Characterization of Coal Pore Structure and Matrix Compressibility by Water Vapor Injection

In China, the exploration and development of low-rank coalbed methane (CBM) resources are in the early stage, and in-situ pyrolysis is an effective technology for mining of low-rank CBM resources. In this paper, N-2 adsorption method and high-pressure mercury injection test were used to study the pore structure characteristics of coal samples by water vapor injection, and the pore size boundaries of the two test methods were determined. From the continuous pore space distribution model, Frenkel-Halsey-Hill model, Menger sponge model, a new method of pore size classification is proposed: (I) (> 10,000 nm), (II) (1000-10,000 nm), (III) (100-1000 nm), (IV) (x (pore diameter boundary)-100 nm), (V) (10-x nm), (VI) (< 10 nm). The results were not inconsistent with the Hodot classification method, indicating that the new pore classification scheme is reliable. Meanwhile, the relationship between pyrolysis temperature and matrix compressibility is discussed, and it was found that transition pores had a significant effect on matrix compressibility. Pyrolysis weakened the connection between coal particles, improved the development of porosity, and led to high matrix compressibility. Furthermore, when pyrolysis temperature was < 400 degrees C and matrix compression effect was dominant, poor pore connectivity resulted in a low level of matrix compressibility; when pyrolysis temperature was > 500 degrees C and pore filling effect was dominant, high level of matrix compressibility was promoted.