CN109064016B - A method for evaluating the effect of hydraulic fracturing in low permeability coal seams - Google Patents

Abstract

The invention relates to a method for evaluating the effect of hydraulic fracturing in low-permeability coal seams, comprising the following steps: (1) determining the applicability of hydraulic fracturing in low-permeability coal seams; (2) determining the fracturing index I ₁ of coal seams; (3) ) to determine the hydraulic fracture expansion index I ₂ ; (4) to determine the hydraulic fracture closure index I ₃ ; (5) to construct a quantitative calculation model for the evaluation index I of the hydraulic fracturing enhancement effect of low-permeability coal seams; (6) to determine the hydraulic fracturing of low-permeability coal seams Evaluation standard of permeability enhancement effect; (7) Evaluate the permeability enhancement effect of hydraulic fracturing in low permeability coal seams. The beneficial effects of the present invention are as follows: the present invention builds a model and method for evaluating the effect of hydraulic fracturing and increasing permeability of low-permeability coal seams based on the mechanism of hydraulic fracturing and increasing permeability of coal seams, and realizes the quantitative evaluation of the effect of hydraulic fracturing and increasing permeability of low-permeability coal seams. It is of great significance to accurately select the hydraulic fracturing and permeability-enhancing area of coal seam and improve the effect of coal seam gas drainage.

Description

A method for evaluating the effect of hydraulic fracturing in low permeability coal seams

technical field

The invention belongs to the technical field of hydraulic fracturing, and in particular relates to a method for evaluating the effect of hydraulic fracturing in low permeability coal seams.

Background technique

Coal seam hydraulic fracturing and permeability enhancement technology relies on the pressure of water injected into the coal seam to overcome the minimum in-situ stress and the tensile strength of the coal rock mass to open, expand and extend the weak surface of the coal seam to form cracks, thereby increasing the coal seam permeability. Due to its multiple functions such as increasing permeability, inhibiting gas gushing, changing coal strength and reducing dust, it has been tested and applied in many mines in my country. The results show that only some mines have good test results, while most mines have poor application results. The reason is that due to the extremely complex coal seam occurrence conditions in my country, hydraulic fracturing and permeability enhancement technology has certain limitations and blindness. Hydraulic fracturing and permeability enhancement technology is a special measure for enhancing permeability of coal seams. If the effect evaluation is not carried out before implementation, it will not only waste a lot of manpower and material resources, but also affect coal seam gas drainage due to the failure of fracturing to achieve the expected goals. , leaving a serious safety hazard to subsequent coal mine production.

At present, researchers have recognized the importance of hydraulic fracturing effect evaluation, and adopted geological strength index or coal structure as the criterion for the applicability of hydraulic fracturing technology. Because coal seam hydraulic fracturing involves multiple processes such as fracturing and drainage, and is also affected by the mechanical properties of coal seams, roof and floor, and in-situ stress, there are many influencing factors and complex influencing mechanisms. Using geological strength index or coal structure to judge the effect of hydraulic fracturing on the one hand is because these indicators are single and cannot cover all the influencing factors of hydraulic fracturing; For example, it will be difficult to identify by on-site technicians, which will inevitably bring certain deviations to the evaluation of hydraulic fracturing and permeability enhancement effects.

SUMMARY OF THE INVENTION

In order to solve the deficiencies in the prior art, according to the mechanism of hydraulic fracturing and permeability enhancement of low-permeability coal seams, the invention divides the hydraulic fracturing and permeability-enhancing process of low-permeability coal seams into three stages of hydraulic fracture initiation, expansion and extension, and closure. , based on the three stages and influencing factors involved in hydraulic fracturing, the coal seam fracturing index, hydraulic fracture expansion index and hydraulic fracture closure index are proposed, and the evaluation model and method of hydraulic fracturing enhancement effect in low-permeability coal seam are established. It can quantitatively evaluate the hydraulic fracturing and permeability-enhancing effect of low-permeability coal seams, improve the accuracy of the evaluation of coal seam hydraulic fracturing effects, provide technical support for the selection of hydraulic fracturing and permeability-enhancing areas in coal seams, and at the same time promote the application of hydraulic fracturing and permeability-enhancing technology. It is of great significance to improve the effect of coal seam gas drainage.

In order to solve the above-mentioned technical problems, the present invention adopts the following technical scheme: a method for evaluating the effect of hydraulic fracturing and permeability enhancement of low-permeability coal seams, comprising the following steps:

(1) Determine the applicability of hydraulic fracturing in low-permeability coal seams;

(2) Determine the fracturing index I ₁ of the coal seam;

(3) Determine the hydraulic fracture propagation index I ₂ ;

(4) Determine the hydraulic fracture closure index I ₃ ;

(5) Using the coal seam fractability index I ₁ obtained in step (2), the hydraulic fracture expansion index I ₂ obtained in step (3), and the hydraulic fracture closure index I ₃ obtained in step (4), the hydraulic pressure of the low-permeability coal seam is constructed. Quantitative calculation model for the evaluation index I of fracturing and permeability enhancement effect;

(6) Determine the evaluation standard of hydraulic fracturing and permeability enhancement effect of low-permeability coal seam;

(7) Evaluate the permeability enhancement effect of hydraulic fracturing in low permeability coal seams.

Further, the specific process of the step (1) is to determine the applicability of the hydraulic fracturing of the low permeability coal seam according to Table 1:

Table 1 Applicability criteria for hydraulic fracturing in low permeability coal seams

If the geological strength index of the low-permeability coal seam is GSI ≥30 and the firmness coefficient f ≥0.4 and the coal body structure is primary structural coal or fractured coal, then the low-permeability coal seam is suitable for the implementation of hydraulic fracturing technology, and can continue to follow the steps ( 2) to step (7) to evaluate the effect of hydraulic fracturing and permeability enhancement of low-permeability coal seams;

If the geological strength index of the low-permeability coal seam is GSI <30 or the firmness coefficient f <0.4 or the coal body structure is fragmented coal or mylonitic coal, then the low-permeability coal seam is not suitable for the implementation of hydraulic fracturing technology, directly according to the steps ( 7) Evaluate the permeability enhancement effect of hydraulic fracturing in low permeability coal seams.

Further, the specific step of the step (2) is that the fractability of the coal body refers to the property that the coal seam can be effectively fracturing to form cracks to increase the permeability, and the mechanical properties of the coal have an effect on the fractability of the coal seam. It has an important role and influence. It is generally believed that the larger the elastic modulus of the coal, the larger the geological strength index of the coal seam, and the smaller the Poisson's ratio, the better the fractability of the coal seam; accordingly, formulas (1) to (4) Calculate the fracturing index I ₁ of the coal seam:

（1）

(1)

（2）

(2)

（3）

(3)

（4）

(4)

In the formula, I ₁ is the fracturing index of the coal seam; _En and v _n are the elastic modulus index and Poisson's ratio index, respectively; E max and E min _are the _maximum and minimum elastic moduli in the coal seam, respectively; v _max and v _min are respectively is the maximum and minimum Poisson’s ratio in the coal seam; GSI is the geological strength index of the coal seam, the value range is 0～100; GSI _n is the geological strength index index of the coal seam; E is the elastic modulus of the coal body, v is the coal body’s Poisson's ratio.

Further, the specific steps of the step (3) are: in the process of hydraulic fracture formation, the fracture toughness, horizontal principal stress difference coefficient and vertical principal stress difference coefficient of the coal body are the main factors affecting the expansion and extension of the hydraulic fracture, and the coal body is The lower the fracture toughness, the more conducive to the expansion and extension of hydraulic fractures; the larger the difference coefficient of horizontal principal stress, the more conducive to the extension of hydraulic fractures, and the larger the range of hydraulic fractures; the smaller the difference coefficient of vertical principal stress, the more It is beneficial to form a network of network fractures; based on this, formula (5) is used to calculate the type I fracture toughness K _IC of the coal body:

（5）

(5)

In the formula, K _IC is the type I fracture toughness; σ _t is the tensile strength of the coal and rock mass; a and b are the coefficients, which can be obtained from the tensile strength and type I fracture toughness K _IC of the coal and rock samples that have been measured in the collection area. , and then use the linear fitting method to determine the coefficients a and b ;

Equation (6) is used to calculate the horizontal principal stress difference coefficient in the coal seam:

（6）

(6)

where K _H is the difference coefficient of horizontal principal stress, σ ₁ is the maximum horizontal principal stress, and σ ₂ is the minimum horizontal principal stress;

Equation (7) is used to calculate the vertical principal stress difference coefficient in the coal seam:

（7）

(7)

where K _V is the vertical principal stress difference coefficient, σ _H is the vertical principal stress, and σ ₂ is the minimum horizontal principal stress;

The hydraulic fracture propagation index I ₂ is obtained by normalizing the type I fracture toughness K _IC , the horizontal principal stress difference coefficient K _H and the vertical principal stress difference coefficient K _V :

（8）

(8)

（9）

(9)

（10）

(10)

（11）

(11)

where K _Icn is the type I fracture toughness index, K _Hn is the horizontal principal stress difference coefficient index, and K _Vn is the vertical principal stress difference coefficient index; K _{IC max} and K _{IC min} are the maximum and minimum type I fracture toughness in the coal seam, respectively. ; K _{H max} and K _{H min} are the maximum and minimum horizontal principal stress difference coefficients in the coal seam respectively; K _{V max} and K _{V min} are the maximum and minimum vertical principal stress difference coefficients in the coal seam respectively; I ₂ is the hydraulic fracture propagation index.

Further, the specific steps of the step (4) are as follows: in the drainage process after fracturing, along with the discharge of water in the coal seam, the pore fluid pressure in the coal seam decreases, the effective stress increases, and the hydraulic fractures in the coal seam will be closed to different degrees. . The main factors affecting the closure of coal seam hydraulic fractures are coal uniaxial compressive strength, coal firmness coefficient, minimum horizontal principal stress and coal seam burial depth. The stronger the ability of fracture closure is, the more unfavorable it is for the closure of hydraulic fractures in the coal seam. The greater the minimum horizontal principal stress and the burial depth of the coal seam, the greater the external force applied to the closure of coal fractures, the more conducive to the closure of hydraulic fractures in the coal seam. Using equations (12) to (16) to calculate the hydraulic fracture closure index I ₃ of the coal seam:

（12）

(12)

（13）

(13)

（14）

(14)

（15）

(15)

（16）

(16)

In the formula, σ _Cn is the uniaxial compressive strength index of the coal body; f _n is the coal body firmness coefficient index; σ _{2 n} is the minimum horizontal principal stress index; H _n is the coal seam burial depth index; I ₃ is the hydraulic fracture closure index ; σ _{C max} and σ _{C min} are the maximum uniaxial compressive strength and minimum uniaxial compressive strength of the coal body, respectively; f _max and f _min are the maximum firmness coefficient and minimum firmness coefficient of the coal body, respectively; σ _2max and σ _2min are the maximum minimum horizontal principal stress and the minimum minimum horizontal principal stress of the coal seam, respectively; H _max and H _min are the maximum and minimum burial depth of the coal seam, respectively; σ _C is the uniaxial compressive strength of the coal body, f is the firmness of the coal body coefficient, σ ₂ is the minimum horizontal principal stress of the coal seam in the hydraulic fracturing area, and H is the burial depth of the coal seam.

Further, the specific steps of the step (5) are: according to the coal seam fracturing index I ₁ in step (2), the hydraulic fracture propagation index I ₂ in step (3) and the hydraulic fracture in step (4) The closure index I ₃ is used to construct a quantitative calculation model for the evaluation index I of hydraulic fracturing and permeability enhancement effect in low-permeability coal seams:

（17）

(17)

In the formula, I is the evaluation index of hydraulic fracturing and permeability enhancement effect of low-permeability coal seam; w ₁ , w ₂ , and w ₃ are the weights of the influence of the coal seam fractability index, hydraulic fracture expansion index and hydraulic fracture closure index on the hydraulic fracturing effect, respectively. , w ₁ + w ₂ + w ₃ =1, w ₁ , w ₂ , and w ₃ can be determined by using the AHP.

Further, the specific steps of the step (6) are: according to the evaluation index value of the hydraulic fracturing and permeability enhancement effect of the low-permeability coal seam calculated in the step (5), combined with the implementation of the hydraulic fracturing of the low-permeability coal seam, the low-permeability coal seam is given. The classification standard of hydraulic fracturing anti-permeability effect grade is shown in Table 2:

Table 2 Classification standard of hydraulic fracturing and permeability enhancement effect in low permeability coal seam

。

.

Further, the specific steps of the step (7) are, according to the determination result of the step (1), if the low-permeability coal seam is suitable for the implementation of the hydraulic fracturing technology, the relevant data of the site to be hydraulic fracturing can be collected, based on the step ( 2) to step (5) to calculate the hydraulic fracturing and permeability-increasing effect evaluation index I of the site where the coal seam hydraulic fracturing is to be carried out. The permeability enhancement effect is evaluated; if the low permeability coal seam is not suitable for the implementation of hydraulic fracturing technology, the hydraulic fracturing permeability enhancement effect of the low permeability coal seam is evaluated as poor.

By adopting the above technical scheme, the beneficial effects of the present invention are: the method of the present invention divides the hydraulic fracturing and permeability enhancement process into three stages of hydraulic crack initiation, expansion and extension and closure according to the mechanism of hydraulic fracturing and permeability enhancement of coal seams, and fully considers each stage. Factors affecting the evolution of hydraulic fractures The coal seam fracturing index, hydraulic fracture expansion index and hydraulic fracture closure index are proposed, and the evaluation model and method for hydraulic fracturing and permeability enhancement in low-permeability coal seams are constructed to realize hydraulic fracturing and permeability enhancement in low-permeability coal seams. Quantitative evaluation of the effect is of great significance for accurately selecting the area for increasing the permeability of coal seam hydraulic fracturing and improving the effect of coal seam gas drainage.

Description of drawings

FIG. 1 is a schematic flow chart of the present invention.

Detailed ways

As shown in FIG. 1, a method for evaluating the effect of hydraulic fracturing and increasing permeability of low permeability coal seams of the present invention includes the following steps:

(1) Determine the applicability of hydraulic fracturing in low-permeability coal seams;

(2) Determine the fracturing index I ₁ of the coal seam;

(3) Determine the hydraulic fracture propagation index I ₂ ;

(4) Determine the hydraulic fracture closure index I ₃ ;

(5) Using the coal seam fractability index I ₁ obtained in step (2), the hydraulic fracture expansion index I ₂ obtained in step (3), and the hydraulic fracture closure index I ₃ obtained in step (4), the hydraulic pressure of the low-permeability coal seam is constructed. Quantitative calculation model for the evaluation index I of fracturing and permeability enhancement effect;

(6) Determine the evaluation standard of hydraulic fracturing and permeability enhancement effect of low-permeability coal seam;

(7) Evaluate the permeability enhancement effect of hydraulic fracturing in low permeability coal seams.

Further, the specific process of the step (1) is to determine the applicability of the hydraulic fracturing of the low permeability coal seam according to Table 1:

Table 1 Applicability criteria for hydraulic fracturing in low permeability coal seams

If the geological strength index of the low-permeability coal seam is GSI ≥30 and the firmness coefficient f ≥0.4 and the coal body structure is primary structural coal or fractured coal, then the low-permeability coal seam is suitable for the implementation of hydraulic fracturing technology, and can continue to follow the steps ( 2) to step (7) to evaluate the effect of hydraulic fracturing and permeability enhancement of low-permeability coal seams;

If the geological strength index of the low-permeability coal seam is GSI <30 or the firmness coefficient f <0.4 or the coal body structure is fragmented coal or mylonitic coal, then the low-permeability coal seam is not suitable for the implementation of hydraulic fracturing technology, directly according to the steps ( 7) Evaluate the permeability enhancement effect of hydraulic fracturing in low permeability coal seams.

Further, the specific step of the step (2) is that the fractability of the coal body refers to the property that the coal seam can be effectively fracturing to form cracks to increase the permeability, and the mechanical properties of the coal have an effect on the fractability of the coal seam. It has an important role and influence. It is generally believed that the larger the elastic modulus of the coal, the larger the geological strength index of the coal seam, and the smaller the Poisson's ratio, the better the fractability of the coal seam; accordingly, formulas (1) to (4) Calculate the fracturing index I ₁ of the coal seam:

（1）

(1)

（2）

(2)

（3）

(3)

（4）

(4)

In the formula, I ₁ is the fracturing index of the coal seam; _En and v _n are the elastic modulus index and Poisson's ratio index, respectively; E max and E min _are the _maximum and minimum elastic moduli in the coal seam, respectively; v _max and v _min are respectively is the maximum and minimum Poisson’s ratio in the coal seam; GSI is the geological strength index of the coal seam, the value range is 0～100; GSI _n is the geological strength index index of the coal seam; E is the elastic modulus of the coal body, v is the coal body’s Poisson's ratio.

Further, the specific steps of the step (3) are: in the process of hydraulic fracture formation, the fracture toughness, horizontal principal stress difference coefficient and vertical principal stress difference coefficient of the coal body are the main factors affecting the expansion and extension of the hydraulic fracture, and the coal body is The lower the fracture toughness, the more conducive to the expansion and extension of hydraulic fractures; the larger the difference coefficient of horizontal principal stress, the more conducive to the extension of hydraulic fractures, and the larger the range of hydraulic fractures; the smaller the difference coefficient of vertical principal stress, the more It is beneficial to form a network of network fractures; based on this, formula (5) is used to calculate the type I fracture toughness K _IC of the coal body:

（5）

(5)

In the formula, K _IC is the type I fracture toughness; σ _t is the tensile strength of the coal and rock mass; a and b are the coefficients, which can be obtained from the tensile strength and type I fracture toughness K _IC of the coal and rock samples that have been measured in the collection area. , and then use the linear fitting method to determine the coefficients a and b ;

Equation (6) is used to calculate the horizontal principal stress difference coefficient in the coal seam:

（6）

(6)

where K _H is the difference coefficient of horizontal principal stress, σ ₁ is the maximum horizontal principal stress, and σ ₂ is the minimum horizontal principal stress;

Equation (7) is used to calculate the vertical principal stress difference coefficient in the coal seam:

（7）

(7)

where K _V is the vertical principal stress difference coefficient, σ _H is the vertical principal stress, and σ ₂ is the minimum horizontal principal stress;

The hydraulic fracture propagation index I ₂ is obtained by normalizing the type I fracture toughness K _IC , the horizontal principal stress difference coefficient K _H and the vertical principal stress difference coefficient K _V :

（8）

(8)

（9）

(9)

（10）

(10)

（11）

(11)

where K _Icn is the type I fracture toughness index, K _Hn is the horizontal principal stress difference coefficient index, and K _Vn is the vertical principal stress difference coefficient index; K _{IC max} and K _{IC min} are the maximum and minimum type I fracture toughness in the coal seam, respectively. ; K _{H max} and K _{H min} are the maximum and minimum horizontal principal stress difference coefficients in the coal seam respectively; K _{V max} and K _{V min} are the maximum and minimum vertical principal stress difference coefficients in the coal seam respectively; I ₂ is the hydraulic fracture propagation index.

Further, the specific steps of the step (4) are as follows: in the drainage process after fracturing, along with the discharge of water in the coal seam, the pore fluid pressure in the coal seam decreases, the effective stress increases, and the hydraulic fractures in the coal seam will be closed to different degrees. . The main factors affecting the closure of coal seam hydraulic fractures are coal uniaxial compressive strength, coal firmness coefficient, minimum horizontal principal stress and coal seam burial depth. The stronger the ability of fracture closure is, the more unfavorable it is for the closure of hydraulic fractures in the coal seam. The greater the minimum horizontal principal stress and the burial depth of the coal seam, the greater the external force applied to the closure of coal fractures, the more conducive to the closure of hydraulic fractures in the coal seam. Using equations (12) to (16) to calculate the hydraulic fracture closure index I ₃ of the coal seam:

（12）

(12)

（13）

(13)

（14）

(14)

（15）

(15)

（16）

(16)

In the formula, σ _Cn is the uniaxial compressive strength index of the coal body; f _n is the coal body firmness coefficient index; σ _{2 n} is the minimum horizontal principal stress index; H _n is the coal seam burial depth index; I ₃ is the hydraulic fracture closure index ; σ _{C max} and σ _{C min} are the maximum uniaxial compressive strength and minimum uniaxial compressive strength of the coal body, respectively; f _max and f _min are the maximum firmness coefficient and minimum firmness coefficient of the coal body, respectively; σ _2max and σ _2min are the maximum minimum horizontal principal stress and the minimum minimum horizontal principal stress of the coal seam, respectively; H _max and H _min are the maximum and minimum burial depth of the coal seam, respectively; σ _C is the uniaxial compressive strength of the coal body, f is the firmness of the coal body coefficient, σ ₂ is the minimum horizontal principal stress of the coal seam in the hydraulic fracturing area, and H is the burial depth of the coal seam; The greater the burial depth, the more favorable it is for the closure of hydraulic fractures in the coal seam.

Further, the specific steps of the step (5) are: according to the coal seam fracturing index I ₁ in step (2), the hydraulic fracture propagation index I ₂ in step (3) and the hydraulic fracture in step (4) The closure index I ₃ is used to construct a quantitative calculation model for the evaluation index I of hydraulic fracturing and permeability enhancement effect in low-permeability coal seams:

（17）

(17)

In the formula, I is the evaluation index of hydraulic fracturing and permeability enhancement effect of low-permeability coal seam; w ₁ , w ₂ , and w ₃ are the weights of the influence of the coal seam fractability index, hydraulic fracture expansion index and hydraulic fracture closure index on the hydraulic fracturing effect, respectively. , w ₁ + w ₂ + w ₃ =1, w ₁ , w ₂ , and w ₃ can be determined by using the AHP.

Further, the specific steps of the step (6) are: according to the evaluation index value of the hydraulic fracturing and permeability enhancement effect of the low-permeability coal seam calculated in the step (5), combined with the implementation of the hydraulic fracturing of the low-permeability coal seam, the low-permeability coal seam is given. The classification standard of hydraulic fracturing anti-permeability effect grade is shown in Table 2:

Table 2 Classification standard of hydraulic fracturing and permeability enhancement effect in low permeability coal seam

。

.

Further, the specific steps of the step (7) are, according to the determination result of the step (1), if the low-permeability coal seam is suitable for the implementation of the hydraulic fracturing technology, the relevant data of the site to be hydraulic fracturing can be collected, based on the step ( 2) to step (5) to calculate the hydraulic fracturing and permeability-increasing effect evaluation index I of the site where the coal seam hydraulic fracturing is to be carried out. The permeability enhancement effect is evaluated; if the low permeability coal seam is not suitable for the implementation of hydraulic fracturing technology, the hydraulic fracturing permeability enhancement effect of the low permeability coal seam is evaluated as poor.

The following is an example of the evaluation of the permeability enhancement effect of a coal seam in a certain mine where hydraulic fracturing is to be carried out:

Step (1): Collect coal samples at the site where hydraulic fracturing is to be performed. After testing, it is found that the coal structure of the low permeability coal seam is fragmented coal, the geological strength index GSI = 70, and the firmness coefficient f = 1.2. According to Table 1, this location is suitable for the implementation of hydraulic fracturing technology, then the effect of hydraulic fracturing and permeability enhancement of low-permeability coal seams can be evaluated according to steps (2) to (7);

Step (2): Collect coal samples at the site where hydraulic fracturing is to be carried out to test the mechanical parameters of coal and rock, and obtain parameters such as coal elastic modulus, geological strength index and Poisson's ratio. E = 1500MPa, GSI =70 and v =0.3 , which is substituted into equations (1) to (4) to calculate the fracturing index I ₁ of the coal seam. where E _max = 2000 MPa, and E _min = 800 MPa; v _max =0.4 and v _min =0.25. After calculation I ₁ =0.65.

Step (3): Collect the measured data of type I fracture toughness and tensile strength of coal and rock mass in the area and perform linear regression analysis to obtain a = 0.03, b = 0.12; The mechanical parameters of coal and rock are tested, and the tensile strength parameters of coal and rock mass are obtained. After measuring σ _t = 2MPa, and substituting it into formula (5), the type I fracture toughness K _IC can be obtained, and K _IC = 0.18MPa can be obtained by calculation.

Collect the data of the maximum horizontal principal stress and the minimum horizontal principal stress at the site where hydraulic fracturing is to be performed. The maximum horizontal principal stress σ ₁ is 12 MPa, and the minimum horizontal principal stress σ ₂ is 8 MPa. Equation (6) is used to calculate the horizontal principal stress difference coefficient K _H . After calculation, K _H = 0.5.

The vertical principal stress of the site to be hydraulic fracturing is collected, the vertical principal stress σ _H is 10MPa, the minimum horizontal principal stress σ ₂ is 8MPa, and the vertical principal stress difference coefficient K _V is calculated by formula (7). After calculation, K _V =0.25.

The hydraulic fracture propagation index I ₂ is obtained by normalizing the type I fracture toughness K _IC , the difference coefficient of horizontal principal stress and the difference coefficient of vertical principal stress K _V by formulas (8) to (11). Wherein, K _{IC max} =0.27, K _{IC min} =0.12; K _{H max} =0.8, K _{H min} =0.2; K _{V max} =0.5, K _{V min} =0.1. After calculation, I ₂ =0.575.

Step (4): Collect coal samples at the proposed hydraulic fracturing site for uniaxial compressive strength test and firmness coefficient test of coal and rock mass, and obtain σ _C =15MPa, f =1.2; according to the situation of the proposed hydraulic fracturing site Collecting relevant data, it can be obtained that σ ₂ =8MPa, H =450m. Substitute it into equations (12) to (16) to calculate the hydraulic fracture closure index I ₃ of the coal seam. where σ _{C max} =20MPa, σ _{C min} =3MPa; f _max =1.5, f _min =0.2; σ _2max =12MPa, σ _2min =5MPa; H _max =800m, H _min =400m. After calculation, I ₃ =0.7.

Step (5): First, use the AHP to determine the weights of the coal seam fractability index, hydraulic fracture expansion index and hydraulic fracture closure index on the effect of hydraulic fracturing, and obtain w ₁ =0.3, w ₂ =0.3, w ₃ = 0.4; then calculate the coal seam fracturing index I ₁ , the hydraulic fracture expansion index I ₂ calculated in step (3) and the hydraulic fracture closure index I ₃ calculated in step (4) according to step (2), using formula (17) ) to calculate the evaluation index I of hydraulic fracturing and permeability enhancement effect in low-permeability coal seams. After calculation, I = 0.648.

Step (6): According to the implementation of hydraulic fracturing in the low-permeability coal seam of the mine, the classification standard of hydraulic fracturing and permeability enhancement effect in the low-permeability coal seam is given (Table 2):

Table 2 Classification standard of hydraulic fracturing and permeability enhancement effect in low permeability coal seam

Step (7): The proposed hydraulic fracturing site is suitable for the implementation of hydraulic fracturing technology. After calculation, the evaluation index of hydraulic fracturing and permeability enhancement effect of low-permeability coal seam is I = 0.655. The antireflection effect was rated as medium.

A hydraulic fracturing test was carried out at the evaluation site. After the test was completed, the holes were sealed and networked for drainage. It was determined that the daily drainage volume could reach 95 cubic meters, which was 3.3 times that of the non-fracturing period. The effect of hydraulic fracturing and permeability enhancement was medium. , which is consistent with the calculation results of the present invention. It shows that the invention has high accuracy, can predict the effect of hydraulic fracturing and permeability enhancement of low-permeability coal seam, and can provide preliminary guidance for the selection of hydraulic fracturing and permeability-enhancing areas of low permeability coal seam.

This embodiment does not limit the shape, material, structure, etc. of the present invention in any form, and any simple modification, equivalent change and modification made to the above embodiment according to the technical essence of the present invention belong to the protection of the technical solution of the present invention scope.

Claims (5)

1. a low permeability coal seam hydraulic fracturing effect evaluation method, is characterized in that: comprise the following steps, (1) Determine the applicability of hydraulic fracturing in low-permeability coal seams; (2) Determine the fracturing index I ₁ of the coal seam; (3) Determine the hydraulic fracture propagation index I ₂ ; (4) Determine the hydraulic fracture closure index I ₃ ; (5) Using the coal seam fractability index I ₁ obtained in step (2), the hydraulic fracture expansion index I ₂ obtained in step (3), and the hydraulic fracture closure index I ₃ obtained in step (4), the hydraulic pressure of the low-permeability coal seam is constructed. Quantitative calculation model for the evaluation index I of fracturing and permeability enhancement effect; (6) Determine the evaluation standard of hydraulic fracturing and permeability enhancement effect of low-permeability coal seam; (7) Evaluate the permeability enhancement effect of hydraulic fracturing in low permeability coal seams; The specific steps of the step (2) are: the fractability of the coal body refers to the property that the coal seam can be effectively fractured to form fractures to increase the permeability, and the mechanical properties of the coal are important to the fractability of the coal seam. It is generally believed that the greater the elastic modulus of the coal, the greater the geological strength index of the coal seam, and the smaller the Poisson's ratio, the better the fractability of the coal seam. Calculate the coal seam fracturing index I ₁ :

(1)

(2)

(3)

(4) In the formula, I ₁ is the fracturing index of the coal seam; _En and v _n are the elastic modulus index and Poisson's ratio index, respectively; E max and E min _are the _maximum and minimum elastic moduli in the coal seam, respectively; v _max and v _min are respectively is the maximum and minimum Poisson’s ratio in the coal seam; GSI is the geological strength index of the coal seam, the value range is 0～100; GSI _n is the geological strength index index of the coal seam; E is the elastic modulus of the coal body, v is the coal body’s Poisson's ratio; The specific steps of the step (3) are as follows: during the formation of hydraulic fractures, the fracture toughness, the horizontal principal stress difference coefficient and the vertical principal stress difference coefficient of the coal body are the main factors affecting the expansion and extension of the hydraulic fracture, and the fracture toughness of the coal body is. The lower it is, the more favorable it is for the expansion and extension of hydraulic fractures; the larger the difference coefficient of horizontal principal stress, the more favorable the extension of hydraulic fractures, and the larger the range of hydraulic fractures; the smaller the difference coefficient of vertical principal stress, the more favorable it is to form a network According to the formula (5), the type I fracture toughness K _IC of the coal body is calculated:

(5) In the formula, K _IC is the type I fracture toughness; σ _t is the tensile strength of the coal and rock mass; a and b are the coefficients, which can be obtained from the tensile strength and type I fracture toughness K _IC of the coal and rock samples that have been measured in the collection area. , and then use the linear fitting method to determine the coefficients a and b ; Equation (6) is used to calculate the horizontal principal stress difference coefficient in the coal seam:

(6) where K _H is the difference coefficient of horizontal principal stress, σ ₁ is the maximum horizontal principal stress, and σ ₂ is the minimum horizontal principal stress; Equation (7) is used to calculate the vertical principal stress difference coefficient in the coal seam:

(7) where K _V is the vertical principal stress difference coefficient, σ _H is the vertical principal stress, and σ ₂ is the minimum horizontal principal stress; The hydraulic fracture propagation index I ₂ is obtained by normalizing the type I fracture toughness K _IC , the horizontal principal stress difference coefficient K _H and the vertical principal stress difference coefficient K _V :

(8)

(9)

(10)

(11) where K _Icn is the type I fracture toughness index, K _Hn is the horizontal principal stress difference coefficient index, and K _Vn is the vertical principal stress difference coefficient index; K _{IC max} and K _{IC min} are the maximum and minimum type I fracture toughness in the coal seam, respectively. ; K _{H max} and K _{H min} are the maximum and minimum horizontal principal stress difference coefficients in the coal seam respectively; K _{V max} and K _{V min} are the maximum and minimum vertical principal stress difference coefficients in the coal seam respectively; I ₂ is the hydraulic fracture propagation index; The specific steps of the step (4) are: during the drainage process after fracturing, along with the discharge of water in the coal seam, the pore fluid pressure in the coal seam decreases, the effective stress increases, and the hydraulic fractures in the coal seam will be closed to different degrees; The main factors affecting the closure of coal seam hydraulic fractures are coal uniaxial compressive strength, coal firmness coefficient, minimum horizontal principal stress and coal seam burial depth. The stronger the ability of fracture closure is, the more unfavorable it is for the closure of hydraulic fractures in the coal seam. The greater the minimum horizontal principal stress and the burial depth of the coal seam, the greater the external force applied to the closure of coal fractures, the more conducive to the closure of hydraulic fractures in the coal seam. Using equations (12) to (16) to calculate the hydraulic fracture closure index I ₃ of the coal seam:

(12)

(13)

(14)

(15)

(16) In the formula, σ _Cn is the uniaxial compressive strength index of the coal body; f _n is the coal body firmness coefficient index; σ _{2 n} is the minimum horizontal principal stress index; H _n is the coal seam burial depth index; I ₃ is the hydraulic fracture closure index ; σ _{C max} and σ _{C min} are the maximum uniaxial compressive strength and minimum uniaxial compressive strength of the coal body, respectively; f _max and f _min are the maximum firmness coefficient and minimum firmness coefficient of the coal body, respectively; σ _2max and σ _2min are the maximum minimum horizontal principal stress and the minimum minimum horizontal principal stress of the coal seam, respectively; H _max and H _min are the maximum and minimum burial depth of the coal seam, respectively; σ _C is the uniaxial compressive strength of the coal body, f is the firmness of the coal body coefficient, σ ₂ is the minimum horizontal principal stress of the coal seam in the hydraulic fracturing area, and H is the burial depth of the coal seam. 2 . The method for evaluating the effect of hydraulic fracturing in low permeability coal seams according to claim 1 , wherein the specific process of the step (1) is, according to Table 1, the application of hydraulic fracturing in low permeability coal seams sex to determine: Table 1 Applicability criteria for hydraulic fracturing in low permeability coal seams

If the geological strength index of the low-permeability coal seam is GSI ≥30 and the firmness coefficient f ≥0.4 and the coal body structure is primary structural coal or fractured coal, then the low-permeability coal seam is suitable for the implementation of hydraulic fracturing technology, and can continue to follow the steps ( 2) to step (7) to evaluate the effect of hydraulic fracturing and permeability enhancement of low-permeability coal seams; If the geological strength index of the low-permeability coal seam is GSI <30 or the firmness coefficient f <0.4 or the coal body structure is fragmented coal or mylonitic coal, then the low-permeability coal seam is not suitable for the implementation of hydraulic fracturing technology, directly according to the steps ( 7) Evaluate the permeability enhancement effect of hydraulic fracturing in low permeability coal seams. 3 . The method for evaluating the effect of hydraulic fracturing and permeability enhancement of low permeability coal seams according to claim 1 , wherein the specific steps of the step (5) are: according to the fracturing index of the coal seam in the step (2) I ₁ , the hydraulic fracture expansion index I ₂ in step (3), and the hydraulic fracture closure index I ₃ in step (4), and a quantitative calculation model for the evaluation index I of hydraulic fracturing and permeability enhancement effect in low-permeability coal seams is constructed:

(17) In the formula, I is the evaluation index of hydraulic fracturing and permeability enhancement effect of low-permeability coal seam; w ₁ , w ₂ , and w ₃ are the weights of the influence of the coal seam fractability index, hydraulic fracture expansion index and hydraulic fracture closure index on the hydraulic fracturing effect, respectively. , w ₁ + w ₂ + w ₃ =1, w ₁ , w ₂ , and w ₃ can be determined by using the AHP. 4 . The method for evaluating the effect of hydraulic fracturing in low permeability coal seams according to claim 1 , wherein the specific steps of the step (6) are: The evaluation index value of fracturing and permeability-enhancing effect, combined with the implementation of hydraulic fracturing in low-permeability coal seams, gives the classification standard of hydraulic fracturing and permeability-enhancing effect in low-permeability coal seams, as shown in Table 2: Table 2 Classification standard of hydraulic fracturing and permeability enhancement effect in low permeability coal seam

. 5 . The method for evaluating the effect of hydraulic fracturing in low permeability coal seams according to claim 1 , wherein the specific steps of step (7) are, according to the judgment result of step (1), such as low permeability The coal seam is suitable for the implementation of hydraulic fracturing technology, and the relevant information of the site to be hydraulic fracturing can be collected, and the evaluation index I of the hydraulic fracturing permeability enhancement effect of the site to be hydraulic fracturing of the coal seam can be calculated based on steps (2) to (5). , according to the classification standard of hydraulic fracturing and permeability-enhancing effect of low-permeability coal seam determined in step (6), evaluate the effect of hydraulic fracturing and permeability-enhancing effect; if the low-permeability coal seam is not suitable for the implementation of hydraulic fracturing technology, the hydraulic fracturing The fracturing and permeability enhancement effect was evaluated as poor.

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