Evaluation of the Technical and Economic Feasibility of CO2 Sequestration and Enhanced Coalbed Methane Recovery in Texas Low-Rank Coals

Abstract

Carbon dioxide (CO2) from energy consumption is a primary anthropogenic greenhouse gas. Injection of CO2 in coalbeds is a plausible method of reducing atmospheric emissions, and it can have the additional benefit of enhancing methane recovery from coal. Most previous studies have evaluated the merits of CO2 disposal in high-rank coals. The objective of this research is to determine the technical and economic feasibility of CO2 sequestration in, and enhanced coalbed methane (ECBM) recovery from, low-rank coals in the Texas Gulf Coast area. Our research included an extensive coal characterization program, deterministic and probabilistic simulation studies, and economic evaluations. We evaluated both CO2 and flue-gas injection scenarios. In this study, coal-core samples and well pressure transient test data were obtained for characterization of Texas low-rank coals. Simulation studies evaluated the effects of well spacing, injectant fluid composition, injection rate, and dewatering on CO2 sequestration and ECBM recovery. Probabilistic simulation of 100% CO2 injection in an 80-ac five-spot pattern indicates that Wilcox Group coals can store 1.27–2.25 bcf of CO2 at depths of 6200 ft (1890 m), with an ECBM recovery of 0.48–0.85 bcf. Simulation results of 50% CO2–50% N2 injection in the same 80-ac five-spot pattern indicate that these coals can store 0.86–1.52 bcf of CO2, with an ECBM recovery of 0.62–1.10 bcf. Simulation results of flue-gas injection (87% N2–13% CO2) indicate that these same coals can store 0.34–0.59 bcf of CO2 with an ECBM recovery of 0.68–1.20 bcf. Economic modeling of CO2 sequestration and ECBM recovery for 100% CO2 injection indicates predominantly negative economic indicators for the reservoir depths and well spacings investigated, using natural gas prices ranging from $2 to $12/mscf and CO2 credits based on carbon market prices ranging from $0.05 to $1.58/mscf CO2 ($1.00 to $30.00/ton CO2). Injection of flue gas (87% N2–13% CO2) results in better economic performance than injection of 100% CO2. Moderate increases in either gas prices or carbon credits could generate attractive economic conditions that, combined with the close proximity of many CO2 point sources near unminable coalbeds, could generate significant CO2 sequestration and ECBM potential in Texas low-rank coals.