Impact of bioconversion on matrix strain response of coal reservoirs: Part 2-Reservoir insights

Abstract Experimental data obtained for the strain response of coal to helium and methane flooding, pre-and post-bioconversion, was used to evaluate the reservoir-scale implications of coal bioconversion. Analysis showed that reservoir parameters, Langmuir-type strain ( ∊ l ) and pressure ( P ∊ ), rebound (P rb ) and recovery (P rc ) pressures decreased post-bioconversion non-linearly with the volume of biogenic methane produced. Also, the solid-matrix compressibility (C S ) increased and shrinkage/swelling (C m ) compressibility decreased post-bioconversion. The effective stress and permeability response of coal to depletion was suppressed after microbial treatment. Finally, bioconversion resulted in softening of the coal matrix as well as ‘excess’ volumetric shrinkage post-methane depletion, enhancing the reservoir permeability. However, the increase was insignificant and was overshadowed by the effect of bioconversion induced swelling, which resulted in a significant reduction in reservoir permeability. The non-linear dependence of C m to pore pressure resulted in its rebound past an inflection pressure (P I ), identifiable for coal treated for longer durations. This was used to define an ideal condition, beyond which there was some improvement in reservoir performance. Effective stress response of coals before and after treatment was then used to rate the suitability of coals to bioconversion from a reservoir perspective.