Shale reservoir anisotropic brittleness index prediction method

Abstract

The invention discloses a shale reservoir anisotropic brittleness index prediction method, and belongs to the technical field of rock mechanics. The method comprises the following steps: preparing a shale test piece containing a horizontal bedding dip angle and a shale test piece containing a vertical bedding dip angle; the elastic modulus and the softening modulus of the shale at the two bedding angles are calculated through the uniaxial compression test of the horizontal bedding test piece and the vertical bedding test piece; performing brittleness evaluation on the horizontal and vertical bedding shale test pieces by adopting a brittleness index BI; predicting the anisotropic brittleness index of the shale reservoir; and calculating to obtain shale brittleness index prediction values at different angles theta, and comparing the shale brittleness index prediction values with calculation results obtained by the brittleness index BI to verify the rationality of the prediction formula. According to the method, the brittleness indexes under different bedding inclination angles can be accurately predicted, the brittleness indexes of other bedding inclination angles are calculated through a mathematical model, and the problem that a large number of experiments and complex calculation are needed in a traditional method is solved.

Description

Technical Field

[0001] The present invention relates to the technical field of rock mechanics, and particularly relates to a method for predicting the anisotropic brittleness index of shale reservoirs. Background Art

[0002] With the gradual depletion of oil and conventional natural gas resources, shale gas, as an unconventional natural gas resource, has received extensive attention from the international community due to its huge reserves and potential. As a layered rock mass, shale exhibits significant differences in physical and mechanical properties in different directions, that is, it shows anisotropy. The anisotropic brittle characteristics of shale have an important impact on related research such as wellbore stability and hydraulic fracture propagation, and are important factors in the exploitation of shale reservoirs. Research shows that some mechanical parameters of layered shale show a U-shaped variation law of first decreasing and then increasing with the increase of bedding angle. At present, common brittle prediction methods include those based on mineral composition method, rock physics modeling, seismic data brittle prediction, etc. These methods establish a multiple regression calculation model between the mineral brittleness index and elastic parameters by analyzing the geological characteristics and engineering geological and mechanical characteristics of shale. However, these methods rely on a large amount of on-site data in application, and the calculation process is relatively cumbersome. Summary of the Invention

[0003] The object of the present invention is to propose a method for predicting the anisotropic brittleness index of shale reservoirs, which can obtain the anisotropic brittleness index of shale through conventional indoor tests by using shale specimens with horizontal bedding dip angle and vertical bedding dip angle, and the method is simple.

[0004] The technical solution adopted by the present invention is: a method for predicting the anisotropic brittleness index of shale reservoirs, comprising the following steps:

[0005] Step S1, preparing a shale specimen with a horizontal bedding dip angle and a shale specimen with a vertical bedding dip angle: drilling a horizontal bedding shale specimen and a vertical bedding shale specimen respectively in layered shale, the bedding dip angle in the horizontal bedding shale specimen is 0°, and the bedding dip angle in the vertical bedding shale specimen is 90°;

[0006] Step S2, through the uniaxial compression tests of the horizontal bedding specimen and the vertical bedding specimen, calculating the elastic modulus and softening modulus of the shale with two bedding angles:

[0007] The calculation method of the elastic modulus in Step S21 is:

[0008] According to the experimental data, respectively draw the uniaxial compression axial stress (σ)-strain (ε1) curves of the two shale specimens;

[0009] The axial stress σ is calculated by the following formula:

[0010]

[0011] Wherein, F represents the axial load (N); A represents the cross-sectional area of the specimen (mm 2 );

[0012] The axial strain ε1 is calculated by the following formula:

[0013]

[0014] Wherein, ε1 represents the axial strain value; △L represents the average axial deformation under each load level (mm); L represents the height of the specimen (mm);

[0015] The elastic modulus is calculated by the following formula:

[0016]

[0017] Wherein, △σ represents the change value of the axial stress in the elastic section before the peak of the stress-strain curve; △ε represents the change value of the axial strain corresponding to the change value of the axial stress in the elastic section before the peak of the stress-strain curve;

[0018] Step S22: The calculation method of the softening modulus is as follows:

[0019] To distinguish from the calculation method of the elastic modulus, in the calculation of the softening modulus, the axial stress in the post-peak section is represented by σ s and the axial strain in the post-peak section is represented by ε s ;

[0020]

[0021] Wherein, △σ s represents the change value of the axial stress in the post-peak section of the stress-strain curve; △ε s represents the change value of the axial strain in the post-peak section of the stress-strain curve;

[0022] Step S3: Use the brittleness index BI to evaluate the brittleness of horizontal and vertical bedding shale specimens, and calculate the brittleness index BI0 of the horizontal bedding of the shale and the brittleness index BI 90 ;

[0023] The calculation formula of the brittleness index BI is:

[0024]

[0025] Wherein, E is the elastic modulus; M is the softening modulus;

[0026] Step S4: Predict the anisotropic brittleness index of the shale reservoir:

[0027] Substitute the two brittleness indices obtained in step S3 into the formula of the initial brittleness index prediction method to obtain the prediction formula of the anisotropic brittleness index of shale for the brittleness index BI; where,

[0028] The formula of the initial brittleness index prediction method is:

[0029]

[0030] In the formula, BU is the predicted value of the brittleness index for the selected brittleness index; B0 is the brittleness index calculated by using the selected brittleness index for shale with a bedding dip angle of 0°; B 90 is the brittleness index calculated by using the selected brittleness index for shale with a bedding dip angle of 90°; θ is the bedding dip angle between 0° and 90°;

[0031] The prediction formula of the anisotropic brittleness index of shale for the brittleness index BI is:

[0032]

[0033] In the formula, BUI is the predicted value of the brittleness index for the brittleness index BI; BI0 is the brittleness index calculated by using the brittleness index BI for shale with a bedding dip angle of 0°; BI 90 is the brittleness index calculated by using the brittleness index BI for shale with a bedding dip angle of 90°; θ is the bedding dip angle between 0° and 90°;

[0034] Step S5: Calculate the predicted values of the brittleness index of shale at different angles θ through formula (7), and compare them with the calculation results obtained from the brittleness index BI.

[0035] As a further improvement of the present invention, in step S2, the loading rate of the uniaxial test is 0.05 - 0.1 mm / min.

[0036] The mechanism of the present invention is as follows: First, through uniaxial tests, the respective elastic moduli and softening moduli of the horizontal bedding shale specimens and the vertical bedding shale specimens are obtained, and based on this, the brittleness index BI obtained on the basis of the tests is established, and from this BI, the respective brittleness values B0 and B of the horizontal bedding shale specimens and the vertical bedding shale specimens are calculated 90 , according to the law that some mechanical parameters of laminated shale show a U-shaped change rule of first decreasing and then increasing with the increase of the bedding angle, the initial brittleness index prediction formula BU is established, and the two values of B0 and B 90 are substituted into the formula of formula BU, and the values of BI at θ = 0 degrees and θ = 90 degrees are calculated, that is, BI0 and BI are obtained 90, substitute these two values ​​into the final brittleness prediction index BUI to form the final prediction formula with θ as the independent variable, and then select 15°, 30°, 45°, 60° and 75° to substitute into the BUI formula respectively to obtain their respective brittleness prediction values. Then use the uniaxial test to perform specimen tests at these angles respectively, and obtain the true brittleness index values ​​of these angles respectively through the BI formula, and compare the true values ​​with the predicted values. The results are quite consistent, thus verifying the rationality and scientificity of the final BUI prediction formula.

[0037] Compared with the prior art, the present invention has the following advantages:

[0038] (1) Simple sample preparation and easy operation: By simply drilling shale specimens with horizontal bedding and vertical bedding inclination and conducting uniaxial compression tests, it does not rely on complex equipment or difficult experimental procedures, and is easy to operate and apply on site;

[0039] (2) Reduce experimental cost and time: This method only needs to obtain the brittleness index of two bedding angles of 0° and 90°, and the brittleness index at other angles can be inferred through mathematical models, avoiding the collection of a large amount of experimental data and complex calculations, significantly reducing the experimental cost and time required;

[0040] (3) Improving the accuracy of brittleness prediction: By accurately measuring and calculating the elastic modulus and softening modulus at different bedding angles, and combining brittleness indicators to evaluate the brittleness in the bedding direction, the anisotropic characteristics of shale can be accurately reflected;

[0041] (4) The comparison results show that the predicted value is highly consistent with the actual brittleness index, ensuring the reliability of the prediction results;

[0042] (5) Strong adaptability and wide application: This method can be applied to the exploration and development of unconventional oil and gas resources such as shale gas and shale oil. It is especially suitable for the anisotropy study of layered shale reservoirs and has important application value in terms of ground stress, wellbore stability and hydraulic fracture expansion.

[0043] (6) Simplified theoretical modeling: The present invention provides a simple and effective brittleness prediction model by combining theory with experiment, avoiding complex geological modeling and reliance on a large amount of field data, making the brittleness assessment of shale reservoirs more intuitive and easy to implement. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] The present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0045] Figure 1 It is a flow chart of the method for predicting the anisotropic brittleness index of shale reservoirs of the present invention;

[0046] Figure 2Schematic diagram of the preparation of shale specimens with horizontal bedding dip angles and vertical bedding dip angles according to the present invention;

[0047] Figure 3 Schematic diagram of the dimensions of the shale specimens according to the present invention;

[0048] Figure 4 Schematic diagram of the uniaxial compression test according to the present invention;

[0049] Figure 5 Curve comparison diagram of the predicted value of the shale brittleness index and the settlement result of the brittleness index according to the present invention;

[0050] In the figure, 1 - laminated shale; 2 - shale specimen with horizontal bedding; 3 - shale specimen with vertical bedding; 4 - bearing plate; 5 - axial load. Detailed implementation manners

[0051] As Figure 1 shown, the method for predicting the anisotropic brittleness index of a shale reservoir according to the present invention includes the following steps:

[0052] Step S1, prepare shale specimens with horizontal bedding dip angles and shale specimens with vertical bedding dip angles, and the process is as follows:

[0053] As Figures 2 - 3 shown, drill a horizontal bedding shale specimen 2 and a vertical bedding shale specimen 3 in the laminated shale 1 respectively. The bedding dip angle in the horizontal bedding shale specimen 2 is 0°, and the bedding dip angle in the vertical bedding shale specimen 3 is 90°. The shapes of the shale specimens are all cylinders, the height of the specimens is 50 mm, and the diameter is 25 mm. The deviation of the specimens does not exceed 0.2 mm.

[0054] Step S2, through the uniaxial compression tests of the horizontal bedding specimens and the vertical bedding specimens, calculate the elastic modulus and the softening modulus of the two types of bedding angle shales.

[0055] As Figure 4As shown, a uniaxial compression test is carried out to calculate the elastic modulus E and softening modulus M of the shale specimens. After installing the horizontal bedding shale specimen 2 with heat shrinkable tubing, it is placed on the bearing plate 4 of the testing machine, and the specimen is clamped with a fixture to ensure that the specimen does not slip or rotate during the loading process. Before the test, a preloading is carried out first, that is, a certain load is applied to make the specimen in full contact with the pressure plate. After contact, a constant-rate axial load 5 is applied to deform the specimen under uniaxial compression. The loading rate is 0.05 - 0.1 mm / min. During the test, the changes in load and displacement are monitored and recorded in real time through the control software of the testing machine. Continue to load until the specimen fails. When it fails, stop loading and record the failure load F and axial strain ε1. The vertical bedding shale specimen 3 is tested according to the same steps, and the uniaxial compression axial stress (σ)-strain (ε1) curves of the two shale specimens are respectively plotted based on the experimental data.

[0056] The axial stress σ is calculated by the following formula:

[0057]

[0058] In the formula, F represents the axial load (N); A represents the cross-sectional area of the specimen (mm2).

[0059] The axial strain ε1 is calculated by the following formula:

[0060]

[0061] In the formula, ε1 represents the axial strain value; △L represents the average axial deformation under each level of load (mm); L represents the height of the specimen (mm).

[0062] The elastic modulus is calculated by the following formula:

[0063]

[0064] In the formula, △σ represents the change value of the axial stress in the elastic section before the peak of the stress-strain curve; △ε represents the change value of the axial strain corresponding to the change value of the axial stress in the elastic section before the peak of the stress-strain curve, and the two are on the same straight line segment.

[0065] The softening modulus is calculated by the following formula:

[0066] To distinguish from the calculation method of the elastic modulus, in the calculation of the softening modulus, the axial stress in the post-peak section is represented by σ s and the axial strain in the post-peak section is represented by ε s for representation.

[0067]

[0068] In the formula, △σ sRepresents the change value of the axial stress in the post-peak section of the stress-strain curve; △ε s Represents the change value of the axial strain in the post-peak section of the stress-strain curve. The two are on the same straight line segment.

[0069] Step S3: Determine a calculation method for the brittleness index of a shale reservoir, and calculate the brittleness indices in the horizontal bedding and vertical bedding directions of the shale under this brittleness index. The method is as follows:

[0070] Use the brittleness index BI to evaluate the brittleness of the horizontal and vertical bedding shale specimens to obtain BI0 and BI 90 . BI0 is the brittleness index calculated for the shale with a bedding dip angle of 0° using the brittleness index BI, and BI 90 is the brittleness index calculated for the shale with a bedding dip angle of 90° using the brittleness index BI.

[0071] The brittleness index BI is calculated using the following formula:

[0072]

[0073] In the formula, E is the elastic modulus; M is the softening modulus.

[0074] Step S4: Determine the prediction method for the anisotropic brittleness index of the shale reservoir based on the results obtained from the above brittleness index calculation method. The method is as follows:

[0075] Substitute the two brittleness indices calculated by the brittleness index BI in the previous step into the formula of the initial brittleness index prediction method to obtain the formula for the prediction method of the anisotropic brittleness index of the shale related to the brittleness index BI.

[0076] The formula of the above initial brittleness index prediction method is:

[0077]

[0078] In the formula, BU is the predicted value of the brittleness index for the selected brittleness index; B0 is the brittleness index calculated for the shale with a bedding dip angle of 0° using the selected brittleness index; B 90 is the brittleness index calculated for the shale with a bedding dip angle of 90° using the selected brittleness index; θ is the bedding dip angle between 0° and 90°.

[0079] The formula for the prediction method of the anisotropic brittleness index of the shale related to the brittleness index BI is as follows:

[0080]

[0081] In the formula, BUI is the predicted value of the brittleness index for the brittleness index BI; BI0 is the brittleness index calculated for the shale with a bedding dip angle of 0° using the brittleness index BI; BI 90The brittleness index calculated for shale with a bedding dip angle of 90° using the brittleness index BI; θ is the bedding dip angle between 0° and 90°.

[0082] Step S5: Through this prediction method, calculate the predicted values of the shale brittleness index at different angles and compare them with the calculation results obtained from the brittleness index. The method is as follows:

[0083] Substitute θ = 15°, 30°, 45°, 60° and 75° into Equation (7) respectively to obtain the predicted values of the shale brittleness index at these bedding dip angles for the brittleness index.

[0084] As Figure 5 shown, using a curve graph, compare the predicted values with the brittleness indices of the same batch of shale with corresponding bedding dip angles that have been evaluated using the brittleness index BI. The comparison results show that there is a high degree of agreement between the predicted values of the shale brittleness index and the brittleness indices calculated using the selected brittleness index.

[0085] In summary, the present invention uses shale specimens with horizontal and vertical bedding dip angles to conduct standard uniaxial compression tests, calculates the elastic modulus and softening modulus of the shale, and calculates the brittleness index based on this data. A prediction formula is established according to the U-shaped law, and the correctness and rationality of the formula are verified through uniaxial tests at multiple angles. This method can accurately predict the brittleness index at different bedding dip angles, and deduce the brittleness indices of other bedding dip angles through a mathematical model, avoiding the problems of requiring a large number of experiments and complex calculations in traditional methods.

[0086] The embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited thereto. All changes that can be made within the knowledge of those skilled in the art without departing from the gist of the present invention fall within the protection scope of the claims of the present invention.

Claims

1. A method for predicting anisotropic brittleness index of shale reservoirs, characterized in that: The following steps are involved: Step S1, preparing a shale specimen with a horizontal bedding dip angle and a shale specimen with a vertical bedding dip angle: drilling a horizontal bedding shale specimen and a vertical bedding shale specimen in layered shale, respectively, wherein the bedding dip angle in the horizontal bedding shale specimen is 0°, and the bedding dip angle in the vertical bedding shale specimen is 90°; Step S2: Calculate the elastic modulus and softening modulus of shale with two bedding angles through uniaxial compression tests on horizontal bedding specimens and vertical bedding specimens: Step S21, the elastic modulus is calculated as follows: According to the experimental data, the uniaxial compression axial stress (σ)-strain (ε1) curves of the two shale specimens were drawn respectively; The axial stress σ is calculated using the following formula: Where, F represents the axial load (N); A represents the cross-sectional area of ​​the specimen; The axial strain ε1 is calculated using the following formula: In the formula, ε1 represents the axial strain value; △L represents the average axial deformation under each level of load; L represents the height of the specimen; The elastic modulus is calculated using the following formula: In the formula, △σ represents the change value of axial stress in the elastic section before the peak of the stress-strain curve; △ε represents the change value of axial stress and axial strain in the elastic section before the peak of the stress-strain curve. The two are the same straight line segment. Step S22: The calculation method of the softening modulus is: In order to distinguish the elastic modulus calculation method, in the softening modulus calculation, the post-peak axial stress adopts σ s The axial strain of the post-peak section is expressed as ε s express, In the formula, △σ s Indicates the change in axial stress in the post-peak section of the stress-strain curve; △ε s It indicates the change value of axial strain in the post-peak section of the stress-strain curve; Step S3: Use the brittleness index BI to evaluate the brittleness of horizontal and vertical bedding shale specimens, and calculate the brittleness index BI0 of the horizontal bedding of the shale and the brittleness index BI0 of the vertical bedding direction under the brittleness index. 90 ; The calculation formula of brittleness index BI is: Where, E is the elastic modulus; M is the softening modulus; Step S4: predicting the anisotropic brittleness index of the shale reservoir: Substitute the two brittleness indices obtained in step S3 into the initial brittleness index prediction method formula to obtain the shale anisotropic brittleness index prediction formula related to the brittleness index BI; wherein, The initial brittleness index prediction method formula is: Where BU is the predicted value of the brittleness index for the selected brittleness index; B0 is the brittleness index of shale with 0° bedding dip angle calculated using the selected brittleness index; B 90 is the brittleness index calculated using the selected brittleness index for shale with a bedding dip angle of 90°; θ is the bedding dip angle between 0° and 90°; The prediction formula of shale anisotropic brittleness index related to brittleness index BI is: Wherein, BUI is the predicted value of the brittleness index of the brittleness index BI; BI0 is the brittleness index of 0° bedding angle shale calculated using the brittleness index BI; BI 90 is the brittleness index of 90° bedding dip shale calculated using the brittleness index BI; θ is the bedding dip between 0° and 90°; Step S5: Calculate the predicted value of shale brittleness index at different angles θ using formula (7).

2. The method for predicting anisotropic brittleness index of shale reservoir according to claim 1, characterized in that: In step S2, the loading rate of the uniaxial test is 0.05-0.1 mm / min.

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