AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
CBM in Illinois? E & P in a Very Unconventional Biogenic Coal Gas Play
(1) Illinois State Geological Survey, Institute of Natural Resource Sustainability, University of Illinois, Champaign, IL.
Limited exploration and production for coalbed methane (CBM) has produced modest returns in the Illinois Basin, an area with 60,000 sq. miles and 284 Billion tons of High Volatile Bituminous coal resource. Finding sweet spots requires some science and luck. Managing costs is especially critical. This poster compiles data and lessons learned by numerous operators and research at the Illinois State Geological Survey in their attempt to understand the origin and distribution of CBM in the Basin.
The targeted Pennsylvanian coals are 3 to 8 ft thick but stacked aggregate thicknesses can total 20 to 30 feet. CBM coal targets range in depth from 300 to 1700 ft and have a typical Ro of 0.55 to 0.65% reflectance. Gas contents range from 50 to over 200 scf/ton (daf basis). The coals may be 60 to 90+% methane saturated compared to their methane isotherms. Pressure transient tests indicate permeabilities range from <1 md to over 100 md. Gas composition ranges from 70 to 98% methane, with lesser, though sometimes significant, amounts of N2 and then CO2. C2+ components in the gas are typically ~ 1%. This dry gas composition, and carbon and deuterium isotopes indicate that the gas is primarily biogenic in origin, likely formed during Quaternary glacial groundwater infiltration.
Drilling peaked in 2005 to 2006, but dropped abruptly with subsequent drop in gas prices. At the present there are only a few operators producing seam gas and one selling mine gas in the basin. Sweet spots occur where there is active biogenic generation. Historic data indicating gas production from stratigraphically deeper Pennsylvanian sands and areas with fresher formation water may point to more prospective, gas-rich areas. Production in areas with New Albany shale gas wells can improve economics of both plays. SW Indiana and SE Illinois appear to be the most prospective.
Optimal stimulation design remains a hurdle to overcome. Most wells have been vertical, conventionally cased, perforated and hydro-fraced in single or multiple coals with varying rates, amounts and sizes of sand proppant. Creative new techniques designed to avoid damaging the coal need to be tried, such as air drilling, nitrogen fracs, and horizontal or multilateral wells. Other future opportunities may include incentives, such as CO2 injection into coal for enhanced CBM recovery, as demonstrated at the Tanquary research site, and potential CO2 sequestration credits.