Assessing the Water Uptake of Alberta Coal and the Impact of CO2 Injection with Low-Field NMR

ABSTRACT

Coal property characterization is an essential step to develop coalbed methane (CBM) recovery processes. In most cases, coal contains free water in the cleats (except dry coal), as well as moisture that forms an integral part of the coal structure. Most CBM production starts with dewatering coalbeds to initialize the gas recovery. Therefore, the wetting behaviour of coal by water is an important aspect in coal property studies. As CO2 has a strong affinity to coal, CO2 injection may change the coal wetting behaviour during a so-called enhanced coalbed methane process (ECBM). Studies on coal wettability are rare. This paper investigates the water uptake by Alberta coal and its wettability alteration due to CO2 injection using low-field nuclear magnetic resonance (NMR).

Low-field NMR is a technique used in logging and in the analysis of fluids contained in reservoir rocks. It measures the hydrogen density in reservoir fluids and distinguishes between ‘free’ bulk water and ‘bound’ surface water. CO2 is invisible to NMR, but its impact can be detected by changes in the water signal.

Experiments on coal samples in the form of dry and moist powder and chunk are used. The water uptake rate can be shown by monitoring the geometrical mean transverse relaxation time. From the spectra of different coal samples, water can be characterized into free, capillary-bound and subsurface-bound (adsorbed) water. These forms of water have different uptake behaviour inside coal. The injection of CO2 will cause coal dewatering, and the effect will increase with elevated CO2 pressure.

Introduction

Coalbed methane (CBM) has evolved into a commercially profitable source of unconventional natural gas. Canada has vast resources of coal and it has been estimated that the total in-place reserves are 36 ? 1012 m3. Over 60% of Canada’s CBM resource is in Alberta(1). Coalbed methane has the potential of contributing a significant portion of Canadian natural gas production in the foreseen future. Using CO2 to enhance methane recovery has been discussed by several researchers(2, 3). This process is called CO2-ECBM. If successful, its implications include CO2 sequestration in deep unmineable coalbeds.

Coal property characterization is an essential step to develop CBM/ECBM recovery processes. In most cases, coal is wet and contains free water in the cleats, as well as moisture that forms an integral part of the coal structure. Some coals found in the Western Canadian Sedimentary Basin’s Horseshoe Canyon Formation are dry coals, which means the coal cleats no longer preserve free water. Most CBM production starts with dewatering coalbeds to initialize gas recovery. Therefore, the wetting behaviour of coal by water is an important aspect in coal property studies. As CO2 has a strong affinity to coal, CO2 injection may change the coal wetting behaviour in the ECBM process. Published studies on coal wettability are very rare to the best of our knowledge.

Low-field NMR is a relatively new technique used in logging and in the analysis of fluids contained in reservoir rocks.

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