Evolution law of microscopic pore fractures of coal after hot steam injection

Coal seam gas occurrence in China has the characteristics of strong adsorption, low permeability and micro-pore.The hot steam injection is a new way to increase the gas extraction rate.However, some basic problems such as gas desorption and pore structure change have not been explored.In order to reveal the effect of hot steam on the pore structure of coal pores, the pore fissure evolution rules of coal samples before and after heat treatment were discussed by means of scanning electron microscopy (SEM),mercury injection and nuclear magnetic resonance (NMR).The experimental results show that the pore structure evolution of coal after thermal steaming action can be summarized into four basic models, namely pore expansion mode, pore connectivity mode, pore fracture mode and fracture penetration mode.After the hot steam treatment, the difference between the inlet and outlet of the two coal samples was larger, and the total pore volume, permeability, porosity and average pore size were significantly improved.It shows that after steaming, the open pores of coal became more and the closed and semi-closed pores decreased.The peak P1、P2, relaxation time and overall peak area of T2 spectrum were significantly improved, indicating that the pore structure of coal splitting was affected by heat after steam action, and the pores changed from medium to small holes to medium and large holes, and pore fracture connectivity also increased.The original pores have been further developed, and the change of pore structure can promote the expansion and permeability of the coal body, which is beneficial to the gas desorption and seepage of gas in coal.It has a good foundation for the further development of technologies such as heat injection and infiltration.The research provides aguidance for improving the permeability of coal seam and the extraction rate of coal seam gas by injecting hot steam.