Journal of Canadian Petroleum Technology
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.
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
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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- Original manuscript received:
2 April 2007
- Meeting paper published:
12 June 2007
- Revised manuscript received:
11 May 2009
- Manuscript approved:
8 June 2009