Quantitative evaluation method based on coal seam isothermal adsorption curve

Abstract

The invention discloses a quantitative evaluation method based on a coal seam isothermal adsorption curve. The method comprises the following steps: collecting coal sample data of at least one well to be evaluated; obtaining the adsorption capacity values of the sample in the coal sample data under different pressures; determining values of coal seam limit accumulated adsorption capacity and coal seam isothermal instantaneous pressure adsorption speed; calculating a curvature K value; calculating a desorption rate K'through a first-order derivative formula of K; calculating a desorption rate change rate K ''through a second derivative formula of K; the method is suitable for the technical field of coal bed gas development, coal samples are taken through well drilling, basic experimental numerical values are obtained, on the basis of the Chen's adsorption equation, the curvature radius of an isothermal adsorption curve is obtained, first-order derivation and second-order derivation are conducted on the curvature radius, and the pressure and desorption rate of the isothermal adsorption curve is obtained. The isothermal adsorption curve is quantitatively described, the initial desorption pressure, the threshold desorption pressure and the peak desorption pressure of the coalbed methane are predicted, the drainage production system is quantitatively guided, and long-term stable output of the coalbed methane well is achieved.

Description

Technical Field

[0001] The invention belongs to the technical field of coalbed methane development, and in particular is a quantitative evaluation method based on coalbed isothermal adsorption curve. Background Art

[0002] Coalbed methane exists in coal reservoirs in various states, such as free, adsorbed and dissolved. Among them, the adsorbed state is the most important form of coalbed methane, and the reservoir pressure is the key factor controlling the amount of adsorbed gas in coal seams.

[0003] Under constant temperature conditions, the adsorption process under different pressures is called isothermal adsorption, and the curve describing the relationship between pressure and adsorption amount during isothermal adsorption is called the isothermal adsorption curve.

[0004] The isothermal adsorption curve of coal is mainly used in the following three aspects: first, to determine the critical desorption pressure of coalbed methane in coal reservoirs; second, to estimate the theoretical gas content of coal reservoirs and determine the saturation of coalbed methane; third, to predict the recovery rate or recoverable resources of coalbed methane in the process of depressurization and desorption of coal reservoirs.

[0005] The adsorption theories of coalbed methane include Langmuir monolayer adsorption theory, BET multilayer adsorption theory and volume filling theory. The most widely used one is Langmuir theory. The Langmuir model (Langmuir, 1916) is established based on the kinetic equilibrium principle of vaporization and condensation. Its equation is simple and practical and has been widely used in the adsorption of coal and other adsorbents to gas.

[0006] In 1918, Langmuir published the famous Langmuir isothermal cumulative adsorption equation, which has been highly valued and widely used. However, it should be pointed out that the Langmuir equation is a nonlinear empirical equation with two constants. a and b are constants related to the physical properties of the gas and temperature, and are empirical constants. Chen Yuanqian et al. published the derivation results of the isothermal adsorption equation and the desorption equation in 2018. At the same time, a complete theoretical derivation of the Langmuir equation was carried out to obtain the Langmuir isothermal instantaneous pressure adsorption equation and the instantaneous pressure desorption equation. Both the Chen equation and the Langmuir equation have two equation constants. Through derivation, the physical meanings of the constants a and b are clarified. The constant A of the Chen equation and the constant a of the Langmuir equation both represent the limiting cumulative adsorption of the sample; the constant B of the Chen equation and the constant b of the Langmuir equation both represent the isothermal instantaneous pressure adsorption decrease rate; the AB of the Chen equation and the ab of the Langmuir equation both represent the maximum initial theoretical adsorption of the sample.

[0007] The Langmuir equation is an empirical equation, while the Chen equation derives the isothermal adsorption equation based on the gas equation and the decline rate equation. The Langmuir equation is less reliable than the Chen equation, and is not as reliable and complete as the Chen equation in theory, and cannot quantitatively describe the change of desorption pressure.

[0008] Therefore, there is an urgent need for a quantitative evaluation method based on the isothermal adsorption curve of coal seams, which can quantitatively guide the drainage and production system and achieve long-term stable output of coalbed methane wells based on the Chen adsorption equation. Summary of the invention

[0009] The purpose of the present invention is to overcome the defects of the prior art and provide a quantitative evaluation method based on the isothermal adsorption curve of coal seams.

[0010] To achieve the above object, the present invention adopts the following technical solutions:

[0011] A quantitative evaluation method based on coal seam isothermal adsorption curve comprises the following steps:

[0012] Step S1, collecting coal sample data of at least one well to be evaluated;

[0013] Step S2, obtaining the adsorption amount of the sample at different pressures in the coal sample data;

[0014] Step S3, determining the value of the coal seam limit cumulative adsorption capacity and the coal seam isothermal transient pressure adsorption rate;

[0015] Step S4, calculating the curvature K value;

[0016] Step S5, calculating the desorption rate K' by the first-order derivative formula of K;

[0017] Step S6, calculating the desorption rate change rate K" by the second-order derivative formula of K;

[0018] Step S7, using a scatter plot to draw a relationship diagram between pressure and desorption rate.

[0019] Preferably, in step S2, an isothermal adsorption test is performed on the coal sample data of each well to obtain the adsorption amount values ​​of the samples in the coal sample data at different pressures.

[0020] Preferably, in step S2, an isothermal adsorption experiment is performed on the coal sample to obtain experimental data including: pressure value, adsorption amount based on equilibrium moisture, adsorption amount based on air drying and P / V (ratio of pressure to adsorption amount), adsorption amount based on dry ash-free and P / V.

[0021] Preferably, in step S3, according to the adsorption amount values ​​of the coal sample data of each well at different pressures obtained in step S2, an adsorption amount is calculated using Chen's equation, and compared with the experimental adsorption amount, the highest fitting degree is obtained to determine the values ​​of the coal seam limit cumulative adsorption amount and the coal seam isothermal transient pressure adsorption rate.

[0022] Preferably, in step S3, calculating the value of the coal seam limit cumulative adsorption capacity and the coal seam isothermal transient pressure adsorption rate according to the iterative trial and error method includes:

[0023] The instantaneous adsorption / desorption linear equation of Chen's isotherm is:

[0024] ln(A / A-Vg)=B(P-Psc);

[0025] The values ​​of A and B in the formula are calculated by iterative trial and error method, and Chen's equation is used Calculate an adsorption amount, compare it with the experimental adsorption amount, and determine the A and B values ​​with the highest fit;

[0026] Among them, A is the ultimate cumulative adsorption capacity of the coal seam; B is the isothermal instantaneous pressure adsorption rate or isothermal instantaneous desorption rate of the coal seam; P is the experimental pressure; Psc is the ground pressure; Vg is the experimental adsorption capacity.

[0027] Preferably, in step S4, the curvature K value, that is, the curvature of the isothermal adsorption curve, is calculated according to the value of the coal seam limit cumulative adsorption capacity and the coal seam isothermal transient pressure adsorption rate, and the calculation formula is:

[0028]

[0029] Among them, K is the curvature, R is the radius of curvature, and Y is the adsorption amount.

[0030] Preferably, in step S5, the desorption rate K' is calculated using the following formula:

[0031]

[0032] Preferably, in step S6, the desorption rate change rate K" is calculated using the following formula:

[0033]

[0034] Preferably, in step S7, a scatter plot is used to draw a relationship diagram between pressure and desorption rate, and the desorption rate change rate and the initial desorption pressure, threshold desorption pressure and peak desorption pressure are predicted to quantitatively guide the drainage production system and achieve long-term stable output of coalbed methane wells.

[0035] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are:

[0036] In the present invention, coal samples are taken through drilling to obtain basic experimental values, and the curvature radius of the isothermal adsorption curve is calculated based on Chen's adsorption equation. The first-order derivative and the second-order derivative of the curvature radius are respectively performed to quantitatively describe the isothermal adsorption curve, predict the initial desorption pressure, threshold desorption pressure and peak desorption pressure of coalbed methane, quantitatively guide the drainage and production system, and realize long-term stable output of coalbed methane wells. The parameter selection is intuitive and reliable, and the operation is simple. Only simple linear conversion is required, which greatly improves the convenience and objectivity of the prediction of desorption rate and desorption pressure. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 It is a flow chart of a quantitative evaluation method based on isothermal adsorption curve of coal seams according to an embodiment of the present invention;

[0038] Figure 2 is a schematic diagram of an isothermal adsorption curve according to an embodiment of the present invention;

[0039] Figure 3 is a schematic diagram of an adsorption amount curve according to an embodiment of the present invention;

[0040] Figure 4 is a schematic diagram of an adsorption amount fitting curve according to an embodiment of the present invention;

[0041] Figure 5 A schematic diagram of a desorption rate curve and a desorption rate change rate curve according to an embodiment of the present invention;

[0042] Figure 6 is a schematic diagram of an isothermal adsorption curve of the XX1 well according to an embodiment of the present invention;

[0043] Figure 7 A schematic diagram of an isothermal adsorption curve of the XX2 well according to an embodiment of the present invention;

[0044] Figure 8 is a relationship diagram between the pressure and desorption rate of the XX1 well according to an embodiment of the present invention;

[0045] Fig. 9 Graph showing the relationship between the pressure and desorption rate of the XX2 well according to an embodiment of the present invention.

[0046] Fig.10 A schematic diagram of a drainage curve of Well XX1 according to an embodiment of the present invention;

[0047] Fig.11 It is a schematic diagram of the production curve of the XX2 well according to an embodiment of the present invention. DETAILED DESCRIPTION

[0048] The specific embodiments of the present invention are described in detail below.

[0049] The "range" disclosed in the present invention is defined in the form of a lower limit and an upper limit, and a given range is defined by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundaries of the particular range. The range defined in this way can be inclusive or exclusive of the end values, and can be arbitrarily combined, that is, any lower limit can be combined with any upper limit to form a range. For example, if a range of 10 to 50 is listed for a specific parameter, it is understood that the range of 10 to 40 and 20 to 50 is also expected. In addition, if the minimum range values ​​1 and 2 are listed, and if the maximum range values ​​3, 4 and 5 are listed, the following ranges can all be expected: 1 to 3, 1 to 4, 1 to 5, 2 to 3, 2 to 4 and 2 to 5. In this application, unless otherwise specified, the range of values ​​"a to b" represents an abbreviation of any real number combination between a and b, where a and b are both real numbers. For example, the range of values ​​"0 to 5" means that all real numbers between "0 to 5" have been fully listed in this article, and "0 to 5" is only an abbreviation of these numerical combinations.

[0050] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0051] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

[0052] If there is no special explanation, all steps of the present application can be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), which means that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), which means that step (c) may be added to the method in any order. For example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0053] If there is no special explanation, the "include" and "comprising" mentioned in this application are open-ended or closed-ended. For example, the "include" and "comprising" may mean that other components not listed may also be included or only the listed components may be included or only the listed components may be included.

[0054] Unless otherwise specified, the reaction is carried out at room temperature and pressure.

[0055] Unless otherwise specified, all parts or percentages are by weight.

[0056] In the present invention, all substances used are known substances and can be purchased or synthesized by known methods.

[0057] In the present invention, the devices or equipment used are all conventional devices or equipment known in the art and are commercially available.

[0058] The following is a further description of a specific implementation of the method for quantitatively evaluating a coal seam isotherm adsorption curve according to the present invention in conjunction with an embodiment. The method for quantitatively evaluating a coal seam isotherm adsorption curve according to the present invention is not limited to the description of the following embodiment.

[0059] The present application discloses a quantitative evaluation method based on coal seam isotherm adsorption curve, which is a method for quantitatively describing coal seam isotherm adsorption curve. Based on Chen's adsorption equation, the curvature radius of the isotherm adsorption curve is obtained, and the first-order and second-order derivatives of the curvature radius are respectively performed to quantitatively describe the isotherm adsorption curve.

[0060] like Figure 1 As shown, a quantitative evaluation method based on coal seam isothermal adsorption curve includes the following steps:

[0061] Step S1, collecting coal sample data of at least one well to be evaluated.

[0062] Specifically, according to the actual situation of the study area, the main focus is on collecting coal sample data from the wells to be evaluated.

[0063] Step S2, performing an isothermal adsorption test on the coal sample data of each well to obtain the adsorption amount of the sample in the coal sample data at different pressures.

[0064] In this step, the target well section is the main coal seam section in the study area. The extracted coal samples are subjected to isothermal adsorption experiments.

[0065] Isothermal adsorption experiments were carried out on coal samples, and the experimental data obtained included: pressure value, adsorption amount based on equilibrium moisture, adsorption amount and P / V based on air drying, and adsorption amount and P / V based on dry ash-free.

[0066] Step S3, according to the adsorption values ​​of the coal sample data of each well at different pressures obtained in step S2, calculate the ultimate cumulative adsorption capacity of the coal seam and the isothermal transient pressure adsorption rate of the coal seam, use Chen's equation to calculate an adsorption amount, compare it with the experimental adsorption amount, and the best fitting degree is determined to determine the ultimate cumulative adsorption capacity of the coal seam and the isothermal transient pressure adsorption rate of the coal seam.

[0067] like Figure 2 and Figure 3 As shown, the instantaneous adsorption / desorption linear equation of Chen's isotherm is:

[0068] ln(A / A-Vg)=B(P-Psc) (1);

[0069] The values ​​of A and B in the formula are calculated by iterative trial and error method, and Chen's equation is used Calculate an adsorption amount, compare it with the experimental adsorption amount, and determine the A and B values ​​with the highest fit. Figure 4 Schematic diagram of the adsorption amount fitting curve.

[0070] Among them, A is the maximum cumulative adsorption capacity of the coal seam; B is the isothermal instantaneous pressure adsorption rate or isothermal instantaneous desorption rate of the coal seam. P is the experimental pressure; Psc is the ground pressure; Vg is the experimental adsorption capacity, and e is a natural constant, the base of the natural function logarithm, and its value is approximately 2.718281828459.

[0071] Step S4, calculating the curvature K value according to the value of the coal seam limit cumulative adsorption capacity and the coal seam isothermal transient pressure adsorption rate.

[0072]

[0073] Among them, K is the curvature and R is the radius of curvature.

[0074] Step S5, calculating the desorption rate K' by the first-order derivative formula of K.

[0075]

[0076] Step S6, calculating the desorption rate change rate K" by the second-order derivative formula of K.

[0077]

[0078] Step S7, using a scatter plot to draw a relationship diagram between pressure and desorption rate, predicting the change rate of desorption rate and the initial desorption pressure, threshold desorption pressure and peak desorption pressure, so as to quantitatively guide the drainage production system and achieve long-term stable output of coalbed methane wells.

[0079] Specifically, a corresponding change curve diagram is made according to the calculation results of K, K', and K".

[0080] The desorption rate curve and desorption rate change rate curve are calculated by using K, K' and K", respectively, which can quantitatively describe the initial desorption pressure, threshold desorption pressure and peak desorption pressure of coalbed methane.

[0081] In the embodiment of the present invention, K' describes the desorption rate, K" describes the rate of change of the desorption rate, and the corresponding desorption pressure. Figure 5 Schematic diagram of a desorption rate curve and a desorption rate change rate curve according to an embodiment of the present invention.

[0082] The following uses the XX block and a single well in the XX block of the Qinshui Basin as examples to illustrate the quantitative evaluation method based on the isothermal adsorption curve of coal seams of the present invention.

[0083] (1) Collect coal samples from wells XX1 and XX2 that have been drilled in the study area.

[0084] (2) The coal sample was subjected to an isothermal adsorption experiment, and the experimental data are shown in Table 1. The experimental data used air-dried adsorption data.

[0085] Table 1

[0086]

[0087] (3) The values ​​of A and B in the formula are calculated using the iterative trial and error method using the Chen isotherm instantaneous adsorption / desorption linear equation: ln(A / A-V_g)=B(P-P_SC). Figure 6 and Figure 7 Schematic diagrams of isothermal adsorption curves of well XX1 and well XX2 respectively.

[0088] By formula:

[0089]

[0090] Among them, K is the curvature and R is the radius of curvature.

[0091] Calculate the K value through the first-order derivative formula of K

[0092]

[0093] Calculate K', the desorption rate, by the second-order derivative formula of K

[0094]

[0095] Calculate K", the rate of change of desorption rate.

[0096] (5) Use a scatter plot to draw a relationship diagram between pressure and desorption rate to achieve the purpose of predicting the change rate of desorption rate and the initial desorption pressure, threshold desorption pressure and peak desorption pressure. Figure 8 is a relationship diagram between the pressure and desorption rate of the XX1 well according to an embodiment of the present invention; Fig. 9 Graph showing the relationship between the pressure and desorption rate of the XX2 well according to an embodiment of the present invention. Fig.10 A schematic diagram of a drainage curve of Well XX1 according to an embodiment of the present invention; Fig.11 It is a schematic diagram of the production curve of the XX2 well according to an embodiment of the present invention.

[0097] Compared with the prior art, the present invention has the following beneficial effects:

[0098] 1. By taking coal samples through drilling, basic experimental values ​​are obtained. Based on Chen's adsorption equation, the radius of curvature of the isothermal adsorption curve is obtained. The first-order and second-order derivatives of the radius of curvature are respectively performed to quantitatively describe the isothermal adsorption curve, predict the initial desorption pressure, threshold desorption pressure and peak desorption pressure of coalbed methane, quantitatively guide the drainage and production system, and achieve long-term stable output of coalbed methane wells.

[0099] 2. The parameter selection is intuitive and reliable, and the operation is simple. Only simple linear conversion is required, which greatly improves the convenience and objectivity of the prediction of desorption rate and desorption pressure.

[0100] The above contents are further detailed descriptions of the present invention in combination with specific preferred embodiments, and it cannot be determined that the specific implementation of the present invention is limited to these descriptions. For ordinary technicians in the technical field to which the present invention belongs, several simple deductions or substitutions can be made without departing from the concept of the present invention, which should be regarded as falling within the protection scope of the present invention.

Claims

1. A quantitative evaluation method based on coal seam isotherm adsorption curve, characterized in that: The steps include: Step S1, collecting coal sample data of at least one well to be evaluated; Step S2, obtaining the value of the adsorption amount of the sample at different pressures in the coal sample data; Step S3, determining the value of the coal seam limit cumulative adsorption capacity and the coal seam isothermal transient pressure adsorption rate; Step S4, calculating the curvature K value; Step S5, calculating the desorption rate K' by the first-order derivative formula of K; Step S6, calculating the desorption rate change rate K" by the second-order derivative formula of K; Step S7, using a scatter plot to draw a relationship diagram between pressure and desorption rate.

2. A quantitative evaluation method based on coal seam isotherm adsorption curve according to claim 1, characterized in that: In the step S2, an isothermal adsorption test is performed on the coal sample data of each well to obtain the adsorption amount of the sample in the coal sample data at different pressures.

3. A quantitative evaluation method based on coal seam isotherm adsorption curve according to claim 1, characterized in that: In step S2, an isothermal adsorption experiment is performed on the coal sample to obtain experimental data including: pressure value, adsorption amount based on equilibrium moisture, adsorption amount based on air drying and P / V (ratio of pressure to adsorption amount), adsorption amount based on dry ash-free and P / V.

4. A quantitative evaluation method based on coal seam isotherm adsorption curve according to claim 1, characterized in that: In step S3, according to the adsorption amount values ​​of the coal sample data of each well at different pressures obtained in step S2, an adsorption amount is calculated using Chen's equation, and compared with the experimental adsorption amount, the highest fitting degree is obtained to determine the values ​​of the coal seam limit cumulative adsorption amount and the coal seam isothermal transient pressure adsorption rate.

5. A quantitative evaluation method based on coal seam isotherm adsorption curve according to claim 1, characterized in that: In step S3, the values ​​of the coal seam limit cumulative adsorption capacity and the coal seam isothermal transient pressure adsorption rate are calculated according to the iterative trial and error method, including: The instantaneous adsorption / desorption linear equation of Chen's isotherm is: ln(A / A-Vg)=B(P-Psc); The values ​​of A and B in the formula are calculated by iterative trial and error method, and Chen's equation is used Calculate an adsorption amount, compare it with the experimental adsorption amount, and determine the A and B values ​​with the highest fit; Among them, A is the ultimate cumulative adsorption capacity of the coal seam; B is the isothermal instantaneous pressure adsorption rate or isothermal instantaneous desorption rate of the coal seam; P is the experimental pressure; Psc is the ground pressure; Vg is the experimental adsorption capacity.

6. A quantitative evaluation method based on coal seam isotherm adsorption curve according to claim 1, characterized in that: In step S4, the curvature K value, that is, the curvature of the isothermal adsorption curve, is calculated according to the value of the coal seam limit cumulative adsorption capacity and the coal seam isothermal transient pressure adsorption rate, and the calculation formula is: Among them, K is the curvature, R is the radius of curvature, and Y is the adsorption amount.

7. A quantitative evaluation method based on coal seam isotherm adsorption curve according to claim 1, characterized in that: In step S5, the desorption rate K' is calculated using the following formula:

8. The quantitative evaluation method based on the isothermal adsorption curve of coal seams according to claim 1, characterized in that: In step S6, the desorption rate change rate K" is calculated using the following formula:

9. The quantitative evaluation method based on the isothermal adsorption curve of coal seams according to claim 1, characterized in that: In step S7, a scatter plot is used to draw a relationship diagram between pressure and desorption rate, and the desorption rate change rate and the initial desorption pressure, threshold desorption pressure and peak desorption pressure are predicted to quantitatively guide the drainage production system and achieve long-term stable output of coalbed methane wells.

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