Progress and trend of key technologies of CBM development and utilization in China coal mine areas

The key to coordinated development of coal mine methane and coal is to coordinate the spatio-temporal relationship between coal mining and coal mine methane development. During the “11th Five-Year Plan” and “13th Five-Year Plan” period, relying on the overall implementation of the national major science and technology project “Development of large oil and gas fields and coal mine methane”, the coordinated development mechanism of coal mine methane and coal in coal mining areas has made great progress. The development mechanism of coal mine methane in coal mine area is formed by “gas drainage”, “coal mine methane extraction”, “combination of ground development and underground extraction” and so on to ensure the safety of coal mine production. By defining the scope of coordinated development of coal mine methane and coal, establishing the evaluation method of coordinated development of coal mine methane and coal, forming the optimization decision-making platform of coordinated development of coal mine methane and coal, the concept of coordinated development of coal mine methane and coal in coal mine area based on “coordinated development, quality and efficiency improvement, technical support, platform decision-making, integrated demonstration” was put forward. Relying on the coordinated development technology system of coal mine methane and coal, as well as a complete set of technologies such as efficient extraction and effective utilization of coal mine methane in coal mining areas, Formation of coal-bed methane mining area in shanxi area “four linkage” global coordination development mode, huaibei mining area coal seam group of coal mine methane (CBM) three-dimensional joint development pattern, the complex soft Songzao mining area coal seam ahead of anti-reflection coordinated development pattern, Xinjiang mining area coal group “trinity” of large dip Angle coordinate development mode, leading to the coal mining area of coal bed methane development practice. The coordinated development mechanism, model and application of coal mine methane and coal in coal mine area have led the scientific and technological innovation of coal mine methane industry in coal mine area, and significantly improved the level of coal mine methane development and utilization in coal mine area. With the adjustment of national energy strategy, the implementation of the strategy of “double carbon” and the new requirements of the new situation, such as construction of ecological civilization coal mining area of coal bed methane development faces new challenges, new demand on promoting fine coal mining area of coal mine methane geological exploration and underground coal bed methane efficient development, abandoned mine coal mine methane extraction, such as coal gas together, a total of mining technology research and development, To construct a new technology body for clean and efficient development of coal mine methane and coal in coal mine are

Review on the progress for physical simulation for gas reservoirs co-production in multi-pressure system

The gas reservoirs co-production in multi-pressure system is one of the important measures to improve the development efficiency of the superposed gas-bearing systems. However, the co-production effect is not ideal due to the special reservoir forming background. The mechanism of co-production and high-efficient development of the multi-pressure system has become an key scientific problem, which restricts the efficient exploration and development of superposed gas-bearing systems. This paper focuses on the gas reservoirs co-production in multi-pressure system, and divides the physical simulation types of co-production into two separate fields: coalbed methane and non coalbed methane. It clarifies the current research status of gas reservoirs co-production in multi-pressure system from the aspects of device functions and characteristics, understanding of co-production, and existing problems. Firstly, the large-scale physical simulation test device can effectively eliminate or weaken the problems of homogeneous single-type reservoir samples, single monitoring data means and single stress loading form caused by paralleling multiple core grippers to build the physical simulation model. The development direction of the physical simulation for co-production in multi-pressure system should be to achieve true three-dimensional heterogeneous complex in-situ stress state of large-scale heterogeneous multi-type reservoir samples. The characteristics of fluid pressure transmission between adjacent reservoirs, the inter-layer crossflow, the multi-phase natural gas symbiosis should be considered. On this basis, the sensitivity of co-production of multi-pressure system to reservoir physical properties was deeply summarized. The differences in inter-layer pressure difference, permeability, effective stress, water saturation and other factors may induce the fluid interference and reservoir gas production damage, and optimizing co-production style may be a way to reduce the fluid interference and reservoir gas production damage. In totally, the next research should focus on exploring the influence of the coupling effect of low porosity and low permeability, gas water two-phase flow, multiphase gas symbiosis and coexistence of multiple types of reservoirs on the dynamic evolution law of reservoir-wellbore flow field induced by co-production fluid interference, clarifying the reservoir damage and its mechanism of different phase fluid intrusions on the reservoir, and revealing the coupling flow characteristics of inter-layer crossflow and wellbore pipe flow considering the fluid interference effect

Research on temporal−spatial relationship between ground fracturing wells and underground drilling for substitution drainage of during extraction exhaustion period

In order to make use of the favorable extraction conditions formed by ground fracturing to carry out the underground borehole replacement extraction in the drainage exhaustion period of ground fracturing wells, and form the advanced treatment of ground fracturing wells + underground borehole replacement extraction mode of underground borehole replacement extraction, the temporal－spatial relationship between ground fracturing wells and underground drilling for substitution extraction of during drainage exhaustion period was studied. Through the analysis of the distribution law of fracturing fractures, the investigation of the effect of ground fracturing on enhancing permeability and promoting extraction, and the numerical simulation of the variation law of coal reservoir parameters, the influence and effective range of ground fracturing wells in Lu ’an Mining Area are mastered. Numerical simulation method is used to analyze and determine the optimal temporal－spatial relationship between ground fracturing wells and underground drilling for substitution extraction. Based on the drainage curve of surface fractured wells and the demand of drilling in the drainage exhaustion period, the definition and division method of the drainage exhaustion period of surface fractured wells are discussed, and the discriminant indexes and key time nodes of the drainage exhaustion period of surface fractured wells are put forward. The results show : The effective influence range of ground fracturing wells is 80 m. In this area, the reservoir pressure can be decreased by about 50%, the permeability coefficient can be increased by 0.77−1.40 times, and the average pure volume of single underground drilling for substitution extraction can reach 53.68−131.67 m3/d, which improves the extraction efficiency of downhole drilling. When the position of ground fracturing wells is fixed along the axis of the underground drilling, the shorter the normal distance between the underground drilling and the ground fracturing wells, the better the extraction effect.When the normal distance between the ground fracturing wells and the underground drilling is constant, the best extraction results are achieved when the ground fracturing wells is located in the middle of the axial direction of the underground drilling.The gas production rate of ground fracturing wells is used as an indicator of drainage exhaustion period.And the critical value of the indicator of drainage exhaustion period of surface fractured wells in the Lu'an mine area is 200 m3/d, and the corresponding failure period time node is 8~10 years

Deformation and failure characteristics of gas drainage drilling-reaming coal mass in non-uniform stress field

The deformation and failure characteristics of coal around gas drainage boreholes in deep soft and low permeability coal seams affect coal seam gas pre-drainage. Based on the condition of non-uniform stress field, the mechanical model of borehole disturbed coal mass was developed, the analytical solutions of stress, strain and displacement in the damaged zone, plastic zone and elastic zone of borehole disturbed coal mass were deduced, the influence law of factors such as lateral pressure coefficient, load condition, cohesion and hole expanding behavior on the “three zone” distribution of disturbed coal mass were analyzed, and the reliability of the theoretical model was verified through engineering examples. The results show that under the condition of non-uniform stress field, the plastic zone and damaged zone of disturbed coal mass are elliptical distribution. With the increase of lateral pressure coefficient, the length of the upper and lower wings of the plastic zone and damaged zone of disturbed coal mass becomes larger and larger, and the radius of the plastic zone and damaged zone in the direction of smaller stress is greater than the radius of the two zones in the direction of large stress. The radius of plastic zone and damaged zone of coal mass increases with the increase of vertical load, and decreases with the increase of initial cohesion and residual cohesion. The influence of vertical load on its shape can be ignored. When the borehole diameter is expanded from 0.1 m to 0.5 m, the coal mass 0−1.0 m away from the borehole center produces a strong disturbance, the coal mass 1.0−4.6 m produces a weak disturbance, and the coal mass after 4.6 m has almost no influence. Through the field example of No.16032 bottom pumping roadway hydraulic reaming in the Guhanshan coal mine, it is observed that the disturbed coal mass in the reaming section has a high degree of damage. Based on the coal output, the reaming diameter is deduced to be 1.5 m, and then the deformation and damage characteristics of drilling reaming coal mass are obtained through theoretical calculation and numerical simulation respectively. The two are in good agreement, so as to verify the reliability of the theoretical model

Hierarchical occurrence law of gas content in deep coal seams and its relationship with outburst prevention

The prediction indexes currently used in China for coal and gas outburst area include gas pressure and gas content, which exhibit varying sensitivity to outbursts in different mining areas. The formulation of rational prediction indexes for outburst risk is based on understanding the mechanism of coal and gas outbursts. According to the “internal gas controlled theory”, it is inferred that under deep conditions, the sensitivity of gas content is higher than that of gas pressure. To verify this inference, this study selects Pingmei No.8 Coal Mine as a typical research site due to its high temperature and high pressure conditions. These conditions result in a different occurrence pattern of gases in deep coal seams compared to shallow ones, where below a “critical depth”, the gas content exhibits negative growth leading to a step-like occurrence pattern, distinct from the linear increase observed in gas pressure. Using a combination of theoretical calculation, experimental analysis, numerical simulation, and on-site verification, the hierarchical occurrence law of gas content in Pingmei No. 8 Mine was first theoretically calculated, and a s solid-flow-heat three-field coupling model considering the competition effect of temperature and pressure was established to analyze the influencing factors of the reverse decrease of gas content with depth. Furthermore, the differential occurrence patterns of gas content and gas pressure during geological exploration/mining periods were compared, verifying the research results of theoretical research and numerical simulation. Additionally, the vertical distribution pattern of outburst energy within the study area was theoretically analyzed while verifying step-like distribution characteristics through positive lateral verification using data on emitted gases amounts and documented outburst accidents cases. Finally,the differences between sensitivities towards outbursts were compared and analyzed between deep conditions for bothgas pressureandgascontent,andthe underlying reasons behind these differences are clarified. The research results of this paper verify the correctness of “internal gas controlled theory” from a macro perspective, and have certain guiding significance for the prevention and control of deep coal gas dynamic disasters

Study on Gas Extraction Technology for Goaf Using L-Shaped Borehole on the Ground

This study aimed to examine gas extraction technology in the goaf of an L-shaped borehole in the mining fissure zone of a short-distance coal seam group. The numerical simulation method was used to analyze the failure law of overlying rock during mining, and a mathematical model was established for gas migration in the mining overburden. Finally, gas extraction tests were performed for the L-shaped borehole in the mining fissure zone. The results showed that as the coal mining project progressed, the damage area of the overlying strata in the goaf became larger, and the plastic damage area of the overlying rock along the strike had a saddle shape, being concave in the middle and convex at both ends. The closer the L-shaped borehole in the mining fissure zone was to the coal seam roof, the greater the amount of air leaking from the working face into the goaf, and the lower the overall gas concentration in the goaf. When the vertical distance of the L-shaped borehole was too high, the ability of the L-shaped borehole to control the gas concentration in the lower goaf was weakened. Moreover, the mining fracture zone was a good space for gas migration and storage. Thus, arranging the L-shaped borehole in this zone can greatly improve the efficiency of borehole gas extraction. According to the overlying rock conditions and mining conditions of Tunlan Mine, the L-shaped borehole was positioned 43 m away from the roof of the coal seam. The extraction rate of the L-shaped borehole reached 9.30 m3∙min−1, and the gas concentration in the corners of the working face was kept below 0.4%, yielding an excellent extraction effect

Undesorbable residual gas in coal seams and its influence on gas drainage

The definition of residual gas can be found in different scenarios, such as the fast and slow desorption methods of measuring gas content and the sorption hysteresis test and gas management of coal mines, however, its meaning varies a lot in different contexts. The main aim of this paper is to discuss the existence of truly undesorbable residual gas in coal seam conditions and its impacts on sorption model and gas drainage efficiency. We believe the undesorbable residual gas does exist due to the observation of the extended slow desorption test and the sorption hysteresis test. The origin of undesorbable residual gas may be because of the inaccessible (closed or semi-closed) pores. Some gas molecules produced during coalification are stored in these inaccessible pores, since the coal is relatively intact in the coal seam condition, these gas molecules cannot escape during natural desorption and then create the undesorbable residual gas. Based on the existing adsorption models, we propose the improved desorption versions by taking into consideration the role of residual gas. By numerically simulating a gas drainage case, the gas contents after different drainage times are studied to understand the influence of residual gas content on gas drainage. The results indicate that the influence starts to be obvious even when the total gas content is at a high level, and the impact becomes more and more apparent with increasing drainage time. Our study shows that the existence of residual gas will impede the gas drainage and the total amount of recoverable coal seam methane may be less than expected

Experimental Study on the Influence of Wettability Alteration on Gas–Water Two-Phase Flow and Coalbed Methane Production

The surface wettability is important in the change in the relative permeability of gas and water. Due to the heterogeneous property of coal, it has a mixed wetting state, which makes it difficult to predict the change in permeability. To investigate the influence of different wettabilities on two-phase flow, a total of three different rank coal samples were collected and were treated with different chemicals. The alteration of the coal’s wettability, characteristics of gas–water flow, and relative permeability of the coal after the chemical treatments were analyzed. The research conclusions suggest that (1) the coal samples treated with SiO2 and H2O2 increased the hydrophilicity of the coal surface, while the coal samples treated with DTAB increased the hydrophobicity of the coal surface. Compared to SiO2, both H2O2 and DTAB can form a uniform wetting surface. (2) The wettability alteration mechanism among the three different chemical reagents is different. (3) All the chemicals can change the gas–water interface. The water migrates more easily through the cleats after H2O2 treatment, while it is more difficult for the water to migrate through cleats after the DTAB treatment. (4) There are two types of flow states of gas and water on different wetting surfaces. A slug flow is formed on a hydrophilic surface, while an annular flow is formed on a hydrophobic surface. (5) The crossover point and the residual water saturation of the relative permeability curves were influenced by the surface wettability

Research on dynamic workflow construction method of coal mine gas control

At present, the key links of coal mine gas control management still need manual supervision. The gas control measures can not achieve 'reliable quality' and 'process traceability'. The backward gas control management mode results in overlapping functions, poor process and low degree of data sharing. In view of the above problems, based on workflow technology, from the perspective of global management, the dynamic workflow construction method of coal mine gas control is studied. Firstly, the workflow, constraint conditions and implementation process of gas control in the minging face and coal uncovering working face are analyzed. The links of gas control flow is divided into two types: test and measure. It is specifically divided into five types of work, including approval of technical documents and reports, drilling construction, sampling, gas parameter measurement, and gas extraction and parameter detection. Secondly, the method reconstructs the last four types of work and further divides the work into 25 basic work units. The method uses Petri Nets to combine basic work units to establish different cross-departmental gas control complex business workflow. Based on the gas control workflow chart, a representation method for the implementation progress of gas control in the working face is established. The method uses the strategy of combining initiative and automation to assign task of the workflow. The method uses description files to dynamically generate and configure workflow networks to meet the requirements of dynamic workflow modeling for gas control. Finally, based on the Flowable workflow engine, the dynamic workflow function of gas control is developed and applied. The results show that the construction of dynamic workflow can make the gas control business process. It is conducive to improving the efficiency of gas control collaborative execution, realizing the rapid flow, tracking and sharing of data. It is conducive to monitoring the overall operation and optimization of gas control work, improving the decision-making capability of coal mine gas control, and innovating the management mode of coal mine gas control

Numerical Study on the Mechanism of Coal and Gas Outburst in the Coal Seam Thickening Area during Mining

Most coal and gas outbursts occur in the coal thickness variation zone. However, it is difficult to illustrate the mechanism of outbursts in coal thickening areas by physical simulation experiments. In this study, a coupled multi-field model, established by considering the stress–strain field, gas transport field and damage field, was used to investigate the evolution of stress, gas pressure and plastic failure zones under different variation gradients and amplitudes of coal thickness. The simulation results show that the stress peak at the coal thickening transition zone caused by mining is higher than that at the constant thickness coal seam. The stress peak at the coal thickening transition zone decreases from 18.8 MPa to 16.9 MPa with the increase in the transition zone from 0 m to 10 m under the constant coal thickness variation from 3 m to 7 m; while it increases from 16.2 MPa to 19.3 MPa with the increase in the transition zone from 2 m to 10 m under the constant coal thickness variation gradient of 45°. Similarly, the plastic deformation volume of the coal seam between the driving face and the coal thickening interface increases with the increase in the coal thickness variation gradient and amplitude. In addition, the gas pressure in the fracture declines slower in the coal thickness variation zone affected by the higher coal thickness variation gradients or amplitudes. The mechanism for outbursts occurring in the increasing coal thickness area was further discussed, and combined with the simulation results for the energy principle of outbursts. Compared with the constant thickness coal seam, the elastic energy increases from 1.85 MJ to 1.94 MJ, and the free gas expansion energy increases from 24.19 MJ to 50.57 MJ when the coal thickness varies from 3 m to 13 m within a 10 m transition zone. The variation of coal thickness causes higher stress, higher gas pressure and low coal strength, which triggers outbursts more easily. The research could provide the theoretical support to prevent and control outbursts in coal seam thickening areas during mining