Karine Schepers, SPE; Anne Oudinot, SPE; Advanced Resources International,
Inc.; Nino Ripepi, SPE; Virginia Center for Coal and Energy Research
SPE International Conference on CO2 Capture, Storage, and Utilization,
10-12 November 2010,
New Orleans, Louisiana, USA
Nitrogen (N2) and carbon dioxide (CO2) injection has been a subject of enhanced
coal bed methane (ECBM) and carbon capture and storage (CCS) research during
the past decade. N2 and CO2 injection produce substantially different recovery
processes. Coal has a higher affinity for CO2 as compared to methane (CH4).
Preferential adsorption of CO2, a larger molecule than methane, onto the coal
surface results in a dramatic decrease in cleat permeability due to coal
swelling. This ultimately induces a loss of injectivity creating a significant
technical hurdle for CCS operations in coal. In contrast, N2 increases cleat
permeability because of its lower coal storage capacity relative to methane. As
a result, injectivity increases during N2-ECBM. Theoretically, the injection of
a mixture of CO2 and N2 will result in ECBM and CCS without a loss of
injectivity. This study presents an investigation of that concept.
To identify key geological and reservoir parameters driving ECBM and
sequestration processes in deep unminable coal seams, a Monte Carlo
probabilistic approach was implemented. Results from tornado plots confirmed
the major role that coal rank (Langmuir isotherms) and pressure-dependent
permeability data play in ECBM processes. As coal rank determines the maximum
gas-in-place that could be stored per volume of coal, average fracture
permeability, matrix and pore compressibility, and differential swelling
factors are predominant in coal capacity to flow water and gas phases,
impacting both incremental methane production as well as injectivity.
Additionally, cleat permeability will vary greatly in response to injected gas
composition during ECBM process. To better understand the consequences of
these permeability changes by coal rank, a parametric study was designed. First
results show that, for a specific coal rank, ECBM can drastically improve by
increase N2 content in the injected gas stream. However, methane incremental
recovery due to high N2 content will increase up to a maximum N2 concentration,
or threshold: besides this threshold, breakthrough occurs too rapidly to
generate additional methane recovery. This N2 threshold varies between coal
ranks, as pressure dependant parameters also vary relative to the rank.
Finally, 100%N2 injection scenarios per coal rank highlight permeability
behaviors easily explained in theory but which would probably need additional
laboratory measurements to better understand their physical meaning while
encountered during “real world” problems.