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

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 CO 2. The permeability to different gases (CH 4, CO 2) 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 CH 4 permeability is relatively smaller than the corresponding He permeability demonstrating a swelling effect of coal by CH 4. The adverse effect of CO 2 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 gains increasing worldwide attention. Using CO 2 for enhanced CBM production or using coal beds for CO 2 sequestration has also received intense research interest in recent years. CO 2 has been proven to have more than twice the affinity to coal than CH 4 (1). Limited studies have shown the concept of CO 2-ECBM to be technically viable. However the mechanisms have not been studied in detail.

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 to methane is crucial for evaluating CBM production rate. When injecting CO 2 into the coalbeds in the ECBM production, the interaction of coal with dense CO 2 can be inferred from the permeability changes. Measuring coal permeability to CO 2 is the fundamental step to investigate the CO 2-ECBM process and potential of CO 2 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), (4), (5) . Permeability is provided to coal by the cleats and depends on cleat orientation, spacing, and aperture. The coal matrix swells and shrinks during adsorption and desorption of gases (6), (7) . As reservoir pressure is depleted, the net stress due to overburden increases, leading to closure of the cleats. At the same time, gas desorption causes coal matrix shrinkage, which leads to cleat opening. Different gases adsorb on coal to different degrees and are expected to cause different degrees of swelling or shrinkage.

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