Experimental Investigation on the Effects of Microwave Irradiation in Pore Structures of LN₂ Freezing Coal for Coalbed Methane Extraction

Liquid nitrogen (LN₂) fracturing, an effective method in stimulating reservoirs, contains freezing and thawing. In general, frozen rocks are air-thawed in past studies. Microwave fracturing not only enhances pore structures but also removes pore water. To investigate the influence of the thawing process, microwave irradiation is used to thaw the coal samples after LN₂ freezing, and is compared with air-thawing. The magnetic resonance imaging (NMR) results illustrate that the increased rates of the seepage pores of the air-thawing samples decrease first from 13.01% (30 min) to 8.76% (120 min) and then gradually increase to 15.49% (270 min) after the freeze–thaw treatments, in which the microwave performs greater than air at the same freezing time. The results of the wave velocity of the samples are similar to that of NMR. The infrared thermal imaging (ITI) results indicate that after microwave thawing, the variance of the surface temperature slowly decreases to the minimum value of 373.9 (120 min) from 636.3 (30 min), and then peaks at 270 min (1243.0). The fractures of the air-thawed samples become more obvious with the increasing freezing time and are further developed after microwave thawing; the development of the fractures influences the distribution of the surface temperature. The SDR permeability variations perform a similar rule to that of the seepage pore. The results can be a possible reference in thawing the LN₂ freezing coal by microwave.