Uncertainty Analysis using Experimental Design Methods for Assessing CO2 Sequestration and Coal Bed Methane Production Potential of Subbituminous Coals of the Nenana Basin, Interior Alaska

Naturally fractured, unmineable coal seam reservoirs are attractive targets for geological sequestration of CO2 because of their high CO2-adsorption capacities and possible cost offsets from enhanced coal bed methane production (ECBM). In this study, we have investigated CO2 sequestration and CH4 production potential of the subbituminous Healy Creek Formation coals through preliminary sensitivity analyses, experimental design methods and fluid flow simulations. Our primary sensitivity analyses indicated that the total cumulative volumes of CO2 sequestered and CH4 produced from the Healy Creek coals are mostly sensitive to bottomhole injection pressure, coal matrix porosity, fracture porosity and permeability, and coal volumetric strain. The results of Plackett-Burman experimental design were used to further constrain the most influential reservoir parameters and generate proxy models for probabilistic reservoir forecasts. Our probabilistic estimates for the mature, subbituminous Healy Creek coals in the entire Nenana basin indicate that it is possible to sequestrate between 0.87 TCF (P10) and 0.2 TCF (P90) of CO2 while producing between 0.29 TCF (P10) and 0.1 TCF (P90) of CH4 at the end of 20-year forecast. Our study demonstrated application of experimental design methods and Monte Carlo analysis in reducing these uncertainties in reservoir properties and quantifying their effect on reservoir performance. In addition, the results of fluid flow scenarios show that the CO2 sequestration through a primary reservoir depletion method is the most effective way to inject CO2 in the coals of the Nenana basin. Including a horizontal well instead of the vertical well resulted in relatively high average gas production rates and subsequent faster production decline. Our CO2 buoyancy scenario suggested that the effect of CO2 buoyancy and the nature of the caprock should be considered when identifying potential geologic sites for CO2 sequestration and in CO2 storage capacity estimates for the coals.