Thermal Cracking And Displacement For Enhancing Methane Extraction By Phase Transition Of Liquid CO₂ Injection In Coal Seam

It is the common characteristics of coal seams with high gas content and low permeability in China.Artificial measures are needed to be put forward for enhancing the efficiency of methane extraction.Based on the features of liquid CO2 with low temperature,elevated pressure,and displacement,thermal cracking and displacement for enhancing methane extraction by phase transition of liquid CO2 injection in coal seam was proposed.A combination of theoretical analysis,experimental test,simulation,and practical application was used in this study.The evalution of pore structure and mechanical failure were analyzed.The aging characteristic of displacement between methane and carbon dioxide was obtained.The mechanism of fracturing and displacement for enhancing methane extraction by liquid CO2 injection in coal seam was presented.There are great theoretical and practical significance for ethancing CH4 extraction from coal seam with high gascontent.The relationships between the thermophysical parameters and moisture content of coals at the range of-50-50 ? were tested by laser thermal conductivity method.Thermal conductivity and specific heat capacity of coals were increased with the rasied temperature,and the inflection point of temperature formed at 0-5 ?.The curves of thermophysical parameters versus temperature showed a linearly increase in each stage.The higher the moisture content of coal,the greater the heat conductivity and specific heat capacity.The temperature field of coal seam during the liquid CO2 injection was simulated and a quadratic function between cooling radius and injecting time was presented.The range of effective cooling radius was 0.102m-0.919 m.Interexchange of heat between coal and liquid carbon dioxide occurred during the injecting process.Five typical stage(phase change boosting,fluid-solid heat exchange,gas-liquid phase transformation,two phase percolation and diffusion,and pressure decay)were experanced in the entire process.The compression and splitting failure of liquid CO2 saturated coals were analyzed based on the characteristics of energy and acoustic emission.The uniaxial compressive strength and the strain were decreased and the load time from beginning to peak point were shortened.The strain and energy of saturated coal in the elastic deformation stage obviously increased and more cracks existed.The peak compressive strength,peak strain,and energy decreased.The failure modes of the saturated coal samples were inclined to ductile fracture as opposed to brittle fracture in the original coal samples.The evalution feature of coal pore structure was tested using nitrogen adsorption and mercury intrusion method.The dimension of coal pores changed to larger ones and new pores generated simultaneously.Average radius and specific area of coals after saturation process enlarged and the total volume decreased.The results of SEM revealed that the more and larger cracks formed with the treated coals.The device of carbon dioxide replacement methane was developed and indexes of injection effect and simulates aging characteristics were proposed.Before the critical time,both the total gas in displacement flow and displacement efficiency increased.Moreover,there is no obvious effect on displacement to the parameters of pressure and flow after the critical time.The greater the displacement flow and displacement pressure,the higher the gas cumulant in displacement flow and displacement efficiency.The mechanisms of improving permeability and enhancing gas extraction are the combination of thermal stress.erosion of pore pressure,and gas displacement during the liquid CO2 injection.It is mainly effected on the thermal stress and erosion of pore pressure.Pressure gradient in seepage channel of coal seam is enhanced and the extraction efficiency is finally improved.In-stu experiments were carried out in the Nantun and Zhangji coal mine.The systematically equipment of improving permeability and enhancing methane extraction by liquid CO2 injection was established and the criterion of effect judgment was proposed.The feability of technology using carbon dioxide fracturing and gas replacement is verified.