A comparative study on the deformation of unconfined coal during the processes of methane desorption with successively decreasing outlet pressure and with constant outlet pressure

Abstract

The shrinkage of coal induced by methane desorption during coalbed methane (CBM) production will change the porosity and permeability of reservoirs. It is of great significance to analyze the deformation characteristics of coal in the process of desorption with successively decreasing outlet pressure, which is conducive to the continuous production of CBM. In this study, five cylindrical samples in Shanxi Province, China were used to reflect the deformation and desorption characteristic in the process of methane desorption with decreasing outlet pressure (simulated the change of bottom hole pressure in the field CBM production) by independently designed instruments. And the results were compared with that in the process of desorption with constant outlet pressure (atmospheric pressure). The results show that the coal swells (shrinks) when methane is absorbed (desorbed). There is a linear relationship between the strain and adsorption/desorption volume. Moreover, the shape of desorption rate-time curves is similar to that of the Langmuir curves in two desorption processes. However, the cumulative desorption rate of desorption with successively decreasing outlet pressure is higher than that of desorption with constant outlet pressure due to the smaller attenuation effect on permeability, which leads to the shrinkage during desorption with successively decreasing outlet pressure is also higher. The shrinkage of the samples is always less than the swelling. There is a negative correlation between the strain differences and the desorption rate. The strain differences in the process of desorption with successively decreasing outlet pressure are lower than that of desorption with constant outlet pressure due to the higher desorption rate.