Study on the disturbance characteristics and control strategies of coordinated exploitation of superimposed resources of coal and oil‐type gas in Ordos Basin

At present, the coordinated exploitation of coal measures‐associated resources is facing many problems, such as wide range, complex technology, and lack of in‐depth theoretical research. To study and solve the coordinated exploitation of coal and oil‐type gas‐associated superimposed resources, taking Huangling No.2 mine in Ordos Basin as an example, a fluid–solid coupling finite element model for coal mining in associated mines is constructed, and the variation laws of axial stress, displacement, stability and oil‐type gas pressure of each gas reservoir at different advancing distances of coal face are numerically simulated and the disturbance characteristics of the mining coal seam to the gas reservoir are obtained. The fluid–solid coupling finite element model of associated mine extraction is constructed, and the variation laws of axial stress, displacement, stability, and oil‐type gas pressure of the gas reservoir and coal seam when extracted from different gas reservoirs are numerically simulated. The disturbance characteristics of the surface‐extracted gas reservoir to the coal seam are obtained. The study results show that the larger the interval between the gas reservoir and the mining coal seam, the smaller the degree of disturbance, and the pressure relief effect of the oil‐type gas in the gas reservoir is positively related to its displacement; the surface drilling extract oil‐type gas, the oil‐type gas in the underlying gas reservoir will migrate and supplement to the extracted layer, and the plastic area around the extraction well basically appears in the mudstone layer and coal seam, and the gas reservoir near the coal seam has good pressure relief effect. Combined with the mutual disturbance characteristics between gas reservoir extraction and coal seam extraction and the field practice, disturbance control strategies for coordinated exploitation of superimposed resources of coal and oil‐type gas in Ordos Basin are proposed.


| INTRODUCTION
The Ordos Basin, spanning the Shanxi Shaanxi Inner Mongolia Ningxia Gansu region, is a typical mining area in China where multiple associated resources coexist in the same basin. 1The basin contains abundant associated mineral resources such as uranium, coal, oil, and natural gas. 2,3he basin has the characteristics of wide area, abundant occurrence of various energy resources, and vertical stacking of associated resources. 4The coal resources in the Ordos Basin are mainly distributed from top to bottom in the Jurassic Yan'an Formation, Triassic Wayaobao Formation, Carboniferous Permian Taiyuan Formation, and Shanxi Formation.The distribution of oil and gas in the basin exhibits characteristics such as "half basin oil, full basin gas" and "south-north oil, upper oil and lower gas." 5 In recent years, with the continuous development of coal resources, the mining contradiction between coal and coal series-associated resources has gradually become prominent. 6,7Co-mining of coal and coal series-associated resources can not only increase the production of coal resources but also effectively develop and utilize other types of associated resources. 8Therefore, Huang et al. proposed the concept of associated mining grade and analyzed the key technical issues of co-mining of coal and coal series associated resources. 6Yuan et al. proposed the concept of precisely coordinated mining of coal and coassociated resources and the technical means of reserving corridor mining. 4,9ompared to other coal series rare metal-associated resources, coal series oil, and gas-associated resources are influenced by their flow characteristics, with a wide range of associated areas, multiple overlapping layers, and extremely rich resource occurrence.Part of the coal and natural gas resources in the Ordos Basin are influenced by the occurrence of petroleum resources, showing the characteristics of rich oil coal and oiltype gas.
9][20][21][22][23] Among them, Zheng and co-workers analyzed the distribution pattern and gas genesis of surrounding rock gas in coal oil gas symbiotic mines. 13,16Xu et al. determined the main controlling factor for gas content in coal oil gas symbiotic mines, 24 and Zhang et al. derived the diffusion law of oil-type gas in excavation tunnels. 257][28] However, there is relatively little research on the multiphase and multifield coupling mechanism for the mutual disturbance and influence of oil-type gas and coal resource extraction.
Therefore, this paper takes the Huangling No.2 mine in the Ordos Basin as an example to study the multiphase and multifield coupling behavior of coordinated mining of coal and oil-type gas, discusses the mutual disturbance characteristics of coordinated development of coal and oil-type gas superimposed associated resources in the Ordos Basin, and proposes coordinated mining control strategies.

| GEOLOGICAL BACKGROUND
The Huangling No.2 mine is located on the southern edge of the Ordos Basin, with the main mining seam being the No.2 coal seam, while the No.3 coal seam is partially minable.The Triassic Yanchang Formation is rich in oil and gas resources, and the oil and gas resource reservoirs are mainly distributed in the Wayaobao Formation, Yongping Formation, Hujiacun Formation, and Tongchuan Formation of the Triassic.However, during the long geological evolution process, the deep oil and gas in the Yanchang Formation gradually migrated upwards to the sandstone layers on the top and bottom of the coal seam under the action of tectonic movement. 15ue to the influence of formation factors, the nitrogen and mercury content of oil-type gas and conventional coalbed methane are different, and the content of these two elements can be used as a method to distinguish between oil-type gas and coalbed methane. 29,30As shown in Figure 1, the comprehensive histogram of coal and oiltype gas-associated reservoirs near the No.2 coal seam of Huangling No.2 mine.
To study the coupling mechanism of coordinated mining between coal and oil-type gas, it is first necessary to understand the multiphase and multifield coupling mechanism of the mutual disturbance characteristics between coal seam mining and oil-type gas extraction.Therefore, the FLAC3D numerical simulation software was used to numerically simulate the disturbance of different advancing distances of coal seam mining on oiltype gas reservoirs, as well as the disturbance characteristics of surface well extraction of different oil-type gas reservoirs on coal seams.CHARACTERISTICS OF COAL SEAM MINING ON GAS RESERVOIRS

| Numerical simulation scheme for coal seam mining
Based on the coal layer histogram shown in Figure 1, a fluid-solid coupling numerical model (240 m × 200 m × 140 m) for coal seam mining in the associated mine was constructed (as shown in Figure 2).From Figure 1, it can be seen that there are two oil-type gas reservoirs on the roof and floor of No.2 coal seam.To facilitate research, the oil-type gas reservoirs are named as cj-1, cj-2, cj-3, and cj-4 from the bottom to top in this paper.Three monitoring points are set up in each of the four oil-type gas reservoirs.Table 1 shows the physical and mechanical parameters of different rock layers.
Based on the actual situation of the mine site, the numerical simulation parameters were simplified and designed.The initial gas pressure of the four oil-type gas reservoirs was uniformly set to 1.78 MPa, and the initial permeability of the reservoir was uniformly set to 0.1 × 10 −3 μm 2 .
This section mainly numerically simulates the influence of coal seam working faces advancing 50, 100, and 150 m, respectively, on the axial stress, axial displacement, elastic-plastic properties, and oil-type gas pressure of oil-type gas reservoirs.

| Gas-solid coupling model of coal and oil-type gas superimposed resources
The disturbance effect of mining coal seams on gas reservoirs and the disturbance effect of extracting gas reservoirs on coal seams are similar to the unloading of protective layers in high gas coal seam mining.Therefore, based on the dynamic model of porosity and permeability of gas-bearing coal rock adopted by Hao et al., 31 as well as the solid-gas coupling model (such as Equations 1-6) considering the deformability of the framework of gas-bearing coal rock mass and the compressibility of gas, this paper has constructed a gas-solid coupling model of coal and oil-type gas superimposed resources, which is used to compare and analyze the geological disturbance characteristics between the mining coal seam and the gas drainage reservoir.
It is assumed that the coal seam is isotropic and homogeneous, the Klinkenberg effect is considered in the flow process of oil-type gas, the oil-type gas is ideal gas, the flow process is approximately an isothermal process, and the flow of oil-type gas in the fracture of coal and rock mass conforms to Darcy seepage law.The dynamic change models of porosity and permeability of coal and rock mass are as follows: where φ e , φ p and k e , k p are the porosity and permeability (m 2 ) of the elastic deformation stage and strain strengthening stage of the oil-type gasbearing coal and rock mass, respectively; φ 0 , k 0 and φ max , k max are the initial porosity and permeability of oil-type gas-bearing coal and rock mass, as well as the porosity and permeability at peak stress; ε v is the volumetric strain of gas containing coal and rock; Δp is the oil-type gas pressure change, Δp = p − p 0 (p 0 is the initial oil-type gas pressure); K s is the bulk modulus of the solid skeleton, MPa; σ i is the stress intensity, MPa; σ s is the yield stress, MPa; σ c is the peak stress, MPa.The control equation for the deformation field of oiltype gas-bearing coal and rock mass is: 1.The equilibrium equation is where σ′ ij is the effective stress, MPa; δ ij is the Kronecher symbol; X i is the volumetric force tensor, N m −3 .

The geometric equation is
where u i is the displacement component, m.
3. The incremental form of the elastoplastic constitutive equation is where E, E′ represents the elastic modulus and the elastic modulus during the strain strengthening stage of oil-type gas-bearing coal and rock mass, respectively, MPa; s′ ij is the deflection stress, MPa; I′ 1 is the first invariant of effective principal stress tensor; α = φ φ 2 sin 3 (3 − sin ) (φ is the internal friction angle of oil-type gas-bearing coal and rock mass).
The gas seepage control equation of oil-type gasbearing coal and rock mass is where K f is the compressibility factor of oil-type gas, a and b are Langmuir adsorption constants, μ is the dynamic viscosity of oil-type gas, and k b is the Klinkenberg factor.Figure 3 shows the axial stress cloud map of the gas reservoir and the stress change curve of the measuring points when the working face is pushed at different distances.In this paper, the stress direction is negative for compression and positive for tension.The nth monitoring point is represented by mp − n.
From Figure 3, it can be seen that as the working face continues to advance, the range of stress unloading on the roof and floor gradually increases.The axial stress of cj-2 and cj-3, which are closer to the coal seam, has been basically unloaded, and they are greatly affected by coal seam mining.The larger the interlayer spacing between the gas reservoir and the coal seam, the smaller the degree of disturbance.The axial stress of cj-4, which is far from the coal seam, decreases slightly and is relatively less disturbed.As the coal seam working face advances, the stress on the roof and floor first sharply decreases, and then slowly increases to a stable pressure.

| Axial displacement of gas reservoirs
Figure 4 shows the axial displacement cloud map of the gas reservoir and the displacement change curve of the measuring points when the working face is pushed at different distances.In this paper, the displacement direction is positive for stretching and negative for compression.
As shown in Figure 4, as the working face gradually advances, the maximum displacement of the coal seam roof and floor gradually increases, and the displacement of the roof is significantly greater than that of the floor.Among them, the displacement of cj-1 is relatively small, while the displacement of cj-2 and cj-3, which are closer to the No.2 coal seam, is relatively large, indicating that coal seam mining has a significant impact on cj-2 and cj-3.With the advancement of coal seam mining, the displacement of cj-1 and cj-2 in the coal seam floor rapidly increases to a certain extent before slowly increasing, while the displacement of cj-3 and cj-4 in the coal seam roof gradually increases with the advancement of coal seam mining.

| Stability of gas reservoirs
Figure 5 shows the elastic-plastic nephograms of the gas reservoir when the working face is pushed at different distances.From Figure 5, it can be seen that as the coal seam working face continues to advance, the plastic zone of the coal seam roof and floor gradually increases, indicating that the gas reservoir of the coal seam roof and floor has produced varying degrees of pore cracks, and oil-type gas can quickly migrate to the goaf through the generated pore cracks.Among them, when the working face advances by 50 m, the plastic zone has not yet appeared in cj-1, and when it advances to 100 m, the plastic zone appears.There is no large-scale plastic zone in cj-4, indicating that the gas reservoir is relatively less affected by coal seam mining disturbance.

| Oil-type gas pressure on the roof and floor of coal seams and gas reservoirs
As shown in Figures 6 and 7, the oil-type gas pressure cloud chart of the gas reservoir and the oil-type gas pressure change curve of the measurement point are displayed when the working face is pushed at different distances.In this paper, the direction of pore pressure is positive for pressure and negative for tension.To monitor the changes in oil-type gas pressure on the roof and floor of the coal seam, a monitoring point has been added to each of the roof and floor.
From Figures 6 and 7, it can be seen that as the coal seam working face advances, the pressure relief effect of each gas reservoir gradually increases, and the roof of the goaf presents a large amount of oil-type gas pressure.This may be because a large amount of oiltype gas in the gas reservoir above the coal seam migrates to the roof of the goaf through a large number of pores and cracks generated by mining.Therefore, in actual production, it is necessary to strengthen the extraction of the overlying roof of the goaf.With the mining of coal seams, the oil-type gas pressure on the roof and floor of the coal seam shows a pattern of first increasing, then decreasing, and finally stabilizing.The pressure relief effect of oil-type gas in gas reservoirs is positively correlated with the displacement variation of gas reservoirs, that is, the larger the displacement, the more obvious the pressure relief effect of oil-type gas.section mainly uses numerical simulation methods to simulate the influence of drilling and extraction of cj-4, cj-3, cj-2, and cj-1 from top to bottom on the axial stress, axial displacement, elastic-plastic properties, and oiltype gas pressure of gas reservoirs and coal seams.

| Axial stress in gas reservoirs and coal seams
As shown in Figures 9 and 10, the cloud map and variation curve of axial stress in the gas reservoir and coal seam during the extraction of different gas reservoirs are presented.From Figure 9, it can be seen that the axial stress decreases slightly within a small range around the extraction well, and the axial stress changes in most areas are not significant.There is a phenomenon of stress concentration at the bottom of the wellbore.From Figure 10, it can be seen that after drilling on the ground (3) ( 4) to the gas reservoir, the axial stress can be unloaded.
After the start of extraction, oil-type gas gushes out to the extraction well, causing a sharp increase in axial stress around the extraction well in a short period of time.As the drilling of the extraction well gradually deepens, the amplitude of axial stress reduction in each gas reservoir gradually increases, and the pressure relief effect is relatively enhanced.At the beginning of the coal seam, there is no oil-type gas, but after drilling to the overlying gas reservoir, the oil-type gas in the nearby gas reservoir gradually moves from the structure or fractures to the coal seam and adsorbs in the coal seam pore fractures, thereby increasing the axial stress of the coal seam.After the start of extraction, the oil-type gas migrates to the extraction well, reducing the axial stress of the coal seam.
As the drilling of the extraction well gradually deepens, the shorter the time for oil-type gas to move to the coal seam, the greater the migration amount, and the peak axial stress of the coal seam gradually increases.

| Axial displacement around surface extraction well
As shown in Figures 11 and 12, the cloud map of the axial displacement of the gas reservoir and coal seam during the extraction from different gas reservoirs and the axial displacement change curve of the measurement points are presented.As shown in Figure 11, due to the radius of the surface extraction well being 0.1 m, the displacement of each gas reservoir and coal seam caused by drilling is small.From Figure 12, it can be seen that due to the close distance between the measurement points and the extraction well, as the surface drilling continues to extract and relieve pressure, the axial displacement of measurement points in cj-3, cj-4, and coal seam is in a stretching state.Due to the small displacement of each gas reservoir and coal seam, it can be considered that the overall displacement remains unchanged.Therefore, affected by the stretching effect of the coal seam and the overlying gas reservoir, the axial displacement of the gas reservoir under the coal seam is in a compression state.The sum of the tensile displacement of measurement points in cj-3, cj-4, and coal seam is basically equal to the sum of the compressive displacement of measurement points in cj-1 and cj-2.

| Stability around surface extraction wells
As shown in Figure 13, it is an elastic-plastic cloud map of the gas reservoir and coal seam during the extraction of different gas reservoirs.From Figure 13, it can be seen that due to drilling and pressure relief, the coal and rock mass within a small range around the extraction well | 3973

| Oil-type gas pressure in gas reservoirs and coal seam
As shown in Figures 14 and 15, the cloud map and change curve of oil-type gas pressure in gas reservoirs and coal seams during the extraction from different gas reservoirs are presented.As shown in Figure 14, with the extraction of surface drilling, the oil-type gas pressure in each reservoir decreases to varying degrees, and with the continuous deepening of the extraction well, the extraction effect becomes more obvious.Moreover, due to the large number of plastic areas near the coal seam, the oil-type gas pressure relief effect of cj-2 and cj-3 is relatively obvious compared to that of cj-1 and cj-4.From Figure 15, it can be seen that during the drilling and extraction of cj-4, the oil-type gas in other gas reservoirs gradually migrates upwards to supplement, causing the oil-type gas pressure in the coal seam to gradually increase, while the oil-type gas pressure in other gas reservoirs gradually decreases.
The decrease rate of oil-type gas pressure during the extraction of cj-4 is relatively slow and the decrease amplitude is relatively small.When extracting cj-1, oiltype gas in each reservoir is extracted, resulting in a faster and relatively larger decrease in oil-type gas pressure in each reservoir, while the increase in coal seam oil-type gas pressure is relatively small.When extracting cj-3, the maximum increase of oil-type gas pressure in the coal seam is observed.The reduced rate of oil-type gas pressure in each reservoir and the rising rate of oil-type gas pressure in coal seams gradually decrease when surface drilling is used to extract oiltype gas.

| DISTURBANCE CONTROL STRATEGY FOR COORDINATED MINING OF COAL AND OIL-TYPE GAS
Based on the multiphase and multifield coupling numerical simulation of coordinated exploitation of coal and oil-type gas, the disturbance characteristics of coal seam mining on gas reservoirs and the disturbance characteristics of extracted oil-type gas on coal seam are obtained.Combined with on-site practice and the concept of precisely coordinated exploitation of coal and co-associated resources, 9 this paper proposes the following disturbance control strategy for coordinated exploitation of coal and oil-type gas-associated superimposed resources in the Ordos Basin:  ( F I G U R 15 Pressure of oil-type gas when extracted from different gas reservoirs.(1) Drainage cj-4, (2) drainage cj-3, (3) drainage cj-2, and (4) drainage cj-1.
6. Gob extraction: After coal seam mining, it is necessary to strengthen the extraction of the goaf to protect the safe and efficient production of the coal mining face.
1.As the coal seam working face gradually advances, the range of stress unloading on the roof and floor gradually increases, the maximum displacement gradually increases, and the range of plastic zone gradually increases.The pressure relief effect of each gas reservoir gradually increases, and the stress on the roof and floor of the coal seam first sharply decreases and then slowly increases to stable pressure.The rate of tensile displacement of the floor gradually decreases, and the compressive displacement of the roof gradually increases.The oil-type gas pressure on the roof and floor first increases and then decreases until it stabilizes.The larger the interlayer spacing between the gas reservoir and the mined coal seam, the smaller the degree of disturbance.The pressure relief effect of oil-type gas in the reservoir is positively correlated with the displacement of the reservoir.2. Surface drilling and extraction result in a small decrease in axial stress, a plastic state, and weak stability in the surrounding area of the extraction well.
There is stress concentration at the bottom of the wellbore, and the displacement of each gas reservoir and coal seam is relatively small.The oil-type gas pressure of each gas reservoir is reduced to varying degrees.Surface drilling produces oil-type gas, and the oil-type gas in the underlying gas reservoir will migrate and supplement to the extraction layer.The decreased rate of oil-type gas pressure in each gas reservoir and the increased rate of oil-type gas pressure in coal seams gradually decreases with the progress of oil-type gas extraction.3.With the gradual deepening of extraction drilling, the axial stress unloading effect of each gas reservoir gradually enhances, and the extraction effect becomes more obvious.When drilling and extracting to cj-2 and cj-1, the plastic area around the coal seam extraction well is relatively large, which is prone to the phenomenon of coal falling or even collapsing on the wellbore.The plastic area around the extraction well mainly appears in the mudstone layer and coal seam.The gas reservoir near the coal seam has good drainage and pressure relief effects.4. Based on the mutual disturbance characteristics between coal seam mining and gas reservoir extraction, the principles of coordinated mining of coal and oil-type gas are proposed, as well as measures such as retaining protective coal pillars for gathering and transportation pipelines, reasonably arranging surface drilling before mining, arranging horizontal wells extraction for coal seam, drilling and extraction oil-type gas of bottom plate in tunnels, and extraction gas of goaf after mining.

F I G U R E 1
Comprehensive histogram of coal and oil-type gasassociated reservoirs in Huangling No.2 mine.

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I G U R E 2 Fluid-solid coupling model for coal mining in associated mines.(1) Numerical model, (2) layout of monitoring points, and (3) initial oil-type gas pressure.T A B L E 1 Physical and mechanical parameters of rock stratum in numerical model.

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I G U R E 3 Axial stress of coal seam roof and floor when the working face advances at different distances.(1) Advanced 50 m, (2) advanced 100 m, (3) advanced 150 m, and (4) stress variation curve of measuring points.

F I G U R E 4
Axial displacement of coal seam roof and floor at different advancing distances of working face.(1) Advanced 50 m, (2) advanced 100 m, (3) advanced 150 m, (4) displacement variation curve of measuring points.F I G U R E 5 Elastoplastic of coal seam roof and floor at different advancing distances of working face.(1) Advanced 50 m, (2) advanced 100 m, and (3) advanced 150 m.

FF 4 . 1 |
I G U R E 6 Pressure of oil-type gas at different advancing distances of working face.(1) Advanced 50 m, (2) advanced 100 m, and (3) advanced 150 m.I G U R E Variation curve of oil-type gas pressure at different advancing distances of working face.(1) Advanced 50 m, (2) advanced 100 m, and (3) advanced 150 m.F I G U R E 8 Fluid-solid coupling model for drainage in associated mines.(1) Surface extraction well model and (2) initial oil-type gas pressure.F I G U R E 9 Axial stress of rock stratum when extracted from different gas reservoirs.(1) Drainage cj-4, (2) drainage cj-3, (3) drainage cj-2, and (4) drainage cj-1.CHARACTERISTICS OF EXTRACTED GAS ON COAL SEAMS Numerical simulation scheme for surface well extraction oil-type gas To study the disturbance characteristics of surface well extraction gas on coal seams, a fluid-solid coupling model for associated mine extraction was constructed as shown in Figure 8, and initial oil-type gas pressure was also set in four gas reservoirs.Due to the limited extraction range of surface extraction well, the numerical model is set to 10 m × 12 m × 140 m, with a radius of 0.1 m set for the extraction well.Arrange measurement points along the extraction well in the four gas reservoirs and coal seams within a relatively close range to the extraction well, to monitor the disturbance effects of the extraction well on the gas reservoirs and coal seams.This

F I G U R 12
Axial displacement of measuring points when extracted from different gas reservoirs.(1) Drainage cj-4, (2) drainage cj-3, (3) drainage cj-2, and (4) drainage cj-1.F I G U R E 13 Elastoplastic of rock stratum when extracted from different gas reservoirs.(1) Drainage cj-4, (2) drainage cj-3, (3) drainage cj-2, and (4) drainage cj-1.exhibits a plastic state with weak stability.Especially when drilling and extracting to cj-2 and cj-1, due to the low tensile strength of the coal body, the plastic area around the extraction well in the No.2 and No.3 coal seams is relatively large, which is prone to the phenomenon of wellbore coal bodies falling or even collapsing.In actual production, precautions need to be taken.Due to the low tensile strength of coal and mudstone, the plastic area around the extraction well mainly appears in the mudstone layer and coal seam.

4 .
Horizontal well extraction: After drilling on the ground to the coal seam, utilizing the relatively strong permeability of the coal seam, horizontal wells are arranged in the coal seam to strengthen the extraction of oil-type gas in the gas reservoirs above and below the coal seam. 5. Bottom plate extraction: During the time period between coal seam excavation and mining, drilling and extraction of the underlying gas reservoir need to be carried out on the roadway bottom plate, to extract the oil-type gas from the underlying gas reservoir before it flows into the coal seam working face. )