Preliminary Understanding of CO2 Sequestration and Enhanced Methane Recovery in Raniganj Coalfield of India by Reservoir Simulation

Abstract

India has a vast reserve of coal (∼ 7% of the world's total proved reserve) and is the third largest producer in the world. Almost 70% of the country's electrical power comes from the coal-fired thermal power plants. A few coalbed methane (CBM) projects have also commenced operations in the central and eastern parts of the country. However, CO2 sequestration especially in geological formations is still at conceptual stage in India. Raniganj coalfield, located in the eastern states of West Bengal of Jharkhand is a repository of large reserves of high-volatile bituminous coals, where both mining and CBM operations are present. A number of coal-based thermal power plants are also located in the area, making this one of the most suitable areas for starting a carbon capture and storage (CCS) pilot project.CCS in unmineable coal seams arguably has the advantage of commercial success through potential release of additional methane when injected CO2 adsorbs into the coal seams, the process known as enhanced coalbed methane (ECBM) recovery. However, a significant concern lies in the loss of injectivity due to reduction in permeability by coal matrix swelling with CO2 adsorption although this effect can be partially be offset with ‘huff and puff’ scheme of cyclic CO2 injection followed by extraction of the released methane.The paper discusses the results of a numerical simulation study carried out with GEM compositional reservoir simulator to evaluate the effects of uncertainties in various reservoir parameters on the overall volume of CO2 storage and additional methane recovery before planning a pilot project in the Raniganj coalfield. A 3-m thick seam at 600 m depth was considered for fluid flow simulation study. While some information on the reservoir setting was obtained through literature and personal communication with the CBM operators, the rest of the information was derived through laboratory studies. The reservoir parameters considered for the study are injection pressure, adsorption capacity, permeability, cleat porosity, sorption time, and initial gas saturation. A 160-acre drainage area with 5-spot vertical well pattern was considered with one central injector and four producers on four corners of the study area.The maximum allowable injection pressure was estimated to be ∼ 850 psi (5856kPa) at the reservoir setting. The injection pressure was varied from 300 psi (2067kPa) to 850 psi (5856kPa) in the simulation and 600 psi (4134kPa) was found to be the optimum injection pressure. A number of adsorption isotherms on coals of Raniganj coalfield were established in the laboratory. The variations in the adsorption parameters observed through the isotherms were considered as uncertainty in the storage capacities. Significant variations were observed due to the variation in adsorption isotherms both for CO2 storage and additional methane recovery. Fracture permeability was varied from 3 md to 40 md, which is the range of permeability observed in the coalfield. The results of simulation indicate a strong influence of permeability on the CO2 storage and ECBM recovery. In the absence of specific information on sorption time, it was theoretically varied from a very fast-desorbing coal (sorption time of 1 day) to an extremely slow-desorbing coal (sorption time 200 days). Since rate of desorption from the coal matrix and fracture permeability are rate-control parameters, for each sorption time a low and a high permeability situation were considered. Fluid flow simulation study shows that variation in sorption time has no significant effect for a low permeability situation while some marginal effect in high permeability situation. Cleat porosity was varied from 0.5% to 1.5%. Within this range of porosities, enhanced methane recovery varied from 70% to 85% relative to the primary recovery but the volume of stored CO2 did not vary significantly. Lastly, the initial gas saturation was varied from 70% to almost 100% and both the CO2 sequestered volume and additional methane recovery were found to increase substantially.