Abstract

To evaluate the technical and economic feasibility of CO₂ sequestration, and enhanced coalbed methane recovery (ECBM) of Texas low-rank coals, we evaluated Wilcox Group coal resources and gas contents in east-central Texas. Data used were well logs, gas desorption analyses, sorption isotherms, and gas compositional analyses collected primarily by Anadarko and secondarily by Texas A&M University as part of a U.S. DOE funded project on CO₂ sequestration and ECBM.

Wilcox Group total coal thickness ranges between 30 and 130 ft in the ~285-mi² area investigated. At outcrop, the Wilcox coal rank is lignite. Thermal maturity increases basinward with coal depth. Vitrinite reflectance is 0.38% at 1,825 ft and 0.67% at 6,382 ft depth.

We desorbed approximately 60 coal samples (primarily coal sidewall cores) from depths between 1,835 and 7,300 ft in six wells. In-situ methane content was low (< 30 scf/t) in the shallow (<2,000 ft) coals. In the deep (>4,000 ft) coals, methane content ranged between 150 and 400 scf/t. Plotting desorbed methane values on sorption isotherms of 4 samples suggests that some deep coals are nearly gas saturated.

We tentatively conclude that there are two coalbed gas systems functioning in the region. The shallow system is typified by fresh formational water and low concentrations of biogenic methane, whereas the deep coalbed gas system is characterized by much higher concentrations of thermogenic gas that, most likely, migrated from deeper source rocks.

Reservoir characterization work suggests that large methane resources are present in Wilcox low-rank coals. To assess the potential for CO₂ sequestration and for ECBM, we are running CO₂, CH₄, and N₂ isotherms and performing field tests of permeability to provide data required for reservoir modeling.