Method for rapidly determining main control factors of coal gas production capacity

Abstract

The invention discloses a method for rapidly determining a main control factor of coal-series gas production capacity, and belongs to the technical field of unconventional energy development. The method comprises the following steps: firstly, determining hydrological and basic geological data of a coal reservoir through exploration and development data, and discussing correlation among factors; then calculating the content of initial adsorbed gas and free gas in the coal seam based on the particularity of the occurrence condition of the deep coal seam; on the basis of the comprehensive geological conditions, constructing a coal gas measure numerical model to perform coal gas measure numerical simulation calculation; using a historical sensitivity analysis method to compare the coal gas production capacity changes under different parameter changes; and finally, comprehensively utilizing a principal component analysis method and an ash content correlation method to carry out coal bed gas productivity influence factor weight calculation, and determining a theoretical thought of the main control factors influencing the coal-series gas productivity based on a weight maximization principle. According to the prediction method for rapidly determining the main control factors of the gas production capacity of the coal series, a good theoretical basis is created for the technical field of unconventional energy development.

Description

Technical Field

[0001] The present invention relates to the technical field of unconventional energy development, and in particular to a method for rapidly determining main controlling factors of coal-measure gas production capacity. Background Art

[0002] After more than 20 years of resource evaluation, development technology trials, and large-scale development, my country has initially established a coalbed methane exploration and development technology system adapted to its geological conditions, forming two major industrial bases in the Qinshui and Ordos Basins. my country's coalbed methane exploration and development has made steady progress, with technological breakthroughs achieved in Shanxi, Shaanxi, Northeast China, North China, Anhui, Yunnan, and Xinjiang.

[0003] However, significant challenges remain in my country's coalbed methane development. The average daily gas production per well is far below the average levels of the United States, Canada, and Australia. Large-scale development is only underway in the Qinshui and Ordos basins, while most basins, despite long-term exploration and development trials, have yet to achieve industrialization. A key factor contributing to the disproportionate growth rate between the number of coalbed methane wells and total production, lagging production capacity, and a limited focus on development basins is a lack of understanding of the dynamic changes in gas and water flow and reservoir parameters during the production process, and a lack of awareness of the impact of these changes on production capacity.

[0004] During coalbed methane (CBM) drainage and production, the directional flow of water and gas causes the coal reservoir's pressure system, stress field, fluid system, and reservoir physical properties to continuously change and exhibit spatial differentiation. The dynamic coupling of these systems or physical fields, accompanied by coordinated changes in the desorption, diffusion, and seepage processes of the coal reservoir, has engineering significance in the form of changes in CBM production capacity. During the dewatering and pressure-reducing CBM development process, the spatial structure, fluid system, and stress field of the coal reservoir continuously change with the extraction of the fluid medium, leading to coordinated changes in the pressure, concentration, desorption, diffusion, and seepage of CBM within the reservoir's pore and fracture structures at different scales.

[0005] With the implementation of coal-measure gas development pilot projects, researchers have gained a deeper understanding of the generation, storage, and migration of coalbed methane. They have also realized the urgent need for an effective tool to perform historical matching of gas and water production data from production wells, adjust and obtain more accurate coalbed methane reservoir parameters, and predict the long-term production dynamics and output of wells. This provides a scientific basis for well pattern layout, well shape selection, well production system optimization, and the most cost-effective coalbed methane project development plan. Against this backdrop, coalbed methane numerical simulation work came into being. Summary of the Invention

[0006] The purpose of the present invention is to provide a method for quickly determining the main controlling factors of coal-bed methane production capacity, which provides a theoretical basis for coal-bed methane production capacity prediction and research on influencing factors.

[0007] In order to achieve the above object, the present invention adopts the following technical solutions:

[0008] A method for quickly determining the main controlling factors of coal-measure gas production capacity, wherein the method quantitatively determines the main controlling factors of coal-measure gas production capacity by combining refined three-dimensional geological modeling and numerical simulation technology, and the method sequentially comprises the following steps:

[0009] a. Based on exploration and development data, clarify the hydrological and basic geological data of the coal reservoir, focusing on the coal seam depth, thickness, porosity, permeability, gas-water relative permeability curve and adsorption characteristics, and explore the correlation between these factors;

[0010] b. Based on the special occurrence conditions of deep coal seams, calculate the initial adsorbed gas and free gas content in deep coal seams;

[0011] c. Based on comprehensive geological conditions, a deep coalbed methane numerical model is constructed to perform deep coalbed methane numerical simulation calculations;

[0012] d. Use historical sensitivity analysis methods to adjust geological and engineering parameters and compare changes in deep coalbed methane production capacity under different parameter changes;

[0013] e. Use principal component analysis and ash correlation method to calculate the weights of factors affecting coalbed methane production capacity, and determine the main controlling factors affecting deep coalbed methane production capacity based on the principle of weight maximization.

[0014] The above-mentioned method for quickly determining the main controlling factors of coal-bearing gas production capacity, in step a, the coal rank type of the target coal seam is determined by coal rock basic component testing, and the pore structure parameters, Langmuir pressure, Langmuir volume and critical desorption pressure of the target coal seam are determined by reservoir physical property characterization testing; the initial permeability, porosity and gas-water two-phase permeability curve of the target coal seam are determined by porosity and permeability testing; the mechanical properties of the coal rock and roof and floor of the target coal seam are revealed by basic mechanical parameter testing, and the compression coefficient of the target coal seam in the study area is derived; the dynamic change test of porosity and permeability is used to determine the cracks and pore compression coefficient of the target coal seam; the physical simulation of gas and water migration is used to reveal the production capacity law mechanism of the target coal seam under the influence of different design schemes; and the permeability of the target coal seam after fracturing is determined by fracturing fracture diversion simulation.

[0015] In the above-mentioned method for quickly determining the main controlling factors of coal-measure gas production capacity, in step b, for coal-measure gas wells that have undergone pressure-maintained coring, the pressure-maintained gas is considered to be the initial free gas content, and the sum of the lost gas, desorbed gas, and residual gas is taken as the initial adsorbed gas content; for coal-measure gas wells that have undergone rope coring, the lost gas is considered to be the initial free gas content, and the sum of the desorbed gas and residual gas is taken as the initial adsorbed gas content.

[0016] The above method for quickly determining the main controlling factors of coal-measure gas production capacity is used to calculate the free gas content in coal-measure gas wells that have not undergone gas content core sampling testing, combining equations (1) to (8):

[0017]

[0018] In formula (1): M g is the molar mass of methane; f P is the reservoir pressure gradient; H is the reservoir depth; P0 is the pressure at point 0 of the reservoir pressure fitting line; ρ c is the apparent density of coal; ρ sc is the density of methane gas under standard conditions; R is the gas constant; Z is the gas compressibility factor; is the coal porosity; is the initial porosity of the coal sample; σ is the average ground stress; υ is the Poisson's ratio of coal; E is the Young's modulus of coal; p is the reservoir pressure; m ar Analyze moisture content for the coal industry; ρ c / ρ w is the ratio of apparent density of coal sample to water density;

[0019]

[0020] In formula (2): q f is the free gas content; P is the in situ pressure; T is the in situ temperature; Z is the gas compressibility factor; P a and T a Represent the standard state pressure and temperature of gas respectively; ρ reservoir density;

[0021]

[0022] In formula (3): G MF is the methane content in the free gas; S W is the water saturation; B is the methane volume coefficient; ρ a is the apparent density of coal; ρ t is the true density of coal;

[0023]

[0024] C y =Φ·P·K·S (5);

[0025] In formula (5): C y is the free gas content of the coal matrix; Φ is the effective pore volume for free gas in the coal matrix; P is the in situ pressure; K is the compressibility coefficient of methane; S is the gas saturation;

[0026]

[0027] In formula (6), T0 is 273K; φ is the ratio of the volume of pores in coal to the total volume; φ w , is the pore volume occupied by water in coal; ρ coal is the density of coal; k is the coefficient used to correct the change of porosity with stress;

[0028]

[0029] In formula (7): V t is the free gas content; S W is water saturation; ρ c B is the apparent density of coal rock; g is the methane volume coefficient;

[0030]

[0031] In formula (8): V free is the amount of free methane; ρ CH4 is the methane density; ρ coal is the density of coal; φ f is the porosity not occupied by water; M CH4 is the molecular weight of methane; M free is the molecular weight of the free gas.

[0032] In the above-mentioned method for quickly determining the main controlling factors of coal-bearing gas production capacity, the specific steps of step c are as follows: assuming that the sealing property of the cap rock is perfectly closed without exceeding the pressure limit, and the boundary condition is set to no flow; using the BHP parameter in the simulation to control the upper limit of the reservoir pressure, which is actually related to parameters such as the fracture pressure that destroy the reservoir sealing property; and performing vertical single-well production capacity simulation and horizontal single-well production capacity simulation respectively.

[0033] The above-mentioned method for quickly determining the main controlling factors of coal-bearing gas production capacity is used to further clarify the basic reservoir physical parameters, fluid parameters, and grid parameters that need to be input into the simulation software based on the information obtained in step a when conducting vertical single-well production capacity simulation, and to construct a deep coalbed methane numerical model.

[0034] The above-mentioned method is a method for quickly determining the main controlling factors of coal-bearing gas production capacity. When conducting horizontal single-well production capacity simulation, ECLIPSE software is used for simulation. After the simulation is successfully run, the gas production and water production results and the changes in the reservoir pressure plane during the production process are output.

[0035] In the above-mentioned method for quickly determining the main controlling factors of coal-bearing gas production capacity, in step d, geological conditions include coal seam thickness, burial depth, reservoir pressure and gas content; engineering conditions include well type and horizontal section length; changes in deep coalbed methane production capacity include changes in maximum gas production, average gas production and stable gas production.

[0036] Compared with the prior art, the present invention brings the following beneficial technical effects:

[0037] (1) The present invention innovatively proposes a method for quickly determining the main controlling factors of coal-measure gas production capacity. This method integrates the advantages of coal-bed methane theoretical calculation models and numerical models. Based on exploration and development data, the coal reservoir hydrology and basic geological data are clarified, focusing on clarifying information such as coal seam burial depth, thickness, porosity, permeability, gas-water relative permeability curve and adsorption characteristics, and exploring the correlation between various factors. Secondly, according to the special conditions of deep coal seam occurrence, the initial adsorbed gas and free gas content in the coal seam are calculated. Then, based on the geological conditions, a coal-measure gas numerical model is constructed to perform coal-measure gas numerical simulation calculations. At the same time, using the historical sensitivity analysis method, the geological conditions (coal seam thickness, burial depth, reservoir pressure, gas content) and engineering conditions (well type, horizontal section length, etc.) parameters are adjusted, and the changes in coal-measure gas production capacity (maximum gas production, average gas production and stable gas production) under different parameter changes are compared. The principal component analysis method, ash correlation method and other methods are comprehensively used to calculate the weights of factors affecting coal-bed methane production capacity, and the main controlling factors affecting coal-measure gas production capacity are determined based on the weight maximization principle. This prediction method makes full use of the reservoir geological data of the original study area and easily obtains a large number of basic geological parameters of the study area, which can provide a good theoretical basis for the subsequent simulation and prediction of coalbed methane production capacity in the study area and the study of changes in coal-measure gas production capacity under the influence of different factors.

[0038] (2) The present invention can quickly determine the prediction method of the main controlling factors of coal-based gas production capacity, creating a good theoretical foundation for the field of unconventional energy development technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] The present invention will be further described below with reference to the accompanying drawings:

[0040] Figure 1 This is a vertical well drainage geological model diagram of the present invention;

[0041] Figure 2 This is a horizontal single well model diagram of the present invention;

[0042] Figure 3 This is the gas-water relative permeability curve of the present invention;

[0043] Figure 4 It is the capillary pressure curve diagram of the present invention;

[0044] Figure 5 is the fluid parameter relationship diagram, Figure 5 (a) shows the adsorption isotherm curve, Figure 5 (b) shows the relationship between water saturation and water permeability. Figure 5 (c) shows the relationship between gas saturation and gas permeability;

[0045] Figure 6The present invention obtains a comparison chart of coalbed methane production under different permeability conditions when the reservoir permeability is 0.2mD. Figure 6 (a) shows the relationship between drainage time and gas production. Figure 6 (b) shows the relationship between reservoir pressure and drainage time; Figure 6 (c) shows the relationship between drainage time and water production;

[0046] Figure 7 This is a comparison chart of coalbed methane production under different permeability conditions obtained by the present invention when the reservoir permeability is 2mD. Figure 7 (a) shows the relationship between drainage time and gas production. Figure 7 (b) shows the relationship between drainage time and reservoir pressure; Figure 7 (c) shows the relationship between drainage time and water production;

[0047] Figure 8 This is a comparison chart of reservoir pressure after 1 year, 5 years and 20 years of production for a single horizontal well;

[0048] Figure 9 This is a comparison chart of gas content after 1, 5, and 20 years of production for a single horizontal well.

[0049] Figure 10 This is the comparison of coalbed methane production with a reservoir permeability of 0.2mD. Figure 10 (a) shows the comparison between drainage time and gas production. Figure 10 (b) shows a comparison diagram of drainage time and reservoir pressure; Figure 10 (c) shows a comparison chart of drainage time and water production;

[0050] Figure 11 Comparison of gas production in coal seams with a reservoir permeability of 2mD. Figure 11 (a) shows the comparison between drainage time and gas production. Figure 11 (b) shows a comparison diagram of drainage time and reservoir pressure; Figure 11 (c) shows a comparison chart of drainage time and water production;

[0051] Figure 12 The following are the reservoir pressure comparison diagrams after 1 year, 5 years and 20 years of drainage when the coal seam thickness is set to 0.5m and 1m respectively;

[0052] Figure 13 The comparison chart of gas content after 1 year, 5 years and 20 years of mining when the coal seam thickness is set to 0.5m and 1m respectively;

[0053] Figure 14 When the porosity is 0.1%, the coalbed methane production is compared. Figure 14 (a) shows the comparison between drainage time and gas production. Figure 14(b) shows a comparison diagram of drainage time and reservoir pressure; Figure 14 (c) shows a comparison chart of drainage time and water production;

[0054] Figure 15 The comparison of coalbed methane production when the porosity is 3% is shown below. Figure 15 (a) shows the comparison between drainage time and gas production. Figure 15 (b) shows a comparison diagram of drainage time and reservoir pressure; Figure 15 (c) shows a comparison chart of drainage time and water production;

[0055] Figure 16 Comparison chart of reservoir pressure after 1 year, 5 years and 20 years of drainage when the porosity is 0.1% and 3%;

[0056] Figure 17 Comparison chart of gas content after 1, 5, and 20 years of drainage when the porosity is 0.1% and 3%;

[0057] Figure 18 The gas content is 5m 3 Comparison chart of coalbed methane production under / t conditions, Figure 18 (a) shows the comparison between drainage time and gas production. Figure 18 (b) shows a comparison diagram of drainage time and reservoir pressure; Figure 18 (c) shows a comparison chart of drainage time and water production;

[0058] Figure 19 The gas content is 10m 3 Comparison chart of coalbed methane production under / t conditions, Figure 19 (a) shows the comparison between drainage time and gas production. Figure 19 (b) shows a comparison diagram of drainage time and reservoir pressure; Figure 19 (c) shows a comparison chart of drainage time and water production;

[0059] Figure 20 The gas content is set to 5m 3 / t、10m 3 / t, the reservoir pressure comparison chart after 1 year, 5 years and 20 years of drainage;

[0060] Figure 21 The gas content is set to 5m 3 / t、10m 3 / t, the comparison chart of gas content after 1 year, 5 years and 20 years of production;

[0061] Figure 22 The Langmuir volume is 10m 3 / t, coalbed methane production comparison, Figure 22 (a) shows the comparison between drainage time and gas production. Figure 22(b) shows a comparison diagram of drainage time and reservoir pressure; Figure 22 (c) shows a comparison chart of drainage time and water production;

[0062] Figure 23 The Langmuir volume is 15m 3 / t, coalbed methane production comparison, Figure 23 (a) shows the comparison between drainage time and gas production. Figure 23 (b) shows a comparison diagram of drainage time and reservoir pressure; Figure 23 (c) shows a comparison chart of drainage time and water production;

[0063] Figure 24 It is the structure diagram of the hierarchical analysis model;

[0064] Figure 25 This is a comparison chart of weight values ​​of the hierarchical analysis method. DETAILED DESCRIPTION

[0065] The present invention proposes a method for quickly determining the main controlling factors of coal-based gas production capacity. In order to make the advantages and technical solutions of the present invention clearer and more specific, the present invention is further described below with reference to specific embodiments.

[0066] The technical solutions in the embodiments of the present application will be described clearly and completely below in conjunction with the drawings in the embodiments of the present application.

[0067] The vertical well drainage geological model in the simulation process of the present invention is as follows Figure 1 As shown, the horizontal single well model is as follows Figure 2 shown.

[0068] The present invention is aimed at coal-bearing gas wells that have not been subjected to gas content core sampling test, and the free gas content is calculated by combining formulas (1) to (8):

[0069]

[0070] In formula (1): M g is the molar mass of methane, g / mol; f P is the reservoir pressure gradient, MPa / hm; H is the reservoir burial depth, hm; p0 is the pressure at point 0 of the reservoir pressure fitting line, MPa; ρ c is the apparent density of coal, kg / m 3 ρ sc is the density of methane gas under standard conditions, kg / m 3 ; R is the gas constant, its value is 8.314 J / (mol·K); Z is the gas compressibility factor; is the coal porosity; is the initial porosity of the coal sample, which is obtained by gas porosity measurement under stress-free conditions. Under this condition, σ0 = 0 MPa, p0 = 0 MPa; σ is the average ground stress, MPa; υ is the Poisson's ratio of coal; E is the Young's modulus of coal; p is the reservoir pressure, MPa; m ar To analyze the moisture content in the coal industry, the raw coal basis is used for characterization; ρ c / ρ w is the ratio of apparent density of coal sample to water density;

[0071]

[0072] In formula (2): q f is the free gas content, m 3 / t; P is the in situ pressure; T is the in situ temperature; Z is the gas compressibility factor; P a and T a Represent the standard state pressure and temperature of gas respectively; ρ reservoir density;

[0073]

[0074] In formula (3): G MF is the methane content in the free gas (air dry basis), cm 3 / g;S W is water saturation, %; B is methane volume coefficient, dimensionless; ρ a is the apparent density of coal, g / cm 3 ρ t is the true density of coal, g / cm 3 ; Z is the gas compressibility factor, dimensionless; p is the reservoir pressure, MPa; T is the reservoir temperature, ℃.

[0075]

[0076] C y =Φ·P·K·S (5);

[0077] In formula (5): C y is the free gas content of coal matrix, cm 3 / g; Φ is the effective pore volume for free gas in the coal matrix, cm 3 / g; P is the in situ pressure, MPa; K is the compressibility coefficient of methane, MPa -1 ; S is gas saturation, %;

[0078]

[0079] In formula (6), p is the pressure in the coal seam, in MPa; T is the temperature in the coal seam, in K; the gas pressure p0 under standard conditions is 0.101 MPa; the absolute temperature T0 under standard conditions is 273 K; the gas compressibility factor Z is used to correct the deviation between the actual gas and the ideal gas; φ is the ratio of the volume of pores in the coal to the total volume; φ w , the pore volume occupied by water in coal; ρ coal , the density of coal, in t / m 3 ; k, coefficient used to correct porosity variation with stress;

[0080]

[0081] In formula (7): V t is the free gas content; S W is water saturation; ρ c B is the apparent density of coal rock; g is the methane volume coefficient;

[0082]

[0083] In formula (8): V free is the amount of free methane gas, in m 3 / t;ρ CH4 is the density of methane, in g / cm 3 ρ coal : Density of coal, in g / cm 3 ;φ f is the porosity not occupied by water; M CH4 is the molecular weight of methane, in g / mol; M free is the molecular weight of the free gas, in g / mol.

[0084] Example 1:

[0085] The standard process for simulating coal-measure gas production capacity is based on the Ningdong coalfield in the Ordos Basin as an example. The key factors controlling coal-measure gas production capacity are quickly determined. The specific steps include the following:

[0086] Step 1: Based on exploration and development data, clarify the hydrological and basic geological data of the coal reservoir, focusing on information such as coal seam depth, thickness, porosity, permeability, gas-water relative permeability curve, and adsorption characteristics, and explore the correlation between various factors. As shown in Table 1, the target coal seam rank type is determined through coal rock basic component testing; the target coal seam pore structure parameters, Lang's pressure, Lang's volume, and critical desorption pressure are determined through reservoir physical property characterization testing; the target coal seam initial permeability, porosity, and gas-water two-phase permeability curve are determined through porosity and permeability testing; the target coal seam coal rock and roof and floor mechanical properties are revealed through basic mechanical parameter testing, and the target coal seam compression coefficient in the study area is derived; the target coal seam fracture and pore compression coefficient are determined through porosity and permeability dynamic change testing; the target coal seam physical simulation is used to reveal the production capacity mechanism of the target coal seam under the influence of different design schemes; and the target coal seam post-fracturing permeability is determined through fracturing fracture conductivity simulation (fracturing fracture conductivity test).

[0087] This study assumes that the caprock is perfectly sealed, provided the pressure limit is not exceeded. Furthermore, boundary conditions are set to no flow. The simulation uses the bottomhole pressure (BHP) parameter to control the upper limit of the reservoir pressure. In practice, this pressure is related to parameters such as the fracture pressure that can disrupt the reservoir's seal. Figure 3 、 4 are the phase permeability curve and capillary pressure used in the numerical simulation.

[0088] Drawing on previous research and achievements, and based on data from coalfield geological exploration, coalbed methane exploration and development, and previous scientific research, this study utilizes a variety of methods and approaches, including geological assessment surveys, sampling, and drilling data, to fully define the basic geology, coal petrology, adsorption reservoirs, reservoir physical properties, and fluid dynamics of the study area. A comprehensive analysis of the study area's overall geological characteristics was conducted. A detailed summary and analysis of the spatial distribution characteristics, gas content, and other conditions of the coal reservoirs in the test area was conducted. Based on the actual needs of the simulation software, relevant parameters were preliminarily determined, aiming to provide practical reference data for subsequent analysis of key controlling factors. Specific parameters are shown in Table 2.

[0089] Step 2: Based on the special conditions of deep coal seams, calculate the initial adsorbed gas and free gas content in the coal seams.

[0090] Taking a well in the deep coal reservoir on the eastern edge of the Ordos Basin as an example, the relevant samples were cored using the pressure-maintaining coring process to obtain the pressure-maintaining gas, loss gas, desorbed gas, residual gas, and total gas content. According to formula (6), the specific relevant parameters are summarized in Table 3. In the table, Z is the gas compressibility factor, which is used to correct the deviation between the actual gas and the ideal gas; p is the pressure in the coal seam; T is the temperature in the coal seam; the gas pressure p0 under standard conditions is 0.101 MPa; the absolute temperature T0 under standard conditions is 273K; φ is the ratio of the volume of pores in the coal to the total volume; φw , the pore volume occupied by water in coal; φ-φ w is the effective porosity, excluding the interference of reservoir water content; ρ coal is the density of coal; k is the coefficient for correcting porosity changes with stress; V s is the free gas content; V total is the total gas content; V a is the adsorbed gas content. This method fully considers relevant factors such as the temperature and pressure conditions in the coal seam, porosity and permeability dynamics, and gas compressibility factors. It can adapt to various realistic conditions of higher temperatures and pressures and complex gas states, and achieves a refined description and quantitative analysis of free gas and adsorbed gas in deep coal reservoirs.

[0091] Step 3: Based on comprehensive geological conditions, a coal-bearing gas numerical model is constructed to perform numerical simulation calculations of coal-bearing gas.

[0092] The basic reservoir conditions of the study area were parameterized and input into the simulation software, and different well type schemes were set to establish the numerical model. The well type scheme set this time was the horizontal well, which is one of the basic well types in the coalbed methane development project.

[0093] (1) Model definition

[0094] According to the coalbed methane production cycle, the coalbed methane production cycle is set to 20 years, and the time step is 30 days.

[0095] (2) Reservoir description

[0096] For this project's coalbed methane numerical simulation, the coal seam depth was set to 800 m, the horizontal deviation was set to 0, the coal seam thickness was set to 5 m, and the well depth was set to 1000 m. Rock mechanical property parameters primarily included fracture porosity, permeability, and matrix compressibility. Reservoir geological parameters primarily included reservoir pressure and gas content. The simulation area was 1000 m × 1000 m, with a grid cell spacing of 50 m.

[0097] (3) Well location setting

[0098] The wellhead coordinates of the horizontal single well are set to (500, 500), the well depth is 1300m, the perforation range is 750-850m, and the coalbed methane production is carried out using a working system of constant bottom hole flow pressure. According to the experience of coalbed methane development, the depletion pressure is 0.7Mpa.

[0099] (4) Fluid characteristics

[0100] The reservoir fluid is set as gas-water two-phase fluid, the desorption time of methane is 10 days, and the relative permeability model parameters are set as follows: Figure 5 shown.

[0101] After completing the above four main parameter settings, the horizontal single well model is as follows: Figure 2As shown in the figure, the results show that the model settings and software operation status are normal, and subsequent simulation work can be carried out.

[0102] Step 4: Use the historical sensitivity analysis method to focus on adjusting the parameters of geological conditions (coal seam thickness, burial depth, reservoir pressure, gas content) and engineering conditions (well type, horizontal section length, etc.), and compare the changes in coal-measure gas production capacity (maximum gas production, average gas production and stable gas production) under different parameter changes.

[0103] In the process of analyzing the main controlling factors of the productivity of a single horizontal well, numerical simulations were performed on different reservoir permeabilities, coal seam thicknesses, porosities, initial gas contents, and Langmuir volumes, and comparative analyses were conducted on the dynamic changes of gas production, reservoir pressure, and reservoir gas content.

[0104] (1) Permeability

[0105] In order to compare the effects of different permeabilities on CBM productivity during horizontal single well development, the reservoir permeabilities were set to 0.2 mD and 2 mD for numerical simulation. Other detailed simulation parameters are the same as those in Table 2.

[0106] The simulation results are as follows Figure 6 、 Figure 7 As shown, combined Figure 6 、 Figure 7 As shown in the figure, higher permeability can bring more significant contribution to gas production, and the trend of reservoir pressure decrease is more obvious. Figure 6 (a) shows the relationship between drainage time and gas production. Figure 6 (b) shows the relationship between reservoir pressure and drainage time; Figure 6 (c) shows the relationship between drainage time and water production; Figure 7 (a) shows the relationship between drainage time and gas production. Figure 7 (b) shows the relationship between drainage time and reservoir pressure; Figure 7 (c) shows the relationship between drainage time and water production. Figure 8 It also shows that during the drainage process, the range and amplitude of reservoir pressure drop are more significant when the permeability is higher. Figure 9 It shows that the intensity and range of the decrease in reservoir gas content during drainage for a reservoir permeability of 2 mD are significantly higher than those for a reservoir permeability of 0.2 mD. Obviously, a higher permeability will bring about stronger reservoir changes and higher gas production.

[0107] (2) Coal seam thickness

[0108] When analyzing the impact of coal seam thickness, the coal seam thickness was set to 0.5 m and 1 m for productivity simulation to compare the impact of coal thickness on vertical well group development. Other detailed simulation parameters are the same as Table 2.

[0109] Combine Figures 10 to 13As shown, the coal seam thickness has no obvious effect on the coalbed methane production, reservoir pressure drop propagation, and gas content change. Figure 10 (a) shows the comparison between drainage time and gas production. Figure 10 (b) shows a comparison diagram of drainage time and reservoir pressure; Figure 10 (c) shows a comparison chart of drainage time and water production; Figure 11 (a) shows the comparison between drainage time and gas production. Figure 11 (b) shows a comparison diagram of drainage time and reservoir pressure; Figure 11 (c) shows a comparison chart of drainage time and water production.

[0110] (3) Porosity

[0111] The vertical well group productivity simulation was carried out with porosity of 0.1% and 3% (other detailed simulation parameters are the same as Table 2) to compare the impact of porosity on the development of vertical well group. Figures 14 to 17 The simulation results show that lower reservoir porosity will bring about a more obvious reservoir pressure drop propagation range and a decrease in gas content as drainage progresses, and the gas production will be more advantageous. Figure 14 (a) shows the comparison between drainage time and gas production. Figure 14 (b) shows a comparison diagram of drainage time and reservoir pressure; Figure 14 (c) shows a comparison chart of drainage time and water production; Figure 15 (a) shows the comparison between drainage time and gas production. Figure 15 (b) shows a comparison diagram of drainage time and reservoir pressure; Figure 15 (c) shows a comparison chart of drainage time and water production.

[0112] (4) Gas content

[0113] Fix other parameters and set the gas content to 5m 3 / t、10m 3 / t, run the simulation to compare the impact of different gas contents on the production capacity of vertical well group development, the simulation parameters are the same as Table 3. Figures 18 to 21 The simulation results show that the higher the reservoir gas content, the higher the gas production at the same drainage time, the larger the reservoir pressure drop propagation range and the decrease amplitude, and the more obvious the decrease amplitude of the reservoir gas content. Figure 18 (a) shows the comparison between drainage time and gas production. Figure 18 (b) shows a comparison diagram of drainage time and reservoir pressure; Figure 18 (c) shows a comparison chart of drainage time and water production; Figure 19 (a) shows the comparison between drainage time and gas production. Figure 19 (b) shows a comparison diagram of drainage time and reservoir pressure; Figure 19 (c) shows a comparison chart of drainage time and water production.

[0114] (5) Langmuir volume

[0115] Set the Langmuir volume to 10m 3 / t、15m 3 / t, the simulation parameters are the same as Table 3. The influence of Langmuir volume on gas production and reservoir changes during the development of vertical well groups is analyzed through numerical simulation.

[0116] Figure 22 、 Figure 23 It shows that the Langmuir volumes are 10m 3 / t and 15m 3 / t, the lower Langmuir volume can have more obvious reservoir pressure drop propagation and reservoir gas content changes during coalbed methane production, which is consistent with the essential characteristic that Langmuir volume represents the adsorption capacity of the matrix for methane. Figure 22 (a) shows the comparison between drainage time and gas production. Figure 22 (b) shows a comparison diagram of drainage time and reservoir pressure; Figure 22 (c) shows a comparison chart of drainage time and water production; Figure 23 (a) shows the comparison between drainage time and gas production. Figure 23 (b) shows a comparison diagram of drainage time and reservoir pressure; Figure 23 (c) shows a comparison chart of drainage time and water production.

[0117] Step 5: Calculate the weights of factors influencing coalbed methane production capacity by comprehensively utilizing principal component analysis, ash correlation method and other methods, and determine the main controlling factors affecting coalbed methane production capacity based on the principle of weight maximization.

[0118] (1) Analytical Hierarchy Process

[0119] The previous article analyzed the relationship between coal seam thickness, gas content, coal seam thickness, porosity, permeability and coalbed methane well productivity, but it was unable to determine the degree of influence of each factor on coalbed methane well productivity. The analytic hierarchy process is now used to construct a hierarchical analysis model, use the "expert scoring method", and combine the relevant literature of predecessors and the specific actual situation of the study area to score the above factors. Based on the comprehensive analysis of coalbed methane productivity and the understanding of the geological characteristics and development engineering factors of the study area, six indicators (such as coal thickness, gas content, coal seam thickness, porosity, permeability, and burial depth) are determined in this hierarchical model. Figure 24 ), and establish a judgment matrix through the “three-scale method” to avoid exaggerating the importance of a certain factor.

[0120] Construct the criterion layer B according to the following formula j and C i The comparison matrix B, C1, C2. Among them:

[0121]

[0122] Factor i is less important than factor j

[0123] From this we can see that the element b in B ij All satisfy 0≤b ij ≤1, b ij +b ji =1, so the comparison matrix B is a complementary matrix. By finding the maximum eigenvalue B of the complementary matrix B λ and the corresponding maximum eigenvector B λ , use the following formula to perform consistency check.

[0124]

[0125] Where: (n is the number of research variables); RI is the average random consistency index.

[0126] When CR < 0.10, the judgment matrix is ​​considered to have acceptable consistency. When CR ≥ 0.10, the judgment matrix needs to be adjusted and modified to meet CR < 0.10 and thus have satisfactory consistency.

[0127] When the judgment matrix passes the consistency test, the eigenvector B λ The matrix obtained after normalization is the weight of the relevant elements.

[0128] Based on the above analysis, we first conducted a pairwise comparison of the geological and engineering factors at level B according to the "three-scale" principle to construct a comparison matrix B. Because the impact of geological and engineering factors on CBM production capacity in vertical and cluster well development is similar and their importance is equal, the matrix is ​​constructed as follows.

[0129]

[0130] The maximum eigenvalue of matrix B is obtained and the consistency test is performed with the help of MATLAB programming. After testing, the matrix CR is less than 0.10, and the weight values ​​of the two first-level indicators are obtained: w a ={0.5 0.5}

[0131] Next, we construct matrices C1 and C2 for the four geological factors and two engineering factors in the C layer according to the aforementioned principles. When constructing matrix C1, the coefficient of variation of coal seam thickness within the block is small, indicating that the coal seams are relatively stable and have little impact on geological factors. Previous research indicates that while burial depth has a certain contribution to coalbed methane production, the correlation is smaller than that of other factors. Gas content and porosity have a relatively greater impact on geological factors. In summarizing the above, we construct matrices C1 and C2 as follows.

[0132]

[0133] Similarly, the maximum eigenvalue of the two matrices was calculated and the consistency test was performed using the MATLAB programming program. After testing, the CR values ​​of the two matrices were both less than 0.10, showing satisfactory consistency. Then the weight values ​​of the four indicators of the C1 matrix were obtained:

[0134] w b1 ={1 / 3 1 / 6 1 / 6 1 / 3}, the C2 matrix obtains the weight values ​​of the two indicators:

[0135] w b2 ={1 / 3 2 / 3}. ;

[0136] The weights of each main controlling factor are calculated:

[0137] w c ={1 / 6 1 / 12 1 / 12 1 / 6 1 / 3 1 / 6};

[0138] The weight value comparison chart of the hierarchical analysis method ( Figure 25 ), it can be clearly seen that coal thickness has the highest weight, which is 1 / 3; permeability, porosity and gas content are second, which are 1 / 6; burial depth and Langmuir volume have the smallest weight, which are 1 / 12.

[0139] (2) Grey correlation analysis

[0140] Based on the AHP approach, the report incorporates grey correlation analysis, using relevant data from a mathematical and statistical perspective to quantitatively determine the correlation between each influencing factor and production capacity. This comparison of the relative magnitude of the correlations among these factors allows for a more intuitive analysis of the primary and secondary importance of these factors, thereby supporting the results of the AHP approach. Given the limited permeability data in the study area, determining grey correlations can result in significant errors, so the permeability correlation calculation was not performed during the analysis.

[0141] Using the numerical simulation data of daily coalbed methane production as the reference series, and the six factors of coal seam thickness, gas content, permeability, porosity, burial depth and Langmuir pressure as the comparison series, the correlation between each factor and daily gas production is calculated. The numerical value reflects the importance of the factor in affecting the production.

[0142] (1) Calculation of correlation coefficient

[0143] Assume that the reference sequence of data changes is {X0(n)}, and the comparison sequence is {X i (n)}. When n=k, {X0(k)} and {X i The correlation coefficient of (k)} can be expressed as ε 0i (k) formula (where {X0(k)} and {X i (k)} is the data processed by averaging based on the original data), that is:

[0144]

[0145] Where: ε 0i (k)—correlation coefficient of two sequences when n=k;

[0146] △ 0i (min)—the minimum value of the absolute differences of the series;

[0147] ζ—resolution coefficient, generally taken as 0.5;

[0148] △ 0i (max)—the maximum value of the absolute differences of the series;

[0149] △ 0i (k)—the absolute difference between the two sequences when n=k, that is, △ 0i (k)=|X0(k)-X i (k)|.

[0150] (2) Correlation calculation

[0151] The correlation between each comparison series and the reference series can be obtained by the following formula:

[0152]

[0153] Where: R 0,i — compare the correlation between sequence i and reference sequence 0;

[0154] n—the length of the sequence, that is, the number of data.

[0155] According to the grey correlation analysis theory, the correlation between each influencing factor and gas production is calculated and shown in Table 4. The factors affecting the productivity of coalbed methane wells are as follows from large to small: permeability > porosity > coal thickness > gas content > Langmuir volume > burial depth.

[0156] △ 0i (max)—the maximum value of the absolute differences of the series;

[0157] △ 0i (k)—the absolute difference between the two sequences when n=k, that is, △ 0i (k)=|X0(k)-X i (k)|.

[0158] Table 1 Types and values ​​of influencing factors in the experiment

[0159]

[0160] Table 2 Numerical simulation parameter values ​​for the test area

[0161]

[0162] Table 3 Theoretical calculation parameter values ​​for the test area

[0163]

[0164] Table 4 Grey correlation analysis results

[0165]

[0166] Parts not described in the present invention can be implemented by referring to the existing technology.

[0167] Those skilled in the art should recognize that the above embodiments are merely intended to illustrate the present application and are not intended to limit the present application. As long as they are within the spirit of the present application, appropriate changes and modifications to the above embodiments are within the scope of protection claimed in the present application.

Claims

1. A method for quickly determining the main controlling factors of coal-measure gas production capacity, characterized in that: The method is to quantitatively determine the main controlling factors of coal-measure gas production capacity by combining refined three-dimensional geological modeling and numerical simulation technology. The method includes the following steps in sequence: a. Based on exploration and development data, clarify the hydrological and basic geological data of the coal reservoir, focusing on the coal seam depth, thickness, porosity, permeability, gas-water relative permeability curve and adsorption characteristics, and explore the correlation between these factors; b. Based on the special occurrence conditions of deep coal seams, calculate the initial adsorbed gas and free gas content in deep coal seams; c. Based on comprehensive geological conditions, a deep coalbed methane numerical model is constructed to perform deep coalbed methane numerical simulation calculations; d. Use historical sensitivity analysis methods to adjust geological and engineering parameters and compare changes in deep coalbed methane production capacity under different parameter changes; e. Use principal component analysis and ash correlation method to calculate the weights of factors affecting coalbed methane production capacity, and determine the main controlling factors affecting deep coalbed methane production capacity based on the principle of weight maximization.

2. The method for rapidly determining the main controlling factors of coal-measure gas production capacity according to claim 1, characterized in that: In step a, the coal rank type of the target coal seam is determined by coal rock basic component testing, and the pore structure parameters, Langmuir pressure, Langmuir volume, and critical desorption pressure of the target coal seam are determined by reservoir physical property characterization testing; the initial permeability, porosity, and gas-water two-phase permeability curve of the target coal seam are determined by porosity and permeability testing; the mechanical properties of the coal rock, roof, and floor of the target coal seam are revealed by basic mechanical parameter testing, and the compression coefficient of the target coal seam in the study area is derived; Through the porosity and permeability dynamic change test to identify the target coal seam cracks and pore compression coefficient; Through physical simulation of gas and water migration, the production capacity mechanism of the target coal seam under the influence of different design schemes is revealed; through fracturing fracture diversion simulation, the permeability of the target coal seam after fracturing is clarified.

3. The method for rapidly determining the main controlling factors of coal-measure gas production capacity according to claim 1, characterized in that: In step b, for coal-bearing gas wells that have undergone pressure-maintained coring, the pressure-maintained gas is considered to be the initial free gas content, and the sum of the lost gas, desorbed gas and residual gas is considered to be the initial adsorbed gas content; for coal-bearing gas wells that have undergone rope coring, the lost gas is considered to be the initial free gas content, and the sum of the desorbed gas and residual gas is considered to be the initial adsorbed gas content.

4. The method for rapidly determining the main controlling factors of coal-measure gas production capacity according to claim 3, characterized in that: For coal-bearing gas wells that have not undergone gas content core sampling testing, the free gas content is calculated by combining equations (1) to (8): In formula (1): M g is the molar mass of methane; f P is the reservoir pressure gradient; H is the reservoir depth; P0 is the pressure at point 0 of the reservoir pressure fitting line; ρ c is the apparent density of coal; ρ sc is the density of methane gas under standard conditions; R is the gas constant; Z is the gas compressibility factor; is the coal porosity; is the initial porosity of the coal sample; σ is the average ground stress; υ is the Poisson's ratio of coal; E is the Young's modulus of coal; p is the reservoir pressure; m ar Analyze moisture content for the coal industry; ρ c / ρ w is the ratio of apparent density of coal sample to water density; In formula (2): q f is the free gas content; P is the in situ pressure; T is the in situ temperature; Z is the gas compressibility factor; P a and T a Represent the standard state pressure and temperature of gas respectively; ρ reservoir density; In formula (3): G MF is the methane content in the free gas; S W is the water saturation; B is the methane volume coefficient; ρ a is the apparent density of coal; ρ t is the true density of coal; C y NΦ·P·K·S (5) In formula (5): C y is the free gas content of the coal matrix; Φ is the effective pore volume for free gas in the coal matrix; P is the in situ pressure; K is the compressibility coefficient of methane; S is the gas saturation; In formula (6), T0 is 273K; φ is the ratio of the volume of pores in coal to the total volume; φ w , is the pore volume occupied by water in coal; ρ coal is the density of coal; k is the coefficient used to correct the change of porosity with stress; In formula (7): V t is the free gas content; S W is water saturation; ρ c B is the apparent density of coal rock; g is the methane volume coefficient; In formula (8): V free is the amount of free methane; ρ CH 4 is the density of methane; ρ coal is the density of coal; φ f is the porosity not occupied by water; M CH4 is the molecular weight of methane; M free is the molecular weight of the free gas.

5. The method for rapidly determining the main controlling factors of coal-measure gas production capacity according to claim 1, characterized in that: The specific steps of step c are as follows: assume that the caprock sealing property is perfectly sealed without exceeding the pressure limit, and set the boundary condition to no flow; use the BHP parameter to control the upper limit of the reservoir pressure in the simulation. In practice, this pressure value is related to parameters such as the fracture pressure that destroy the reservoir sealing property; perform vertical single well productivity simulation and horizontal single well productivity simulation respectively.

6. The method for rapidly determining the main controlling factors of coal-measure gas production capacity according to claim 5, characterized in that: When conducting vertical single-well productivity simulation, the basic reservoir physical parameters, fluid parameters, and grid parameters required to be input into the simulation software are further clarified based on the information obtained in step a, and a deep coalbed methane numerical model is constructed.

7. The method for rapidly determining the main controlling factors of coal-measure gas production capacity according to claim 5, characterized in that: When conducting horizontal single-well productivity simulation, ECLIPSE software is used for simulation. After successful simulation, the output results of gas production, water production and reservoir pressure plane changes during the production process are output.

8. The method for rapidly determining the main controlling factors of coal-measure gas production capacity according to claim 1, characterized in that: In step d, geological conditions include coal seam thickness, burial depth, reservoir pressure and gas content; engineering conditions include well type and horizontal section length; changes in deep coalbed methane production capacity include changes in maximum gas production, average gas production and stable gas production.

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