Gas Desorption And Seepage In Low Rank Coal Seam And Its Control Mechanism For Gas Drainage

Coal is the main energy source in China,accounting for 76% and 64% of the primary energy production and consumption structure respectively.The “National Energy Development Strategy 2030-2050” pointed out that,in 2050,consumption of coal in China is still approximately 2.5~3 billion tons.However,the proportion of low-degree metamorphic(lowrank)coal accounts for coal resources in China is about 46%.Therefore,the development of low-rank coal has important significance for Chinese energy supply.Gas(coalbed methane)is a concomitant of coal,and formed during the coal formation and deposited in coal reservoirs.In China,coalbed methane resources are very rich.However,low coal rank coalbed methane resources account for 43% of the total coalbed methane,which mainly distributed in the early Jurassic coal basins in the western China.Gas is a kind of clean energy,also is a source of disasters for coal mine safety and a strong greenhouse gas.However,the development of low-rank coal reservoir gas in China is still at a preliminary stage,and relevant scientific issues need to be further studied.In this dissertation,based on the structural characteristics and gas occurrence characteristics of the Jurassic low rank coal reservoirs of Binchang mining area in western China.Focusing on the current difficulties of gas drainage in low-rank coal reservoirs and using a combination of experimental research,theoretical analysis,numerical calculation and engineering practice.This study illuminates the microstructural characteristics of low-rank coal in Jurassic period and its control mechanism for gas desorption,seepage and drainage in the reservoirs.A new gas drainage model of “up-down wells stereo coupling” based on the principle of low rank coal gas desorption was proposed.The main research findings are as follows:Based on nitrogen adsorption,nuclear magnetic resonance and infrared spectroscopy,the microstructure and pore connectivity of Jurassic low-rank coal were obtained.The connection of macropores and micro-cracks of low rank coal is better,which is beneficial to fluid migration.But limited by the pore diameter effect of “semi-closed pores” that the control mechanism of gas adsorption and desorption on low rank coals is staged,mean that the release of gas under medium-high pressure in the open slot pores are rapid,but the gas desorption and flow difficultly under low pressure in the ink bottle-shaped semi-closed pores.Meanwhile,The surface of low rank coal is mainly hydroxyl and carboxyl groups,which results in the relatively weak affinity for methane of low rank coal.The low-rank coal in the Binchang mining area is affected by the bedding structure,and exhibits obvious anisotropic characteristics in the mechanical properties and gas seepage process.The bedding structure is the key to the internal compressive and tensile strength of the coal body being greater than the interlayer strength.At the same time,the presence of moisture in the coal body will lead to weakening of the compressive tensile strength of the coal body and even induce damage,and the presence of low-order coal microporous water also significantly restricts the desorption and permeability of the coal microporous gas.Based on the pore structure characteristics of low rank coal and its dynamic characteristics of gas desorption and diffusion,combined with the mechanical characteristics of coal body,a multiphysics theoretical coupling model of rock deformation,gas diffusion and seepage of lowrank coal is constructed.The effective control range of gas pre-drilling holes in low-rank coal reservoirs and its dynamic evolution law with extraction time,drilling space location and mining impact are obtained.Based on the structural characteristics of low rank coal and its coupling mechanism with gas in Binchang mining area,a new gas drainage model of “up-down wells stereo coupling” was proposed.Through the coupling of multi-type ground well drainage and underground networked drilled holes,which represented as a combination of up-down wells in space and regionally pre-drainage and distributed progressively in time.The stereo networked gas seepage advantageous channel was formed by the arrangement of underground pre-drainage holes and effectively penetrates the fractured zone of the surface well.The theoretical model of gas well production prediction for surface wells in Binchang mining area was established,and the optimal control method for gas extraction and downhole gas pre-pumping in the ground well was obtained.The research results are helpful for understanding the changing law of gas desorption and seepage in low rank coal reservoirs and its control mechanism for gas drainage in Binchang mining area,which provide theoretical support and practice guidance for the engineering application of high-efficiency gas drainage in low rank coal reservoirs.