Experimental investigation on pores and permeability variation characteristics of coal treated by chemical solvents

Enhancing coal permeability by adopting chemical solvents treatment is one of the most effective methods to improve coalbed methane production efficiency in deep coal reservoirs with low permeability. However, coal pores and permeability variation characteristics affected by chemical solvents is not quantitatively determined. In this study, after being treated with carbon disulfide (CS2), acetone (C3H6O) and N-methyl pyrrolidone (NMP), changes in coal ultrasonic characteristics were investigated. Relationship between ultrasonic parameter variations and coal fracture development was revealed. Meanwhile, through adopting nuclear magnetic resonance (NMR) and permeability testing equipment, coal pore structure, porosity and permeability changes were studied. Results show that after treated by three chemical solvents, ultrasonic velocity in coal reduces by 7.7%–51.2%, while ultrasonic attenuation coefficient increases by 4.22dB/m-123.8 dB/m, indicating that coal fractures experience good development. Moreover, NMR signal quantity of treated coal under saturated state rises significantly, with big growth in mesopore and macropore peaks. Compared with NMP and acetone, CS2-treated coal porosity experiences the maximum increase of 7.71%, while the corresponding permeability growth is also the largest of 69.565 × 10−15m2. Permeability changes obtained from NMP calculation model is basically consistent with that acquired by lab measurement. Furthermore, changes in extraction rate, ultrasonic parameters, pore fracture characteristics and permeability results of solvent-treated coal are mutually consistent. This verifies that chemical solvents could dissolve molecular substances in coal, effectively increase primary fractures and change small pores to mesopores and macropores, as well as improving pore connectivity. As a result, coalbed methane extraction performance is expected to be enhanced due to increased coal permeability.