Journal of Canadian Petroleum Technology
Volume 48, Number 7, July 2009, 40-46

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.

© 2009. Petroleum Society of Canada (now Society of Petroleum Engineers)

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History

• Original manuscript received: 2 April 2007
• Meeting paper published: 12 June 2007
• Revised manuscript received: 11 May 2009
• Manuscript approved: 8 June 2009