Investigation of Adsorption–Desorption, Induced Strains and Permeability Evolution During N2–ECBM Recovery

In this study, the adsorption–desorption/induced strains/permeability characteristics of seven columnar coal samples with Ro.ran ranging from 0.42 to 3.23% were monitored dynamically during N2-enhanced coalbed methane (N2–ECBM) recovery. The results showed that N2 injection and sealing (N2–IS) is an important step in N2–ECBM recovery, with average desorption of 60.25%. During desorption with stepwise depressurization (DSD), the desorption volume increased linearly with time. The breakthrough point of gas production was in the first gas drainage phase, and the low-rank coal reservoirs entered the depletion well stage earlier. The samples were subjected to N2–IS and DSD, the cumulative desorption of the samples was in the 88.03–100.96% range, and the average desorption of the medium-rank coals was 100.40%. The residual volume strain was negative in medium-rank coals and caused by pore–fracture structural damage. The induced strain behavior was independent of gas composition, but with the gas volume being polynomial, it had a predictive effect. The permeability recovery percentage (PRP) during N2–IS was in the order of medium- > high- > low-rank coal, and the PRP of medium-rank coal was up to 90%. The strain sensitivity coefficient of low-rank coal increased with increase in volume strain, that of medium-rank was independent of volume strain, and that of high-rank coal decreased with increase in volume strain. This work revealed that, by injecting N2 into the reservoir and sealing the well for a certain period of time while maintaining high reservoir pressure, CH4 desorption induced coal volume shrinkage, the fracture aperture was restored, and some new fractures were induced, increasing permeability; in addition, DSD can be more consistent with the gas production process.