A review of experimental research on Enhanced Coal Bed Methane (ECBM) recovery via CO2 sequestration

There is a significant diversity in the global coal basins in terms of their rank, age, geological features and fracturing characteristics. A section of such coal basins is often not economically minable either due to their occurrence at greater depth or their inherent low grades. The coal seams, however, are characteristically porous, generally permeable and may contain naturally occurring methane gas, a potential source of unconventional energy. The extraction of methane gas from coal seams poses environmental issues and extraction of gas from underground can disturb natural equilibrium. Injection of CO2 into the coal seams has been proposed to be one of the most effective methods to solve the problem. The injected CO2 forces the free as well as adsorbed methane gas to flow along a preferable direction, which makes the methane extraction operations easier and at the same time, allows the coal seam to remain or act as a long term storage of CO2. Enhanced Coal Bed Methane (ECBM) recovery via CO2 sequestration involves the following three key processes: i) adsorption of CO2 and desorption of methane from pores, ii) adsorption induced swelling/shrinkage of coal matrix and iii) flow of gases through the altered dimension of cleats, depending on the compressibility of cleats. These processes operate at microscale, and are sensitive to a range of physical parameters, such as temperature, pressure, moisture content, type of coals. Laboratory experiments are found to be an effective approach in characterizing and quantifying the micro processes of combined ECBM and CO2 sequestration. A considerable number of applications are nowadays being employed based on the proposed laboratory data and suggestions. However, a significant range of results have been obtained globally, which have led to the foundation of multiple school of thoughts. This paper presents a comprehensive summary of the state of the art knowledge, underpins the science associated with gas adsorption/desorption, swelling/shrinkage in coals and discusses petrophysical properties (porosity, permeability, etc.) as functions of pressures, temperatures, moisture content and coal rank. A part of this article highlights the applications of recently developed X-ray tomography used to evaluate various petrophysical properties of coal, directly related to ECBM and CO2 sequestration. We finally identify potential research areas for future projects in strengthening the experimental data and our better understanding of ECBM technology, in general.