CN110006738B - A method for evaluating rock brittleness based on stress-strain curve and scratch testing - Google Patents

Abstract

The invention provides a rock brittleness evaluation method based on a stress-strain curve and a scratch test, which comprises the following steps: evaluating the rock from the ground stress environment and the pore pressure of the rock to obtain the stratum condition of the rock; selecting rock mechanics experiments which accord with the environment of the rock sample to be tested, including conventional triaxial compression experiments or triaxial compression experiments considering pore pressure, and scratch tests; acquiring a pre-peak strain energy density value, a crack initiation stress value, a peak stress value, a residual stress value, and strain magnitude and crack line density corresponding to the peak stress value and the residual stress value; and determining the value range of the effective stress coefficient alpha according to the pore pressure of the tested rock sample, and substituting the pre-peak strain energy value, the initiation stress value, the peak stress value, the residual stress value, the strain values corresponding to the residual stress value and the crack line density into a brittleness index calculation formula to obtain the brittleness index of the rock to be tested. The brittleness evaluation method improves the accuracy and the applicability of rock brittleness evaluation.

Description

A method for evaluating rock brittleness based on stress-strain curve and scratch testing

technical field

The invention relates to the technical field of deep rock mechanics, in particular to a rock brittleness evaluation method based on stress-strain curve and scratch test.

Background technique

The classic view is that rock undergoes little or no permanent deformation to brittle fracture before fracture. For deep rocks, they generally have high brittleness and are often accompanied by a complex environment of high temperature and high pressure. Brittleness is an important property of rock, and its evaluation has important guiding significance for rock engineering. For example, in petroleum engineering, brittleness is an important index for evaluating the geomechanical characteristics of reservoirs and the evaluation of hydraulic fracturing crack propagation; while in deep rock mass engineering under complex stress conditions, the brittleness of rock mass is an important factor affecting engineering disasters such as rock bursts. important internal factors. At present, scholars at home and abroad have not formed a unified evaluation standard for rock brittleness. For example, a coal-rock brittleness evaluation method disclosed in the Chinese invention patent application with publication number CN108827774A is established by establishing a power function distribution rock damage constitutive model, and considering Coal rock mechanical properties and cleat, fracture system distribution characteristics, this method is relatively complex and only suitable for coal rock; a method for fast calculation of rock brittleness based on logging data disclosed in Chinese Patent Application Publication No. CN106547034A. The analysis and interpretation of well data will bring certain errors, and the reliability of the test results is questionable. Because the brittleness of rock is closely related to its mechanical properties, the analysis of the mechanical properties of the rock in the environment can accurately evaluate the brittleness of the rock.

The Chinese invention patent application with publication number CN106908322A provides a rock brittleness index evaluation method based on the total stress-strain curve, the method includes: evaluating the geological environment where the rock sample to be measured is located from the aspects of in-situ stress, temperature, water pressure, etc. ;Select rock mechanics experiments that conform to the geological environment of the rock sample to be tested, including uniaxial compression experiments, triaxial compression experiments considering temperature, and triaxial compression experiments considering water pressure; obtain each characteristic stress value and its corresponding strain during the experiment According to the proposed calculation method of brittleness index, the characteristic stress value and strain are brought into the calculation to obtain the brittleness index of the rock to be tested. However, this evaluation method is not suitable for the brittleness evaluation of fractured rocks, and the accuracy and reliability are low, and there is currently no scientific evaluation method for the brittleness of fractured rocks.

SUMMARY OF THE INVENTION

In order to solve the technical problem in the prior art that there is currently no scientific evaluation method for fractured rock brittleness, the present invention provides a rock brittleness evaluation method based on stress-strain curve and scratch test, the method is based on rock triaxial compression test The whole process and scratch test of the medium full stress-strain curve can be analyzed, and the brittleness evaluation results of the rock can be obtained conveniently and accurately.

In order to achieve the above object, the present invention provides a method for evaluating rock brittleness based on stress-strain curve and scratch test, including the following steps performed in sequence:

(1) Evaluate the rock from the in-situ stress environment and pore pressure where the rock is located, and obtain the stratum conditions where the rock is located;

(2) Select rock mechanics experiments that conform to the environment of the rock samples to be tested, including conventional triaxial compression experiments or triaxial compression experiments considering pore pressure, and scratch tests;

(3) obtaining the pre-peak strain energy density value in the experimental process, as well as the crack initiation stress value, the peak stress value and the residual stress value and their corresponding strain sizes; obtain the crack line density D _F according to the scratch test result;

(4) Determine the value range of the effective stress coefficient α from the pore pressure of the test rock sample, and then calculate the pre-peak strain energy value, the crack initiation stress value, the peak stress value, the residual stress value and their corresponding values according to the calculation method of the brittleness index. The strain value and fracture linear density are brought into the calculation to obtain the brittleness index of the rock to be tested;

The calculation method of the brittleness index B is:

B=B ₁ +B ₂

in,

B ₂ =log( _DF +1)

in:

σ _p is the peak stress, ε _p is the peak strain, σ _r is the residual stress, ε _r is the residual strain, α is the effective stress coefficient, P _P is the pore pressure, U _D is the pre-peak strain energy density, ε is the The strain value corresponding to this section of the curve at the peak point, σ is the stress value on the curve corresponding to ε, V is the core sample volume, and D _F is the crack line density, obtained from scratch testing.

Preferably, for tight rocks, conventional triaxial compression experiments and scratch tests must be selected for rock mechanics experiments, and confining pressures need to be set according to formation conditions in the setting of triaxial compression experimental conditions.

In any of the above solutions, it is preferable that, for fractured rocks, the rock mechanics experiment must select triaxial compression experiment and scratch test considering pore pressure.

In any of the above schemes, it is preferable that, in the conventional triaxial compression experiment and the triaxial compression experiment considering pore pressure, the confining pressure is set as formation depth/100Mpa according to the formation conditions. If the formation depth is 4000 meters, then the corresponding confining pressure is set. Set as 4000/100MPa=40MPa.

In any of the above solutions, preferably, the pore pressure is zero, and the effective stress coefficient α in the brittleness index calculation method is zero.

In any of the above solutions, preferably, the pore pressure is not zero, and the effective stress coefficient α in the calculation method of the brittleness index is 0.3-0.5.

In any of the above solutions, preferably, for a low-porosity and low-permeability reservoir, the porosity is between 10-15%, the permeability is 5-50×10 ^-3 μm ² , and the effective stress coefficient is 0.3.

In any of the above solutions, it is preferred that, for reservoirs with high porosity and permeability, such as unconsolidated sandstone reservoirs, the porosity reaches >25% and the permeability is >500×10 ^-3 μm ² . This effective stress coefficient can be Take 0.4-0.5.

The rock brittleness evaluation method based on the stress-strain curve and the scratch test of the present invention is a comprehensive calculation method of the brittleness index based on the full stress-strain curve of the triaxial compression experiment according to the geological environment where the rock is located, and is not only suitable for compact rocks It is also suitable for the brittleness evaluation of fractured rocks, with high accuracy, strong reliability and wide applicability.

Description of drawings

FIG. 1 is a schematic flowchart of a preferred embodiment of a method for evaluating rock brittleness based on stress-strain curves and scratch testing according to the present invention.

Figure 2 is a schematic diagram of the calculation method of the pre-peak strain energy density.

3 is a stress-strain curve of a rock mechanics experiment of a preferred embodiment of the method for evaluating rock brittleness based on stress-strain curve and scratch test according to the present invention.

FIG. 4 is a scratch test result of the embodiment shown in FIG. 3 .

5 is a stress-strain curve of a rock mechanics experiment of another preferred embodiment of the method for evaluating rock brittleness based on stress-strain curve and scratch test according to the present invention.

FIG. 6 is a scratch test result of the embodiment shown in FIG. 5 .

7 is a stress-strain curve of a rock mechanics experiment according to another preferred embodiment of the method for evaluating rock brittleness based on stress-strain curve and scratch test according to the present invention.

FIG. 8 is a scratch test result of the embodiment shown in FIG. 7 .

Detailed ways

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. The described embodiments are only some, but not all, embodiments of the present invention. In the following embodiments, the rock samples used for the triaxial compression test are cylinders with a diameter of 25 mm and a length of 50 mm, and the test rock samples used for the scratch test are all core columns with a length of 200 mm.

Example 1

To evaluate the brittleness index of tight carbonate rocks with different burial depths, the method steps are shown in Figure 1, and the details are as follows:

(1) Evaluation of the rock from the in-situ stress environment and pore pressure of the rock: Evaluation of the geological environment of the tight carbonate rock: The burial depth is widely distributed, from the surface to the underground 4000 meters, the rock is extremely dense, and the interior is almost does not contain fluids;

(2) Select a rock mechanics experiment that matches the environment of the rock sample to be tested: Since the geological environment where the rock sample is located, the main influencing factor is the in-situ stress, so the conventional triaxial compression experiment is selected, the number of experiments is 4, and the confining pressure is set They are 10MPa, 20MPa, 30MPa and 40MPa respectively. For extremely tight reservoirs, the effective stress coefficient is 0. The complete stress-strain curves of 4 samples were obtained by the RTR-1500 triaxial experimental system, as shown in Figure 3; the scratch test results of the dense carbonate rock were obtained by the scratch tester, as shown in Figure 4;

(3) Obtain the pre-peak strain energy density value during the experiment, as well as the crack initiation stress value, the peak stress value and the residual stress value and their corresponding strain values; according to the scratch test results, obtain the crack line density _DF : pre-peak strain The relationship between energy density and stress-strain curve is shown in Figure 2, and the stress-strain results of this embodiment are shown in Figure 3. Combining Figures 2 and 3, the stress-strain curves of the four rock samples can be obtained. The term stress value, strain value and pre-peak strain energy density are shown in Table 1.

Table 1 Various stress values, strain values and pre-peak strain energy density values of tight carbonate rock

At the same time, according to Fig. 4, the number of fractures in the tight carbonate rock is 4, which is converted into a linear fracture density _DF of 20 fractures/m.

(4) Determine the value range of the effective stress coefficient based on the pore pressure of the test rock sample. In this embodiment, since the rock is a tight carbonate rock, it hardly contains fluid, and the pore pressure is close to zero, so the effective stress coefficient is taken as a value. is 0, then according to the calculation method of brittleness index, the pre-peak strain energy value, crack initiation stress value, peak stress value, residual stress value and its corresponding strain value and crack linear density are brought into the calculation to obtain the brittleness of the rock to be tested. index;

The calculation formula of the brittleness index B is:

B=B ₁ +B ₂

B ₂ =log( _DF +1)

After calculation, the brittleness indices of tight carbonate rock at 10MPa, 20MPa, 30MPa and 40MPa are 2.9578, 2.7464, 2.6643 and 2.3475. It can be seen that for tight carbonate rock, under different confining pressure conditions, the relative brittleness degree is: 10MPa>20MPa>30MPa>40MPa.

Example 2

The brittleness index was evaluated for the shale in a low-porosity and low-permeability reservoir (porosity between 10-15%, permeability 5-50* ^10-3 μm ² ) in a certain area, as shown in Fig. 1. The brittleness index calculation method provided by the embodiment of the invention, the steps are as follows:

(1) Evaluate the geological environment of mud shale: the burial depth is 4000 meters to 4200 meters, the rock has the characteristics of low porosity and low permeability, and the regional pore pressure varies greatly, and the pressure coefficient ranges from 1.1 to 1.5.

(2) The main influencing factors are in-situ stress and pore pressure in the geological environment where the rock sample is located. Therefore, the triaxial compression experiment was selected, the number of experiments was 4, the confining pressure was set to 80MPa, and the pore pressures were 45MPa, 50MPa, and 55MPa respectively. and 60MPa. For low-porosity and low-permeability reservoirs, the effective stress coefficient is taken as 0.3. The complete stress-strain curves of 4 samples were obtained through the RTR-1500 triaxial experimental system, as shown in Figure 5;

(3) According to Figure 5, the stress-strain curves of the four rock samples obtained various stress values, strain values and pre-peak strain energy density of the rock samples, as shown in Table 2.

Table 2 Various stress values, strain values and pre-peak strain energy density values of shale in a low-porosity and low-permeability reservoir

Meanwhile, according to FIG. 6 , the number of fractures in the shale of the low-porosity and low-permeability reservoir in this embodiment is 10, which is converted into a linear fracture density D _F of 50 fractures/m.

(4) Determine the value range of the effective stress coefficient according to the pore pressure of the test rock sample. In this embodiment, since the pore pressure is not zero, for low-porosity and low-permeability reservoirs, the effective stress coefficient is 0.3. Then, the pre-peak strain energy value, the crack initiation stress value, the peak stress value, the residual stress value and its corresponding strain value and crack linear density are brought into the calculation to obtain the brittleness index of the rock to be tested;

The calculation method of brittleness index B is:

B=B ₁ +B ₂

B ₂ =log( _DF +1)

The brittleness indices of shale under the confining pressure of 80MPa and different pore pressures of 45MPa, 50MPa, 55MPa and 60MPa are 3.6157, 3.5357, 3.3509 and 3.0108. It can be seen that for mud shale, under the condition of 80MPa confining pressure and different pore pressures, the relative brittleness degree is as follows: pore pressure 45MPa>pore pressure 50MPa>pore pressure 55MPa>pore pressure 60MPa.

Example 3

The brittleness index evaluation of tight carbonate rock, the method steps are shown in Figure 1, and the details are as follows:

(1) Evaluate the geological environment where the carbonate rock is located: the carbonate rock in this section is relatively dense, there is no fluid in the underground environment, and its burial depth is about 4000m;

(2) Setting the conditions for the triaxial compression experiment according to the environment where the rock is located: Since there is no fluid in the geological environment where the rock sample is located, there is no need to consider the influence of fluid pore pressure. The main influencing factor is in-situ stress, so conventional Triaxial compression experiment.

In this example, the number of experiments is 1, and the confining pressure is set to 40 MPa according to the burial depth of the rock. In this example, since there is no fluid, the void pressure is zero, so the effective stress coefficient is taken as zero. The complete stress-strain curve of the sample was obtained by the RTR-1500 triaxial experimental system, as shown in Figure 7.

Scratch test is carried out on it, and the experimental results are shown in Figure 8. (3) Obtain the pre-peak strain energy density value during the experiment, as well as the crack initiation stress value, peak stress value and residual stress value and their corresponding strain values; According to the scratch test results, the crack line density _DF is obtained: the relationship between the pre-peak strain energy density and the stress-strain curve is shown in Figure 2. The stress-strain results of this embodiment are shown in Figure 7. Combining Figures 2 and 7, it can be obtained From the stress-strain curve of the rock sample, the stress values, strain values and pre-peak strain energy density of the rock samples are obtained as shown in Table 3.

Table 3 Various stress values, strain values and pre-peak strain energy density values of tight carbonate rock

At the same time, according to Figure 8, the length of the experimental core section is 20cm. According to the continuous strength test results, it can be observed that the number of fractures in the carbonate rock is 5, so the linear density is converted to 25/m.

(4) Determine the value range of the effective stress coefficient from the pore pressure of the test rock sample. In this embodiment, since the rock is a tight carbonate rock, there is no fluid, and the pore pressure is zero, the effective stress coefficient is taken as 0, and then according to the calculation method of brittleness index, the pre-peak strain energy value, crack initiation stress value, peak stress value, residual stress value and its corresponding strain value and crack linear density are brought into the calculation to obtain the brittleness index of the rock to be tested. ;

Bring in the calculation method of the brittleness index B,

B=B _l +B ₂

B ₂ =log( _DF +1)

The brittleness index of the carbonate rock at 40MPa is 2.278.

The accuracy and reliability of the brittleness index evaluation method provided by the present invention are compared with the evaluation methods in the prior art. The results show that the evaluation methods in the prior art all have some stress-strain curves that meet certain characteristics. , which makes it impossible to accurately evaluate its brittleness, and there will be abnormal conclusions such as different curves but the same results are obtained; and the brittleness evaluation method in the present invention can avoid abnormal conclusions and accurately evaluate. Therefore, the method of the present invention is more accurate and more reliable.

The specific embodiments described above further describe the purpose, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above-mentioned specific embodiments are only specific embodiments of the present invention, and are not intended to limit the scope of the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims (8)

1. A method for evaluating rock brittleness based on stress-strain curve and scratch testing, comprising the following steps performed in sequence: (1) Evaluate the rock from the in-situ stress environment and pore pressure where the rock is located, and obtain the stratum conditions where the rock is located; (2) Selecting rock mechanics experiments that conform to the environment where the rock samples to be tested are located, including triaxial compression experiments and scratch tests, where the triaxial compression experiments include conventional triaxial compression experiments or triaxial compression experiments considering pore pressure; (3) obtaining the pre-peak strain energy density value in the experimental process, as well as the crack initiation stress value, the peak stress value and the residual stress value and their corresponding strain sizes; obtain the crack line density D _F according to the scratch test result; (4) Determine the value range of the effective stress coefficient α from the pore pressure of the test rock sample, and then calculate the pre-peak strain energy value, the crack initiation stress value, the peak stress value, the residual stress value and their corresponding values according to the calculation method of the brittleness index. The strain value and fracture linear density are substituted into the calculation to obtain the brittleness index of the rock to be tested; The calculation method of the brittleness index B is: B=B ₁ +B ₂ in,

B ₂ =log( _DF +1) in: σ _p is the peak stress, ε _p is the peak strain, σ _r is the residual stress, ε _r is the residual strain, α is the effective stress coefficient, P _P is the pore pressure, U _D is the pre-peak strain energy density, ε is the The strain value corresponding to this section of the curve at the peak point, σ is the stress value on the curve corresponding to ε, V is the core sample volume, and D _F is the crack line density, obtained from scratch testing. 2. The evaluation method as claimed in claim 1 is characterized in that, for compact rock, conventional triaxial compression experiment and scratch test must be selected for rock mechanics experiment, and in the setting of triaxial compression experiment conditions, it is necessary to set according to formation conditions. Fixed confining pressure. 3. The evaluation method according to claim 1, characterized in that, for fractured rock, triaxial compression test and scratch test considering pore pressure must be selected for rock mechanics test. The evaluation method according to claim 2 or 3, characterized in that, in a conventional triaxial compression experiment or a triaxial compression experiment considering pore pressure, the confining pressure is set as formation depth/100Mpa according to formation conditions. 5 . The evaluation method according to claim 4 , wherein the pore pressure is zero, and the effective stress coefficient α in the brittleness index calculation method is zero. 6 . 6 . The evaluation method according to claim 4 , wherein the pore pressure is not zero, and the effective stress coefficient α in the brittleness index calculation method is 0.3-0.5. 7 . 7 . The evaluation method according to claim 6 , wherein, for a reservoir with a porosity of 10-15% and a permeability of 5-50×10 ^-3 μm ² , the effective stress coefficient is 0.3. 8 . 8 . The evaluation method according to claim 6 , wherein, for a reservoir with a porosity of >25% and a permeability of >500×10 ⁻³ μm ² , the effective stress coefficient is 0.4-0.5. 9 .

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