Coalbed- and Shale-Gas Reservoirs

Abstract

Distinguished Author Series articles are general, descriptive representations that summarize the state of the art in an area of technology by describing recent developments for readers who are not specialists in the topics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and present specific details only to illustrate the technology. Purpose: to inform the general readership of recent advances in various areas of petroleum engineering. Introduction Annual natural-gas production from coalbed- and shale-gas reservoirs in the US is approximately 2.7 Tscf, which represents 15% of total natural-gas production. Approximately 1.7 Tscf of this gas comes from more than 40,000 coalbed gas wells completed in at least 20 different basins. The remaining 1.0 Tscf comes from more than 40,000 shale gas wells completed in five primary basins. While the pace of coalbed-gas drilling is starting to slow, shale gas continues to be one of the hottest plays in the US, and drilling is expanding rapidly, especially in the south-central US (the Barnett shale and its equivalents), the Appalachian basin, and numerous Rocky Mountain basins. Outside the US, more than 40 countries have investigated the potential of coalbed gas, resulting in commercial projects in Australia, Canada, China, and India. No commercial shale-gas projects currently exist outside of the US, but work continues to identify both new shale-gas reservoirs and to add incremental shale-gas production in existing reservoirs. Given that worldwide coalbed-gas resources are estimated to exceed 9,000 Tscf and shale-gas resources are estimated to exceed 16,000 Tscf, it is clear that tremendous potential exists for future growth (Kawata and Fujita 2001). Reservoir Fundamentals Coals are sedimentary rocks containing more than 50 wt% organic matter, whereas shales contain less than 50 wt% organic matter. Methane is generated from the transformation of this organic matter by bacterial (biogenic gas) and geochemical (thermogenic gas) processes during burial. The gas is stored by multiple mechanisms including free gas in the micropores and sorbed gas on the internal surfaces of the organic matter. Nearly all coalbed gas is considered to be sorbed gas, whereas shale gas is a combination of sorbed gas and free gas. Coalbed-gas reservoirs contain an orthogonal fracture set called cleats that are oriented perpendicular to the bedding and provide the primary conduit for fluid flow. Gas diffuses from the matrix into the cleats and flows to the wellbore. In shale-gas reservoirs, gas is sometimes produced through more-permeable sand or silt layers interbedded with the shale, through natural fractures, or from the shale matrix itself. In some cases, natural fractures are healed by a mineral filling and must be forced open by hydraulic-fracture stimulation. It also is possible to have both shales and coals interbedded in a single reservoir, resulting in gas contributions from both lithologies.