A calculation method for rock brittleness index of tight sandstone reservoirs

Abstract

The invention discloses a method for calculating the brittleness index of a tight sandstone reservoir rock, relates to the field of unconventional oil and gas exploration and development, and relates to a method for calculating the brittleness index of formation rocks, in particular to a method for calculating the brittleness index of a tight sandstone reservoir rock, comprising the following steps: 1) preparing a tight sandstone sample required for a stress-strain experiment; 2) conducting a stress-strain experiment until the rock sample is destroyed; 3) processing the destroyed rock sample to prepare a casting thin section; 4) analyzing crack distribution characteristics in the casting thin section and calculating fractional dimensions; 5) analyzing fractional dimension data and estimating the rock brittleness index; 6) correcting the rock brittleness index estimated in step 5 to obtain a calculation model for the brittleness index of a tight sandstone reservoir rock; the invention effectively improves the calculation accuracy and reliability of the brittleness index of a tight sandstone reservoir rock, and effectively solves the problem that the residual strength surface is not obvious in a brittleness index evaluation method based on elastic strain.

Description

Technical Field

[0001] The invention relates to the field of unconventional oil and gas exploration and development, and to a method for calculating the brittleness index of formation rocks, in particular to a method for calculating the brittleness index of tight sandstone reservoir rocks. Background Art

[0002] The brittleness index of rock is one of the important factors that should be considered in the design of tight sandstone oil and gas volume fracturing. It is generally believed that the brittleness index is closely related to the rock mineral composition, rock mechanical properties, etc. The higher the rock brittleness index, the easier it is to break and form complex cracks.

[0003] At present, the rock brittleness evaluation methods at home and abroad can be divided into three categories: evaluation methods based on elastic strain, evaluation methods based on rock elastic parameters and evaluation methods based on rock mineral composition. For unconventional reservoirs, the brittleness indexes calculated by different brittleness evaluation methods are often quite different. The existing brittleness index evaluation methods are not reliable and their accuracy needs to be further improved.

[0004] The brittleness evaluation method based on elastic strain is one of the main means to study the brittleness index of rocks in the laboratory. According to the stress-strain experiment analysis, when the stress is loaded from a certain initial elastic state to the peak strength, it will suddenly drop to the residual strength surface (such as the attached Figure 1 ). Based on this characteristic of rock stress drop, the brittleness index B1 can be expressed as B1=((ba)-(cb)) / (ba). However, in actual operation, the problem of lack of residual strength or unclear residual strength is often encountered. Figure 2 ), the evaluation method based on elastic strain cannot accurately calculate the brittleness index. Summary of the invention

[0005] The purpose of the present invention is to improve the calculation accuracy and reliability of the rock brittleness index of tight sandstone reservoirs, effectively solve the problem that the residual strength surface is not obvious in the brittleness index evaluation method based on elastic strain, and propose a method for calculating the rock brittleness index of tight sandstone reservoirs with reasonable design, strong versatility, and accurate and reliable calculation results.

[0006] The present invention provides a method for calculating the rock brittleness index of a tight sandstone reservoir, comprising the following steps:

[0007] 1) Prepare dense sandstone samples required for stress-strain experiments;

[0008] 2) Conduct stress-strain experiments until the rock sample fails;

[0009] 3) Processing the destroyed rock samples to prepare casting thin sections;

[0010] 4) Analyze the crack distribution characteristics in the casting thin section and calculate the fractional dimension;

[0011] 5) Analyze the fractional dimension data and estimate the rock brittleness index. The estimation model is brittleness index B=(DD min ) / (D max -D min ), where D max is the maximum fractional dimension calculated in step 4), D min is the minimum fractional dimension calculated by step 4), and D is the fractional dimension of a certain sample;

[0012] 6) Correct the rock brittleness index estimated in step 5 to obtain the calculation model of rock brittleness index of tight sandstone reservoir B0=(DD 0min ) / (D 0max -D 0min ), where D 0min is the minimum fractional dimension after correction, D 0max is the maximum fractional dimension after correction;

[0013] D 0min and D 0max The calculation of is as follows:

[0014] F(D 0min , D 0max )= ,

[0015] Where 1<D 0min <D 0max <2, B i is the brittleness index of the i-th rock sample calculated from the fractional dimension of the casting thin section, B1 i is the brittleness index of the i-th rock sample calculated based on elastic strain,

[0016] When F(D 0min , D 0max ) takes the minimum value as the objective function, and obtains the corrected D 0min and D 0max .

[0017] Preferably, in step 4), the fractional dimension calculation method is as follows: an optical microscope is used to observe the casting thin slice and take a picture, the picture is processed to obtain a crack distribution map, and the fractional dimension is calculated according to the crack distribution map.

[0018] Preferably, in step 4), the grid covering method is used to calculate the crack fractional dimension, specifically, the center of the casting thin slice is used as the reference point, a square grid is drawn, and the crack length is selected as the crack feature to obtain the side length of R The total length of cracks in the square grid L , fitting ln in double logarithmic coordinatesL Same as ln R After linear fitting, we get ; where c is the proportionality constant and D is the fracture fractional dimension calculated based on the fracture length.

[0019] Preferably, in step 4), the grid covering method is used to calculate the crack fractional dimension, specifically, the center of the casting thin slice is used as the reference point, a square grid is drawn, the number of cracks is selected as the crack feature, and the number of cracks in the square grid with a side length of R is obtained. N , fitting ln in double logarithmic coordinates L Same as ln N After linear fitting, we get ln L =c+Dln N ; where c is the proportionality constant and D is the fracture fractional dimension calculated based on the number of fractures.

[0020] Preferably, in step 4), the grid covering method is used to calculate the crack fractional dimension, specifically, the center of the casting thin slice is used as the reference point, a square grid is drawn, the crack area is selected as the crack feature, and the crack area in the square grid with a side length of R is obtained. S , fitting ln in double logarithmic coordinates L Same as ln S After linear fitting, we get ln L =c+Dln S ; where c is the proportionality constant and D is the fracture fractional dimension calculated based on the fracture area.

[0021] Or preferably, in step 3), the rock sample after destruction is dyed with blue fluorescent resin to mark the initial shape of the cracks, cut, polished, and packaged to finally form a casting thin section for observation under an optical microscope.

[0022] Preferably, the stress-strain experiment in step 2) is a uniaxial compression experiment or a triaxial compression experiment, the sample is loaded into a rubber sleeve, placed in the center of the pressure plate of the press, and loaded at a loading rate of 0.5 MPa / s until the sample is damaged.

[0023] Preferably, the stress-strain experiment in step 2) is a tensile experiment, a shear experiment, or a cutting experiment.

[0024] The present invention effectively improves the calculation accuracy and reliability of the rock brittleness index of tight sandstone reservoirs, and effectively solves the problem that the residual strength surface is not obvious in the brittleness index evaluation method based on elastic strain.

[0025] When the residual strength surface is not obvious, the brittleness evaluation method based on elastic strain fails, but the calculation method based on fractional dimension of the present invention can still quantitatively evaluate the brittleness. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 The stress-strain curve of dense sandstone with residual strength.

[0027] Figure 2 The stress-strain curve for dense sandstone without residual strength.

[0028] Figure 3 This is the rock sample after destruction.

[0029] Figure 4 The present invention processes the destroyed rock sample to obtain a casting thin slice.

[0030] Figure 5 This is a schematic diagram of calculating the fractional dimension of cracks using the grid covering method in the present invention.

[0031] Figure 6 It is a comparison diagram of the brittleness index calculation result of the present invention and the brittleness index calculation result based on elastic strain. DETAILED DESCRIPTION

[0032] The present invention provides a method for calculating the rock brittleness index of a tight sandstone reservoir, comprising the following steps:

[0033] 1) Prepare dense sandstone samples required for stress-strain experiments;

[0034] 2) Conduct stress-strain experiments until the rock sample fails;

[0035] 3) Processing the destroyed rock samples to prepare casting thin sections;

[0036] 4) Analyze the crack distribution characteristics in the casting thin section and calculate the fractional dimension;

[0037] 5) Analyze the fractional dimension data and estimate the rock brittleness index. The estimation model is brittleness index B=(DD min ) / (D max -D min ), where D max is the maximum fractional dimension calculated in step 4), D min is the minimum fractional dimension calculated by step 4), and D is the fractional dimension of a certain sample;

[0038] 6) Correct the rock brittleness index estimated in step 5 to obtain the calculation model of rock brittleness index of tight sandstone reservoir B0=(DD 0min ) / (D 0max -D 0min ), where D 0min is the minimum fractional dimension after correction, D 0max is the maximum fractional dimension after correction;

[0039] D 0min and D 0max The calculation of is as follows:

[0040] F(D 0min , D 0max )= ,

[0041] Where 1<D 0min <D 0max <2, B i is the brittleness index of the i-th rock sample calculated from the fractional dimension of the casting thin section, B1 i is the brittleness index of the i-th rock sample calculated based on elastic strain,

[0042] When F(D 0min , D 0max ) takes the minimum value as the objective function, and obtains the corrected D 0min and D 0max .

[0043] In step 4), the fractional dimension calculation method is as follows: an optical microscope is used to observe the casting thin slice and take pictures, the pictures are processed to obtain a crack distribution map, and the fractional dimension is calculated based on the crack distribution map.

[0044] In step 4), the grid covering method is used to calculate the fractional dimension of the crack. Specifically, the center of the casting slice is used as the reference point, a square grid is drawn, and the crack length is selected as the crack feature. The side length is R The total length of cracks in the square grid L , fitting ln in double logarithmic coordinates L Same as ln R After linear fitting, we get ; where c is the proportionality constant and D is the fracture fractional dimension calculated based on the fracture length.

[0045] In step 4), the grid covering method is used to calculate the fractional dimension of the cracks. Specifically, a square grid is drawn with the center of the casting slice as the reference point, and the number of cracks is also selected as the crack feature to obtain the number of cracks in the square grid with a side length of R. N , fitting ln in double logarithmic coordinates L Same as ln N After linear fitting, we get ln L =c+Dln N ; where c is the proportionality constant and D is the fracture fractional dimension calculated based on the number of fractures.

[0046] In step 4), the grid covering method is used to calculate the fractional dimension of the crack. Specifically, a square grid is drawn with the center of the casting slice as the reference point. The crack area can also be selected as the crack feature to obtain the crack area in the square grid with a side length of R. S , fitting ln in double logarithmic coordinates L Same as ln S After linear fitting, we get ln L =c+Dln S ; where c is the proportionality constant and D is the fracture fractional dimension calculated based on the fracture area.

[0047] In step 3), the rock sample after destruction is stained with blue fluorescent resin to mark the initial shape of the cracks, cut, polished, and packaged to finally form a casting thin section for observation under an optical microscope.

[0048] In step 2), the stress-strain test is a uniaxial compression test or a triaxial compression test. The sample is loaded into a rubber sleeve and placed in the center of the pressure plate of the press. The sample is loaded at a loading rate of 0.5 MPa / s until the sample is damaged.

[0049] The stress-strain test in step 2) may also be a tensile test, a shear test, or a cutting test.

[0050] Tensile test: Select a cylindrical rock sample and then perform uniaxial stretching on a universal material testing machine until it breaks.

[0051] Shear test: Select a cylindrical rock sample and apply shear force on a universal material testing machine until it breaks.

[0052] Cutting experiment: The cutting experiment is carried out on a rock cutting test bench. During the experiment, the sample is fixed with a rock clamp, the height of the cutting teeth is adjusted so that the height of the cutting teeth is lower than the top of the sample, and the motor is started to move the sample toward the cutting teeth until the sample is broken. Example

[0053] Select a core of a specific depth in a dense sandstone formation from the core library, use a sleeve drill bit with a diameter of 25 mm to drill a small core, cut the end face of the small core, and obtain a cylindrical sample with a diameter of 25 mm and a height of 50 mm;

[0054] The rock sample was destroyed by uniaxial compression test. The sample was put into a rubber sleeve and placed in the center of the pressure plate of the press. The sample was loaded at a loading rate of 0.5 MPa / s until the sample was destroyed.

[0055] The rock samples after destruction were dyed with blue fluorescent resin, the initial crack morphology was marked, and then they were cut, polished, and packaged to form casting thin-section specimens for observation under an optical microscope, such as Figure 4As shown, under an optical microscope, the cracks caused by the compression experiment appear in blue.

[0056] To analyze the crack distribution characteristics in the casting thin slice, the grid covering method is used. The center of the casting thin slice is used as the reference point to draw a square grid. The crack length is used as the crack characteristic. R The total length of cracks in the square grid L like Figure 5 As shown, fitting ln in double logarithmic coordinates L Same as ln R After linear fitting, we get ; where c is the proportional constant, and D is the fracture fractional dimension calculated based on the fracture length. Finally, the fractional dimension data set (1.22, 1.29, 1.34, 1.34, 1.35, 1.36, 1.41, 1.42, 1.43, 1.65) was obtained.

[0057] The maximum fractional dimension D calculated by the statistical step max (1.65) and the minimum division dimension D min (1.22), the brittleness index is estimated using the formula B=(D-1.22) / (1.65-1.22); the brittleness index data set is estimated to be (0, 0.16, 0.27, 0.29, 0.31, 0.32, 0.45, 0.46, 0.50, 1).

[0058] The rock brittleness index estimated by the fracture fractional dimension in the previous step was corrected using the brittleness index data set (0.2, 0.28, 0.32, 0.32, 0.41, 0.41, 0.45, 0.51, 0.76) calculated based on elastic strain, and the calculation model of the rock brittleness index of tight sandstone reservoirs based on fracture fractional dimension was obtained. 0min ) / (D 0max -D 0min ), where D 0min is the minimum fractional dimension after correction, D 0max is the maximum fractional dimension after correction;

[0059] D 0min and D 0max The calculation of is as follows:

[0060] F(D 0min , D 0max )= ,

[0061] Where 1<D 0min <D 0max <2, B iis the brittleness index of the i-th rock sample calculated from the fractional dimension of the casting thin section, B1 i is the brittleness index of the i-th rock sample calculated based on elastic strain,

[0062] When F(D 0min , D 0max ) takes the minimum value as the objective function, and obtains the corrected D 0min and D 0max ;

[0063] In this embodiment, by gradually increasing D max Value and gradually reduce D min The value combination method is used to solve F(D 0min , D 0max ) takes the maximum fractional dimension D of the minimum value max With the minimum fractional dimension D min , then the maximum fractional dimension D max is the maximum fractional dimension D after correction 0max , the minimum fractional dimension D min is the minimum fractional dimension D after correction 0min , 1<D should be ensured in the solution process min <D max <2.

[0064] MATLAB program for the calibration process:

[0065] clear, close all, clc

[0066] D=[1.22,1.29,1.34,1.34,1.35,1.36,1.41,1.42,1.43,1.65];

[0067] B=[0.2, 0.28, 0.32, 0.32, 0.41, 0.41, 0.41, 0.45, 0.51, 0.76];

[0068] f=zeros(23,36);

[0069] m=0;

[0070] for mina=1.22:-0.01:1

[0071] m=m+1;

[0072] n=0;

[0073] for maxa=1.65:0.01:2

[0074] n=n+1;

[0075] err=0;

[0076] for i=1:10

[0077] err=err+((((D(i)-mina)) / (maxa-mina))-B(i))^2;

[0078] end

[0079] f(m,n)=err;

[0080] end

[0081] end

[0082] [m, n]=find(f==min(min(f)));

[0083] min_real=1.22-(m-1)*0.01

[0084] max_real=1.65+(n-1)*0.01)

[0085] The maximum fractional dimension D obtained after correction in this embodiment 0max is 1.83, the minimum division dimension D 0min is 1.07, and the brittleness index calculation method based on fractal dimension is finally determined as B=(D-1.07) / (1.83-1.07);

[0086] In addition, the model can be corrected using a brittleness index calculated based on rock elastic parameters or rock minerals.

[0087] Figure 6 The figure shows the comparison between the brittleness index calculation result of the present invention and the brittleness index calculation result based on elastic strain. It can be seen that the brittleness index calculated by the present invention is basically consistent with the calculation result of the brittleness index calculated by the conventional method, i.e., based on elastic strain. More importantly, the method of the present invention can quantitatively calculate the brittleness index in the absence of residual strength, providing a new means for evaluating the compressibility of sandstone reservoirs.

Claims

1. A method for calculating the rock brittleness index of a tight sandstone reservoir, characterized in that: The following steps are involved: 1) Prepare dense sandstone samples required for stress-strain experiments; 2) Conduct stress-strain experiments until the rock sample fails; 3) Processing the destroyed rock samples to prepare casting thin sections; 4) Analyze the crack distribution characteristics in the casting thin section and calculate the fractional dimension; 5) Analyze the fractional dimension data and estimate the rock brittleness index. The estimation model is brittleness index B=(DD min ) / (D max -D min ), where D max is the maximum fractional dimension calculated by step 4), D min is the minimum fractional dimension calculated by step 4), and D is the fractional dimension of a certain sample; 6) Correct the rock brittleness index estimated in step 5) to obtain the calculation model of rock brittleness index of tight sandstone reservoir B0=(DD 0min ) / (D 0max -D 0min ), where D 0min is the minimum fractional dimension after correction, D 0max is the maximum fractional dimension after correction; D 0min and D 0max The calculation of is as follows: F(D 0min ,D 0max )= , Where 1<D 0min <D 0max <2, B i is the brittleness index of the i-th rock sample calculated from the fractional dimension of the casting thin section, B1 i is the brittleness index of the i-th rock sample calculated based on elastic strain, When F(D 0min , D 0max ) takes the minimum value as the objective function, and obtains the corrected D 0min and D 0max ; By gradually increasing D max Value and gradually reduce D min The value combination method is used to solve F(D 0min , D 0max ) takes the maximum fractional dimension D of the minimum value max With the minimum fractional dimension D min , then the maximum fractional dimension D max is the maximum fractional dimension D after correction 0max , the minimum fractional dimension D min is the minimum fractional dimension D after correction 0min , ensure that 1<D during the solution process min <D max <2.

2. A method for calculating rock brittleness index of tight sandstone reservoir according to claim 1, characterized in that: In step 4), the fractional dimension calculation method is as follows: an optical microscope is used to observe the casting thin slice and take pictures, the pictures are processed to obtain a crack distribution map, and the fractional dimension is calculated based on the crack distribution map.

3. A method for calculating rock brittleness index of tight sandstone reservoir according to claim 2, characterized in that: In step 4), the grid covering method is used to calculate the fractional dimension of the crack. Specifically, the center of the casting slice is used as the reference point, a square grid is drawn, and the crack length is selected as the crack feature. The side length is R The total length of cracks in the square grid L , fitting ln in double logarithmic coordinates L Same as ln R After linear fitting, we get ; where c is the proportionality constant and D is the fracture fractional dimension calculated based on the fracture length.

4. A method for calculating rock brittleness index of tight sandstone reservoir as claimed in claim 3, characterized in that: In step 3), the rock sample after destruction is stained with blue fluorescent resin to mark the initial shape of the cracks, cut, polished, and packaged to finally form a casting thin section for observation under an optical microscope.

5. A method for calculating rock brittleness index of tight sandstone reservoir according to claim 4, characterized in that: In step 2), the stress-strain test is a uniaxial compression test or a triaxial compression test. The sample is loaded into a rubber sleeve and placed in the center of the pressure plate of the press. The sample is loaded at a loading rate of 0.5 MPa / s until the sample is damaged.

6. A method for calculating rock brittleness index of tight sandstone reservoir according to claim 5, characterized in that: The stress-strain experiment in step 2) is a tensile experiment, a shear experiment, or a cutting experiment.

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