Walter B. Ayers1
(1) Schlumberger Holditch - Reservoir Technologies, College Station, TX
Abstract: Methane production from thermally immature coal, Fort Union Formation, Powder River Basin
The Tongue River Member of the Paleocene Fort Union Formation contains more than 1 trillion short tons of coal. This coal was considered to have little potential for commercial coalbed methane production. Yet, gas reserves were recently reported to exceed 7 Tcf exist in the current play, there are several active projects in the basin, hundreds of wells have been drilled, and million of dollars are being invested in pipelines and facilities.
Tongue River coal occurs in 2 to 24 subbituminous seams that range up to 100 ft thick. Total coal thickness is as great as 300 ft. From outcrops, coal beds extend to the depths greater than 2,000 ft. At the east margin of basin, face cleats strike east-west, which facilitates ground water recharge. Coal beds and interbedded sandstones are aquifers. Water is fresh. Coalbed gas content is 23 and 70 Scf/t; the gas is approximately 90% methane and 8% carbon dioxide. The methane is isotopically light, which indicates a biogenic origin.
Early coalbed methane drilling was concentrated around Gillette, where dewatering has occurred in association surface mines. However, exploration has expanded to the northern and northwest parts of the basin. As many as 840 wells were drilled to depths between 300-1,500 ft in 1997-8. Drilling, completion, and facility costs for the wells range between $65,000 and $95,000. Peak production rate of Tongue River wells is typically 130-150 Mcf/d. Average EUR is 300 MMcf/well on 80-ac spacing.
The Fort Union coalbed gas play is flourishing in the post-tax-credit era because new pipelines provide market access; drilling, completions, and water handling, costs are low; and geologic characteristics of the coal are favorable. Commercial production of coalbed gas from Fort Union subbituminous coal indicates that we should reassess viability of economic projects in low-rank coal beds of other regions.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana