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

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 15 ft (0.6–4.6 m) and aggregate thickness ranges from 64 to 163 ft (19.5–49.7 m). 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 6000 ft (610–1579 m). Gas content ranges from 115 to 263 scf/t or 3.59–6.92 cm3/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 990 BTU/ft3. The coals are relatively clean (average ash content is 15 wt.%) and have fast diffusivity (63% of total gas desorbs in < 16 h). A valid pressure transient test in a coal seam at 3140 ft (957 m) depth showed permeability to be 14 mD (un-stimulated). The coal reservoir was normally‐pressured reservoir (0.44 psi/ft or 9.7 kPa/m) but was heavily damaged (skin of + 23). Another coal seam located at 4100 ft (1250 m) had a modeled permeability of 2.6 to 4.6 mD, which was estimated from long-term water production. Potential recoverable resource is thought to be 700 BCF (billion cubic feet or almost 20 billion m3) with wells drilled on 80-acre spacing, which translates to a gross EUR of 1.4 BCF 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 320 MCF/D (9014 m3 per day) after 36 months, and production should remain at that level for 12 months 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 2000 ft (526 m). Few wells drilled decades ago, having perforations in these sands, showed IP (initial production) rates ranging from 500 MCF/D (14,084 m3 per day) to 12 MMCF/D (338,028 m3 per day). A 24-inch (60 cm) diameter pipeline that has an operating pressure of 800–1000 psig, a tap, and a capacity of 275 MMCF/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.