Experimental study on the evolution of pore-fracture structures and mechanism of permeability enhancement in coal under cyclic thermal shock

Abstract

Heat injection is a new method of recovering enhanced coalbed methane (ECBM). In this study, the impact of a sudden increase in ambient temperature on the permeability of coal was studied, and the application prospects of multiple thermal effects in coal fracturing and permeability enhancement were explored. The permeability and development of pore-fracture structures in coal were tested before and after cyclic thermal shock. The evolution characteristics of the pore distribution were evaluated, and an empirical formula for estimating the permeability growth of coal was established. Digital image processing technology and fractal theory were used to analyse changes in macroscopic fractures. The results indicates that cyclic thermal shock could effectively promote the generation and expansion of permeability pores, effectively connect the relatively independent fracture structures, and break and expand the microfractures and pore groups to form an interwoven fracture network. The increase in cyclic thermal shock resulted in an increase of 0.92%, 1.13%, and 16.15% in total porosity increment, the porosity increment of seepage pores and the proportion increment of seepage pores, respectively, after seven thermal shocks. The permeability of coal exhibited a logarithmic growth pattern with an increase in the number of thermal shock cycles; the average permeability increased by 100.02% after seven cycles. Based on the theory of thermodynamics, the influencing mechanism of cyclic thermal shock on the evolution of pore-fracture structures was explored. Differences were observed in the thermal expansion coefficients of minerals in coal, and thermal stress was generated under cyclic thermal shock, which may lead to damage, promoting fracture formation and increasing permeability. With an increase in the number of thermal shock cycles, the efficiency of permeability enhancement gradually decreased, which suggests that the reservoir conditions should be combined and mining plan should be reasonably set in ECBM extraction using multiple thermal injection techniques.