Bing Qin

To investigate the evolutionary behavior of the physical and chemical properties of reservoirs following the injection of supercritical CO2, a custom-built constant temperature and pressure supercritical CO2-water-rock heat flow curing coupling experimental system was used to conduct supercritical CO2 soaking experiments on siltstone. The changes in cation concentration within the reaction solution were analyzed, and the evolution of the porosity, permeability, and water/gas saturation of the reservoir core samples was examined before and after the reaction; the experimental results were compared with numerical simulations. The reaction rate constant and activation energy were adjusted, and a method for eliminating the error associated with the time effect on the reservoir’s physical and chemical properties during CO2 geological sequestration was proposed. Additionally, the long-term changes in the physical and chemical properties of the reservoir during prolonged supercritical CO2 sealing were examined. The results indicate that with prolonged exposure to supercritical CO2, the pH of the reaction solution decreases from 7.19 to 5.68. Calcite had the fastest dissolution rate, followed by potassium feldspar and, finally, Illite. During CO2 injection, both the porosity and permeability of the reservoir increased rapidly, but the increase was smaller after CO2 injection ceased, eventually stabilizing. The time-dependent behavior of the physical and chemical properties of siltstone under supercritical CO2 conditions was determined, providing valuable insights into the pore permeability characteristics of reservoirs under long-term mineralization.

Based on the engineering background of coalbed methane block in Liujia District of Fuxin with complex geological conditions, a seepage mathematical model considering the dynamic changes of porosity and permeability of coal reservoir caused by coal matrix shrinkage under the conditions of gas adsorption, desorption, and stress change is established. A geological model of heterogeneous coalbed methane reservoir is established, which includes igneous rock and fault geological structure. The influence of permeability on coalbed methane productivity and reservoir pressure under complex geological conditions is studied by numerical simulation. The results show that the permeability of coal seam is an important factor affecting the yield of coalbed methane. The higher the permeability, the faster the output rate of coalbed methane, the higher the output, the faster the pressure drop of coal reservoir, and the faster the desorption rate of coalbed methane. Under the premise that the horizontal wellbore is perpendicular to the maximum permeability direction of the coal seam, the productivity of the well arrangement method is higher than that of the parallel maximum permeability direction. However, the length of the horizontal well in the Liujia block is limited by the dense igneous rock distribution, and the desorption area of the coalbed methane is reduced and the production is reduced when the vertical arrangement is arranged. Therefore, the horizontal wells in the Liujia block should be parallel to the maximum permeability direction and set along the direction of igneous rock. The greater the vertical permeability of the coal seam, the faster the reservoir pressure decreases, and the better the horizontal well productivity.

In order to obtain the minimum mining height that can play an effective protective role in the mining of the non-full coal protective layer in the Hongyang No. 3 coal mine and improve its economic benefits, the relationship between the mining height and the pressure relief of the protected layer is studied. Theoretical analysis is used to establish a calculation model of the goaf stress distribution law, with the mining height as a variable. The calculation model research results show that the mining height adjusts the goaf stress distribution by adjusting the range of the “three zones”. The force of the falling zone and the frustration zone on the goaf is approximately trapezoidal geostatic stress, and the roof stress in the vertical projection area of the trapezoidal waistline is not transmitted to the goaf. The development heights of the “two zones” are different at different mining heights, and the corresponding pressure-relief ranges are different from the waistline vertical projection. The curved subsidence zone transmits stress to the goaf through the fissure zone and the caving zone below, which can be calculated by the elastic foundation beam model. The falling zone is the elastic foundation, and different mining heights have different foundation coefficients. With the increase in mining heights, the foundation coefficients first decrease and then tend to be stable. The pressure-relief range of the stress transmitted from the curved subsidence zone to the goaf first increases and then tends to be stable. According to the calculation model, the minimum mining height for effective pressure relief of the upper protective layer of thin coal in the Hongyang No. 3 coal mine is 2.5 m, which can effectively relieve the pressure of the protected layer with the floor layer spacing of 48 m.

For extra-thick coal seams, slice mining is a safer mining method than top coal mining, which can effectively reduce the strong mine pressure behavior caused by mining. However, in the slice mining of high-gas and extra-thick coal seams, the gas in the lower slice flows into the goaf, which increases the gas control difficulty on the upper slice working face. It is easy to cause the gas transfinite at the upper corner in the upper slice and reduce the mining efficiency. Therefore, it is of a great significance to carry out the research on gas control technology in slice mining of the extra-thick coal seam. There are some problems in the gas control of slice mining, such as a single gas control method, low control efficiency, and unclear gas migration law. Therefore, it is necessary to study the gas migration law and propose a targeted prevention and control the technical scheme. In order to improve the gas control efficiency of the extra-thick coal seam, the evolution law of permeability of the lower slice is obtained under mining through experimental research. The liquid-solid coupling seepage-flow model for gas migration is established in the lower slice. Comsol Multiphysics software is used to study the migration law of pressure relief gas in the lower slice. Based on the gas migration law, the gas extraction and cut flow technology for the lower slice long borehole is proposed. Through this technology, the amount of gas flowing into the upper slice goaf and the gas content of the lower slice are reduced, and the drilling horizon is optimized. The research results show that the determination of the optimal drilling horizon of the lower slice needs to balance the amount of gas flowing into the goaf and the total amount of gas extraction. The range of 3-7 m horizon in the lower slice is appropriate to the boreholes arranged. When the borehole is located in the lower slice -3 m horizon, the 360 day gas emission quantity of goaf can be reduced to 51.2% of the nondrilled emission quantity, and the total extraction amount is 1143 m³. When the borehole is located in the lower slice -7 m horizon, the 360 day gas emission quantity of goaf can be reduced to 95.31% of the nondrilled emission quantity, and the total extraction amount is 1461 m³. Considering the gas emission capacity of the upper slice and ensuring that the total extraction volume of the lower slice is maximized and the boreholes in the lower slice are not damaged, the boreholes are located in the -6 m horizon of the lower slice.

The co-mining of coal and gas is the inevitable future direction of the mining of coal resources. Taking coal mining and gas extraction as the two subsystems of the coal and gas co-mining system, to reveal the mechanism of action between coal mining and gas extraction is the premise of orderly co-mining. On the basis of a similar simulation experiment of coal and gas co-mining, by obtaining the gas migration law during the mining process and collecting a large amount of data on the coal production and gas extraction, it is found that the two subsystems of coal extraction and gas extraction in the coal and gas co-mining system promote and restrict each other. The control parameters for coal mining and gas extraction that affect co-mining are identified. To coordinate the process connection between coal mining and gas extraction, the optimal synergistic relationship of co-mining should be found. The recovery rate and economic benefit of coal and gas resources are taken as the optimization objective function of coal and gas co-mining. Taking the safety production laws, regulations, and production technology-level restrictions of coal mining and gas drainage as constraints, by constituting a nonlinear model for the collaborative optimization of coal and gas co-mining, the method of determining the optimal advancing speed and optimal gas drainage volume of the working face is proposed. By optimizing variables, such as coal mining advancement, coal mining time, gas extraction time, and gas extraction volume, the co-mining of coal and gas is ensured to be safe and efficient, and the output of coal and gas resources is optimized. The time connection and the process succession of the two subsystems are attained. An overall orderly structure is formed between the coal mining system and the gas extraction system, and the mechanism of the cooperative co-mining of coal and gas is revealed. This research has important significance with regard to improving the basic theoretical system of coal and gas co-mining. The control variables of the co-mining working face in the Shaqu mine are optimized. After optimization, the profit is increased by 16.3%, and the gas extraction rate is increased by 2.6%. The drilling spacing is optimized according to the optimization results. The simulation shows that 7 m is the optimal drilling spacing of the working face.