Experimental study on the petrophysical variation of different rank coals with microwave treatment

To maximize coalbed methane recovery, the reservoir is often stimulated because of its low permeability. An exploratory study on improving coal porosity and permeability by microwave treatment was proposed. Pore size distribution of four unconstrained coals (lignite, subbituminous, bituminous, and anthracite), before and after microwave treatment, were evaluated using the nuclear magnetic resonance. With continuous exposure to microwaves, the pore size distribution of coals (especially the lignite coal) extends, and the pore volume and connectivity increase. In addition, coal porosity and permeability evaluated based on the Schlumberger Doll Research model increase by 33–72% and 73–181%, respectively. The mechanism was revealed by combining P-wave tests, thermal imaging and X-ray computed tomography scanner. The moisture and minerals bounded in pores are selectively heated and then detached, and, as a result, the pore structure is opened. Continued exposure to microwaves rapidly converts the mobilized moisture into super-heated steam. Under the steam pressure, pores and fractures generally expand. Borrowing from microwave-assist oil recovery, we presented a conceptual design of a CBM extraction borehole with microwave irradiation. •The evolution of pore structure during microwave treatment was obtained.•NMR T2 spectrum of fluids within the pore structure is coal rank sensitive.•Porosity and permeability increase with increasing microwave treatment time.•Microwave has more significant effect on lower rank coals than higher rank coals.