Laboratory Investigation on the Permeability of Coal During Primary and Enhanced Coalbed Methane Production

Abstract

Abstract Coalbed Methane (CBM) has grown into an important natural gas resource in North America in recent decades. The economical development of CBM heavily relies on coal permeability. The coal permeability is provided through a network of coal cleats and natural fractures. It is greatly affected by the confining stresses and the coal matrix response during gas adsorption and desorption in primary and enhanced recovery processes. This paper is focused on a laboratory investigation of coal permeability variability with different operating parameters. Core flood experiments in coal have been conducted with methane production and methane displacement by CO2. The permeability to different gases (CH4, CO2) under equilibrium conditions was measured at different confining stresses. The coal permeability to helium, which was reported in previous work, provides a comparison baseline. The permeability of a coal core to gases shows a strong dependence on net confining pressure and exhibits strong hysteresis. The CH4 permeability is relatively smaller than the corresponding He permeability, demonstrating a swelling effect of coal by CH4. The adverse effect of CO2 on coal permeability is also shown. The paper focuses on the experimental results obtained to date. Introduction Coalbed methane (CBM) is an unconventional natural gas resource that is gaining increasing worldwide attention. Successful CBM production depends on two factors: coal gas content and reservoir permeability. Coal gas content means the amount of gas presented in coal, and the reservoir permeability reflects the rate of the gas that could be produced. Unlike conventional gas reservoirs, methane is stored by adsorption onto the subsurface of the coal seams. The ability of coal to store gas is a function of coal rank, reservoir temperature and pressure which is related to coal depth. The primary CBM production is to deplete the reservoir pressure to desorb the methane gas from coal. Using CO2 for enhanced CBM production or using coal beds for CO2 sequestration has also received intense research interest in recent years. CO2 has been proven to have more than twice the affinity to coal than CH4(1). Limited studies have shown the concept of CO2-ECBM to be technically viable. However, the mechanisms have not been fully understood. Permeability of coal is recognized as the most important parameter controlling CBM and ECBM (enhanced CBM) production. Coal cleats are responsible for the permeability of coal. Flow through the cleats is dominated by Darcy flow that relates flow rate to permeability and pressure gradient(2). Methane is the major component of the coalbed gas. Measuring coal permeability tomethane is crucial for evaluating CBM production rate. When injecting CO2 into the coalbeds in ECBM production, the interaction of coal with dense CO2 can be inferred from the permeability changes. Measuring coal permeability to CO2 is the fundamental step to investigate the CO2-ECBM process and the potential of CO2 sequestration. Measuring meaningful coal permeability is more difficult than measuring reservoir rock permeability. Only a limited number of research groups have measured the permeability of coal by core flooding, and have pointed out some of the associated difficulties(3–5).