Variable saturation simulation of desorption during CO2 - injected CH4 gas production from CBM reservoirs at Southeastern Jharia, India

In 1997, Exploratory drilling program led by Oil and Natural Gas Corporation (ONGC) substantiated that coal seams ranging from 180 m to 1300 m in the southeastern Jharia, India indicate high CH₄ gas saturations within reservoir - quality. A comprehensive analysis and evaluation of collected samples from the prospective well sites measure vitrinite reflectance between 0.70 - 1.61%, which indicates reservoirs are of post-mature stage exhibiting good potential for the commercial coalbed methane (CBM) exploitation.

By constraining the saturations obtained from gas adsorption and desorption isotherms and thicknesses data of CBM bearing coal seams, cylindrical three dimensional (3D) r-z reservoir models were numerically modelled using commercial reservoir simulator. The generation potential inferred from the experimental analyses are used to delineate the reservoirs conditions for the reservoir simulation. Variable saturation simulation of CH₄ gas production from the prospective coal seams were investigated adopting desorption at a constant bottom hole pressure (BHP) and CO₂ - injected CH₄ gas recovery method.

Obtained results from the simulations reveal that these CBM reservoirs are positively produced, with sustained high intrinsic reservoir - quality. Adopting depressurization at a constant BHP and CO₂ - induced CH₄ recovery method. CH₄ gas production displays rates can reach ̴ 3Mscf/day from that of ̴ 70629.8 scf/day i.e. average current production rate. Simulation results predict that cumulative gas volumes of ̴ 25 - 30 BScf can be drained from the reservoirs over 10 years. In addition, Results suggests that CO₂ induced - CH₄ recovery from interbedded coal and shale deposits might be desirable hotspots when compared to that of regular deposits. Confined pressure conditions are effectively considered in regard to the permeability anisotropy on gas production.

The outcomes of this simulation dictate that heterogeneous, inter-bedded coal and shale deposit are better candidates for the CO₂ induced - CH₄ recovery when compared to that of regular deposits for the prospective CBM reservoir.