Abstract
To overcome the low efficiency of extracting gas in coal reservoirs with a low gas permeability, some boreholes were drilled for gas extraction in No. 2 coal reservoir of Wangjialing Coalmine in Shanxi Province, China and reasonably sealed. Aiming at shortfalls such as rapid attenuation of volume for extracted gas as well as low gas permeability when using boreholes in the No. 2 coal reservoir, the traditional COMSOL MultiphysicsMT Earth Science Module was used to couple the three governing equations (Darcy-Brinkman–Navier-Stokes) for fluids. On this basis, numerical simulation on the seepage law along the directions of roadways and boreholes was carried out. The simulation results indicated that when the hole-sealing length was within the width range of fractures in roadways, the negative pressure not only led the gas in surrounding rock masses to flow to the boreholes, but also made the air flow in roadways to permeate into coal walls. As a result, gas and air flows both entered into the boreholes through the loosening zone containing fractures, resulting in seepage of air in roadway to the boreholes. The seepage velocity along the roadway direction under condition with a hole-sealing length of 12 m was obviously slower than that when the hole-sealing length was 8 m. While, the method by simply increasing the length of the hole-sealing section for boreholes failed to effectively stop the air flow in roadways from permeating into the coal wall and then entering the boreholes. Moreover, the increase in the hole-sealing length brought about much more difficulties to the hole-sealing construction. So, the method is not operable in practical condition of the coal mine. Therefore, it is necessary to improve the traditional hole-sealing technology based on foamed macromolecular materials which are mainly made of polyurethane (PU) and use the fluid wall-type hole-sealing technology based on solid-liquid coupling. Then, the effects of gas extraction before and after using the fluid wall-type hole-sealing technology based on solid-liquid coupling to increase the hole-sealing length to 12 m were compared. The comparison results revealed that the pure extraction amount of gas from a single borehole in the No. 2 coal reservoir of Wangjialing Coalmine was improved by 4∼6 times. In addition, the concentration of extracted gas increased from less than 1% under the traditional hole-sealing mode to 20%∼25%, with an increase of more than 20 times. The extraction effect of the No. 2 coal reservoir of the coal mine was significantly enhanced by employing the fluid-wall-type hole-sealing technology based on solid-liquid coupling.
Highlights
Restricted by gas permeability and natural conditions, coal reservoirs generally have a low efficiency in extracting gas, especially the coal reservoirs of low-gas permeability
A numerical simulation was conducted in the research on gas seepage along the boreholes for gas extraction and the coal wall of roadways by taking essential data of gas in Wangjialing Coalmine as the basis based on research findings relating gas seepage obtained by numerous scholars in the world
By utilizing COMSOL Multiphysics numerical simulation, the seepage and flow laws of gas near the boreholes for gas extraction and discharge can be analyzed by amplifying the gas flows around the boreholes and the inner of boreholes in the coal reservoir
Summary
Restricted by gas permeability and natural conditions, coal reservoirs generally have a low efficiency in extracting gas, especially the coal reservoirs of low-gas permeability. A numerical simulation was conducted in the research on gas seepage along the boreholes for gas extraction and the coal wall of roadways by taking essential data of gas in Wangjialing Coalmine as the basis based on research findings relating gas seepage obtained by numerous scholars in the world. On this basis, we attempted to further analyze the migration law of gas seepage along the boreholes for gas extraction and the coal wall of roadways. EQUATION FOR CONTROLLING GAS SEEPAGE AROUND THE BOREHOLES FOR GAS EXTRACTION AND WALL OF ROADWAYS
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