Middle-deep layer coal bed gas horizontal well step-by-step depressurization drainage method

Abstract

The invention discloses a middle-deep layer coal bed gas horizontal well step-by-step depressurization drainage and mining method. The method comprises the following steps that S1, basic data are collected; s2, deep analysis and calculation are conducted on the basic data collected in the step S1 through an old horizontal well productivity index equation, the value interval of the anisotropic parameter beta value and the value interval of the equivalent wellbore radius of the horizontal well are determined through the old horizontal well productivity index equation, and the reasonability of the drainage and production well system is verified; and S3, according to the second-order derivative isothermal adsorption curve, the drainage and mining stage of the coal-bed gas well is preliminarily divided into six stages. According to the method, the reasonable production pressure difference is determined, the targets of sand control, powder control and water control are achieved, and the continuous drainage and mining time is prolonged; an effective pressure drop funnel is formed, and high and stable yield of the middle-deep layer coal bed gas horizontal well is achieved.

Description

Technical Field

[0001] The invention relates to a step-by-step pressure reduction and drainage method for a medium-deep coalbed methane horizontal well. Background Art

[0002] Coalbed methane is a clean new energy source. The coalbed methane industry has developed rapidly in the United States, Canada, Australia, Russia, the United Kingdom, the Czech Republic, Germany, India and other countries, and has become another emerging natural gas industry after conventional natural gas. At the same time, the misunderstanding and prejudice of "coalbed methane" as "harmful gas in coal" has been gradually corrected. The scientific and rational use of coalbed methane has been formed, and the use of coal resources has been replaced, which can achieve "promoting coal production safety, reducing environmental pollution, and making full use of the advantages of coalbed methane natural resources to obtain clean energy."

[0003] Coalbed methane is a natural gas that is stored in coal seams and their adjacent rock formations in a self-generated and self-stored manner. It includes adsorbed gas in coal seams, free gas outside coal seams and water-soluble gas. Its main components are methane and heavy hydrocarbon gas, 98% of which is methane gas.

[0004] Medium-deep coalbed methane refers to unconventional natural gas resources buried at a depth of 1,000-2,000 meters.

[0005] At present, there are more than 10 trillion tons of deep coal fields below 800 meters in my country, and most of them are in the stage of being unable to be mined. The significant characteristic of medium-deep coalbed methane is that the coal reservoir has low permeability, making it difficult to form an effective pressure drop funnel. Due to the large burial depth, the lifting process of medium-deep coalbed methane is relatively complex compared to shallow coalbed methane, and the difficulty of controlling pressure and powder is increased, resulting in increased difficulty in stable and continuous drainage. Therefore, how to formulate a reasonable drainage method for medium-deep coalbed methane has become a technical problem that needs to be solved urgently. Summary of the invention

[0006] Purpose of the invention: In order to solve the deficiencies of the above-mentioned prior art, the present invention discloses a method for step-by-step depressurization and drainage of medium-deep coalbed methane horizontal wells. The present invention achieves continuous and stable drainage by finely dividing the drainage stages, clarifying the control range of production pressure difference in different drainage stages, and constructing a new method for step-by-step depressurization and efficient drainage and gas production of medium-deep coalbed methane wells.

[0007] Technical solution: A method for step-by-step pressure reduction and drainage of medium-deep coalbed methane horizontal wells, the specific steps are as follows:

[0008] S1: Collect basic data;

[0009] S2: Using the Chen-type horizontal well productivity index equation, the basic data collected in step S1 are deeply analyzed and calculated. Using the Chen-type horizontal well productivity index equation, the value range of the anisotropic parameter β and the equivalent wellbore radius of the horizontal well are determined to verify the rationality of the drainage and production well system;

[0010] S3: Based on the second-order conductance isotherm adsorption curve, the drainage and production stage of the coalbed methane well is preliminarily divided into six stages.

[0011] Furthermore, the method further comprises step S4: based on the relationship between the anisotropy parameter β and the gas production, water production and bottom hole pressure, finely dividing the six stages of the drainage and production phase of the coalbed methane well preliminarily divided in step S3.

[0012] Furthermore, the basic data in step S1 includes coal sample analysis data, core data and horizontal well production data.

[0013] Furthermore, the basic data in step S1 include permeability, horizontal section length, viscosity, volume coefficient, and wellbore radius.

[0014] Furthermore, the Chen-style horizontal well productivity index equation in step S2 is as follows:

[0015]

[0016] Where: q——output, unit is m 3 ;

[0017] Δp——reservoir pressure difference, in MPa;

[0018] K h ——horizontal permeability, in mD;

[0019] h——reservoir height, in m;

[0020] μ——viscosity, unit is mPa·s;

[0021] B——volume coefficient, dimensionless;

[0022] r——wellbore radius, in m;

[0023] a——liquid supply radius, in m;

[0024] L——horizontal section length, in m;

[0025] β——anisotropy parameter, dimensionless.

[0026] Furthermore, the calculation formula of the anisotropy parameter in step S2 is as follows:

[0027]

[0028] Where: β is anisotropy parameter, dimensionless;

[0029] K h ——horizontal permeability, in mD;

[0030] K v ——Vertical permeability, in mD.

[0031] Furthermore, the specific steps of step S3 are as follows:

[0032] S31: draw the second-order conductance isotherm adsorption curve;

[0033] S32: Based on the five pressure characteristic points of the second-order derivative isothermal adsorption curve, the drainage stage of the coalbed methane well is initially divided into six stages, including:

[0034] The five characteristic point values ​​of the second-order conductance isotherm adsorption curve are as follows: saturation pressure value, initial desorption pressure value, threshold desorption pressure value, peak desorption pressure value, and constant rate desorption pressure value according to the pressure value from high to low.

[0035] Furthermore, the steps of step S31 are as follows:

[0036] Step S311, collecting coal sample data of at least one coalbed methane well to be mined;

[0037] Step S312, obtaining the value of adsorption amount of the sample under different pressures in the coal sample data, wherein:

[0038] In the step S312, 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;

[0039] In the step S312, 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;

[0040] Step S313, determining the value of the coal seam limit cumulative adsorption capacity and the coal seam isothermal transient pressure adsorption rate;

[0041] Step S314: Calculate the curvature K value, where:

[0042] According to the coal seam limit cumulative adsorption capacity and the coal seam isothermal transient pressure adsorption rate, the curvature K value, that is, the curvature of the isothermal adsorption curve, is calculated by the following formula:

[0043]

[0044] Among them, K is the curvature, R is the radius of curvature, and Y is the adsorption amount.

[0045] Step S315: Calculate the desorption rate K' by the first-order derivative formula of K, where the calculation formula of the desorption rate K' is:

[0046]

[0047] Step S316: Calculate the desorption rate change rate K'' by the second-order derivative formula of K, where:

[0048] The calculation formula of desorption rate change rate K" is:

[0049]

[0050] Step S317: Use the scatter plot to draw a relationship diagram between pressure and desorption rate, that is, obtain a second-order derivative isotherm adsorption curve.

[0051] Furthermore, in step S313, according to the adsorption amount values ​​of the coal sample data of each well at different pressures obtained in step S312, an adsorption amount is calculated using Chen's equation, and compared with the experimental adsorption amount, the fitting degree is the highest, and the value of the coal seam limit cumulative adsorption amount and the coal seam isothermal transient pressure adsorption rate is determined, wherein:

[0052] Chen's equation in:

[0053] A is the maximum cumulative adsorption capacity of the coal seam;

[0054] B is the isothermal instantaneous pressure adsorption rate or isothermal instantaneous desorption rate of the coal seam;

[0055] P is the experimental pressure;

[0056] Psc is the ground pressure;

[0057] Vg is the experimental adsorption amount, and / or

[0058] In step S313, the values ​​of the coal seam limit cumulative adsorption amount and the coal seam isothermal transient pressure adsorption rate are calculated according to the iterative trial and error method, including:

[0059] The instantaneous adsorption / desorption linear equation of Chen's isotherm is:

[0060] ln(A / A-Vg)=B(P-Psc);

[0061] 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;

[0062] 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.

[0063] Furthermore, in step S317, a pressure and second-order derivative isothermal adsorption curve is drawn using a scatter plot to obtain a saturated pressure value and a constant rate desorption pressure value, and to predict an initial desorption pressure value, a threshold desorption pressure value and a peak desorption pressure value.

[0064] Furthermore, the coalbed methane well drainage stage described in step S32 is initially divided into six stages including:

[0065] Stage 1: Initial depressurization stage: The working condition in this stage is the single-phase flow state of water. The main purpose of this stage is to obtain the formation fluid supply capacity and control the reasonable pressure difference range. In this stage, the water phase seepage channel is improved, the water production is increased, and the value of the anisotropic parameter β tends to be stable as the bottom hole flow pressure decreases, showing an exponential trend, and entering the transition depressurization stage;

[0066] Stage 2: Transitional depressurization stage. The working condition in this stage is the single-phase flow state of water. As the formation pressure decreases, the bottom hole flow pressure is lower than the saturation pressure P1, the state of coal matrix gas molecules changes, from adsorbed gas to free gas, and the value of the anisotropy parameter β suddenly increases. The main purpose of this stage is to control the value of the anisotropy parameter β to be stable and to reasonably control the pressure difference. As the formation pressure decreases and approaches the initial desorption pressure, the value of the anisotropy parameter β suddenly drops. In this stage, the water phase seepage channel is further improved, the water production reaches the peak, and enters the stable depressurization stage. As the flow pressure decreases, the fluctuation is small, and the drainage parameters need to be adjusted to control the pressure difference within a reasonable range to adapt to the changes in coalbed methane output.

[0067] Stage 3: Stable pressure reduction stage. The working condition in this stage is the three-phase flow state of water. As the formation pressure decreases, the bottom hole pressure is lower than the initial desorption pressure P2, the coal matrix gas molecules begin to migrate, and no continuous air flow channel is formed. At the same time, the gas-liquid flow in the coal seam drives the migration of coal powder proppant, and gas-liquid-solid three-phase flow appears in the formation. As the pressure decreases, the coal seam begins to desorb in large quantities. The main purpose of this stage is to control a reasonable pressure difference, prevent the occurrence of velocity-sensitive effects, damage the seepage channel, and keep the value of the anisotropy parameter β stable between 0-4. The bottom hole pressure gradually drops to the threshold pressure P3 and enters the slug flow stage. At this time, the desorption rate is the largest. As the bottom hole pressure decreases, the fluctuation range is small, and the water production is controlled to remain stable.

[0068] Stage 4 and Stage 5 - Slug Flow Stage: The wellbore operating conditions are: gas-liquid-solid three-phase flow, and the permeability of the water phase and gas phase changes, resulting in fluctuations in the value of the anisotropy parameter β, where:

[0069] In stage 4, the value of anisotropy parameter β increases, the fluctuation range increases, and the value of anisotropy parameter β gradually changes from the anisotropy of water phase permeability to the anisotropy of gas phase permeability. The production efficiency of coalbed methane begins to increase and reaches the peak desorption pressure P4, entering the half-amplitude point of the slug flow stage. At this time, the water phase permeability is dominant, and some coal powder and sand are carried out of the well.

[0070] The gas phase permeability is dominant in stage five, and the water phase permeability gradually decreases compared with stage four.

[0071] Stage 6: High-yield and stable production stage, bottom hole pressure and water production steadily decline, gas phase permeability dominates, high gas-liquid ratio, and the fluctuation range of the anisotropy parameter β tends to be stable, indicating that the coalbed methane well has entered a new gas production cycle. In this stage, it is necessary to closely monitor the changes in bottom hole pressure and gas production, and the drainage intensity should be gradually reduced. When the bottom hole pressure reaches the constant desorption pressure P5, it is necessary to ensure the continuous and stable production of coalbed methane until it is completely stable. This stage lasts for a long time.

[0072] Through the refined control of the above six stages, the recovery rate of coalbed methane can be effectively improved, the production life of coalbed methane wells can be extended, and the efficient development of coalbed methane resources can be achieved.

[0073] The innovation of the present invention lies in:

[0074] First: Through the characteristics of the second-order conductance isotherm adsorption curve, the drainage and production stages are finely divided to accurately grasp the production dynamic characteristics of different drainage and production stages;

[0075] Second: Using the Chen-type horizontal well productivity index equation and the quantitative relationship between the anisotropy parameter β and the production, a reasonable production pressure difference is determined to form an effective pressure drop funnel to ensure stable and high production capacity of medium-deep coalbed methane horizontal wells.

[0076] Beneficial effects: The step-by-step pressure reduction and drainage method for medium-deep coalbed methane horizontal wells disclosed in the present invention has the following beneficial effects:

[0077] 1. Strictly control the specific recovery rate indicators such as the maximum gas production and gas production time of horizontal wells, extend the stable and high production time of horizontal wells, and increase the recovery rate of horizontal wells.

[0078] 2. By drilling and sampling coal from parameter wells, basic experimental values ​​are obtained, and combined with the corresponding second-order derivative isothermal adsorption curve, the saturation pressure, initial desorption pressure, threshold desorption pressure, peak desorption pressure, and constant rate desorption pressure are quickly determined. Combined with the production capacity formula, the drainage and production stages are reasonably divided, and a step-by-step pressure reduction drainage and production system is formulated. This method significantly improves the mining efficiency of medium-deep coalbed methane wells, reduces operating costs, ensures the stable and high production capacity of medium-deep coalbed methane wells, provides a new way for the sustainable development of medium-deep coalbed methane resources, and shows good application prospects;

[0079] 3. Through the application of the present invention, medium-deep coalbed methane wells can achieve a significant increase in production, long-term stability of production capacity, and a continuous extension of gas production time, thereby providing strong technical support and theoretical guidance for the sustainable development of medium-deep coalbed methane resources. BRIEF DESCRIPTION OF THE DRAWINGS

[0080] Figure 1 The present invention discloses a flow chart of a method for step-by-step pressure reduction and drainage of medium-deep coalbed methane horizontal wells.

[0081] Figure 2 It is a scatter plot of the isothermal adsorption curve and the rate of change of the analytical rate, the anisotropy parameter β and the bottom hole flow pressure in the embodiment.

[0082] Figure 3 This is the relationship between the water production and anisotropy parameter β in stage one.

[0083] Figure 4 This is the relationship between the water production and anisotropy parameter β in stage 2.

[0084] Figure 5 This is the relationship between the water production and anisotropy parameter β in stage three.

[0085] Figure 6 This is the relationship between the gas production and the anisotropy parameter β in the initial gas production stage.

[0086] Figure 7 This is the relationship between gas production and anisotropy parameter β.

[0087] Figure 8 This is the relationship diagram between the bottom hole flowing pressure and the anisotropy parameter β in stage six.

[0088] Fig. 9 This is the drainage curve diagram of LXX-X well. DETAILED DESCRIPTION

[0089] The specific embodiments of the present invention are described in detail below.

[0090] If not otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0091] 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.

[0092] 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.

[0093] 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.

[0094] Unless otherwise specified, the reaction is carried out at room temperature and pressure.

[0095] Unless otherwise specified, all parts or percentages are by weight.

[0096] In the present invention, all substances used are known substances and can be purchased or synthesized by known methods.

[0097] In the present invention, the devices or equipment used are all conventional devices or equipment known in the art and are commercially available.

[0098] In this application: the five characteristic point values ​​of the second-order conductance isotherm adsorption curve are respectively: saturation pressure value, initial desorption pressure value, threshold desorption pressure value, peak desorption pressure value, and constant rate desorption pressure value according to the pressure value from high to low, where:

[0099] The saturation pressure value refers to the pressure corresponding to the maximum gas adsorption of coalbed methane in the coal matrix. In this state, the free gas molecules in the coal matrix and the adsorbed gas molecules in the coal matrix reach a dynamic equilibrium.

[0100] The initial desorption pressure value refers to the lowest pressure corresponding to when the gas molecules in the coal matrix begin to change from an adsorbed state to a non-adsorbed state during the desorption process. This pressure marks the starting point of the desorption process.

[0101] The threshold desorption pressure value refers to the pressure at which the desorption rate is fastest during the desorption process. This pressure value is usually a critical point. When the desorption rate is lower than this value, the desorption rate is still increasing, while when the desorption rate is higher than this value, the desorption rate will decrease.

[0102] The peak desorption pressure value refers to the fastest desorption rate during the desorption process, indicating that the desorption pressure reaches its peak value and casing begins to appear at the wellhead.

[0103] The constant desorption pressure value refers to the stable pressure value required to maintain a constant desorption rate during the desorption process. At this pressure, the gas molecules in the coal matrix are desorbed from the coal matrix at a relatively constant rate, so that the desorption process can continue without significant rate fluctuations.

[0104] The LXX-X well adopts a large-scale volume fracturing process, and the original daily gas production is not good. Now, a medium-deep coalbed methane horizontal well step-by-step pressure reduction and drainage method of the present invention is applied to the LXX-X well to perform corresponding operations.

[0105] A method for step-by-step pressure reduction and drainage of medium-deep coalbed methane horizontal wells, the specific steps are as follows:

[0106] S1: Collect basic data;

[0107] S2: Using the Chen-type horizontal well productivity index equation, the basic data collected in step S1 are deeply analyzed and calculated. Using the Chen-type horizontal well productivity index equation, the value of the anisotropy parameter β and the value range of the equivalent wellbore radius of the horizontal well are determined to verify the rationality of the drainage and production well system;

[0108] S3: Based on the second-order conductance isotherm adsorption curve, the drainage and production stage of the coalbed methane well is preliminarily divided into six stages.

[0109] The method further includes step S4: based on the relationship between the anisotropy parameter β and the gas production, water production and bottom hole pressure, finely dividing the six stages of the coalbed methane well drainage and production stage preliminarily divided in step S3.

[0110] Furthermore, the basic data in step S1 includes coal sample analysis data, core data and horizontal well production data.

[0111] Furthermore, the basic data in step S1 include permeability, horizontal section length, viscosity, volume coefficient, and wellbore radius.

[0112] Furthermore, the Chen-style horizontal well productivity index equation in step S2 is as follows:

[0113]

[0114] Where: q——output, unit is m 3 ;

[0115] Δp——reservoir pressure difference, in MPa;

[0116] K h ——horizontal permeability, in mD;

[0117] h——reservoir height, in m;

[0118] μ——viscosity, unit is mPa·s;

[0119] B——volume coefficient, dimensionless;

[0120] r——wellbore radius, in m;

[0121] a——liquid supply radius, in m;

[0122] L——horizontal section length, in m;

[0123] β——anisotropy parameter, dimensionless.

[0124] Furthermore, the calculation formula of the anisotropy parameter in step S2 is as follows:

[0125]

[0126] Where: β is anisotropy parameter, dimensionless;

[0127] K h ——horizontal permeability, in mD;

[0128] K v ——Vertical permeability, in mD.

[0129] Furthermore, the specific steps of step S3 are as follows:

[0130] S31: draw the second-order conductance isotherm adsorption curve;

[0131] S32: Based on the five pressure characteristic point values ​​of the second-order derivative isotherm adsorption curve, the drainage and production stages of the coalbed methane well are initially divided into six stages.

[0132] Furthermore, the coalbed methane well drainage stage described in step S32 is initially divided into six stages including:

[0133] Stage 1: Initial depressurization stage: The working condition in this stage is the single-phase flow state of water. The main purpose of this stage is to obtain the formation fluid supply capacity and control the reasonable pressure difference range. In this stage, the water phase seepage channel is improved, the water production is increased, and the value of the anisotropic parameter β tends to be stable as the bottom hole flow pressure decreases, showing an exponential trend, and entering the transition depressurization stage;

[0134] Stage 2: Transitional depressurization stage. The working condition in this stage is the single-phase flow state of water. As the formation pressure decreases, the bottom hole flow pressure is lower than the saturation pressure P1, the state of coal matrix gas molecules changes, from adsorbed gas to free gas, and the value of the anisotropy parameter β suddenly increases. The main purpose of this stage is to control the value of the anisotropy parameter β to be stable and to reasonably control the pressure difference. As the formation pressure decreases and approaches the initial desorption pressure, the value of the anisotropy parameter β suddenly drops. In this stage, the water phase seepage channel is further improved, the water production reaches the peak, and enters the stable depressurization stage. As the flow pressure decreases, the fluctuation is small, and the drainage parameters need to be adjusted to control the pressure difference within a reasonable range to adapt to the changes in coalbed methane output.

[0135] Stage 3: Stable pressure reduction stage. The working condition in this stage is the three-phase flow state of water. As the formation pressure decreases, the bottom hole pressure is lower than the initial desorption pressure P2, the coal matrix gas molecules begin to migrate, and no continuous air flow channel is formed. At the same time, the gas-liquid flow in the coal seam drives the migration of coal powder proppant, and gas-liquid-solid three-phase flow appears in the formation. As the pressure decreases, the coal seam begins to desorb in large quantities. The main purpose of this stage is to control a reasonable pressure difference, prevent the occurrence of velocity-sensitive effects, damage the seepage channel, and keep the value of the anisotropy parameter β stable between 0-4. The bottom hole pressure gradually drops to the threshold pressure P3 and enters the slug flow stage. At this time, the desorption rate is the largest. As the bottom hole pressure decreases, the fluctuation range is small, and the water production is controlled to remain stable.

[0136] Stage 4 and Stage 5 - Slug Flow Stage: The wellbore operating conditions are: gas-liquid-solid three-phase flow, and the permeability of the water phase and gas phase changes, resulting in fluctuations in the value of the anisotropy parameter β, where:

[0137] In stage 4, the value of anisotropy parameter β increases, the fluctuation range increases, and the value of anisotropy parameter β gradually changes from the anisotropy of water phase permeability to the anisotropy of gas phase permeability. The production efficiency of coalbed methane begins to increase and reaches the peak desorption pressure P4, entering the half-amplitude point of the slug flow stage. At this time, the water phase permeability is dominant, and some coal powder and sand are carried out of the well.

[0138] The gas phase permeability is dominant in stage five, and the water phase permeability gradually decreases compared with stage four.

[0139] Stage 6: High-yield and stable production stage, bottom hole pressure and water production steadily decline, gas phase permeability dominates, high gas-liquid ratio, and the fluctuation range of the anisotropy parameter β tends to be stable, indicating that the coalbed methane well has entered a new gas production cycle. In this stage, it is necessary to closely monitor the changes in bottom hole pressure and gas production, and the drainage intensity should be gradually reduced. When the bottom hole pressure reaches the constant desorption pressure P5, it is necessary to ensure the continuous and stable production of coalbed methane until it is completely stable. This stage lasts for a long time.

[0140] The five characteristic point values ​​of the second-order conductance isotherm adsorption curve are as follows: saturation pressure value, initial desorption pressure value, threshold desorption pressure value, peak desorption pressure value, and constant rate desorption pressure value according to the pressure value from high to low.

[0141] Furthermore, the steps of step S31 are as follows:

[0142] Step S311, collecting coal sample data of at least one coalbed methane well to be mined;

[0143] Step S312, obtaining the value of the adsorption amount of the sample at different pressures in the coal sample data, wherein:

[0144] In the step S312, 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;

[0145] In the step S312, 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;

[0146] Step S313, determining the value of the coal seam limit cumulative adsorption capacity and the coal seam isothermal transient pressure adsorption rate;

[0147] Step S314: Calculate the curvature K value, where:

[0148] According to the coal seam limit cumulative adsorption capacity and the coal seam isothermal transient pressure adsorption rate, the curvature K value, that is, the curvature of the isothermal adsorption curve, is calculated by the following formula:

[0149]

[0150] Among them, K is the curvature, R is the radius of curvature, and Y is the adsorption amount.

[0151] Step S315: Calculate the desorption rate K' by the first-order derivative formula of K, where the calculation formula of the desorption rate K' is:

[0152]

[0153] Step S316: Calculate the desorption rate change rate K'' by the second-order derivative formula of K, where:

[0154] The calculation formula of desorption rate change rate K" is:

[0155]

[0156] Step S317: Use the scatter plot to draw a relationship diagram between pressure and desorption rate, that is, obtain the second-order conductance isotherm adsorption curve, as shown in the following figure: Figure 2shown.

[0157] Furthermore, in step S313, according to the adsorption amount values ​​of the coal sample data of each well at different pressures obtained in step S312, an adsorption amount is calculated using Chen's equation, and compared with the experimental adsorption amount, the fitting degree is the highest, and the value of the coal seam limit cumulative adsorption amount and the coal seam isothermal transient pressure adsorption rate is determined, wherein:

[0158] Chen's equation in:

[0159] A is the maximum cumulative adsorption capacity of the coal seam;

[0160] B is the isothermal instantaneous pressure adsorption rate or isothermal instantaneous desorption rate of the coal seam;

[0161] P is the experimental pressure;

[0162] Psc is the ground pressure;

[0163] Vg is the experimental adsorption amount, and / or

[0164] In step S313, the values ​​of the coal seam limit cumulative adsorption amount and the coal seam isothermal transient pressure adsorption rate are calculated according to the iterative trial and error method, including:

[0165] The instantaneous adsorption / desorption linear equation of Chen's isotherm is:

[0166] ln(A / A-Vg)=B(P-Psc);

[0167] 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;

[0168] 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.

[0169] Furthermore, in step S317, a pressure and second-order derivative isothermal adsorption curve is drawn using a scatter plot to obtain a saturated pressure value and a constant rate desorption pressure value, and to predict an initial desorption pressure value, a threshold desorption pressure value and a peak desorption pressure value.

[0170] For the LXX-X well, according to the horizontal well productivity formula:

[0171]

[0172] It is known that the horizontal section length of the well is L = 743m. During the dewatering and pressure reduction stage, K is inverted by numerical simulation software. h=15mD, water viscosity coefficient μ = 0.6mPa·s water volume coefficient B w =1 gas volume coefficient B g =0.0042Horizontal section length L = 743mReservoir thickness h = 6mEquivalent wellbore radius ɑ = 461m

[0173] Specific collection system:

[0174] from Figure 2 It can be seen that in the relationship diagram between bottom hole pressure and anisotropy parameter β, in stages ①, ②, and ③, we can find that there is an exponential relationship. In stage ②, the anisotropy parameter β rises as a whole, and the water production increases in this stage, indicating that during the drainage and pressure reduction process, the seepage channel is dredged, and the ΔP pressure drop amplitude is controlled according to the change characteristics of the anisotropy parameter β:

[0175] The first initial pressure reduction stage (stage 1): determine the formation fluid supply capacity, and control the pressure difference at 100KPa-150KPa.

[0176] from Figure 3 It can be seen that in stage 1, the bottom hole pressure changes from 14.38 to 10.7 MPa. At this time, the value of the anisotropy parameter β changes more dramatically and has a larger range. It can be inferred that before the initial desorption, the formation is slowly dredged by continuous drainage and pressure reduction, and the liquid supply radius is gradually increasing.

[0177] The second transitional pressure reduction stage (stage 2): the pressure difference is controlled at 100KPa.

[0178] Steady drainage and pressure reduction to reach the initial desorption pressure. At this time, the value of the anisotropy parameter β does not change much, and the fluctuation range is between 0 and 10. At this time, the reservoir pressure is close to the initial analysis, and the anisotropy of the formation shows a stable trend according to the continuous drainage and pressure reduction.

[0179] The third stable pressure reduction stage (stage three): the pressure difference is controlled at 50KPa-70KPa.

[0180] When the initial desorption pressure is reached, the formation begins to desorb a small amount of gas, and gradually reaches the threshold desorption pressure. The adsorbed gas migrates into the reservoir seepage channel, resulting in a gradual decrease in the correlation between β and water production.

[0181] The ④ and ⑤ stages (stages four and five) are slug flow stages respectively: the pressure difference is controlled at 10KPa-20KPa.

[0182] This stage is also the plug flow stage where water, gas and coal powder are produced at the same time. The bottom hole pressure and water and gas production begin to change. With the change of water-gas relative permeability, the value of anisotropy parameter β fluctuates violently, and the water production is relatively stable. It is necessary to adjust the water production and the expansion speed of the pressure drop funnel in time to ensure the smooth desorption of coalbed methane. At the same time, it is necessary to pay close attention to the changes in water quality and the output of coal powder solid phase particles, stabilize the liquid level in time, control the bottom hole pressure relationship, and control the water quality and coal powder content. For details, see Figure 6-Figure 7 .

[0183] Stage ⑥ (Stage 6): High and stable production stage: The pressure difference is controlled at 10KPa-20KPa.

[0184] During this stage, the bottom hole flowing pressure decreases steadily, the value of the anisotropy parameter β decreases, and the fluctuation range decreases. The main concern is to maintain a stable gas production and minimize the fluctuation of the anisotropy parameter β.

[0185] The specific operation includes: after ensuring that the coalbed methane desorption reaches its peak, the bottom hole flow pressure is finely adjusted to achieve stable control of gas production. At the same time, by monitoring the changes in the value of the anisotropic parameter β, the drainage parameters are adjusted to maintain the stability of the value of the anisotropic parameter β and reduce its fluctuation. At this stage, since the desorption and production of coalbed methane have reached a relative equilibrium state, the correlation between the value of the anisotropic parameter β and the water production is further weakened, and the fluctuation range of the value of the anisotropic parameter β is minimized, thereby ensuring the efficiency and stability of the drainage process. For details, see Figure 8

[0186] In summary, the operation of each stage of the medium-deep coalbed methane step-by-step pressure reduction and drainage technology requires fine control to achieve high and stable production and sustainable mining of coalbed methane. In order to ensure the effective implementation of the technology, the following points should also be noted:

[0187] First, strengthen geological exploration and research on coal seam characteristics, accurately grasp key parameters such as coal seam reservoir characteristics, permeability, desorption characteristics, etc., and provide a basis for formulating scientific drainage and production plans.

[0188] Secondly, optimize the selection and configuration of drainage equipment to ensure that the performance of the equipment meets the drainage needs, while improving the reliability and stability of the equipment, reducing downtime, and improving drainage efficiency.

[0189] In addition, a complete monitoring system has been established to monitor key parameters such as formation pressure, water quality, water production, and gas production in real time, so as to promptly detect and handle abnormal situations and ensure the smooth progress of the production and drainage process.

[0190] At the same time, strengthen on-site management and personnel training, improve the skill level and safety awareness of operators, and ensure the safety and controllability of the drainage and production process.

[0191] Finally, we will focus on technological innovation and research and development, constantly explore new production technologies and methods, improve the efficiency and economic benefits of coalbed methane extraction, and make greater contributions to the development of the coalbed methane industry.

[0192] Through the implementation of the above measures, the effect of the step-by-step pressure reduction and drainage technology for medium and deep coalbed methane can be further improved, and the efficient development and utilization of coalbed methane resources can be achieved.

[0193] Finally, the drainage curve of the production well is as follows: Fig. 9 As shown, through the step-by-step pressure reduction and drainage method for medium-deep coalbed methane horizontal wells of the present invention, the daily gas production of the LXX-X well has reached 15,000 cubic meters.

[0194] The above describes the embodiments of the present invention in detail. However, the present invention is not limited to the above embodiments, and various changes can be made within the knowledge of ordinary technicians in the relevant technical field without departing from the purpose of the present invention.

Claims

1. A method for step-by-step pressure reduction and drainage of coalbed methane in medium-deep horizontal wells, characterized in that: The specific steps are as follows: S1: Collect basic data; S2: Using the Chen-type horizontal well productivity index equation, the basic data collected in step S1 are deeply analyzed and calculated. Using the Chen-type horizontal well productivity index equation, the value range of the anisotropic parameter β and the equivalent wellbore radius of the horizontal well are determined to verify the rationality of the drainage and production well system; S3: Based on the second-order conductance isotherm adsorption curve, the drainage and production stage of the coalbed methane well is preliminarily divided into six stages.

2. A method for step-by-step pressure reduction and drainage of medium-deep coalbed methane horizontal wells as claimed in claim 1, characterized in that: The method further includes step S4: based on the relationship between the anisotropy parameter β and the gas production, water production and bottom hole pressure, finely dividing the six stages of the coalbed methane well drainage and production stage preliminarily divided in step S3.

3. A method for step-by-step pressure reduction and drainage of medium-deep coalbed methane horizontal wells as claimed in claim 1, characterized in that: The basic data in step S1 include coal sample analysis data, core data and horizontal well production data, and / or The basic data in step S1 include permeability, horizontal section length, viscosity, volume coefficient, and wellbore radius.

4. A method for step-by-step pressure reduction and drainage of medium-deep coalbed methane horizontal wells as claimed in claim 1, characterized in that: The Chen-style horizontal well productivity index equation in step S2 is as follows: Where: q——output, unit is m 3 ; Δp——reservoir pressure difference, in MPa; K h ——horizontal permeability, in mD; h——reservoir height, in m; μ——viscosity, unit is mPa·s; B——volume coefficient, dimensionless; r——wellbore radius, in m; a——liquid supply radius, in m; L——horizontal section length, in m; β——anisotropy parameter, dimensionless; The calculation formula of the anisotropy parameter β in step S2 is as follows: Where: β is anisotropy parameter, dimensionless; K h ——horizontal permeability, in mD; K v ——Vertical permeability, in mD.

5. The method for step-by-step pressure reduction and drainage of medium-deep coalbed methane horizontal wells according to claim 1, characterized in that: The specific steps of step S3 are as follows: S31: draw the second-order conductance isotherm adsorption curve; S32: Based on the five pressure characteristic points of the second-order derivative isothermal adsorption curve, the drainage stage of the coalbed methane well is initially divided into six stages, including: The five characteristic point values ​​of the second-order conductance isotherm adsorption curve are as follows: saturation pressure value, initial desorption pressure value, threshold desorption pressure value, peak desorption pressure value, and constant rate desorption pressure value according to the pressure value from high to low.

6. A method for step-by-step pressure reduction and drainage of medium-deep coalbed methane horizontal wells as claimed in claim 5, characterized in that: The steps of step S31 are as follows: Step S311, collecting coal sample data of at least one coalbed methane well to be mined; Step S312, obtaining the value of the adsorption amount of the sample at different pressures in the coal sample data, wherein: In the step S312, 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; In the step S312, 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; Step S313, determining the value of the coal seam limit cumulative adsorption capacity and the coal seam isothermal transient pressure adsorption rate; Step S314: Calculate the curvature K value, where: According to the coal seam limit cumulative adsorption capacity and the coal seam isothermal transient pressure adsorption rate, the curvature K value, that is, the curvature of the isothermal adsorption curve, is calculated by the following formula: Among them, K is the curvature, R is the radius of curvature, and Y is the adsorption amount; Step S315: Calculate the desorption rate K' by the first-order derivative formula of K, where the calculation formula of the desorption rate K' is: Step S316: Calculate the desorption rate change rate K'' by the second-order derivative formula of K, where: The calculation formula of desorption rate change rate K" is: Step S317: Use the scatter plot to draw a relationship diagram between pressure and desorption rate, that is, obtain a second-order derivative isotherm adsorption curve.

7. A method for step-by-step pressure reduction and drainage of medium-deep coalbed methane horizontal wells as claimed in claim 5, characterized in that: In step S313, according to the adsorption amount values ​​of the coal sample data of each well at different pressures obtained in step S312, an adsorption amount is calculated using Chen's equation, and compared with the experimental adsorption amount, the fitting degree is the highest, and the value of the coal seam limit cumulative adsorption amount and the coal seam isothermal transient pressure adsorption rate is determined, wherein: Chen's equation in: 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 amount, and / or In step S313, the values ​​of the coal seam limit cumulative adsorption amount 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; Wherein, 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 / or In step S317, a pressure and second-order derivative isothermal adsorption curve is drawn using a scatter plot to obtain a saturated pressure value and a constant rate desorption pressure value, and to predict an initial desorption pressure value, a threshold desorption pressure value, and a peak desorption pressure value.

8. A method for step-by-step pressure reduction and drainage of medium-deep coalbed methane horizontal wells as claimed in claim 5, characterized in that: The coalbed methane well drainage stage described in step S32 is initially divided into six stages including: Stage 1: Initial depressurization stage: The working condition in this stage is the single-phase flow state of water. The main purpose of this stage is to obtain the formation fluid supply capacity and control the reasonable pressure difference range. In this stage, the water phase seepage channel is improved, the water production is increased, and the value of the anisotropic parameter β tends to be stable as the bottom hole flow pressure decreases, showing an exponential trend, and entering the transition depressurization stage; Stage 2: Transitional depressurization stage. The working condition in this stage is the single-phase flow state of water. As the formation pressure decreases, the bottom hole flow pressure is lower than the saturation pressure P1, the state of coal matrix gas molecules changes from adsorbed gas to free gas, and the value of anisotropy parameter β suddenly increases. The main purpose of this stage is to control the value of anisotropy parameter β to be stable and to reasonably control the pressure difference. Stage 3: Stable pressure reduction stage. The working condition in this stage is the three-phase flow state of water. As the formation pressure decreases, the bottom hole flow pressure is lower than the initial desorption pressure P2, the coal matrix gas molecules begin to migrate, and no continuous air flow channel is formed. At the same time, the gas-liquid flow in the coal seam drives the migration of coal powder proppant, and gas-liquid-solid three-phase flow appears in the formation. As the pressure decreases, the coal seam begins to desorb in large quantities. The main purpose of this stage is to control a reasonable pressure difference, prevent the occurrence of velocity-sensitive effects, damage the seepage channel, and keep the value of the anisotropy parameter β stable between 0-4. The bottom hole flow pressure gradually drops to the threshold pressure P3, entering the slug flow stage; Stage 4 and Stage 5 - Slug Flow Stage: The wellbore operating conditions are: gas-liquid-solid three-phase flow, and the permeability of the water phase and gas phase changes, resulting in fluctuations in the value of the anisotropy parameter β, where: In stage 4, the value of anisotropy parameter β increases, the fluctuation range increases, and the value of anisotropy parameter β gradually changes from the anisotropy of water phase permeability to the anisotropy of gas phase permeability. The production efficiency of coalbed methane begins to increase and reaches the peak desorption pressure P4, entering the half-amplitude point of the slug flow stage. At this time, the water phase permeability is dominant, and some coal powder and sand are carried out of the well. In stage five, the gas phase permeability is dominant, and the water phase permeability gradually decreases compared with stage four; Stage 6: High and stable production stage, bottom hole pressure and water production steadily decrease, gas permeability dominates, high gas-liquid ratio, and the fluctuation range of the anisotropy parameter β tends to be stable, indicating that the coalbed methane well has entered a new gas production cycle.

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