Test methods for determining the self-healing effect of damaged salt rock

Abstract

The present invention discloses a test method for determining the self-healing effect of damaged salt rock, which belongs to the field of geological exploration technology and includes the following steps: obtaining initial damage to rock salt, obtaining pore distribution data of damaged salt rock through nuclear magnetic resonance test, and calculating the pore fractal dimension of initial damaged salt rock; performing static test on self-healing of damaged salt rock, and determining the fractal dimension of static self-healing salt rock; calculating the self-healing effect of damaged salt rock, and evaluating the self-healing grade of damaged salt rock. The present invention performs nuclear magnetic resonance test on damaged salt rock, collects test data and analyzes the changes of cracks before and after self-healing, and calculates the fractal dimension before and after healing to obtain the self-healing effect of damaged salt rock. The present invention quantitatively describes the self-healing effect of damaged salt rock, which is of great significance for in-depth research on the self-healing characteristics of underground salt rock.

Description

Technical Field

[0001] The invention belongs to the technical field of geological exploration, and in particular relates to a test method for measuring the self-healing effect of damaged salt rock. Background Art

[0002] Salt rock is considered an ideal medium for energy storage and waste disposal due to its low porosity, low permeability, self-healing of damage and good creep properties. However, the surrounding rock of underground salt cavern reservoirs will be damaged under the pressure cycle of injection and production. If the operating pressure is too high, the average stress of the salt rock will increase. When the upper limit is exceeded, the salt rock will stretch the damage zone, and the internal cracks will continue to expand, causing the permeability of the surrounding rock to decrease, causing the working gas to leak, thereby affecting the sealing and stability of the gas storage. However, salt rock has good self-healing properties, and damage can be restored under conditions such as pressure closure, diffusion effect and recrystallization. Therefore, the self-healing research on damaged salt rock must be able to accurately measure its self-healing effect, which is of great significance to the long-term stability of the salt cavern and the overall life prediction of the reservoir.

[0003] At present, the self-healing effect of salt rock is measured by the changes in macroscopic indicators such as strength, permeability, and ultrasound before and after damage. However, the damage process of salt rock is essentially the process of internal microcracks generation, nucleation, and penetration, while the healing process is the filling of microcracks and crack closure caused by grain growth. Therefore, the macroscopic indicator measurement method cannot truly reflect its internal recovery, making the self-healing effect measurement results inaccurate. Summary of the invention

[0004] In order to solve the above problems, the present invention provides a test method for measuring the self-healing effect of damaged salt rock.

[0005] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0006] A test method for determining the self-healing effect of damaged salt rock comprises the following steps:

[0007] S100, set the initial damage of rock salt;

[0008] S200, conduct nuclear magnetic resonance test to obtain pore distribution data of damaged salt rock, calculate pore fractal dimension of initial damaged salt rock; divide the initial damaged salt rock into four types according to pore radius: micro cracks, small cracks, medium cracks, and large cracks, calculate the pore radius proportion of the four types of cracks , , , The fractal dimension of , , , ;

[0009] S300. Conduct the self - healing static test on the damaged salt rock;

[0010] S400. Measure the fractal dimension of the static self - healing salt rock: Obtain the proportion of the pore radii of the self - healing salt rock after static state using the procedure in step S200 , , , of the fractal dimension , , , ;

[0011] S500. Calculate the self - healing effect of the damaged salt rock:

[0012] The formula for the self - healing effect of the damaged salt rock is: ; where, represents the fractal dimension benchmark before self - healing, represents the fractal dimension benchmark after self - healing;

[0013] After normalization processing, the modified self - healing effect formula is denoted as: ;

[0014] S600. Calculate the average self - healing effect;

[0015] S700. Evaluate the self - healing grade of the damaged salt rock.

[0016] Furthermore, the pore radius division of the four types of fissures is as follows:

[0017] Micro - fissure: r ≤ 0.5 nm; Small fissure: 0.5 nm < r ≤ 2.0 nm; Medium fissure: 2.0 nm < r ≤ 50.0 nm; Large fissure: r > 50.0 nm.

[0018] Furthermore, in step S100, select a complete salt rock specimen and conduct a uniaxial compression test on the specimen to cause initial damage.

[0019] Furthermore, in step S200, the process of obtaining the pore fractal dimension of the initially damaged salt rock is as follows:

[0020] 1). Vacuum - saturate the specimen with water, then conduct a nuclear magnetic resonance test and collect the T2 - spectrum test data;

[0021] 2). According to the test data, obtain the T2 - spectrum at each position inside the salt rock through Fourier transform: ;

[0022] In the formula: T2 is the transverse relaxation time, is the transverse surface relaxation intensity, is the specific surface area of ​​a single pore;

[0023] 3) Relationship between pore radius R and specific surface area: , substitute into the formula in step 2) and we get , where F s is the pore geometry factor (spherical pore , columnar pores );

[0024] 4) According to fractal theory, the relationship between the number of pores N with the same radius and the pore radius R is obtained: , where a is the proportionality constant and D is the fractal dimension of the pore structure;

[0025] 5) The geometric formula of pore distribution is: S v = [ R R max ] 3 − D ,in is the cumulative volume fraction of pores with a radius smaller than R, is the maximum pore radius.

[0026] 6) Take the logarithm of both sides of the formula in step 5) to obtain the expression of the fractal dimension of the pore structure:

[0027]

[0028] The pore radius of salt rock is divided into four types: micro-cracks, small cracks, medium cracks, and large cracks. According to the number of pores N, the proportion of each pore radius is calculated. , , , According to the fractal theory, the fractal dimensions of different pore radii of initially damaged salt rock are obtained and recorded as , , , ;

[0029] Before self-healing, the internal fracture network of the damaged salt rock is complex and highly connected, with a high fractal dimension.

[0030] After self-healing, the internal cracks of the damaged salt rock gradually closed, the number of cracks decreased, and the fractal dimension decreased.

[0031] Further, in step S300, the contents of the damaged salt rock self-healing static test are as follows:

[0032] Three groups of damaged salt rock self-healing tests were conducted under the conditions of confining pressure of 12 MPa and temperatures of 30°C, 50°C and 70°C, respectively, and the pressure was maintained for 30 days.

[0033] Further, in step S600, the self-healing effects of damaged salt rock at temperatures of 30°C, 50°C, and 70°C are calculated as follows: , , , the average self-healing effect is: .

[0034] Further, in step S700, the degree of self-healing of the damaged salt rock is between 0 and 1, and the degree of self-healing of the damaged salt rock is divided into three levels, as follows:

[0035] Level I: When , the self-healing degree of damaged salt rock is low;

[0036] Level II: When , the degree of self-healing of damaged salt rock is moderate;

[0037] Level III: When , the damaged salt rock has a high degree of self-healing.

[0038] Compared with the prior art, the present invention has the following technical advances:

[0039] The present invention conducts nuclear magnetic resonance tests on damaged salt rock, collects test data and analyzes the changes in cracks before and after self-healing, and calculates the fractal dimension before and after healing to obtain the self-healing effect of damaged salt rock. The present invention can quantitatively describe the self-healing effect of damaged salt rock, which is of great significance for in-depth research on the self-healing characteristics of underground salt rock. DETAILED DESCRIPTION

[0040] The technical solutions in the embodiments of the present invention are clearly and completely described below in conjunction with specific embodiments. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0041] An experimental method for determining the self-healing effect of damaged salt rock provided by an embodiment of the present invention comprises the following steps:

[0042] S100, set the initial damage of rock salt;

[0043] A complete salt rock sample was selected and subjected to uniaxial compression to produce initial damage to the sample.

[0044] S200, performing a nuclear magnetic resonance test to obtain pore distribution data of damaged salt rock and calculate the pore fractal dimension of the initial damaged salt rock;

[0045] 1) The sample is vacuumed and saturated with water, and then the nuclear magnetic resonance test is carried out to collect T2 spectrum test data;

[0046] 2) Based on the test data, the T2 spectrum at each location inside the salt rock is obtained by Fourier transform: ;

[0047] In the formula: T2 is the transverse relaxation time, is the transverse surface relaxation intensity, is the specific surface area of a single pore;

[0048] 3) Relationship between pore radius R and specific surface area: , substituting into the formula in step 2), we get , where F s is the pore geometry factor (for spherical pores , and for cylindrical pores );

[0049] 4) According to the fractal theory, the relationship between the number of pores N with the same radius and the pore radius R is obtained: , where a is the proportionality constant and D is the fractal dimension of the pore structure;

[0050] 5) The geometric formula for pore distribution is: S v = [ R R max ] 3 − D , where is the cumulative volume fraction of pores with a radius less than R, is the maximum pore radius.

[0051] 6) Taking the logarithm of both sides of the formula in step 5), the expression for the fractal dimension of the pore structure is obtained:

[0052]

[0053] The pore radius of salt rock is divided into four types: microcracks, small cracks, medium cracks, and large cracks. The pore radii of the four types of cracks are: for microcracks, r ≤ 0.5 nm; for small cracks, 0.5 nm < r ≤ 2.0 nm; for medium cracks, 2.0 nm < r ≤ 50.0 nm; for large cracks, r > 50.0 nm; According to the number of pores N, calculate the proportion of pore radii of the four types of cracks , , , ; According to the fractal theory, the fractal dimensions of different pore radii of initially damaged salt rock are denoted as , , , ;

[0054] Before self-healing, the internal crack network of damaged salt rock is complex and has strong connectivity, and the fractal dimension is relatively high.

[0055] S300. Conduct a self-healing static test on damaged salt rock; The content of the self-healing static test on damaged salt rock is as follows:

[0056] The existing salt cavern storage is at a depth of 1000m, with a temperature of about 50℃ and a confining pressure of 12MPa. In order to effectively cover the engineering requirements of salt cavern storage, three groups of damaged salt rock self-healing tests were set with a confining pressure of 12MPa and temperatures of 30℃, 50℃, and 70℃, respectively, and the pressure was maintained for 30 days.

[0057] The present invention can truly reflect the self-healing situation of salt rock by conducting static tests on damaged salt rock self-healing under a variety of different temperature conditions, and provide an accurate data basis for subsequent salt rock self-healing assessment.

[0058] S400, determining the fractal dimension of the static self-healing salt rock: using step S200 to obtain the proportion of each pore radius of the static self-healing salt rock , , , The fractal dimension of , , , .

[0059] After self-healing, the internal cracks of the damaged salt rock gradually closed, the number of cracks decreased, and the fractal dimension decreased.

[0060] S500, calculate the self-healing effect of damaged salt rock:

[0061] The formula for the self-healing effect of damaged salt rock is: ;in, It is expressed based on the fractal dimension before self-healing. It represents the fractal dimension benchmark after self-healing;

[0062] After normalization, the modified self-healing effect formula is recorded as: ;

[0063] The present invention can accurately reflect the difference in self-healing effects between different salt rocks by normalizing the self-healing effects of salt rocks, which is convenient for subsequent in-depth research and evaluation and analysis of the self-healing characteristics of underground salt rocks.

[0064] S600, calculating the average self-healing effect;

[0065] The self-healing effect of damaged salt rock at temperatures of 30°C, 50°C, and 70°C is calculated as follows: , , , the average self-healing effect is: .

[0066] The present invention further improves the accuracy of the salt rock self-healing assessment result by conducting a static test of the damaged salt rock self-healing under a variety of different temperature conditions and comprehensively processing the corresponding salt rock self-healing effects under different temperature conditions.

[0067] S700, assess the self-healing level of damaged salt rock. The self-healing level of damaged salt rock ranges from 0 to 1. The self-healing level of damaged salt rock is divided into three levels, as follows:

[0068] Level I: When , the self-healing degree of damaged salt rock is low;

[0069] Level II: When , the degree of self-healing of damaged salt rock is moderate;

[0070] Level III: When , the damaged salt rock has a high degree of self-healing.

[0071] The present invention divides the self-healing degree of damaged salt rock into multiple levels according to the interval of self-healing effect value, thereby realizing the division of self-healing levels of damaged salt rock, and providing a classification basis for the classification study of the self-healing characteristics of underground salt rock.

[0072] Fractal dimension and pore ratio are key parameters for describing the self-healing effect of salt rock. Fractal dimension reflects the complexity of pore structure. The larger the value, the more complex the pore structure. Before self-healing, the pore structure of salt rock increases due to damage, and the fractal dimension is high, indicating that the pore distribution is uneven and diverse. After self-healing, the pores gradually close and the fractal dimension decreases, indicating that the pore structure tends to be simple and uniform, and the integrity of the material is restored. The pore ratio indicates the proportion of the number of pores of each type. Before self-healing, the proportion of larger pores is high, indicating that the material is more seriously damaged. After self-healing, the proportion of smaller pores increases, indicating that the material has successfully repaired larger pores and restored density through the self-healing process. Therefore, the decrease in fractal dimension and the change in pore ratio jointly reflect the self-healing effect of salt rock and describe the degree of damage repair and structural recovery.

[0073] The following are relevant test data obtained by conducting self-healing effect tests on three different salt rocks using the technical solution provided by the present invention:

[0074] Salt rock 1: H = 0.2498, low degree of self-healing.

[0075]

[0076] Salt rock 2: H=0.4967, medium self-healing degree.

[0077]

[0078] Salt rock 3: H=0.7863, high degree of self-healing.

[0079]

[0080] At present, the self-healing effect of salt rock is measured by the changes in macroscopic indicators such as strength and ultrasound before and after damage. For example, ultrasound evaluates the material integrity after self-healing of salt rock by measuring parameters such as wave velocity attenuation. However, the resolution of ultrasound is limited, making it difficult to identify tiny pores and unable to accurately quantify the microscopic changes in the pore structure. The fractal dimension test proposed in the present invention is a quantitative analysis method based on the microstructure of the material. By conducting a nuclear magnetic resonance test on the damaged salt rock, collecting pore nuclear magnetic resonance T2 spectrum data, and using fractal dimension to represent the changes in the pore structure of the salt rock before and after self-healing, the degree of self-healing of the damaged salt rock can be effectively evaluated. In summary, the present invention quantitatively describes the self-healing effect of damaged salt rock from a microscopic perspective, which is of great significance for in-depth research on the self-healing characteristics of underground salt rock.

[0081] Finally, it should be noted that the above is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art can still modify the technical solutions described in the aforementioned embodiments or replace some of the technical features therein with equivalents. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the scope of protection of the claims of the present invention.

Claims

1. A test method for determining the self-healing effect of damaged salt rock, characterized in that: It includes the following steps: S100. Set the initial damage of the salt rock; S200, conduct nuclear magnetic resonance test to obtain pore distribution data of damaged salt rock, calculate pore fractal dimension of initial damaged salt rock; divide the initial damaged salt rock into four types according to pore radius: micro cracks, small cracks, medium cracks, and large cracks, calculate the pore radius proportion of the four types of cracks , , , The fractal dimension of , , , ; The pore radius divisions of the four types of fractures are as follows: Micro-fractures: r ≤ 0.5 nm; small fractures: 0.5 nm < r ≤ 2.0 nm; medium fractures: 2.0 nm < r ≤ 50.0 nm; large fractures: r > 50.0 nm; The process of obtaining the pore fractal dimension of the initially damaged salt rock is as follows: 1) Vacuum the sample and saturate it with water, then perform the NMR test and collect T 2 Spectrum test data; 2) According to the test data, the internal positions of the salt rock are obtained by Fourier transform. T 2 spectrum: ; Where: T 2 is the transverse relaxation time, is the transverse surface relaxation strength, is the specific surface area of ​​a single pore; 3) Pore radius R Relationship with specific surface area: , substitute into the formula in step 2) and we get , where F s is the pore geometry factor; 4) According to fractal theory, the number of pores with the same radius is obtained N and pore radius R Relationship: , where a is the proportionality constant, D is the fractal dimension of the pore structure; 5) The geometric formula of pore distribution is: ,in The pore radius is less than R The cumulative volume fraction of is the maximum pore radius; 6). Take the logarithm of both sides of the formula in step 5), and the expression of the pore structure fractal dimension is obtained as: ； The pore radius of salt rock is divided into four types: micro-cracks, small cracks, medium cracks, and large cracks. N , calculate the proportion of each pore radius , , , According to the fractal theory, the fractal dimensions of different pore radii of initially damaged salt rock are obtained and recorded as , , , ; S300. Conduct the self-healing static test on the damaged salt rock; S400, determining the fractal dimension of the static self-healing salt rock: using step S200 to obtain the proportion of each pore radius of the static self-healing salt rock , , , The fractal dimension of , , , ; S500. Calculate the self-healing effect of the damaged salt rock; The formula for the self-healing effect of damaged salt rock is: ;in, It is expressed based on the fractal dimension before self-healing. It represents the fractal dimension benchmark after self-healing; After normalization, the modified self-healing effect formula is recorded as: ; S700. Evaluate the self-healing grade of the damaged salt rock; The self-healing degree of the damaged salt rock is between 0 and 1, and the self-healing grade of the damaged salt rock is divided into three grades, the details are as follows: Level I: When , the self-healing degree of damaged salt rock is low; Level II: When , the degree of self-healing of damaged salt rock is moderate; Level III: When , the damaged salt rock has a high degree of self-healing.

2. A test method for determining the self-healing effect of damaged salt rock according to claim 1, characterized in that: In step S100, select a complete salt rock specimen and conduct a uniaxial compression test on the specimen to cause initial damage to the specimen.

3. A test method for determining the self-healing effect of damaged salt rock according to claim 1, characterized in that: In step S300, the content of the self-healing static test on the damaged salt rock is as follows: Set three groups of self-healing tests on the damaged salt rock under the test conditions of confining pressure of 12 MPa and temperatures of 30 °C, 50 °C, and 70 °C respectively, and keep the pressure and stand still for 30 days.

4. A test method for determining the self-healing effect of damaged salt rock according to claim 3, characterized in that: It also includes S600, calculating the average self-healing effect; calculating the self-healing effect of damaged salt rock at temperatures of 30°C, 50°C, and 70°C respectively: , , , the average self-healing effect is: .

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