Gas Permeability of Coal at In-Situ Conditions

Abstract

Depletion of conventional resources, and increasing demand for clean energy, forces India to hunt for alternatives to conventional energy resources. Intense importance has been given for finding out more and more energy resources; specifically, non-conventional ones like CBM, shale gas and gas hydrates, as gas is less polluting compared to oil or coal. CBM is considered to be one of the most viable alternatives to combat the situation. With growing demand and rising oil and gas prices, CBM is definitely a feasible alternative supplementary energy source.CBM is a remarkably clean fuel if utilized efficiently. Production of CBM is initiated by water depletion method i.e. reduction in pressure in the coal bed as water is being pumped out from the reservoir. Desorption of methane creates voids in the coal matrix which help in the CO2 storage. Commercially, the production of methane depends on proximate and ultimate analyses parameters, petrographic parameters, microstructural analysis, mechanical properties, adsorption characteristics and permeability characteristics of the coal samples. Gas permeability affects the most in the determination of gas flow from the matrix to well and vice versa and is usually affected by any changes in the mechanical properties. Therefore, it is important to determine these parameters in order to evaluate the changes in the permeability with varying confinement and gas pressures.
In this research work, investigations were carried out to characterize coal properties, adsorption, and gas permeability characteristics of coal from 200 to 690 m depth from Raniganj coalfield. Proximate and ultimate analyses, petrographic analysis, microstructural analysis and mechanical properties have been evaluated for the characterization of the coal bed reservoir. The variation of the proximate analysis parameters with depth like moisture content, volatile matter and ash content confirms coal from medium to low volatile bituminous coal. Reduction in moisture content from 1.95 % to 0.4 % and higher fixed carbon from 50.36 % to 56.72 % with increase in depth from 200 to 690 m confirms higher porosity and gas adsorption sites in coal matrix. Ash content was 22.61 % at 690 m which displays increase in hydrocarbon content with depth. The variation in ultimate analysis parameters like carbon, hydrogen, nitrogen and sulphur confirms the coal samples are bituminous in nature. The variation of carbon and hydrogen content exhibits enrichment of vitrinite macerals and methane generation in the coals seams with depth. Increase in the virtrinite macerals confirms the increase in methane content of coal seam with depth. The calculated vitrinite reflectance ranges from 0.52 % to 0.77 % which lies between the threshold value and thus reflects the commercial prospects of CBM in study area. The FE-SEM images indicate the presence of primary and secondary pores in coal. The distribution of pores was dispersed or aggregated, circular, elliptical and lenticular or strip indicates strong adsorption capacity of hydrocarbons. Inter particle as well as intra particle pores were distributed throughout the coal samples significantly impacting hydrocarbon storage. The presence of micro fractures at the edges confirms the existence of flow channels between micropores and macropores for flow of gas. Unconfined compressive strength, elastic modulus and Poisson’s ratio varied from 2.31 MPa to 5.56 MPa, 0.407 GPa to 0.535 GPa and 0.319 to 0.371.
Maximum storage capacity of 25.85 cc/gm for CO2 and 13.4 cc/gm for CH4 was observed at 690 m. Increase in gas storage with depth is due to reduced moisture content and ash content in coal, resulting in more sites available for gas adsorption. The adsorption capacity of CO2 was higher than that of CH4 because of small molecular size of CO2 as compared to CH4. Higher CO2 adsorption capacity of coal confirms promising CBM potential in Raniganj coalfield. The CO2 permeability varies from 0.92 to 1.72 mD with increase in depth whereas the CH4 permeability varies from 0.48 to 0.96 mD. Permeability reduced rapidly from 1 to 4 MPa. The reduction in permeability is due to crushing of grains as well as narrowing and closure of fracture. The cleat width of coal specimen exhibited 12 to 47 % reduction from 200 to 690 m depth respectively due to gas pressure.
The findings consist characterization of coal samples based on the laboratory experimentation, determination of adsorption capacity of coal and its variation with pressure, variation of permeability with confinement, gas pressure and mean gas pressure, which is useful and needed information for industrial and commercial CBM production.