Assessing the Water Uptake of Alberta Coal and the Impact of CO2 Injection With Low-field NMR

摘要

Coal property characterization is an essential step to develop Coalbed Methane (CBM) recovery processes.Coal contains free water in the cleats 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 behavior 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 behavior during a so-called Enhanced Coalbed Methane process (ECBM).Studies on coal wettability are rare.This paper investigates the water uptake by the 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 by 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 behavior inside coal.The injection of CO2 will cause coal de-watering,and the effect is increasing with elevated CO2 pressure.