CN107092032B - A method of utilizing well-log information quantitative assessment coal-bed gas exploitation complexity - Google Patents

Abstract

A method of using well-log information quantitative assessment coal-bed gas exploitation complexity, first carries out coal bed gas extraction water yield and build coal bed gas extraction water yield prediction model with log parameter correlation analysis：Coal Pore Structure index, the completion qualitative index of prediction of coal reservoir, calculating coal-bed gas exploitation complexity evaluation number are calculated again, it is final to determine that coal-bed gas exploitation complexity evaluation criterion divides the evaluation of coal-bed gas exploitation complexity：The present invention had not only fully considered influence of the mining water yield to coal-bed gas exploitation complexity, but also had taken into account the influence of Coal Pore Structure and completion qualitative index, and the coal-bed gas exploitation complexity evaluated more is coincide with coal-bed gas exploitation practical condition.

Description

A Method for Quantitatively Evaluating the Difficulty of Coalbed Gas Exploitation Using Well Logging Data

technical field

The invention relates to a logging quantitative evaluation technology in the process of coal bed methane mining, in particular to a method for quantitatively evaluating the difficulty of coal bed methane mining by using logging data.

Background technique

In order to exploit coalbed methane efficiently, it is necessary to evaluate the difficulty of coalbed methane mining. Generally speaking, if the water output after coal reservoir fracturing is small, the pressure reduction and desorption will be fast, which is beneficial to the production of coalbed methane, and the mining of coalbed methane will be easier; if the coal structure is structural coal such as crushed coal and mylonitic coal, Moreover, if the brittleness index is low and the completion quality index is small, it is difficult to successfully fracturing, and even coal powder blocks the fracture channel, making gas extraction more difficult.

As of now, there is no method to quantitatively evaluate the difficulty of coalbed methane mining by using well logging data at home and abroad. In the reports of existing research results, they are only limited to the research on the impact of water drainage and coal reservoir fracturing on the development of coalbed methane. In fact, the difficulty of coalbed methane mining is not only related to the amount of water produced, but also related to the coal body structure and the completion quality index. In the existing evaluation of the difficulty of coalbed methane mining, the coalbed methane logging data has not been fully utilized to calculate the drainage and production water volume, coal body structure and completion quality index, and then quantitatively evaluate the difficulty of coalbed methane mining. Bring inconvenience to the development of coalbed methane.

Contents of the invention

In order to overcome the deficiencies of the above-mentioned existing methods, the object of the present invention is to provide a method for quantitatively evaluating the difficulty of coalbed methane mining by using well logging data. The quantitative evaluation model and standard for the difficulty of coalbed methane mining are established, and the evaluation standard is used to divide the difficulty of coalbed methane mining, which will provide technical support for the efficient development of coalbed methane.

In order to achieve the above object, technical scheme of the present invention is:

A method for quantitatively evaluating the difficulty of mining coalbed methane by using logging data, comprising the following steps:

Step 1. Correlation analysis between coalbed methane drainage production water production and logging parameters: use logging parameters and actual coalbed methane drainage production water production to conduct correlation analysis, and screen out logging parameters that are more sensitive to coalbed methane drainage production water production;

Step 2: Construct the prediction model of coalbed methane drainage production water production: based on step 1, the water production of coalbed methane roof and floor has a good correlation with sandstone thickness, porosity and the distance between sandstone and coal seam. Based on this, the following coal seam roof is constructed Baseplate water discharge prediction model:

In the formula: Q _wtb is the water produced by coalbed methane drainage, m ³ /d; H _tb is the thickness of roof and floor strata, m; Φ is the porosity of sandstone, %; S is the distance between sandstone and coal seam, m; W ₁ , W _2. W ₃ is the weight coefficient of sandstone thickness, porosity and distance from sandstone to coal seam, dimensionless.

From the research on the sensitivity parameters of coal seam drainage itself, it can be seen that the density, acoustic time difference, resistivity and coal seam thickness are closely related to the coal seam water yield. Therefore, the self-drainage of coal seam shown in formula (2) is constructed by using this set of parameters. Water output prediction model:

Q _wc =-7.518-0.375×ρ _b +0.021×Δt-0.184×log(Rt)+0.128×H _c R ² =0.739 (2)

In the formula: Q _wc is the amount of water produced by the coal seam itself, m ³ /d; ρ _b is the bulk density of the coal seam, g/cm ³ ; Δt is the acoustic time difference of the coal seam, μs/m; Rt is the resistivity of the coal seam, Ω .m; H _c is the thickness of the coal seam, m; the dimensions of other parameters are the same as before;

Based on the roof and floor of the coal seam and its own water production, the water production of coalbed methane drainage shown in equation (3) can be obtained.

Q _W ＝Q _wtb +Q _wc (3)

In the formula: Q _W is the total drainage and production volume, m ³ /d; other parameters are the same as above;

Step 3. Calculating the coal structure index: the integrity coefficient of the coal seam can reflect the coal structure to a certain extent, so the integrity coefficient is introduced when constructing the calculation model of the coal structure index. Accordingly, the definition shown in formula (4) Coal structure index logging calculation model.

In the formula: I _CS is the coal structure index, dimensionless; K _v is the integrity coefficient of the coal seam, dimensionless; V _p is the sound velocity of the longitudinal wave of the rock mass, which can be replaced by the sound velocity of the well logging longitudinal wave, m/s; V _r is the The theoretical longitudinal wave sound velocity of the frame, m/s; the dimensions of other parameters are the same as before;

The larger the coal structure index I _CS , the closer the coal rock is to the primary structure coal; the smaller the coal structure index I _CS , the closer the coal rock is to the crushed coal and mylonitic coal.

Step 4. Predict the completion quality index of the coal reservoir: Poisson’s ratio reflects the fracture ability of coal rock under stress, and the elastic modulus reflects the support ability of coal rock after fracture. The higher the elastic modulus, the higher the Poisson’s ratio. The lower the ratio is, the stronger the brittleness of the coal rock is. Therefore, the brittleness index of the coal rock is calculated using formulas (6) to (8).

In the formula: I _BE and I _Bμ are the brittleness index calculated by Young’s modulus and Poisson’s ratio method, %; I _B is the brittleness index of the coal seam, %; E is the Young’s modulus of the coal seam, 10 ⁴ MPa; μ is Poisson's ratio of the coal seam; Δt, Δt _s are the time difference between longitudinal and shear waves of the coal seam, μs/m; the physical meanings of other parameters are the same as before;

The Young's modulus calculated by the P-S wave time difference and density logging data is compared with the Young's modulus of the rock skeleton to characterize the fracture development degree of the coal seam. The calculation model of the fracture development degree index is shown in equation (11):

In the formula: R _F is the fracture development index of the coal seam; E _tma is the Young's modulus value of the coal seam without cracks, MPa; other parameter dimensions are as shown above;

The calculation equation for the horizontal principal stress difference between the coal seam and the roof and floor is shown in formula (12).

Δσ = σ _s - σ _c (12)

In the formula: Δσ is the in-situ stress difference between the coal seam and its roof and floor, MPa; σ _s is the minimum horizontal principal stress of the roof and floor, MPa; σ _c is the minimum horizontal principal stress of the coal seam, MPa; σ _v is the vertical in-situ stress , MPa; a is the Biot coefficient, dimensionless; P _p is the formation pore pressure, MPa; β is the structural stress coefficient, dimensionless; the dimensions of other parameters are as shown above;

Equation (14) is used to calculate the horizontal stress difference coefficient inside the coal seam:

In the formula: K _H is the difference coefficient of the horizontal principal stress of the coal seam, dimensionless; σ ₁ is the maximum horizontal principal stress of the coal seam, MPa; σ ₂ is the minimum horizontal principal stress of the coal seam, MPa.

In favor of the brittleness index, fracture development coefficient, interlayer in-situ stress difference and horizontal stress difference coefficient of the coal seam, the coal seam completion quality index prediction model shown in equation (15) is constructed:

In the formula: I _CP is the completion quality index of the coal seam, dimensionless; the dimensions of other parameters are as shown above.

Step 5. Calculating the evaluation index of the difficulty of coalbed methane mining: based on the calculated water yield, coal structure index and well completion quality index in steps 2 to 4, after normalizing them, considering that 1m has 8 The effects of logging sampling data points, coal seam thickness and roof and floor thickness, and considering that the increase of water content will increase the difficulty of coalbed methane mining, and the coal structure index and completion quality index are high, the successful fracturing is easy, and the equation Quantitative calculation model of evaluation index of coalbed methane mining difficulty shown in (16):

In the formula: I _ER is the evaluation index of coalbed methane mining difficulty, dimensionless; i is the number of logging data points to be calculated, dimensionless; I _CSN , I _CPN , and Q _WN are the normalized coal structure indexes respectively , Completion quality index and water production volume, dimensionless;

Step 6. Determining the evaluation criteria for the degree of difficulty of coalbed methane mining: According to the evaluation index value of the degree of difficulty of coalbed methane mining calculated in step 5, and on the basis of systematically comparing the actual development data of coalbed methane, the coalbed methane shown in Table 1 is given. Difficulty classification standard for gas extraction:

Table 1 CBM mining difficulty evaluation grade classification table

Coalbed methane mining difficulty type Coalbed methane mining difficulty index I _ER easy _IER > 0.8 easier 0.6<I _ER ≤0.8 more difficult 0.4<I _ER ≤0.6 Disaster _IER≤0.4

Step 7. Evaluation of coalbed methane mining difficulty: based on the calculation model of each evaluation index of coalbed methane mining difficulty in steps 2 to 4, and on the basis of compiling processing and interpretation programs, calculate water output, coal structure index and complete Well quality index, and then use the model in Scheme 5 to calculate the evaluation index of coalbed methane mining difficulty, and finally determine the evaluation of coalbed methane mining difficulty according to the evaluation standard of coalbed methane mining difficulty shown in Scheme 6.

For the first time, the present invention proposes a method for quantitatively evaluating the difficulty of coalbed methane mining, which can effectively use logging data to calculate the three indicators of the difficulty of coalbed methane mining, in order to provide a comprehensive analysis of the coalbed methane production process. The development provides technical support for borehole logging, which not only fully considers the influence of water drainage and production on the difficulty of coalbed methane mining, but also takes into account the influence of coal body structure and completion quality index. The evaluated difficulty of coalbed methane mining It is more consistent with the actual production situation of coalbed methane mining.

Description of drawings

Fig. 1 is a flow chart of the method for quantitatively evaluating the ease of mining coalbed methane in the present invention.

Fig. 2 is a graph showing the relationship between the daily water production of coalbed methane and the thickness of sandstone in the present invention.

Fig. 3 is a graph showing the relationship between the daily water production of coalbed methane and the distance from the sandstone to the coal seam in the present invention.

Fig. 4 is a graph showing the relationship between the daily water production of coalbed methane and the porosity in the present invention.

Fig. 5 is a graph showing the relationship between the daily water production and density of the coal seam itself in the present invention.

Fig. 6 is a graph showing the relationship between the daily water production of the coal seam itself and the time difference of sound waves in the present invention.

Fig. 7 is a graph showing the relationship between the daily water production of the coal seam itself and the resistivity in the present invention.

Fig. 8 is a graph showing the relationship between the daily water production of the coal seam itself and the thickness of the coal seam in the present invention.

Fig. 9 is an intersection diagram of borehole diameter and resistivity for identifying coal body structure in the present invention.

Fig. 10 is a cross diagram of density and acoustic wave time difference for identifying coal body structure in the present invention.

Fig. 11 is a diagram of quantitative evaluation results of coalbed methane mining difficulty in the present invention.

Detailed ways

The technical solution of the present invention will be described in detail below in conjunction with the accompanying drawings.

Referring to Figure 1, an evaluation method for quantitatively evaluating the difficulty of coalbed methane exploitation includes the following steps:

Step 1. Correlation analysis between coalbed methane drainage water production and logging parameters: when the immediate roof and floor of the coal seam are sandstone, the physical properties are better, and the thicker the sandstone is, the stronger the water content of the roof and floor sandstone is. The floor sandstone has a large amount of drainage and production water. Taking full account of the mapping ability of geophysical logging technology to the water production of coalbed methane drainage, water production analysis is carried out from the roof and floor and the coal seam itself. Referring to Figures 2 to 5, based on the analysis of the correlation between sandstone thickness, distance from sandstone to coal seam, and porosity and daily water production, it can be concluded that the daily drainage water production is highly sensitive to parameters such as sandstone thickness and porosity, so this set of parameters is used to A prediction model for the water yield of the roof and floor strata during drainage is constructed. Referring to Figures 6 to 9, the analysis of the correlation between coal seam density, acoustic time difference and resistivity, and coal seam thickness and daily water production shows that the relationship between acoustic time difference, coal seam thickness and coal seam water yield is relatively close, and coal seam volume density and resistivity have a significant impact on The water content of the coal seam is also sensitive to a certain extent, so this set of parameters is used to construct the water yield prediction model of the coal seam itself.

Step 2: Construct the prediction model of coalbed methane drainage production water production: based on step 1, the water production of coalbed methane roof and floor has a good correlation with sandstone thickness, porosity and distance from sandstone to coal seam. Based on this, the following coal seam roof and floor water yield prediction model is constructed:

In the formula: Q _wtb is the water produced by coalbed methane drainage, m ³ /d; H _tb is the thickness of the roof and floor rock layers, m; Φ is the porosity of sandstone, %; S is the distance between sandstone and coal seam, m; W ₁ , W ₂ and W ₃ are the weight coefficients of sandstone thickness, porosity and distance from sandstone to coal seam, dimensionless.

From the first step, it is known that the density, acoustic time difference, resistivity and coal seam thickness are closely related to the water yield of the coal seam. Therefore, using this set of parameters, a prediction model for the water yield of the coal seam itself is constructed as shown in formula (2).

Q _wc =-7.518-0.375×ρ _b +0.021×Δt-0.184×log(Rt)+0.128×H _c R ² =0.739 (2)

In the formula: Q _wc is the amount of water produced by the coal seam itself, m ³ /d; ρ _b is the bulk density of the coal seam, g/cm ³ ; Δt is the acoustic time difference of the coal seam, μs/m; Rt is the resistivity of the coal seam, Ω .m; H _c is the thickness of the coal seam, m; other parameters are the same as before.

Based on the roof and floor of the coal seam and its own water production, the water production of coalbed methane drainage shown in equation (3) can be obtained.

Q _W ＝Q _wtb +Q _wc (3)

In the formula: Q _W is the total drainage and production volume, m ³ /d; other parameters are the same as before.

Step 3: Calculating the index of coal structure: structural coal has low mechanical strength, loose coal structure, and cannot be brittlely cracked, so it is difficult to form cracks. When the fracture wall is formed during fracturing, a large amount of coal powder that breaks away and peels off will block the fractures, and the permeability of the coal seam will not be improved. Referring to Fig. 10 and Fig. 11, the resistivity curve of primary structural coal is generally medium-high amplitude, high density, and low acoustic time difference; while the density of structural coal decreases, the resistivity is medium-low, and the acoustic time difference increases. By systematically analyzing the logging response characteristics of primary structure coal, fragmented coal, crushed coal, and mylonitic coal in the study area, it is found that as the coal body structure transitions from primary structure coal to mylonitic coal, the density logging value and resistivity The value decreases, while the sonic moveout and borehole diameter increase. Since the resistivity, density and acoustic time difference are all affected by diameter expansion, the well diameter parameter may not be introduced when constructing the coal structure index logging evaluation model. The integrity coefficient of the coal seam can reflect the coal structure to a certain extent, so the integrity coefficient is introduced when constructing the calculation model of the coal structure index. Accordingly, the coal structure index shown in formula (4) is defined.

In the formula: I _CS is the coal structure index, dimensionless; K _v is the integrity coefficient of the coal seam, dimensionless; V _p is the sound velocity of the longitudinal wave of the rock mass, which can be replaced by the sound velocity of the well logging longitudinal wave, m/s; V _r is the The theoretical longitudinal wave sound velocity of the frame, m/s; the dimensions of other parameters are the same as before.

The larger the coal structure index I _CS , the closer the coal rock is to the primary structure coal; the smaller the coal structure index I _CS , the closer the coal rock is to the crushed coal and mylonitic coal.

Step 4. Predict the completion quality index of the coal reservoir: Poisson's ratio reflects the fracture ability of the coal rock under stress, and the elastic modulus reflects the support ability of the coal rock after fracture. The higher the elastic modulus and the lower the Poisson's ratio, the stronger the brittleness of coal rock. Therefore, formulas (6) to (8) are used to calculate the brittleness index of coal rock.

In the formula: I _BE and I _Bμ are the brittleness index calculated by Young’s modulus and Poisson’s ratio method, %; I _B is the brittleness index of the coal seam, %; E is the Young’s modulus of the coal seam, 10 ⁴ MPa; μ is Poisson's ratio of the coal seam; Δt and Δt _s are the time difference between longitudinal and shear waves of the coal seam, μs/m; the physical meanings of other parameters are the same as before.

The Young's modulus calculated by the compressional and shear wave transit time and density logging data is compared with the Young's modulus of the rock skeleton to characterize the fracture development degree of the coal seam. The calculation model of fracture development degree index is shown in Equation (11).

In the formula: R _F is the fracture development index of the coal seam; E _tma is the Young's modulus value of the rock without fractures, MPa; the dimensions of other parameters are as shown above.

The calculation equation for the horizontal principal stress difference between the coal seam and the roof and floor is shown in formula (12).

Δσ = σ _s - σ _c (12)

In the formula: Δσ is the in-situ stress difference between the coal seam and its roof and floor, MPa; σ _s is the minimum horizontal principal stress of the roof and floor, MPa; σ _c is the minimum horizontal principal stress of the coal seam, MPa; σ _v is the vertical in-situ stress , MPa; a is the Biot coefficient, dimensionless; P _p is the formation pore pressure, MPa; β is the structural stress coefficient, dimensionless; the dimensions of other parameters are as shown above.

Equation (14) is used to calculate the horizontal stress difference coefficient inside the coal seam.

In the formula: K _H is the difference coefficient of the horizontal principal stress of the coal seam, dimensionless; σ ₁ is the maximum horizontal principal stress of the coal seam, MPa; σ ₂ is the minimum horizontal principal stress of the coal seam, MPa.

The fracturing effect of the coal reservoir is directly proportional to the brittleness index of the coal seam and the degree of fracture development; when the in-situ stress difference between the coal seam and its roof and floor is large, the fracturing fractures are easy to be controlled inside the coal seam, and will not communicate with the aquifer of the roof and floor; the coal seam The smaller the difference coefficient of horizontal principal stress, the easier it is for fracturing to form complex network fractures inside the coal seam, which in turn facilitates the drainage and pressure reduction of the coal seam. Based on this understanding, the coal seam completion quality index prediction model shown in Equation (15) was constructed in favor of the coal seam brittleness index, fracture development coefficient, interlayer in-situ stress difference and horizontal stress difference coefficient.

In the formula: I _CP is the completion quality index of the coal seam, dimensionless; the dimensions of other parameters are as shown above.

Step 5. Calculating the evaluation index of the difficulty of coalbed methane mining: based on the calculated water yield, coal structure index and well completion quality index in steps 2 to 4, after normalizing them, considering that 1m has 8 The effects of logging sampling data points, coal seam thickness and roof and floor thickness, and considering that the increase of water content will increase the difficulty of coalbed methane mining, and the coal structure index and completion quality index are high, the successful fracturing is easy, and the equation Quantitative calculation model of evaluation index of coalbed methane mining difficulty shown in (16):

In the formula: I _ER is the evaluation index of coalbed methane mining difficulty, dimensionless; i is the number of logging data points to be calculated, dimensionless; I _CSN , I _CPN , and Q _WN are the normalized coal structure indexes respectively , well completion quality index and water production volume, dimensionless.

Step 6. Determining the evaluation criteria for the degree of difficulty of coalbed methane mining: According to the evaluation index value of the degree of difficulty of coalbed methane mining calculated in step 5, and on the basis of systematically comparing the actual development data of coalbed methane, the coalbed methane shown in Table 1 is given. Difficulty classification standard for gas extraction:

Table 1 CBM mining difficulty evaluation grade classification table

Coalbed methane mining difficulty type Coalbed methane mining difficulty index I _ER easy _IER > 0.8 easier 0.6<I _ER ≤0.8 more difficult 0.4<I _ER ≤0.6 Disaster _IER≤0.4

Step 7. Evaluation of coalbed methane mining difficulty: based on the calculation model of each evaluation index of coalbed methane mining difficulty in steps 2 to 4, and on the basis of compiling processing and interpretation programs, calculate water output, coal structure index and complete Well quality index, and then use the model in Scheme 5 to calculate the evaluation index of coalbed methane mining difficulty, and finally determine the evaluation of coalbed methane mining difficulty according to the evaluation standard of coalbed methane mining difficulty shown in Scheme 6.

The present invention is tested in the actual coal field. In the application of the quantitative evaluation of the difficulty of coalbed methane mining in Well X, referring to Figure 11, the main coalbed methane reservoir section of this well is 573.5-577.4m, the thickness is 3.9m, and there is no obvious gangue in the layer. The coalbed methane reservoir upper 573.5-576.0m section, the logging curve shows that the coal quality is good, and the gas content of the coal core analysis is 8.9-19.4m ³ /t, which shows that the coalbed methane enrichment section. However, the coal structure index and well completion quality index calculated for this coal seam section are small, and the daily water production predicted by well logging is relatively high. The coalbed methane mining difficulty index calculated by the method described in this invention is between 0.4 and 0.6 time, indicating that it is difficult to exploit coalbed methane in this well section. In actual production, the coal seam section was fractured, but after more than three months of drainage, no gas was released. The monitoring of the fracturing effect and the dynamics of drainage show that due to the poor completion quality of the coal seam section, a large amount of coal powder that fell off during the fracturing construction blocked the fractures. The 576.0-577.4m well section in the lower part of the coalbed methane reservoir has low fixed carbon content and high ash content. The gas content calculated by well logging is 6.3-10.2m ³ /t, and the quality of the coalbed methane reservoir is relatively poor; but The coal structure index and completion quality index of this coal seam section are higher than those of the upper coal seam section, and the predicted daily water production is relatively low. The coalbed methane mining difficulty index calculated by the method described in this invention is greater than 0.8, indicating that this well section is easy to mine. CBM. After the fracturing of the upper coal seam section failed, the lower coal seam section was re-fractured. After more than 20 days of drainage after fracturing, the daily gas production was 873 cubic meters. This fully shows that the degree of difficulty of coalbed methane mining quantitatively evaluated in this study is in good agreement with the actual production characteristics.

This method not only fully considers the influence of drainage and production water on the difficulty of coalbed methane mining, but also takes into account the influence of coal body structure and completion quality index. match. Each evaluation index in this method can be obtained from coalfield borehole logging data, and almost all coalfields have a large number of borehole logging data. Therefore, the logging quantitative evaluation method for the ease of coalbed methane exploitation described in the present invention has a good application prospect and value.

Those skilled in the art should understand that since the logging data is easily affected by the drilling environment such as diameter expansion, in order to more accurately evaluate the difficulty of coalbed methane production, it is very necessary to correct the logging data for environmental impact. In addition, the rock mechanics parameters involved in the evaluation of the completion quality index must undergo dynamic and static conversion, so that the quantitative evaluation results of the difficulty of coalbed methane extraction can have high accuracy.

Claims (1)

1. a kind of method using well-log information quantitative assessment coal-bed gas exploitation complexity, which is characterized in that including following step Suddenly：

Step 1: coal bed gas extraction water yield and log parameter correlation analysis：Utilize log parameter and practical coal bed gas extraction Water yield carries out correlation analysis, filters out the more sensitive log parameter of coal bed gas extraction water yield；

Step 2: structure coal bed gas extraction water yield prediction model：Based on step 1 it is found that the water yield of coal bed gas roof and floor with Sandstone thickness, porosity and sandstone have good correlation away from coal seam distance, accordingly, construct following Seam Roof And Floor water outlet Measure prediction model：

In formula：Q_wtbFor coal bed gas extraction water yield, m³/d；H_tbFor adjoining rock thickness, m；Φ is sandstone porosity, %；S For distance of the sandstone away from coal seam, m；W₁、W₂、W₃To be respectively the weight coefficient of sandstone thickness, porosity and sandstone away from coal seam distance, Dimensionless；

It is studied it is found that density, interval transit time, resistivity and coal seam thickness and coal by coal seam mining itself water yield responsive parameter Layer water outlet magnitude relation is more close, then, constructs coal seam itself mining water yield shown in formula (2) using this group of parameter and predicts Model：

Q_wc=-7.518-0.375 × ρ_b+0.021×Δt-0.184×log(Rt)+0.128×H_c R²=0.739 (2)

In formula：Q_wcFor coal seam itself mining water yield, m³/d；ρ_bFor the bulk density in coal seam, g/cm³；Δ t is the sound wave in coal seam The time difference, μ s/m；Rt is the resistivity in coal seam, Ω .m；H_cFor the thickness in coal seam, m；Other parameters dimension is the same；

Based on Seam Roof And Floor and itself mining water yield, coal bed gas extraction water yield shown in equation (3) can be obtained；

Q_W=Q_wtb+Q_wc (3)

In formula：Q_WFor total mining water yield, m³/d；Other parameters dimension is the same；

Step 3: calculating Coal Pore Structure index：The integrity factor in coal seam can reflect Coal Pore Structure to a certain extent, then Integrity factor is introduced when building Coal Pore Structure index computation model, accordingly, Coal Pore Structure index shown in definition (4) is surveyed Well computation model；

In formula：I_CSFor Coal Pore Structure index, dimensionless；K_vFor the integrity factor in coal seam, dimensionless；V_pFor the longitudinal wave sound of rock mass Speed can be replaced, m/s with well logging longitudinal wave velocity；V_rFor the theoretical longitudinal wave velocity of rock matrix, m/s；Other parameters dimension is the same；

Coal Pore Structure index I_CSIt is bigger, show that coal petrography more levels off to primary structure coal；Coal Pore Structure index I_CSIt is smaller, show coal petrography More level off to granulated coal and rotten rib coal；

Step 4: the completion qualitative index of prediction of coal reservoir：Poisson's ratio reflects breakage of the coal petrography under stress, and Elasticity modulus reflects the enabling capabilities after coal petrography rupture, and elasticity modulus is higher, Poisson's ratio is lower, and the brittleness of coal petrography is stronger, in It is that the brittleness index of coal petrography is calculated using formula (6)~formula (8)；

In formula：I_BE、I_BμThe brittleness index that respectively Young's modulus and Poisson's ratio method calculate, %；I_BRefer to for the brittleness in coal seam Number, %；E be coal seam Young's modulus, 10⁴MPa；μ is the Poisson's ratio in coal seam；Δt,Δt_sFor the P-wave And S time difference in coal seam, μ s/ m；Other parameters physical significance is the same；

It is relatively characterized compared with the Young's modulus of rock matrix with the Young's modulus of the longitudinal and shear wave time difference and density log material computation The development degree of micro cracks in oil in coal seam, shown in development degree of micro cracks in oil index computation model such as equation (11)：

In formula：R_FFor the fracture development index in coal seam；E_tmaFor the Young's modulus value in free from flaw coal seam, MPa；Other parameters dimension is such as Shown in preceding；

Shown in coal seam and the horizontal deviator stress accounting equation of roof and floor interlayer such as formula (12)；

Δ σ=σ_s-σ_c (12)

In formula：Ground stress deviations of the Δ σ between coal seam and its roof and floor, MPa；σ_sFor the minimum horizontal principal stress of roof and floor, MPa；σ_c For the minimum horizontal principal stress in coal seam, MPa；σ_vFor vertical crustal stress, MPa；α is Biot coefficients, dimensionless；P_pFor formation pore Pressure, MPa；β is tectonic stress coefficient, dimensionless；Other parameters dimension is as previously shown；

Horizontal stress coefficient of variation inside coal seam is calculated using equation (14)：

In formula：K_HFor the different coefficient of the horizontal deviator stress in coal seam, dimensionless；σ₁For the maximum horizontal principal stress in coal seam, MPa；σ₂For coal The minimum horizontal principal stress of layer, MPa；

Conducive to the brittleness index in coal seam, fracture development coefficient, interlayer ground stress deviation and horizontal stress coefficient of variation, equation is constructed (15) completion qualitative index prediction model in coal seam shown in：

In formula：I_CPFor coal seam completion qualitative index, dimensionless；Other parameters dimension is as previously shown；

Step 5: calculating coal-bed gas exploitation complexity evaluation number：Water yield based on the calculating in step 2~step 4, Coal Pore Structure index and completion qualitative index, after being normalized, it is contemplated that 1m have 8 well logging sampled data points, The influence of coal seam thickness and roof and floor thickness, and in view of water content increase can increase coal-bed gas exploitation difficulty, Coal Pore Structure refers to Number and completion qualitative index value are easy to successfully pressure break when high, construct coal-bed gas exploitation complexity shown in equation (16) and evaluate The quantitative calculation of index：

In formula：I_ERFor coal-bed gas exploitation complexity evaluation number, dimensionless；I is that log data to be calculated is counted, immeasurable Guiding principle；I_CSN、I_CPN、Q_WNCoal Pore Structure index, completion qualitative index and mining water yield after respectively normalizing, dimensionless；

Step 6: determining coal-bed gas exploitation complexity evaluation criterion：According to the coal-bed gas exploitation difficulty or ease journey calculated in step 5 Evaluation number value is spent, on the basis of system coal seam correlation gas actual development data, it is difficult to give coal-bed gas exploitation shown in table 1 The easy intensity grade criteria for classifying：

1 coal-bed gas exploitation complexity opinion rating of table divides table

Coal-bed gas exploitation difficulty type Coal-bed gas exploitation complexity index I_ER Easily I_ER> 0.8 It is easier to 0.6<I_ER≤0.8 It is more difficult 0.4<I_ER≤0.6 It is difficult I_ER≤0.4

Step 7: coal-bed gas exploitation complexity is evaluated：It is each based on the coal-bed gas exploitation complexity in step 2~step 4 A evaluation index computation model calculates water yield, Coal Pore Structure index and completion product on the basis of Directorate Of Organization manages interpretive program Matter index, and then coal-bed gas exploitation complexity evaluation number is calculated using the model in step 5, it is last according to step 6 Shown in coal-bed gas exploitation complexity evaluation criterion, determine the evaluation of evaluated coal-bed gas exploitation complexity.