CN109064016B - Method for evaluating hydraulic fracturing permeability-increasing effect of low-permeability coal seam - Google Patents
Method for evaluating hydraulic fracturing permeability-increasing effect of low-permeability coal seam Download PDFInfo
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Abstract
The invention relates to a method for evaluating the hydraulic fracturing permeability-increasing effect of a low-permeability coal seam, which comprises the following steps of: (1) judging the applicability of the hydraulic fracturing of the low-permeability coal seam; (2) determining coal bed fracability indexI 1 (ii) a (3) Determination of hydraulic fracture propagation indexI 2(ii) a (4) Determination of hydraulic fracture closure indexI 3(ii) a (5) Evaluation index for building permeability-increasing effect of low permeability coal seam hydraulic fracturingIA quantitative calculation model; (6) determining an evaluation standard of the hydraulic fracturing permeability increasing effect of the low-permeability coal seam; (7) and evaluating the hydraulic fracturing permeability-increasing effect of the low-permeability coal seam. The invention has the beneficial effects that: according to the coal seam hydrofracture permeability-increasing mechanism, the model and the method for evaluating the permeability-increasing effect of the low-permeability coal seam hydrofracture are constructed, quantitative evaluation of the permeability-increasing effect of the low-permeability coal seam hydrofracture is achieved, and the method and the device have important significance for accurately selecting a coal seam hydrofracture permeability-increasing area and improving the gas extraction effect of the coal seam.
Description
Technical Field
The invention belongs to the technical field of hydraulic fracturing, and particularly relates to a method for evaluating the permeability increasing effect of low-permeability coal seam hydraulic fracturing.
Background
The coal seam hydraulic fracturing permeability-increasing technology is a method measure for overcoming the minimum ground stress and the tensile strength of a coal rock body by means of the pressure of water injected into a coal seam to enable the weak face of the coal seam to be opened, expanded and extended to form a crack so as to increase the permeability of the coal seam. The result shows that the test effect of only part of mines is better, while the application effect of most of mines is poorer, because the occurrence conditions of coal beds in China are extremely complex, the hydraulic fracturing permeability-increasing technology has certain limitation and blindness. The hydraulic fracturing permeability-increasing technology is a special measure for enhancing permeability-increasing of a coal bed, if effect evaluation is not carried out before implementation, a large amount of manpower and material resources are wasted, and because fracturing does not reach an expected target, coal bed gas extraction is influenced, and serious potential safety hazards are left for subsequent coal mine production.
At present, researchers have recognized the importance of hydraulic fracturing effect evaluation, and adopt a geological strength index or a coal body structure as a criterion for the applicability of a hydraulic fracturing technology. Because the coal seam hydraulic fracturing relates to a plurality of processes such as fracturing, drainage and the like, and is influenced by the mechanical properties of the coal seam, the top and bottom plates and the ground stress, a plurality of influencing factors and a complex influencing mechanism exist. The effect of judging the hydraulic fracturing by adopting the geological strength indexes or the coal body structure is that on one hand, the indexes are single and cannot cover all the influence factors of the hydraulic fracturing; on the other hand, the indexes belong to qualitative description, the judgment needs higher professional technical knowledge and experience, for example, the identification by field technicians has certain difficulty, and certain deviation is inevitably brought to the evaluation of the permeability increasing effect of the hydraulic fracturing.
Disclosure of Invention
In order to solve the defects in the prior art, the method divides the hydraulic fracturing permeability-increasing process of the low-permeability coal seam into three stages of initiation, expansion extension and closure of hydraulic fractures according to the hydraulic fracturing permeability-increasing mechanism of the low-permeability coal seam, provides a coal seam fracturing index, a hydraulic fracture expansion index and a hydraulic fracture closure index based on the three stages and influence factors related to hydraulic fracturing, establishes a low-permeability coal seam hydraulic fracturing permeability-increasing effect evaluation model and a low-permeability coal seam hydraulic fracturing permeability-increasing effect evaluation method, can quantitatively evaluate the hydraulic fracturing permeability-increasing effect of the low-permeability coal seam, improves the hydraulic fracturing effect evaluation precision of the coal seam, provides technical support for selection of a coal seam hydraulic fracturing permeability-increasing area, and has important significance for popularization and application of a hydraulic fracturing permeability-increasing technology and improvement of coal seam gas extraction effect.
In order to solve the technical problems, the invention adopts the following technical scheme: a method for evaluating the hydraulic fracturing permeability-increasing effect of a low-permeability coal seam comprises the following steps,
(1) judging the applicability of the hydraulic fracturing of the low-permeability coal seam;
(2) determining the fracability index of the coal seamI 1 ；
(3) Determining hydraulic fracture propagation indexI 2；
(4) Determining hydraulic fracture closure indexI 3；
(5) And (3) utilizing the coal bed fracturing index obtained in the step (2)I 1The hydraulic fracture propagation index obtained in the step (3)I 2And the hydraulic fracture closure index obtained in step (4)I 3And constructing evaluation index of permeability-increasing effect of low permeability coal seam hydraulic fracturingIA quantitative calculation model;
(6) determining an evaluation standard of the hydraulic fracturing permeability increasing effect of the low-permeability coal seam;
(7) and evaluating the hydraulic fracturing permeability-increasing effect of the low-permeability coal seam.
Further, the specific process of the step (1) is to judge the applicability of the hydraulic fracturing of the low-permeability coal seam according to table 1:
TABLE 1 suitability criterion for hydraulic fracturing of low permeability coal seams
If the geological strength index of the low permeability coal seamGSICoefficient of firmness of not less than 30fThe coal body structure is primary structure coal or cracked coal, so that the low-permeability coal bed is suitable for implementation of a hydraulic fracturing technology, and the hydraulic fracturing permeability-increasing effect of the low-permeability coal bed can be evaluated continuously according to the steps (2) to (7);
if the geological strength index of the low permeability coal seamGSI< 30 or a factor of robustnessfIf the coal body structure is less than 0.4 or the coal body structure is broken coal or minced coal, the low-permeability coal bed is not suitable for implementation of a hydraulic fracturing technology, and the hydraulic fracturing permeability-increasing effect of the low-permeability coal bed is directly evaluated according to the step (7).
Further, the concrete step of step (2) is that the fracturability of the coal body means that the coal seam has the property of being effectively fractured to form cracks so as to increase permeability, and the mechanical characteristics of the coal have the fracturability of the coal seamThe method has important functions and influences, and generally, the larger the elastic modulus of coal is, the larger the geological strength index of a coal bed is, the smaller the Poisson ratio is, and the better the crushability of the coal bed is; calculating the coal bed fracturing index by adopting the formulas (1) to (4)I 1：
In the formula,I 1is the coal bed fracturable index;E n andv n respectively, elastic modulus index and Poisson ratio index;E maxandE minmaximum and minimum elastic moduli within the coal seam, respectively;v maxandv minmaximum and minimum poisson's ratio in the coal seam, respectively;GSIthe value range of the geological strength index of the coal seam is 0-100;GSI n the geological strength index of the coal bed is obtained;Eis the modulus of elasticity of the coal body,vis the poisson ratio of the coal body.
Further, the concrete step in the step (3) is that, in the hydraulic fracture forming process, fracture toughness, a horizontal principal stress difference coefficient and a vertical principal stress difference coefficient of the coal body are main factors influencing the hydraulic fracture expansion and extension, and the lower the fracture toughness of the coal body is, the more beneficial the hydraulic fracture expansion and extension is; the larger the difference coefficient of the horizontal principal stress is, the more beneficial the extension of the hydraulic fracture is, and the larger the range of the hydraulic fracture is; the smaller the difference coefficient of the vertical principal stress is, the moreThe formation of a network of network cracks is facilitated; the type I fracture toughness of the coal body is calculated by adopting the formula (5)K IC ：
In the formula,K IC type I fracture toughness;σ t the tensile strength of the coal rock mass is shown;a、bthe tensile strength and the type I fracture toughness of the coal rock sample measured in the collecting area can be used as coefficientsK IC Then determining the coefficients using a linear fitting methodaAndb；
the horizontal principal stress difference coefficient within the coal seam is calculated using equation (6):
in the formula,K H is the coefficient of difference of the horizontal principal stresses,σ 1in order to be the maximum horizontal principal stress,σ 2is the minimum horizontal principal stress;
the vertical principal stress difference coefficient within the coal seam is calculated using equation (7):
in the formula,K V is the coefficient of the difference in the vertical principal stresses,σ H in order to be the vertical principal stress,σ 2is the minimum horizontal principal stress;
toughness to type I fractureK IC Coefficient of horizontal principal stress differenceK H Coefficient of difference from vertical principal stressK V Normalization processing is carried out to obtain the hydraulic fracture propagation indexI 2：
In the formula,K Icn is a type I fracture toughness index,K Hn is an index of the coefficient of difference of the horizontal principal stresses,K Vn is a vertical principal stress difference coefficient index;K ICmaxandK ICminmaximum and minimum type i fracture toughness in the coal seam, respectively;K HmaxandK Hminmaximum and minimum horizontal principal stress difference coefficients within the coal seam, respectively;K VmaxandK Vminmaximum and minimum vertical principal stress difference coefficients in the coal seam, respectively;I 2is the hydraulic fracture propagation index.
Further, the concrete step of the step (4) is that along with the discharge of water in the coal bed in the drainage process after fracturing, the pore fluid pressure in the coal bed is reduced, the effective stress is increased, and hydraulic fractures in the coal bed can be closed to different degrees. The main factors influencing the closing of the coal seam hydraulic fracture comprise coal body uniaxial compressive strength, a coal firmness coefficient, minimum horizontal principal stress and coal seam burial depth, the larger the coal body uniaxial compressive strength and the coal firmness coefficient are, the stronger the resistance of the coal body to fracture closure is represented, the more adverse the closing of the coal seam hydraulic fracture is, the larger the minimum horizontal principal stress and the coal seam burial depth are, the larger external force applied to the coal body fracture closure is represented, and the more favorable the closing of the coal seam hydraulic fracture is; calculating the closing index of the coal seam hydraulic fracture by adopting the formulas (12) to (16)I 3：
In the formula,σ Cn the coal body uniaxial compressive strength index;f n the factor index of the firmness of the coal body is obtained;σ n2is the minimum level principal stress index;H n the coal seam burial depth index is obtained;I 3is hydraulic fracture closure index;σ Cmaxandσ Cminthe maximum uniaxial compressive strength and the minimum uniaxial compressive strength of the coal body are respectively;f maxandf minrespectively a maximum firmness coefficient and a minimum firmness coefficient of the coal body;σ 2maxandσ 2minrespectively the maximum minimum horizontal principal stress and the minimum horizontal principal stress of the coal bed;H maxandH minrespectively the maximum buried depth and the minimum buried depth of the coal bed; sigmaCThe coal body has the uniaxial compressive strength,fis the coal solidity coefficient, σ2The minimum horizontal principal stress of the coal seam in the hydraulic fracturing area,His the depth of the coal seam.
Further, the specific step of the step (5) isAccording to the coal bed fracability index in the step (2)I 1The hydraulic fracture propagation index in the step (3)I 2And hydraulic fracture closure index in step (4)I 3And constructing evaluation index of permeability-increasing effect of low permeability coal seam hydraulic fracturingIA quantitative calculation model:
in the formula,Ievaluating indexes of the hydraulic fracturing permeability-increasing effect of the low-permeability coal seam;w 1、w 2、w 3respectively the weight of the coal bed fracturing index, the hydraulic fracture expansion index and the hydraulic fracture closure index on the influence of the hydraulic fracturing effect,w 1+w 2+w 3=1，w 1、w 2、w 3can be determined by adopting an analytic hierarchy process.
Further, the specific step of the step (6) is to provide a classification standard of the permeability increasing effect level of the low-permeability coal seam hydraulic fracturing according to the index value of the permeability increasing effect of the low-permeability coal seam hydraulic fracturing calculated in the step (5) and by combining the implementation situation of the low-permeability coal seam hydraulic fracturing, as shown in table 2:
TABLE 2 grading Standard for permeability-increasing Effect of hydraulic fracturing of low permeability coal seam
Further, the specific step of the step (7) is that according to the determination result of the step (1), if the low permeability coal seam is suitable for implementation of the hydraulic fracturing technology, relevant data of a place where hydraulic fracturing is to be performed can be collected, and an evaluation index of the permeability increasing effect of the hydraulic fracturing of the place where the hydraulic fracturing of the coal seam is to be performed is calculated based on the steps (2) to (5)IEvaluating the hydraulic fracturing permeability increasing effect according to the classification standard of the hydraulic fracturing permeability increasing effect grade of the low-permeability coal seam determined in the step (6); for example, low permeability coal seams are not suitable for hydraulic fracturing technologyAnd (3) evaluating the hydraulic fracturing permeability-increasing effect of the low-permeability coal seam as poor.
By adopting the technical scheme, the invention has the beneficial effects that: according to the method, the hydraulic fracturing permeability-increasing process is divided into three stages of hydraulic fracture initiation, expansion extension and closure according to the coal seam hydraulic fracturing permeability-increasing mechanism, factors influencing hydraulic fracture evolution in each stage are fully considered, the coal seam fracturing index, the hydraulic fracture expansion index and the hydraulic fracture closure index are provided, a low-permeability coal seam hydraulic fracturing permeability-increasing effect evaluation model and method are constructed, quantitative evaluation of the low-permeability coal seam hydraulic fracturing permeability-increasing effect is achieved, and the method has important significance for accurately selecting a coal seam hydraulic fracturing permeability-increasing area and improving the coal seam gas extraction effect.
Drawings
FIG. 1 is a schematic flow diagram of the present invention.
Detailed Description
As shown in FIG. 1, the method for evaluating the permeability increasing effect of the hydraulic fracturing of the low-permeability coal seam comprises the following steps,
(1) judging the applicability of the hydraulic fracturing of the low-permeability coal seam;
(2) determining the fracability index of the coal seamI 1 ；
(3) Determining hydraulic fracture propagation indexI 2；
(4) Determining hydraulic fracture closure indexI 3；
(5) And (3) utilizing the coal bed fracturing index obtained in the step (2)I 1The hydraulic fracture propagation index obtained in the step (3)I 2And the hydraulic fracture closure index obtained in step (4)I 3And constructing evaluation index of permeability-increasing effect of low permeability coal seam hydraulic fracturingIA quantitative calculation model;
(6) determining an evaluation standard of the hydraulic fracturing permeability increasing effect of the low-permeability coal seam;
(7) and evaluating the hydraulic fracturing permeability-increasing effect of the low-permeability coal seam.
Further, the specific process of the step (1) is to judge the applicability of the hydraulic fracturing of the low-permeability coal seam according to table 1:
TABLE 1 suitability criterion for hydraulic fracturing of low permeability coal seams
If the geological strength index of the low permeability coal seamGSICoefficient of firmness of not less than 30fThe coal body structure is primary structure coal or cracked coal, so that the low-permeability coal bed is suitable for implementation of a hydraulic fracturing technology, and the hydraulic fracturing permeability-increasing effect of the low-permeability coal bed can be evaluated continuously according to the steps (2) to (7);
if the geological strength index of the low permeability coal seamGSI< 30 or a factor of robustnessfIf the coal body structure is less than 0.4 or the coal body structure is broken coal or minced coal, the low-permeability coal bed is not suitable for implementation of a hydraulic fracturing technology, and the hydraulic fracturing permeability-increasing effect of the low-permeability coal bed is directly evaluated according to the step (7).
Furthermore, the concrete step of step (2) is that the fracturability of the coal body means that the coal seam has the property of being effectively fractured to form cracks so as to increase permeability, the mechanical properties of the coal have important effects and influences on the fracturability of the coal seam, and generally, the larger the elastic modulus of the coal is, the larger the geological strength index of the coal seam is, the smaller the poisson's ratio is, the better the fracturability of the coal seam is; calculating the coal bed fracturing index by adopting the formulas (1) to (4)I 1：
In the formula,I 1is the coal bed fracturable index;E n andv n respectively, elastic modulus index and Poisson ratio index;E maxandE minmaximum and minimum elastic moduli within the coal seam, respectively;v maxandv minmaximum and minimum poisson's ratio in the coal seam, respectively;GSIthe value range of the geological strength index of the coal seam is 0-100;GSI n the geological strength index of the coal bed is obtained;Eis the modulus of elasticity of the coal body,vis the poisson ratio of the coal body.
Further, the concrete step in the step (3) is that, in the hydraulic fracture forming process, fracture toughness, a horizontal principal stress difference coefficient and a vertical principal stress difference coefficient of the coal body are main factors influencing the hydraulic fracture expansion and extension, and the lower the fracture toughness of the coal body is, the more beneficial the hydraulic fracture expansion and extension is; the larger the difference coefficient of the horizontal principal stress is, the more beneficial the extension of the hydraulic fracture is, and the larger the range of the hydraulic fracture is; the smaller the difference coefficient of the vertical main stress is, the more favorable the formation of a reticular fracture network is; the type I fracture toughness of the coal body is calculated by adopting the formula (5)K IC ：
In the formula,K IC type I fracture toughness;σ t the tensile strength of the coal rock mass is shown;a、bthe tensile strength and the type I fracture toughness of the coal rock sample measured in the collecting area can be used as coefficientsK IC Then determining the coefficients using a linear fitting methodaAndb；
the horizontal principal stress difference coefficient within the coal seam is calculated using equation (6):
in the formula,K H is the coefficient of difference of the horizontal principal stresses,σ 1in order to be the maximum horizontal principal stress,σ 2is the minimum horizontal principal stress;
the vertical principal stress difference coefficient within the coal seam is calculated using equation (7):
in the formula,K V is the coefficient of the difference in the vertical principal stresses,σ H in order to be the vertical principal stress,σ 2is the minimum horizontal principal stress;
toughness to type I fractureK IC Coefficient of horizontal principal stress differenceK H Coefficient of difference from vertical principal stressK V Normalization processing is carried out to obtain the hydraulic fracture propagation indexI 2：
In the formula,K Icn is a type I fracture toughness index,K Hn is an index of the coefficient of difference of the horizontal principal stresses,K Vn is the coefficient of difference of vertical principal stressAn index;K ICmaxandK ICminmaximum and minimum type i fracture toughness in the coal seam, respectively;K HmaxandK Hminmaximum and minimum horizontal principal stress difference coefficients within the coal seam, respectively;K VmaxandK Vminmaximum and minimum vertical principal stress difference coefficients in the coal seam, respectively;I 2is the hydraulic fracture propagation index.
Further, the concrete step of the step (4) is that along with the discharge of water in the coal bed in the drainage process after fracturing, the pore fluid pressure in the coal bed is reduced, the effective stress is increased, and hydraulic fractures in the coal bed can be closed to different degrees. The main factors influencing the closing of the coal seam hydraulic fracture comprise coal body uniaxial compressive strength, a coal firmness coefficient, minimum horizontal principal stress and coal seam burial depth, the larger the coal body uniaxial compressive strength and the coal firmness coefficient are, the stronger the resistance of the coal body to fracture closure is represented, the more adverse the closing of the coal seam hydraulic fracture is, the larger the minimum horizontal principal stress and the coal seam burial depth are, the larger external force applied to the coal body fracture closure is represented, and the more favorable the closing of the coal seam hydraulic fracture is; calculating the closing index of the coal seam hydraulic fracture by adopting the formulas (12) to (16)I 3：
In the formula,σ Cn the coal body uniaxial compressive strength index;f n the factor index of the firmness of the coal body is obtained;σ n2is the minimum level principal stress index;H n the coal seam burial depth index is obtained;I 3is hydraulic fracture closure index;σ Cmaxandσ Cminthe maximum uniaxial compressive strength and the minimum uniaxial compressive strength of the coal body are respectively;f maxandf minrespectively a maximum firmness coefficient and a minimum firmness coefficient of the coal body;σ 2maxandσ 2minrespectively the maximum minimum horizontal principal stress and the minimum horizontal principal stress of the coal bed;H maxandH minrespectively the maximum buried depth and the minimum buried depth of the coal bed; sigmaCThe coal body has the uniaxial compressive strength,fis the coal solidity coefficient, σ2The minimum horizontal principal stress of the coal seam in the hydraulic fracturing area,Hthe burial depth of the coal seam; the larger the compression strength of the coal body and the firmness coefficient of the coal are, the more unfavorable the closure of the hydraulic fracture of the coal bed is, the larger the minimum horizontal principal stress and the coal bed burial depth are, and the more favorable the closure of the hydraulic fracture of the coal bed is.
Further, the concrete step of the step (5) is that the fracability index of the coal bed in the step (2) is determinedI 1The hydraulic fracture propagation index in the step (3)I 2And hydraulic fracture closure index in step (4)I 3And constructing evaluation index of permeability-increasing effect of low permeability coal seam hydraulic fracturingIA quantitative calculation model:
in the formula,Ievaluating indexes of the hydraulic fracturing permeability-increasing effect of the low-permeability coal seam;w 1、w 2、w 3respectively fracturing the coal seamThe number, the hydraulic fracture propagation index and the hydraulic fracture closure index are the weight of the influence on the hydraulic fracturing effect,w 1+w 2+w 3=1，w 1、w 2、w 3can be determined by adopting an analytic hierarchy process.
Further, the specific step of the step (6) is to provide a classification standard of the permeability increasing effect level of the low-permeability coal seam hydraulic fracturing according to the index value of the permeability increasing effect of the low-permeability coal seam hydraulic fracturing calculated in the step (5) and by combining the implementation situation of the low-permeability coal seam hydraulic fracturing, as shown in table 2:
TABLE 2 grading Standard for permeability-increasing Effect of hydraulic fracturing of low permeability coal seam
Further, the specific step of the step (7) is that according to the determination result of the step (1), if the low permeability coal seam is suitable for implementation of the hydraulic fracturing technology, relevant data of a place where hydraulic fracturing is to be performed can be collected, and an evaluation index of the permeability increasing effect of the hydraulic fracturing of the place where the hydraulic fracturing of the coal seam is to be performed is calculated based on the steps (2) to (5)IEvaluating the hydraulic fracturing permeability increasing effect according to the classification standard of the hydraulic fracturing permeability increasing effect grade of the low-permeability coal seam determined in the step (6); and if the low-permeability coal seam is not suitable for implementing the hydraulic fracturing technology, evaluating the permeability increasing effect of the low-permeability coal seam as poor.
The following description will be given by taking the evaluation of the permeability increasing effect of a hydraulic fracturing site of a certain coal seam of a certain mine as an example:
step (1): collecting coal samples at a place where hydraulic fracturing is to be carried out, testing to obtain a coal body structure of a low-permeability coal layer, wherein the coal body structure is cracked coal and has geological strength indexesGSI=70, coefficient of robustnessf= 1.2. According to the table 1, the site is suitable for implementation of a hydraulic fracturing technology, and the hydraulic fracturing permeability-increasing effect of the low-permeability coal seam can be evaluated according to the steps (2) to (7);
step (2): collecting coal samples at the place where hydraulic fracturing is to be carried outTesting coal rock mechanical parameters to obtain coal elastic modulus, geological strength index and Poisson's ratio, and measuringE=1500MPa，GSI=70 andvand (4) calculating the coal bed fracturable index by substituting the formula (1) to the formula (4) =0.3I 1. Wherein,E max=2000MPa, andE min=800MPa；v max=0.4 andv min= 0.25. Through calculationI 1=0.65。
And (3): the data of the I-type fracture toughness and the tensile strength of the coal rock mass measured in the collecting region are subjected to linear regression analysis to obtaina=0.03，b= 0.12; then collecting coal samples at the place where hydraulic fracturing is to be carried out to carry out coal-rock mechanical parameter testing to obtain coal-rock body tensile strength parameters, and measuringσ t (ii) =2MPa, and substituting the same into the formula (5) can obtain type I fracture toughnessK IC Can be obtained by calculationK IC =0.18MPa。
Collecting data of maximum horizontal principal stress and minimum horizontal principal stress of a place to be subjected to hydraulic fracturing, wherein the maximum horizontal principal stressσ 1Is 12MPa, minimum horizontal principal stressσ 2At 8MPa, the horizontal principal stress difference coefficient is calculated by the formula (6)K H . The calculation results show that the method can be used,K H =0.5。
collecting the vertical principal stress of the site to be subjected to hydraulic fracturingσ H Is 10MPa, minimum horizontal principal stressσ 2At 8MPa, the vertical principal stress difference coefficient is calculated by the formula (7)K V . The calculation results show that the method can be used,K V =0.25。
the type I fracture toughness is measured by adopting formulas (8) to (11)K IC Horizontal principal stress difference coefficient and vertical principal stress difference coefficientK V Normalization processing is carried out to obtain the hydraulic fracture propagation indexI 2. Wherein,K ICmax=0.27，K ICmin=0.12；K Hmax=0.8，K Hmin=0.2；K Vmax=0.5，K Vmin= 0.1. The calculation results show that the method can be used,I 2=0.575。
and (4): collecting coal samples at the place where hydraulic fracturing is to be carried out, and carrying out coal rock uniaxial compressive strength test and firmness coefficient test to obtainσ C =15MPa，f= 1.2; collecting relevant data according to the situation of the place to be subjected to hydraulic fracturing to obtainσ 2=8MPa，H=450 m. Substituting the values into the formulas (12) to (16) can calculate the closing index of the coal seam hydraulic fractureI 3. Whereinσ Cmax=20MPa，σ Cmin=3MPa；f max=1.5，f min=0.2；σ 2max=12MPa，σ 2min=5MPa；H max=800m，H min=400 m. The calculation results show that the method can be used,I 3=0.7。
and (5): firstly, determining the weight of the influence of the coal bed fracturing index, the hydraulic fracture expansion index and the hydraulic fracture closure index on the hydraulic fracturing effect by adopting an analytic hierarchy process to obtainw 1=0.3、w 2=0.3、w 3= 0.4; then calculating the coal bed fracturing index according to the step (2)I 1And (3) calculating the hydraulic fracture propagation indexI 2And the hydraulic fracture closure index calculated in the step (4)I 3Calculating the evaluation index of the permeability increasing effect of the hydraulic fracturing of the low-permeability coal seam by adopting the formula (17)I. The calculation results show that the method can be used,I=0.648。
and (6): according to the implementation situation of the hydraulic fracturing of the low-permeability coal seam of the mine, giving a grading standard of the hydraulic fracturing permeability-increasing effect of the low-permeability coal seam (table 2):
TABLE 2 grading Standard for permeability-increasing Effect of hydraulic fracturing of low permeability coal seam
And (7): the place to be subjected to hydraulic fracturing is suitable for hydraulic fracturing technologyThe implementation of the technology is calculated, and the evaluation index of the permeability increasing effect of the low-permeability coal seam hydraulic fracturingI=0.655, the permeability increase effect of the site to be subjected to hydraulic fracturing can be evaluated to be moderate in comparison with table 2.
The hydraulic fracturing test is carried out at an evaluation site, after the test is finished, the hole sealing networking is carried out for extraction, the daily extraction amount can reach 95 cubic meters after the test is carried out, the daily extraction amount is 3.3 times of the daily extraction amount when the hole sealing is not fractured, the hydraulic fracturing permeability increasing effect is moderate, and the calculation result is identical with the calculation result of the method. The method has higher accuracy, can predict the hydraulic fracturing permeability-increasing effect of the low-permeability coal seam, and can provide early-stage guidance for selecting the hydraulic fracturing permeability-increasing area of the low-permeability coal seam.
The present embodiment is not intended to limit the shape, material, structure, etc. of the present invention in any way, and any simple modification, equivalent change and modification made to the above embodiments according to the technical spirit of the present invention are within the scope of the technical solution of the present invention.
Claims (5)
1. A method for evaluating the hydraulic fracturing permeability-increasing effect of a low-permeability coal seam is characterized by comprising the following steps of: comprises the following steps of (a) carrying out,
(1) judging the applicability of the hydraulic fracturing of the low-permeability coal seam;
(2) determining the fracability index of the coal seamI 1 ；
(3) Determining hydraulic fracture propagation indexI 2；
(4) Determining hydraulic fracture closure indexI 3；
(5) And (3) utilizing the coal bed fracturing index obtained in the step (2)I 1The hydraulic fracture propagation index obtained in the step (3)I 2And the hydraulic fracture closure index obtained in step (4)I 3And constructing evaluation index of permeability-increasing effect of low permeability coal seam hydraulic fracturingIA quantitative calculation model;
(6) determining an evaluation standard of the hydraulic fracturing permeability increasing effect of the low-permeability coal seam;
(7) evaluating the hydraulic fracturing permeability-increasing effect of the low-permeability coal seam;
the steps areThe concrete step of the step (2) is that the fracturability of the coal body means that the coal seam has the property of being effectively fractured to form cracks so as to increase permeability, the mechanical property of the coal has important effect and influence on the fracturability of the coal seam, and generally, the larger the elastic modulus of the coal is, the larger the geological strength index of the coal seam is, the smaller the poisson ratio is, and the better the fracturability of the coal seam is; calculating the coal bed fracturing index by adopting the formulas (1) to (4)I 1：
In the formula,I 1is the coal bed fracturable index;E n andv n respectively, elastic modulus index and Poisson ratio index;E maxandE minmaximum and minimum elastic moduli within the coal seam, respectively;v maxandv minmaximum and minimum poisson's ratio in the coal seam, respectively;GSIthe value range of the geological strength index of the coal seam is 0-100;GSI n the geological strength index of the coal bed is obtained;Eis the modulus of elasticity of the coal body,vthe Poisson ratio of the coal body;
the concrete step of the step (3) is that in the hydraulic fracture forming process, the fracture toughness, the horizontal principal stress difference coefficient and the vertical principal stress difference coefficient of the coal body are main factors influencing the hydraulic fracture expansion and extension, and the fracture toughness, the horizontal principal stress difference coefficient and the vertical principal stress difference coefficient of the coal body are main factors influencing the hydraulic fracture expansion and extensionThe lower the fracture toughness, the more beneficial to the expansion and extension of the hydraulic fracture; the larger the difference coefficient of the horizontal principal stress is, the more beneficial the extension of the hydraulic fracture is, and the larger the range of the hydraulic fracture is; the smaller the difference coefficient of the vertical main stress is, the more favorable the formation of a reticular fracture network is; the type I fracture toughness of the coal body is calculated by adopting the formula (5)K IC ：
In the formula,K IC type I fracture toughness;σ t the tensile strength of the coal rock mass is shown;a、bthe tensile strength and the type I fracture toughness of the coal rock sample measured in the collecting area can be used as coefficientsK IC Then determining the coefficients using a linear fitting methodaAndb；
the horizontal principal stress difference coefficient within the coal seam is calculated using equation (6):
in the formula,K H is the coefficient of difference of the horizontal principal stresses,σ 1in order to be the maximum horizontal principal stress,σ 2is the minimum horizontal principal stress;
the vertical principal stress difference coefficient within the coal seam is calculated using equation (7):
in the formula,K V is the coefficient of the difference in the vertical principal stresses,σ H in order to be the vertical principal stress,σ 2is the minimum horizontal principal stress;
toughness to type I fractureK IC Coefficient of horizontal principal stress differenceK H And perpendicular principal stressCoefficient of differenceK V Normalization processing is carried out to obtain the hydraulic fracture propagation indexI 2：
In the formula,K Icn is a type I fracture toughness index,K Hn is an index of the coefficient of difference of the horizontal principal stresses,K Vn is a vertical principal stress difference coefficient index;K ICmaxandK ICminmaximum and minimum type i fracture toughness in the coal seam, respectively;K HmaxandK Hminmaximum and minimum horizontal principal stress difference coefficients within the coal seam, respectively;K VmaxandK Vminmaximum and minimum vertical principal stress difference coefficients in the coal seam, respectively;I 2is the hydraulic fracture propagation index;
the concrete step of the step (4) is that along with the discharge of water in the coal bed in the drainage process after fracturing, the pressure of pore fluid in the coal bed is reduced, the effective stress is increased, and hydraulic fractures in the coal bed can be closed to different degrees;
the main factors influencing the closing of the coal seam hydraulic fracture comprise coal body uniaxial compressive strength, coal firmness coefficient, minimum horizontal principal stress and coal seam burial depth, the larger the coal body uniaxial compressive strength and the coal firmness coefficient are, and the representationThe stronger the resistance of the coal body to fracture closure, the more unfavorable the closure of the coal seam hydraulic fracture, the larger the minimum horizontal principal stress and the coal seam burial depth are, the larger the external force representing the closure of the coal seam fracture is, the more favorable the closure of the coal seam hydraulic fracture is; calculating the closing index of the coal seam hydraulic fracture by adopting the formulas (12) to (16)I 3：
In the formula,σ Cn the coal body uniaxial compressive strength index;f n the factor index of the firmness of the coal body is obtained;σ n2is the minimum level principal stress index;H n the coal seam burial depth index is obtained;I 3is hydraulic fracture closure index;σ Cmaxandσ Cminthe maximum uniaxial compressive strength and the minimum uniaxial compressive strength of the coal body are respectively;f maxandf minrespectively a maximum firmness coefficient and a minimum firmness coefficient of the coal body;σ 2maxandσ 2minrespectively the maximum minimum horizontal principal stress and the minimum horizontal principal stress of the coal bed;H maxandH minrespectively the maximum buried depth and the minimum buried depth of the coal bed; sigmaCThe coal body has the uniaxial compressive strength,fis the coal solidity coefficient, σ2The minimum horizontal principal stress of the coal seam in the hydraulic fracturing area,His the depth of the coal seam.
2. The method for evaluating the hydraulic fracturing permeability-increasing effect of the low-permeability coal seam according to claim 1, wherein the method comprises the following steps: the concrete process of the step (1) is that the applicability of the hydraulic fracturing of the low-permeability coal seam is judged according to the following table 1:
TABLE 1 suitability criterion for hydraulic fracturing of low permeability coal seams
If the geological strength index of the low permeability coal seamGSICoefficient of firmness of not less than 30fThe coal body structure is primary structure coal or cracked coal, so that the low-permeability coal bed is suitable for implementation of a hydraulic fracturing technology, and the hydraulic fracturing permeability-increasing effect of the low-permeability coal bed can be evaluated continuously according to the steps (2) to (7);
if the geological strength index of the low permeability coal seamGSI< 30 or a factor of robustnessfIf the coal body structure is less than 0.4 or the coal body structure is broken coal or minced coal, the low-permeability coal bed is not suitable for implementation of a hydraulic fracturing technology, and the hydraulic fracturing permeability-increasing effect of the low-permeability coal bed is directly evaluated according to the step (7).
3. The method for evaluating the hydraulic fracturing permeability-increasing effect of the low-permeability coal seam according to claim 1, wherein the method comprises the following steps: the concrete step of the step (5) is that the fracturable index of the coal bed in the step (2) is determinedI 1The hydraulic fracture propagation index in the step (3)I 2And hydraulic fracture closure index in step (4)I 3And constructing evaluation index of permeability-increasing effect of low permeability coal seam hydraulic fracturingIA quantitative calculation model:
in the formula,Ievaluating indexes of the hydraulic fracturing permeability-increasing effect of the low-permeability coal seam;w 1、w 2、w 3respectively the weight of the coal bed fracturing index, the hydraulic fracture expansion index and the hydraulic fracture closure index on the influence of the hydraulic fracturing effect,w 1+w 2+w 3=1，w 1、w 2、w 3can be determined by adopting an analytic hierarchy process.
4. The method for evaluating the hydraulic fracturing permeability-increasing effect of the low-permeability coal seam according to claim 1, wherein the method comprises the following steps: the concrete steps of the step (6) are that according to the evaluation index value of the hydraulic fracturing permeability-increasing effect of the low-permeability coal seam calculated in the step (5), the classification standard of the hydraulic fracturing permeability-increasing effect of the low-permeability coal seam is given by combining the implementation situation of the hydraulic fracturing of the low-permeability coal seam, and the classification standard is shown in table 2:
TABLE 2 grading Standard for permeability-increasing Effect of hydraulic fracturing of low permeability coal seam
5. The method for evaluating the hydraulic fracturing permeability-increasing effect of the low-permeability coal seam according to claim 1, wherein the method comprises the following steps: the concrete step of the step (7) is that according to the judgment result of the step (1), if the low-permeability coal seam is suitable for implementation of the hydraulic fracturing technology, relevant data of a place where hydraulic fracturing is to be performed can be collected, and the evaluation index of the anti-reflection effect of the hydraulic fracturing of the place where the hydraulic fracturing of the coal seam is to be performed is calculated based on the steps (2) to (5)IEvaluating the hydraulic fracturing permeability increasing effect according to the classification standard of the hydraulic fracturing permeability increasing effect grade of the low-permeability coal seam determined in the step (6); for example, low permeability coal seams are not suitable for implementation of hydraulic fracturing technologyAnd evaluating the hydraulic fracturing permeability-increasing effect of the low-permeability coal seam as poor.
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