Coalbed methane potential of the Paleocene Fort Union coals in south-central Wyoming, USA

Abstract

The Paleocene-age Wasatch and Fort Union Formations in the study area contain numerous coal seams that are interbedded with shale, sandstone, and conglomerates. Individual seam thickness ranges from 2 to 15ft (0.6–4.6m) and aggregate thickness ranges from 64 to 163ft (19.5–49.7m). The coals are of sub-bituminous to high-volatile C/B bituminous rank (Ro,ran=0.47 to 0.59%) and are found at depths from <2000 to 6000ft (610–1579m). Gas content ranges from 115 to 263scf/t or 3.59–6.92cm3/g (ARB), which is higher than expected for the coal rank. The coals appear to be close to full saturation, based on a comparison of adsorption isotherms with desorption data. The coals pinch-out updip in the subsurface forming a stratigraphic trap, which may explain the high gas content and the mixed biogenic–thermogenic origin supported by isotopic analysis. Gas composition is 97% methane with 3% inerts, and the gas has a heating value of 990BTU/ft3. The coals are relatively clean (average ash content is 15wt.%) and have fast diffusivity (63% of total gas desorbs in <16h). A valid pressure transient test in a coal seam at 3140ft (957m) depth showed permeability to be 14mD (un-stimulated). The coal reservoir was normally‐pressured reservoir (0.44psi/ft or 9.7kPa/m) but was heavily damaged (skin of +23). Another coal seam located at 4100ft (1250m) had a modeled permeability of 2.6 to 4.6mD, which was estimated from long-term water production. Potential recoverable resource is thought to be 700BCF (billion cubic feet or almost 20 billion m3) with wells drilled on 80-acre spacing, which translates to a gross EUR of 1.4BCF per well for the 500 well locations identified in the developable acreage. Based on numerical simulation using the simplistic FeketeTM model, peak coal gas rate is expected to be 320MCF/D (9014m3 per day) after 36months, and production should remain at that level for 12months before declining at a rate of 10% per year. Conventional gas potential also exists in numerous intervals within the Upper Fort Union and Wasatch Formations sandstones at depths shallower than 2000ft (526m). Few wells drilled decades ago, having perforations in these sands, showed IP (initial production) rates ranging from 500MCF/D (14,084m3 per day) to 12MMCF/D (338,028m3 per day). A 24-inch (60cm) diameter pipeline that has an operating pressure of 800–1000psig, a tap, and a capacity of 275MMCF/D (7.74 million m3 per day) crosses the study area. The pipeline is connected to the Colorado Interstate Gas mainline, which provides gas to markets in the eastern United States.