the shale content percentage of each rock stratum, the brittle mineral content percentage of each rock stratum is obtained.

[0015] According to a method for judging the compressibility of a rock stratum provided by the present invention, the obtaining of the elastic modulus of each rock stratum based on in-situ borehole testing includes:

[0016] Based on the borehole penetration method, the rock mass pressure-displacement curves of each rock stratum are obtained;

[0017] Based on the rock mass pressure-displacement curves of each rock stratum, the elastic modulus of each rock stratum is obtained.

[0018] According to a method for judging the compressibility of a rock stratum provided by the present invention, the obtaining of the fracture pressure and tensile strength of each rock stratum based on in-situ borehole testing includes:

[0019] Based on the hydraulic fracturing method, the fracture pressure, maximum horizontal principal stress, and minimum principal stress of each rock stratum are obtained;

[0020] Based on the fracture pressure, maximum horizontal principal stress, and minimum principal stress of each rock stratum, the tensile strength of each rock stratum is obtained.

[0021] According to a method for judging the compressibility of a rock stratum provided by the present invention, the obtaining of the standardized parameters of the brittle mineral composition percentage, elastic modulus, fracture pressure, and tensile strength of each rock stratum based on the brittle mineral composition percentage, elastic modulus, fracture pressure, and tensile strength of each rock stratum includes:

[0022] Based on the range variation, the parameters of the brittle mineral composition percentage, elastic modulus, fracture pressure, and tensile strength of each rock stratum are unified to obtain the standardized parameters of the brittle mineral composition percentage, elastic modulus, fracture pressure, and tensile strength of each rock stratum.

[0023] A method for judging the compressibility of rock formations provided by the present invention, wherein the standardized parameter of the percentage of brittle mineral components in each rock formation is the decimal value of the percentage of brittle mineral components in each rock formation.

[0024] A method for judging the compressibility of rock formations provided by the present invention, wherein obtaining the compressibility coefficient of each rock formation based on the standardized parameters of the percentage of brittle mineral components, elastic modulus, fracture pressure, and tensile strength of each rock formation and the weights of each index includes:

[0025] Obtaining the comprehensive index of the percentage of brittle mineral components, elastic modulus, fracture pressure, and tensile strength of each rock formation based on the product of the standardized parameters of the percentage of brittle mineral components, elastic modulus, fracture pressure, and tensile strength of each rock formation and their weight values in the weights of each index;

[0026] Obtaining the compressibility coefficient of each rock formation based on the sum of the comprehensive indexes of the percentage of brittle mineral components, elastic modulus, fracture pressure, and tensile strength of each rock formation.

[0027] A method for judging the compressibility of rock formations provided by the present invention, judging the fracturing feasibility degree of each rock formation based on the compressibility coefficient of each rock formation and the feasible reference range.

[0028] The method for judging the compressibility of rock formations provided by the present invention drills holes in the surrounding rock, and combines the corresponding tests on the influence changes generated by the holes on the surrounding rock, so as to obtain the percentage b of brittle mineral components, elastic modulus E, and fracture pressure P b and tensile strength σ t , as the conditional basis for judging the compressibility of each rock formation, standardizes the obtained percentage b of brittle mineral components, elastic modulus E, fracture pressure P b and tensile strength σ t for parameter standardization, and uses the analytic hierarchy process to analyze the weights of the percentage b of brittle mineral components, elastic modulus E, fracture pressure P b and tensile strength σ t which are the influencing factors for judging the compressibility of rock formations, so as to obtain the weight values of the percentage b of brittle mineral components, elastic modulus E, fracture pressure P b and tensile strength σ t , form the weights of each index, and finally comprehensively consider the standardized parameters S b of the percentage b of brittle mineral components, elastic modulus E, fracture pressure P t and tensile strength σ i of each rock formation and the weights of each index to obtain the compressibility coefficient FI of each rock formation.

[0029] The method for judging the compressibility applicable to coal-bearing sedimentary strata provided by the present invention can quantitatively judge the compressibility of each stratum. By testing a number of in-situ data in boreholes and adopting a comprehensive evaluation method to judge the compressibility of rock formations at different horizons, the testing and evaluation efficiency is greatly improved. At the same time, the description of the rock formation and its distribution characteristics based on the testing results of boreholes is more accurate than the comprehensive columnar section in the prior art, and it can more reasonably select the fracturing horizons, conduct parameter design, process optimization, and equipment selection to achieve a better fracturing effect.

[0030] In addition to the technical problems solved by the present invention, the technical features of the technical solutions constituted, and the advantages brought by these technical features described above, the other technical features of the present invention and the advantages brought by these technical features will be further described in conjunction with the drawings or understood through the practice of the present invention. Brief Description of the Drawings

[0031] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0032] Figure 1 It is a schematic flow chart of the method for judging the compressibility of a rock formation provided by an embodiment of the present invention. Detailed Embodiments

[0033] The following will further describe in detail the embodiments of the present invention in conjunction with the drawings and embodiments. The following embodiments are used to illustrate the present invention, but cannot be used to limit the scope of the present invention.

[0034] In the description of the embodiments of the present invention, it should be noted that the orientation or positional relationship indicated by the terms "center", "longitudinal", "transverse", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc. is based on the orientation or positional relationship shown in the drawings, and is only for the convenience of describing the embodiments of the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation to the embodiments of the present invention. In addition, the terms "first", "second", "third" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance.

[0035] In the description of the embodiments of the present invention, it should be noted that unless otherwise clearly defined and limited, the terms "connected" and "coupled" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be directly connected or indirectly connected through an intermediate medium. For those of ordinary skill in the art, the specific meanings of the above terms in the embodiments of the present invention can be understood according to specific situations.

[0036] In the embodiments of the present invention, unless otherwise clearly defined and limited, the first feature being "on" or "under" the second feature may be that the first and second features are in direct contact, or the first and second features are indirectly in contact through an intermediate medium. Moreover, the first feature being "above", "over" and "on top of" the second feature may be that the first feature is directly above or obliquely above the second feature, or merely means that the first feature has a higher horizontal height than the second feature. The first feature being "under", "below" and "beneath" the second feature may be that the first feature is directly below or obliquely below the second feature, or merely means that the first feature has a lower horizontal height than the second feature.

[0037] In addition, in the description of the embodiments of the present invention, unless otherwise specified, the meanings of "a plurality of", "multiple roots", "multiple groups" are two or more, and the meanings of "several", "several roots", "several groups" are one or more.

[0038] In the description of this specification, the descriptions with reference to the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples", etc. mean that the specific features, structures, materials, or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the embodiments of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described can be combined in a suitable manner in any one or more embodiments or examples. In addition, without contradiction, those skilled in the art can combine and combine the different embodiments or examples described in this specification and the features of different embodiments or examples.

[0039] Relevant research at home and abroad shows that the brittleness of rocks is an important essential characteristic for characterizing the initiation and propagation of rock fractures. Brittleness can be characterized by parameters such as brittle mineral composition, elastic modulus, and Poisson's ratio. The greater the brittleness, the stronger the compressibility, that is, the greater the proportion of brittle mineral composition, the greater the elastic modulus, and the smaller the Poisson's ratio, the stronger the compressibility. The initiation pressure represents the difficulty of crack generation in the rock formation under its in-situ stress environment. The greater the initiation pressure, the more difficult it is for cracks to initiate and the worse the compressibility. Fracture toughness is a parameter for characterizing the difficulty of crack propagation forward, but fracture toughness needs to be obtained through laboratory tests. A large number of studies show that fracture toughness, especially mode I fracture toughness, has a strong correlation with tensile strength, and tensile strength can be used to replace fracture toughness to characterize the difficulty of crack extension. The greater the tensile strength, the more difficult it is for cracks to extend and the worse the compressibility.

[0040] Based on the above summary of relevant research results on the compressibility of rock formations at home and abroad, combined with years of experience in coal mine roof rock formation fracturing design and construction, four parameters that can be obtained through in-situ testing, namely the content of brittle mineral composition, elastic modulus, breakdown pressure, and tensile strength, are selected for compressibility evaluation.

[0041] As Figure 1 shown, the method for judging the compressibility of a rock formation provided by an embodiment of the present invention includes:

[0042] Based on in-situ borehole testing, obtain the percentage b of brittle mineral composition, elastic modulus E, breakdown pressure P b and tensile strength σ t ;

[0043] Based on the percentage b of brittle mineral composition, elastic modulus E, breakdown pressure P b and tensile strength σ t of each rock formation, obtain the standardized parameter S b of the percentage b of brittle mineral composition, elastic modulus E, breakdown pressure P t and tensile strength σ i of each rock formation;

[0044] Based on the analytic hierarchy process, obtain the weight of each index;

[0045] Based on the standardized parameter S b of the percentage b of brittle mineral composition, elastic modulus E, breakdown pressure P t and tensile strength σ i of each rock formation and the weight of each index, obtain the compressibility coefficient FI of each rock formation.

[0046] The method for judging the compressibility of a rock formation according to an embodiment of the present invention drills a borehole in the surrounding rock and combines the corresponding tests on the influence changes generated by the borehole on the surrounding rock to obtain the percentage b of brittle mineral composition, elastic modulus E, breakdown pressure Pb and the tensile strength σ t , as the conditional basis for judging the compressibility of each rock stratum, for the obtained percentage b of brittle mineral components, elastic modulus E, fracture pressure P of each rock stratum b and the tensile strength σ t Perform parameter standardization, and use the analytic hierarchy process to analyze the weights of the percentage b of brittle mineral components, elastic modulus E, fracture pressure P b and the tensile strength σ t as the influencing factors for judging the compressibility of rock strata, so as to obtain the percentage b of brittle mineral components, elastic modulus E, fracture pressure P b and the tensile strength σ t The weight value W i , form the weights of each index, and finally synthesize the percentage b of brittle mineral components, elastic modulus E, fracture pressure P of each rock stratum b and the tensile strength σ t The standardized parameter S i and the weights of each index to obtain the compressibility coefficient FI of each rock stratum.

[0047] The compressibility judgment method applicable to coal-bearing sedimentary strata provided by the present invention can quantitatively judge the compressibility of each stratum. By testing multiple in-situ data in the borehole and using a comprehensive evaluation method to judge the compressibility of rock strata at different horizons, the testing and evaluation efficiency can be greatly improved. At the same time, the description of the testing results of the borehole for the rock strata and distribution characteristics is more accurate than the comprehensive columnar diagram of the prior art, and it can more reasonably select the fracturing horizon, carry out parameter design, process optimization and equipment selection to achieve a better fracturing effect.

[0048] In this embodiment, the weight value W i of the above influencing factors is obtained by using the analytic hierarchy process. The four indexes of brittle mineral content b, elastic modulus E, fracture pressure P b and the tensile strength σ t are used as judgment indexes for comprehensive evaluation. It is very difficult to complete the compressibility decision through the comparison evaluation index of a single compressibility influencing factor. There is an interaction between various factors. Considering the reliability and independence of comprehensive data, finally, the four indexes of brittle mineral content b, elastic modulus E, fracture pressure P b , tensile strength σ t are selected as evaluation indexes to establish a comprehensive judgment hierarchical model of the compressibility of rock strata.

[0049] According to the analytic hierarchy process, establish a hierarchical structure, construct a pairwise comparison judgment matrix, calculate the eigenvalue and eigenvector respectively. Compare any two judgment indexes item by item, and assign values according to the importance degree of the judgment indexes. Compare the compressibility judgment indexes pairwise to obtain the judgment matrix. The scale and meaning of the judgment matrix are shown in Table 1 below.

[0050] Table 1

[0051]

[0052] For example, when the elastic modulus E and the fracture pressure P b For the judgment matrix composed of judgment indexes, after calculating the characteristic roots and eigenvectors, A is obtained ij is 5, which proves that the elastic modulus E is more important than the fracture pressure P b much more important

[0053] Solve the eigenvector corresponding to the maximum eigenvalue of the judgment matrix by the sum-product method or the square root method, and perform a consistency test on the judgment matrix according to the maximum eigenvalue. When the consistency is not satisfactory, the judgment matrix needs to be readjusted until the required accuracy of the consistency test is met (generally 0.1, and the judgment matrix is considered acceptable when the test standard is less than 0.1). After normalizing the eigenvector, it is the weight vector.

[0054] According to an embodiment provided by the present invention, based on in-situ borehole testing, the percentage b of brittle mineral components of each rock layer is obtained, including:

[0055] Based on natural gamma testing, the gamma intensity value of each rock layer is obtained;

[0056] Based on the test results of laboratory rock mineral components, the gamma intensity values of each rock layer are calibrated to obtain the percentage a of shale content of each rock layer;

[0057] Based on the percentage a of shale content of each rock layer, the percentage b of brittle mineral content of each rock layer is obtained.

[0058] In this embodiment, the gamma intensity value of each rock layer is obtained by natural gamma testing, and is calibrated according to the test results of laboratory rock mineral components to obtain the percentage a of shale content of the rocks of different rock layers. Then, the percentage b of brittle mineral content of each rock layer = 1 - a.

[0059] According to an embodiment provided by the present invention, based on in-situ borehole testing, the elastic modulus E of each rock layer is obtained, including:

[0060] Based on the borehole penetration method, the rock mass pressure-displacement curve of each rock layer is obtained;

[0061] Based on the rock mass pressure-displacement curve of each rock layer, the elastic modulus E of each rock layer is obtained.

[0062] In this embodiment, the rock mass pressure-displacement curves of different rock layers are obtained by the borehole penetration method, and then the elastic modulus E of different rock layers is obtained by fitting the rock mass pressure-displacement curves.

[0063] According to an embodiment provided by the present invention, based on in-situ borehole testing, the fracture pressure P of each rock stratum is obtained. b and the tensile strength σ t including:

[0064] Based on the hydraulic fracturing method, the fracture pressure P of each rock stratum is obtained. b 、the maximum horizontal principal stress σ H and the minimum principal stress σ h ;

[0065] Based on the fracture pressure P of each rock stratum b 、the maximum horizontal principal stress σ H and the minimum principal stress σ h , the tensile strength σ of each rock stratum is obtained. t .

[0066] In this embodiment, the fracture pressure P of the rock stratum, the maximum horizontal principal stress σ b , and the minimum principal stress σ H can be obtained by the hydraulic fracturing method. According to the formula fracture pressure P h = 3σ b -σ h +σ H +σ t , the tensile strength σ can be further calculated. t .

[0067] According to an embodiment provided by the present invention, based on the percentage b of brittle mineral components, elastic modulus E, fracture pressure P b and tensile strength σ t of each rock stratum, the standardized parameters S of the percentage b of brittle mineral components, elastic modulus E, fracture pressure P b and tensile strength σ t of each rock stratum are obtained. i including:

[0068] Based on the range variation, the parameters of the percentage b of brittle mineral components, elastic modulus E, fracture pressure P b and tensile strength σ t of each rock stratum are unified to obtain the standardized parameters S of the percentage b of brittle mineral components, elastic modulus E, fracture pressure P b and tensile strength σ t of each rock stratum. i .

[0069] In this embodiment, the percentage b of brittle mineral components, elastic modulus E, fracture pressure P b and tensile strength σ tAs judgment indicators, they all have their own units and dimensions and need to participate in calculations after parameter standardization. Since the judgment indicators are divided into positive and reverse types, positive means the larger the indicator value, the better, and reverse means the smaller the indicator value, the better. Therefore, the percentage of brittle mineral components b and the elastic modulus E in the judgment indicators are positive indicators, and the fracture pressure P b and the tensile strength σ t are reverse indicators. For the convenience of calculation, the range variation is used to unify the parameters of the percentage of brittle mineral components b and the elastic modulus E, which are positive indicators, and the fracture pressure P b and the tensile strength σ t .

[0070] After the range variation parameter unification of the elastic modulus E, the standardized parameter B1 of the elastic modulus E is obtained as B1=(E - E min ) / (E max -E min ), where E max is the maximum value of the elastic modulus in the rock strata exposed by this borehole, and E min is the minimum value. Alternatively, according to the evaluation range, the maximum and minimum values of the elastic modulus of this mine, this mining area, or even a larger area can be selected; for the fracture pressure P b , after the range variation parameter unification, the standardized parameter C1 of the fracture pressure P b is obtained as C1=(P bmax -P b ) / (P bmax -P bmin ), where P bmax and P bmin are the maximum and minimum values of the fracture pressure respectively; for the tensile strength σ t , after the range variation parameter unification, the standardized parameter D1 of the tensile strength σ t is obtained as D1=(σ tmax -σ t ) / (σ tmax -σ tmin ); σ tmax and σ tmin are the maximum and minimum values of the tensile strength respectively.

[0071] According to an embodiment provided by the present invention, the standardized parameter A1 of the percentage of brittle mineral components b in each rock stratum is the decimal value of the percentage of brittle mineral components b in each rock stratum. In this embodiment, the parameter unification of the percentage of brittle mineral components b in each rock stratum can directly use a decimal to represent its value A1. For example, when the percentage of brittle mineral components b is 50%, the standardized parameter A1 of the percentage of brittle mineral components b after parameter unification is 0.5, and when the percentage of brittle mineral components b is 89%, the standardized parameter A1 of the percentage of brittle mineral components b after parameter unification is 0.89.

[0072] According to an embodiment provided by the present invention, based on the brittle mineral composition percentage b, elastic modulus E, breakdown pressure P of each rock stratum b , tensile strength σ t of the standardized parameter S i and the weight of each index, obtaining the compressibility coefficient FI of each rock stratum includes:

[0073] Based on the standardized parameter A1 of the brittle mineral composition percentage b of each rock stratum, the standardized parameter B1 of the elastic modulus E, the breakdown pressure P b of the standardized parameter C1, the tensile strength σ t of the standardized parameter D1 and its weight value W i in the weight of each index, obtaining the comprehensive index of the brittle mineral composition percentage b, elastic modulus E, breakdown pressure P b and tensile strength σ t of each rock stratum;

[0074] Based on the sum of the comprehensive indexes of the brittle mineral composition percentage b, elastic modulus E, breakdown pressure P b and tensile strength σ t of each rock stratum, obtaining the compressibility coefficient FI of each rock stratum.

[0075] In this embodiment, the standardized parameter S i is weighted with the weight value W i to obtain the compressibility coefficient FI of the rock stratum, that is FI is the compressibility coefficient, dimensionless; S i is the standardized value of the judgment index of each rock stratum; W i is the weight value of the judgment index of the rock stratum; n is the number of parameters.

[0076] According to an embodiment provided by the present invention, based on the compressibility coefficient FI of each rock stratum and the feasibility reference range, judge the fracturing feasibility degree of each rock stratum.

[0077] According to the implementation process, monitoring data, implementation effect, etc. of the hydraulic fracturing project, obtain the statistical relationship between the compressibility coefficient FI of the rock strata at different horizons of the roof of the same coal seam in a certain coal mine or mining area and the fracturing implementation effect, so as to obtain the empirical classification standard of the compressibility coefficient FI of the rock strata as shown in Table 2 below.

[0078] Table 2

[0079] FI ≤a <![CDATA[a < FI ≤ b > b < FI < c ≥c Compressibility Poor Medium Good Excellent

[0080] The compressibility coefficient FI model can quantitatively calculate the fracturable coefficient of the roof rock strata, and can also select the fracturing interval by using the compressibility coefficient FI according to the distribution characteristics of different parameters in the depth of the test borehole.

[0081] To solve problems such as high mine pressure, a certain mine plans to implement hydraulic fracturing weakening construction of roof strata, and in-situ testing methods are used for compressibility evaluation before construction. The main process is as follows:

[0082] (1) Drill holes with a depth of 40 m (the length of the drill holes should reach or be greater than the height of the difficult-to-collapse rock strata estimated according to relevant geological data); a total of 7 rock strata are exposed.

[0083] (2) Use natural gamma testing to obtain the gamma intensity values of the rock strata, and calibrate them according to the test results of laboratory rock mineral compositions. The percentage of clay content of the rocks in different rock strata is inversely obtained as a, and the percentage of brittle mineral content is b = 1 - a. The results are shown in Table 3 below:

[0084] Table 3

[0085]

[0086] (3) Use the borehole penetration method to obtain the pressure-displacement curves of rock masses at different horizons, and obtain the in-situ strength and elastic modulus E of rock masses at different horizons, as shown in Table 4 below.

[0087] Table 4

[0088] Rock layer number Compressive strength (MPa) Elastic modulus (GPa) 1 65.1 13.20 2 73.8 22.17 3 42.9 6.46 4 55.4 8.29 5 85.3 17.85 6 34.2 10.04 7 116.8 32.58

[0089] (4) Use the hydraulic fracturing method to obtain the fracture pressure P b , the maximum horizontal principal stress σ H , the minimum horizontal principal stress σ h . According to the formula P b = 3σ h - σ H + σ t , the tensile strength σ t is obtained, as shown in Table 5 below;

[0090] Table 5

[0091]

[0092] (5) Use the analytic hierarchy process to take the brittle mineral content b, elastic modulus E, fracture pressure P b , and tensile strength σ t as four evaluation indicators for comprehensive evaluation.

[0093] According to the above method, parameter standardization is carried out to obtain the normalized values of each parameter, as shown in Table 6 below.

[0094] Table 6

[0095]

[0096] Based on a comprehensive reference to existing research results and combined with engineering experience, the judgment matrix shown in Table 7 is determined. That is, the brittle mineral content is the most important, slightly more important than the elastic modulus and the fracture pressure, and the elastic modulus and the fracture pressure are of the same importance. Considering that the tensile strength mainly affects the degree of crack propagation, it can be improved through technical means such as increasing the fracturing flow rate and pressure, and optimizing the fracturing fluid. Therefore, the brittle mineral content is defined as being stronger important than the tensile strength.

[0097] Table 7 Judgment Matrix of Evaluation Indexes

[0098] Influencing factors Brittle mineral content Elastic modulus Fracture pressure Tensile strength Brittle mineral content 1 3 3 5 Elastic modulus 1 / 3 1 1 3 Fracture pressure 1 / 3 1 1 3 Tensile strength 1 / 5 1 / 3 1 / 3 1

[0099] Use the sum-product method or the square root method to solve the eigenvector corresponding to the maximum eigenvalue of the judgment matrix, and conduct a consistency test on the judgment matrix according to the maximum eigenvalue. When the consistency is not satisfactory, the judgment matrix needs to be readjusted until the required accuracy of the consistency test is met (generally 0.1, and the judgment matrix is considered acceptable when the test standard is less than 0.1). After normalizing the eigenvector, it is the weight vector. The calculation results are shown in Table 8.

[0100] Table 8 Calculated Weights of Evaluation Indexes

[0101]

[0102] Using the square root method for calculation, the weights of the four indexes of brittle mineral content, elastic modulus, fracture pressure, and tensile strength are 0.520490138, 0.200959887, 0.200959887, and 0.077590089 respectively. The CR value of the consistency index is 0.016 < 0.1, indicating that the judgment matrix meets the consistency test and is acceptable.

[0103] The weighted sum of the standardized values and the weight coefficients is the rock layer compressibility coefficient, and its calculation formula is:

[0104]

[0105] In the formula, FI is the compressibility coefficient; S i is the standardized value of each rock layer evaluation parameter; W i is the weight coefficient of each rock layer evaluation parameter; n is the number of parameters, and here n is 4.

[0106] Perform a weighted sum on the evaluation indexes of each rock layer exposed by this borehole, and the results are shown in Table 9.

[0107] Table 9 Evaluation Results of Rock Layer Compressibility Index

[0108] Rock layer number Compressibility index 1 0.543024 2 0.488855 3 0.517912 4 0.517156 5 0.618368 6 0.57118 7 0.90423

[0109] Based on the microseismic monitoring of fracturing events during the fracturing construction process, through comprehensive judgment from aspects such as the number of events, event energy, and fracturing radius, the approximate classification of the fracturability indicators of different rock formations can be obtained as shown in Table 10 below:

[0110] Table 10

[0111] FI ≤0.3 0.3 < FI ≤ 0.5 0.5 < FI < 0.7 ≥0.7 Compressibility Poor Medium Good Excellent

[0112] For subsequent fracturing projects in mines and similar conditions, on the basis of the fracturability evaluation, this empirical value can be combined for evaluation, and engineering parameter design, equipment and process selection and matching, etc. can be carried out.

[0113] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements for some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for judging the compressibility of a rock formation, characterized in that: Including: Based on in-situ borehole tests, obtain the percentage of brittle mineral components, elastic modulus, fracture pressure, and tensile strength of each rock stratum; Based on the percentage of brittle mineral components, elastic modulus, fracture pressure, and tensile strength of each rock stratum, obtain the standardized parameters of the percentage of brittle mineral components, elastic modulus, fracture pressure, and tensile strength of each rock stratum; Based on the analytic hierarchy process, obtain the weights of each index; Based on the standardized parameters of the percentage of brittle mineral components, elastic modulus, fracture pressure, and tensile strength of each rock stratum and the weights of each index, obtain the compressibility coefficient of each rock stratum; The obtaining of the standardized parameters of the percentage of brittle mineral components, elastic modulus, fracture pressure, and tensile strength of each rock stratum based on the percentage of brittle mineral components, elastic modulus, fracture pressure, and tensile strength of each rock stratum includes: Based on the range variation, unify the parameters of the percentage of brittle mineral components, elastic modulus, fracture pressure, and tensile strength of each rock stratum to obtain the standardized parameters of the percentage of brittle mineral components, elastic modulus, fracture pressure, and tensile strength of each rock stratum; The obtaining of the compressibility coefficient of each rock stratum based on the standardized parameters of the percentage of brittle mineral components, elastic modulus, fracture pressure, and tensile strength of each rock stratum and the weights of each index includes: Based on the product of the standardized parameters of the percentage of brittle mineral components, elastic modulus, fracture pressure, and tensile strength of each rock stratum and their weight values in each index weight, obtain the comprehensive index of the percentage of brittle mineral components, elastic modulus, fracture pressure, and tensile strength of each rock stratum; Based on the sum of the comprehensive indexes of the percentage of brittle mineral components, elastic modulus, fracture pressure, and tensile strength of each rock stratum, obtain the compressibility coefficient of each rock stratum; The standardized parameter of the percentage of brittle mineral components of each rock stratum is the decimal value of the percentage of brittle mineral components of each rock stratum.

2. The method for judging the compressibility of a rock formation according to claim 1, characterized in that: The obtaining of the percentage of brittle mineral components of each rock stratum based on in-situ borehole tests includes: Based on natural gamma tests, obtain the gamma intensity values of each rock stratum; Calibrate the gamma intensity values of each rock stratum based on the test results of laboratory rock mineral components to obtain the percentage of shale content of each rock stratum; Based on the percentage of shale content of each rock stratum, obtain the percentage of brittle mineral content of each rock stratum.

3. The method for judging the compressibility of a rock formation according to claim 1, characterized in that: The obtaining of the elastic modulus of each rock stratum based on in-situ borehole tests includes: Based on the borehole penetration method, obtain the rock mass pressure-displacement curve of each rock stratum; Based on the rock mass pressure-displacement curve of each rock stratum, obtain the elastic modulus of each rock stratum.

4. The method for judging the compressibility of a rock formation according to claim 1, characterized in that: The obtaining of the fracture pressure and tensile strength of each rock stratum based on in-situ borehole tests includes: Based on the hydraulic fracturing method, obtain the fracture pressure, maximum horizontal principal stress, and minimum principal stress of each rock stratum; Based on the fracture pressure, maximum horizontal principal stress, and minimum principal stress of each rock stratum, obtain the tensile strength of each rock stratum.

5. The method for judging the compressibility of a rock formation according to any one of claims 1 to 4, characterized in that: Based on the compressibility coefficient of each rock stratum and the feasibility reference range, judge the fracturing feasibility degree of each rock stratum.

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