CN111353218B - Logging quantitative evaluation method for coal bed gas-dense gas reservoir compaction property - Google Patents

Abstract

A logging quantitative evaluation method for coalbed methane-dense gas reservoir compaction property comprises the following steps: calculating the brittleness index difference between the sand layer and the coal bed; step two: calculating the minimum horizontal ground stress difference between the sand layer and the coal bed; step three: calculating the tensile strength difference between the sandstone and the coal rock; step four: calculating the Young modulus difference of the sandstone and the coal rock; and finally, evaluating the compressibility of the coal bed gas-dense gas reservoir by using the four evaluation indexes according to the brittleness index difference between the sand layer and the coal bed, the minimum horizontal main stress difference between the sandstone and the coal rock, the tensile strength difference between the sandstone and the coal rock and the Young modulus difference between the sand layer and the coal bed, so that the logging technical support is preferably provided for the fracturing layer of the coal bed gas-dense gas reservoir while the quantitative evaluation precision of the compressibility logging of the coal bed gas-dense gas reservoir is improved, and the method has the characteristics of simplicity and practicability.

Description

A well logging quantitative evaluation method for the compressibility of coalbed methane-tight gas reservoirs

Technical Field

The invention relates to the technical field of well logging evaluation, and in particular to a well logging quantitative evaluation method for the combined pressure of coalbed methane-tight gas reservoirs.

Background Art

In the process of joint development of coalbed methane and tight gas, fracturing and other production-increasing measures are often used. The evaluation of the compressibility of coalbed methane and tight gas reservoirs has become an important task in formulating fracturing plans. Geophysical logging data contains a lot of information such as coalbed methane and tight gas reservoir mechanics, ground stress, pressure, etc. Based on this, logging data can be used to evaluate the compressibility of coalbed methane and tight gas reservoirs.

The existing logging evaluation methods for reservoir compressibility mostly use brittleness index and fracture pressure values to classify compressibility. However, there are large differences in brittleness index, Young's modulus and tensile strength between coalbed methane-tight gas reservoirs and tight gas reservoirs, and the ground stress between coalbed methane-tight gas reservoirs will also have a great impact on fracturing. Secondly, how to evaluate the compressibility of coalbed methane-tight gas reservoirs as a system has not been reported yet. From the perspective of existing compressibility evaluation methods, there is no method to quantitatively evaluate the compressibility of coalbed methane-tight gas reservoirs using logging data, which brings inconvenience to the optimization of fracturing layers during the joint development of coalbed methane-tight gas.

Summary of the invention

In order to overcome the shortcomings of the above-mentioned prior art, the purpose of the present invention is to provide a logging quantitative evaluation method for the compressibility of coalbed methane-tight gas reservoirs, which uses logging data to determine the brittleness index difference between sandstone and coal rock, the minimum horizontal principal stress difference between sandstone and coal rock, the tensile strength difference between sandstone and coal rock, and the Young's modulus difference between sand layer and coal layer, and uses these four evaluation indicators to evaluate the compressibility of coalbed methane-tight gas reservoirs. While improving the accuracy of quantitative logging evaluation of the compressibility of coalbed methane-tight gas reservoirs, it will provide logging technology support for the optimization of fracturing positions for coalbed methane-tight gas reservoirs, and has the characteristics of simple method and practicality.

In order to achieve the above object, the technical solution adopted by the present invention is:

A well logging quantitative evaluation method for the compressibility of coalbed methane-tight gas reservoirs, the steps are as follows:

Step 1: Calculate the brittleness index difference between sand layer and coal layer

The brittleness index difference between sand layer and coal layer is determined by formula (1):

ΔI _B ＝I _BS -I _BC (I)

Where: ΔI _B is the difference in brittleness index between sand layer and coal layer, %; I _BS and I _BC are the brittleness index of sand layer and coal layer respectively, %;

in:

Where: 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, E _max and E _min are the Young's modulus, the maximum Young's modulus and the minimum Young's modulus ratio of the coal seam, 10 ⁴ MPa; μ, μ _max and μ _min are the Poisson's ratio, the maximum Poisson's ratio and the minimum Poisson's ratio of the coal seam, dimensionless; ρ _b is the bulk density, g/cm ³ ; Δt and Δt _s are the longitudinal and transverse wave time differences of the coal seam, μs/ft;

Step 2: Calculate the minimum horizontal stress difference between the sand layer and the coal seam

Based on the determination of the in-situ stress of the coal seam and sandstone using the logging data, the minimum horizontal principal stress difference between the coal seam and its roof and floor is determined using formula (2):

Δσ＝σ _s -σ _c (2)

Where: Δσ is the minimum horizontal principal stress difference between the sand layer and the coal seam, MPa; _σs is the minimum horizontal principal stress of the top and bottom plates of the coal seam, MPa; _σc is the minimum horizontal principal stress of the coal seam, MPa;

in

σ _s or

Where: σ _v is vertical geostress, MPa; α is Biot coefficient, dimensionless; P _p is formation pore pressure, MPa; β ₁ is tectonic stress coefficient in the direction of minimum horizontal geostress, dimensionless; ρ _o is the average density value of the formation without logging density depth section, g/cm ³ ; H _o is the starting depth of density logging, m; H is the depth of the calculation point, m; Δt _ma is the acoustic time difference of the coal rock skeleton, μs/ft; A and B are regional coefficients, dimensionless; the physical meanings of other parameters are the same as above;

Step 3: Calculate the difference in tensile strength between sandstone and coal rock

Based on the shale content and Young's modulus of the logging data, the tensile strength of the sand layer and the coal layer is calculated, and then the tensile strength difference between the sand layer and the coal layer is determined:

ΔC＝ _Cs - _Cc (3)

ΔC is the difference in tensile strength between the sand layer and the coal layer, MPa; _Cs and _Cc are the tensile strengths of the sand layer and the coal layer, MPa respectively;

in:

_Cs or _Cc = 0.0045E(1- _Vsh ) + 0.008E· _Vsh

Where: E is the Young's modulus of the formation, 10 ⁴ MPa; _Vsh is the shale content of the formation, %;

Step 4: Calculate the difference in Young's modulus between sandstone and coal rock

The Young's modulus of the sand layer and the coal layer is calculated using the logging data, and then the Young's modulus difference between the sand layer and the coal layer is determined:

ΔE＝E _s -E _c (4)

Where: ΔE is the difference in Young's modulus between the sand layer and the coal layer, 10 ⁴ MPa; _Es and _Ec are the Young's modulus of the sand layer and the coal layer, 10 ⁴ MPa respectively;

Step 5: Evaluation of the compressibility of sandstone and coal rock

Based on the results of the above steps, the classification standard for the evaluation of the compressibility of coalbed methane-tight gas reservoirs is obtained as shown in Table 1:

Table 1 Classification of evaluation levels of coalbed methane-tight gas reservoir compressibility

As can be seen from Table 1, the pressure compressibility evaluation of coalbed methane-tight gas reservoirs is divided into three categories: Category I indicates good pressure compressibility evaluation and strong pressure compressibility; Category II indicates medium pressure compressibility evaluation and certain pressure compressibility; Category III indicates poor pressure compressibility evaluation and difficulty in successful fracturing.

Compared with the prior art, the beneficial effects of the present invention are as follows: the coalbed methane-tight gas reservoir pressure compressibility quantitative evaluation method of the present invention can effectively use logging data to evaluate the coalbed methane-tight gas reservoir pressure compressibility, and organically combines four evaluation indicators, namely, the brittleness index difference between sandstone and coal rock, the minimum horizontal principal stress difference between sandstone and coal rock, the tensile strength difference between sandstone and coal rock, and the Young's modulus difference between sand layer and coal layer. While improving the accuracy of the coalbed methane-tight gas reservoir pressure compressibility quantitative evaluation logging, it also provides strong logging technology support for the optimization of fracturing layers, opens up a new way to evaluate the coalbed methane-tight gas reservoir pressure compressibility using logging data, has the characteristics of simple method and practicality, and has good promotion and application value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a flow chart of the well logging quantitative evaluation method for the combined pressure of coalbed methane and tight gas reservoirs in the present invention.

FIG. 2 is a diagram showing the quantitative evaluation results of the combined pressure logging of the coalbed methane-tight gas reservoir according to the present invention.

DETAILED DESCRIPTION

The present invention is further described in detail below with reference to the embodiments.

1, a well logging quantitative evaluation method for the compressibility of coalbed methane-tight gas reservoirs includes the following steps:

A well logging quantitative evaluation method for the compressibility of coalbed methane-tight gas reservoirs, the steps are as follows:

Step 1: Calculate the brittleness index difference between sand layer and coal layer

Sand layers and coal layers have different brittleness. Generally, sand layers are more brittle. If the brittleness index is different, the sand layer can be successfully fractured, but it is difficult to form fractures in the coal layer.

The brittleness index difference between sand layer and coal layer is determined by formula (1):

ΔI _B ＝I _BS -I _BC (I)

Where: ΔI _B is the difference in brittleness index between sand layer and coal layer, %; I _BS and I _BC are the brittleness index of sand layer and coal layer respectively, %;

in

In the formula: I _BE and I _Bμ are the brittleness index calculated by Young's modulus and Poisson's ratio method, respectively, %; I _B is the brittleness index of the coal seam, %; E, E _max and E _min are the Young's modulus, maximum Young's modulus and minimum Young's modulus ratio of the coal seam, respectively, 10 ⁴ MPa; μ, μ _max and μ _min are the Poisson's ratio, maximum Poisson's ratio and minimum Poisson's ratio of the coal seam, respectively, dimensionless; ρ _b is the bulk density, g/cm ³ ; Δt and Δt _s are the longitudinal and shear wave time differences of the coal seam, respectively, μs/ft.

Step 2: Calculate the minimum horizontal stress difference between the sand layer and the coal seam

The horizontal principal stress of the sand layer is generally greater than that of the coal seam. The horizontal principal stress generated by tectonic action is greater in the sandstone layer, resulting in a larger difference in the minimum horizontal geostress between the sand layer and the coal seam. When the difference in the minimum horizontal geostress between the sand layer and the coal seam increases, the fracture height gradually decreases, the fracture is completely confined in the coal seam, the fracture length in the coal seam increases steadily, and the high geostress of the sand layer will hinder the extension of the fracture into the sand layer.

Based on the determination of the in-situ stress of the coal seam and sandstone using the logging data, the minimum horizontal principal stress difference between the coal seam and its roof and floor is determined using formula (2):

Δσ＝σ ₅ -σ _c (2)

Where: Δσ is the minimum horizontal principal stress difference between the sand layer and the coal seam, MPa; _σs is the minimum horizontal principal stress of the top and bottom plates of the coal seam, MPa; _σc is the minimum horizontal principal stress of the coal seam, MPa;

in

σ _s or

Wherein: σ _v is vertical geostress, MPa; α is Biot coefficient, dimensionless; P _p is formation pore pressure, MPa; β ₁ is tectonic stress coefficient in the direction of minimum horizontal geostress, dimensionless; ρ _o is the average density value of the formation without logging density depth section, g/cm ³ ; H _o is the starting depth of density logging, m; H is the depth of the calculation point, m; Δt _ma is the acoustic time difference of the coal rock skeleton, μs/ft; A and B are regional coefficients, dimensionless; the physical meanings of other parameters are the same as above.

Step 3: Calculate the difference in tensile strength between sandstone and coal rock

As the tensile strength of the sand layer increases, the fracture height decreases, and the fracture length and width gradually increase; the sand layer with low tensile strength will cause the crack to expand rapidly in the fracture height direction; the greater the tensile strength, the greater the expansion resistance in the fracture length direction, and the pressure in the fracture is easy to accumulate, resulting in the occurrence of layer penetration in the fracture height direction.

Based on the shale content and Young's modulus of the logging data, the tensile strength of the sand layer and the coal layer is calculated, and then the tensile strength difference between the sand layer and the coal layer is determined:

ΔC＝ _Cs - _Cc (3)

in:

_Cs or _Cc = 0.0045E(1- _Vsh ) + 0.008E· _Vsh

Where: ΔC is the difference in tensile strength between the sand layer and the coal layer, MPa; _Cs and _Cc are the tensile strengths of the sand layer and the coal layer, MPa; E is the Young's modulus of the formation, 10 ⁴ MPa; _Vsh is the mud content of the formation, %.

Step 4: Calculate the difference in Young's modulus between sandstone and coal rock

As the elastic modulus of the sand layer increases, the profile of the fracture in the direction of fracture height becomes more "thin and tall", the fracture width gradually decreases, the fracture height increases, and there is a very obvious decreasing trend in the fracture length direction; the sandstone layer with a large elastic modulus limits the growth of the fracture width, and the smaller the pressure at the fracture tip in the fracture length direction, the more difficult it is to expand in the fracture length direction.

The Young's modulus of the sand layer and the coal layer is calculated using the logging data, and then the Young's modulus difference between the sand layer and the coal layer is determined:

ΔE＝E _{s -EC} (4)

Where: ΔE is the difference in Young's modulus between the sand layer and the coal layer, 10 ⁴ MPa; _Es and _Ec are the Young's modulus of the sand layer and the coal layer, 10 ⁴ MPa respectively.

Step 5: Evaluation of the compressibility of sandstone and coal rock

According to the results of the above steps and on the basis of actual production verification, the classification standard of coalbed methane-tight gas reservoir compressibility evaluation is obtained as shown in Table 1:

Table 1 Classification of evaluation levels of coalbed methane-tight gas reservoir compressibility

As can be seen from Table 1, the pressure compressibility evaluation of coalbed methane-tight gas reservoirs is divided into three categories: Category I indicates good pressure compressibility evaluation and strong pressure compressibility; Category II indicates medium pressure compressibility evaluation and certain pressure compressibility; Category III indicates poor pressure compressibility evaluation and difficulty in successful fracturing.

Based on the above-mentioned logging calculation model for evaluating the compressibility of coalbed methane-tight gas reservoirs, and on the basis of compiling processing and interpretation procedures, logging processing and interpretation were carried out on the compressibility of the main coalbed methane-tight gas reservoirs in each well in the study area.

Figure 2 is a quantitative evaluation result of the well logging of the combined pressure of the coalbed methane-tight gas reservoir in Well X. The well has a tight gas reservoir section of 1073.5-1079m with a thickness of 5.5m; the coalbed methane reservoir section is 1082.2-1085.3 with a thickness of 3.1m. The brittleness index difference of the coalbed methane-tight gas reservoir is 28, the minimum horizontal principal stress difference is 0.5MPa, the sand tensile strength difference is 3.4MPa, and the Young's modulus difference is 5.2GPa. The comprehensive evaluation of the combined pressure of the coalbed methane-tight gas reservoir is Class I, indicating that the combined pressure is strong. The coalbed methane-tight gas reservoir was combined pressured, and the microseismic monitoring results after fracturing showed that the coalbed methane-tight gas reservoir had formed complex fractures with long radial length and wide vertical length, and the daily gas production after fracturing was 13,000 cubic meters. This fully demonstrates that the compressibility evaluation of this study is consistent with the actual fracturing monitoring and drainage, and further shows that the accuracy of the quantitative evaluation of compressibility logging plays a key role in the optimization of coalbed methane fracturing layers. This method fully exploits the compressibility information of coalbed methane-tight gas reservoirs contained in the logging data, and the evaluation can meet the requirements of coalbed methane-tight gas reservoir fracturing layer optimization.

Those skilled in the art should understand that since coalbed methane logging is seriously affected by environmental factors, in order to ensure the effectiveness and feasibility of the method, it is necessary to ensure that the environmental impact correction effect of the logging data is good, and the four evaluation indicators, namely the brittleness index difference between sandstone and coal rock, the minimum horizontal principal stress difference between sandstone and coal rock, the tensile strength difference between sandstone and coal rock, and the Young's modulus difference between sand layer and coal layer, are calculated more accurately, so that the quantitative evaluation results of coalbed methane-tight gas reservoir combined pressure logging will have higher accuracy.

Claims (4)

1. A well logging quantitative evaluation method for the compressibility of coalbed methane-tight gas reservoirs, characterized in that the steps are as follows: Step 1: Calculate the brittleness index difference between sand layer and coal layer The brittleness index difference between sand layer and coal layer is determined by formula (1): ΔI _B =I _BS -I _BC (1) Where: ΔI _B is the difference in brittleness index between sand layer and coal layer, %; I _BS and I _BC are the brittleness index of sand layer and coal layer respectively, %; Step 2: Calculate the minimum horizontal stress difference between the sand layer and the coal seam Based on the determination of the in-situ stress of the coal seam and sandstone using the logging data, the minimum horizontal principal stress difference between the coal seam and its roof and floor is determined using formula (2): Δσ＝σ _s -σ _c (2) Where: Δσ is the minimum horizontal principal stress difference between the sand layer and the coal seam, MPa; _σs is the minimum horizontal principal stress of the top and bottom plates of the coal seam, MPa; _σc is the minimum horizontal principal stress of the coal seam, MPa; Step 3: Calculate the difference in tensile strength between sandstone and coal rock Based on the shale content and Young's modulus of the logging data, the tensile strength of the sand layer and the coal layer is calculated, and then the tensile strength difference between the sand layer and the coal layer is determined: ΔC＝ _Cs - _Cc (3) Where: ΔC is the difference in tensile strength between the sand layer and the coal layer, MPa; _Cs and _Cc are the tensile strengths of the sand layer and the coal layer, MPa respectively; Step 4: Calculate the difference in Young's modulus between sandstone and coal rock The Young's modulus of the sand layer and the coal layer is calculated using the logging data, and then the Young's modulus difference between the sand layer and the coal layer is determined: ΔE＝E _s -E _c (4) Where: ΔE is the difference in Young's modulus between the sand layer and the coal layer, 10 ⁴ MPa; _Es and _Ec are the Young's modulus of the sand layer and the coal layer, 10 ⁴ MPa respectively; Step 5: Evaluation of the compressibility of sandstone and coal rock Based on the results of the above steps, the classification standard for the evaluation of the compressibility of coalbed methane-tight gas reservoirs is obtained as shown in Table 1: Table 1 Classification of evaluation levels of coalbed methane-tight gas reservoir compressibility

As can be seen from Table 1, the compressibility evaluation of coalbed methane-tight gas reservoirs is divided into three categories: Category I indicates good compressibility evaluation and strong compressibility; Category II indicates medium compressibility evaluation and certain compressibility; Category IⅡ indicates poor compressibility evaluation and difficulty in successful fracturing. 2. The well logging quantitative evaluation method for the compressibility of coalbed methane-tight gas reservoirs according to claim 1 is characterized in that: In step one,

In the formula: I _BE and I _Bμ are the brittleness index calculated by Young's modulus and Poisson's ratio method, respectively, %; I _B is the brittleness index of the coal seam, %; E, E _max and E _min are the Young's modulus, maximum Young's modulus and minimum Young's modulus ratio of the coal seam, respectively, 10 ⁴ MPa; μ, μ _max and μ _min are the Poisson's ratio, maximum Poisson's ratio and minimum Poisson's ratio of the coal seam, respectively, dimensionless; ρ _b is the bulk density, g/cm ³ ; Δt and Δt _s are the longitudinal and shear wave time differences of the coal seam, respectively, μs/ft. 3. The method for quantitative evaluation of the compressibility of coalbed methane-tight gas reservoirs by logging according to claim 1, characterized in that: In step 2, σ _s or

In the formula: σ _v is the vertical geostress, MPa; μ and μ are the Poisson's ratio of the coal seam; α is the Biot coefficient, dimensionless; P _p is the formation pore pressure, MPa; β ₁ is the tectonic stress coefficient in the direction of minimum horizontal geostress, dimensionless; ρ _o is the average density value of the formation without logging density depth section, g/cm ³ ; H _o is the starting depth of density logging, m; H is the depth of the calculation point, m; Δt _ma is the acoustic time difference of the coal rock skeleton, μs/ft; A and B are regional coefficients, dimensionless. 4. The method for quantitative evaluation of the compressibility of coalbed methane-tight gas reservoirs by logging according to claim 1, characterized in that: In step three, _Cs or _Cc = 0.0045E(1- _Vsh ) + 0.008E· _Vsh E is the Young's modulus of the formation, 10 ⁴ MPa; _Vsh is the shale content of the formation, %.

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