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Unlike conventional reservoirs, coal seams are the source, trap, and reservoir for CBM. A comparison of the two reservoir types shows profound differences in reservoir properties, storage mechanisms, flow mechanisms, and production profiles. CBM reservoirs are layered and contain an orthogonal fracture set called cleats, which are perpendicular to bedding. Because the coal matrix has essentially no permeability, CBM can be produced economically only if there is sufficient fracture permeability. Relative to conventional gas reservoirs, coal seam permeabilities are generally low and may vary by three orders of magnitude in wells separated by distances of less than 500 m.
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.
Unlike conventional reservoirs, coal seams are the source, trap, and reservoir for coalbed methane (CBM). A comparison of the two reservoir types shows profound differences in reservoir properties, storage mechanisms, flow mechanisms, and production profiles. Understanding the reservoir differences is key to successful evaluation and operation of a CBM project. Coal is a chemically complex, combustible solid consisting of a mixture of altered plant remains. Organic matter constitutes more than 50% of coal by weight and more than 70% by volume. Type refers to the variety of organic constituents.
Abstract In cooperation with China United Coalbed Methane Corporation Ltd. (CUCBM), Texaco China BV (Texaco) is evaluating the Huaibei coal basins in northern Anhui Province for coalbed methane development. Texaco's initial exploration will focus on the Sunan and Nanping synclines where Permian-age coals are known to be within optimum maturity levels. Based on recent test drilling data, gas contents of objective coal seams are also shown to be quite attractive. However, the tectonic history of the Huaibei region presents some concern about the in-situ gas saturation levels in the coals. Even though the region certainly has some challenging technical risks, the initial technical evaluation indicates a promising potential. A multi-well core and test program has been designed to quickly and efficiently evaluate specific reservoir properties prior to critical decisions for pilot testing and full-scale commercial development. Key reservoir properties to be determined include coal thickness, permeability, porosity, gas content and gas saturation levels. Reservoir model studies using a coalbed methane simulator will then be used to examine the productive potential of the area. The variability and sensitivity of these key reservoir parameters in regard to production rates is discussed along with Texaco's exploration and development strategy. The paper also discusses the operational plans and overall strategy of managing risk by utilizing a phased approach. P. 517
Abstract Indonesia has thick, low-rank coal deposits that are prospective for coalbedmethane (CBM) development but remain untested. Conventional oil and gas wellsthat drill through these coal seams experience gas kicks and blow outs, goodCBM indicators. We analyzed petroleum and coal mining data to perform acomprehensive assessment of Indonesia's CBM resources. We identified 12.7trillion m3 (450 Tcf) of prospective CBM resources within eleven onshore coalbasins. Full-cycle development costs in high-graded areas are estimated at$0.70/Mcf. These potential CBM reservoirs could be tested at low cost usingcoreholes or production "wells of opportunity." Introduction Coalbed methane (CBM), which is methane desorbed and produced from deep coalseams, has become a significant source of natural gas supply in the USA. Fromhumble beginnings in the 1980's, CBM production has steadily grown to thecurrent 127 million m/day (4.5 Bcfd), nearly 10% of total USAnatural gas production. CBM reserves at the end of 2002 stood at 18.5 Tcf.Cumulative reserve additions, including past production, total nearly 30 Tcf.Much of this reserve is low-cost natural gas with all-in supply costs of under$1.00/Mcf. Outside the USA, CBM is undergoing initial commercial development inAustralia and Canada, while exploration is underway in China, India, SouthAfrica and several other coal-rich countries. Indonesia has extensive coal deposits distributed in eleven onshore coalbasins (Figure 1), yet has not yet experienced significant CBM testing .There has been a perception by CBM operators that Indonesia's coal deposits aretoo shallow and too low rank to be prospective. After all, Indonesia's coalmining sector produces primarily lignite or sub-bituminous coal from open-pitmines that have no significant methane control issues. However, this perception is changing due to several factors:The successof low-rank CBM development in the Powder River Basin, Wyoming, USA, where gasproduction is more than 28 million m/day (1.0 Bcfd) and increasingrapidly; improved understanding that the shallow coal seams mined at thesurface in Indonesia dip basinward and become gas charged at CBM target depths(100–1,500 m) over broad areas; and strong and nearly ubiquitous gas kicksthat are recorded on petroleum well mud logs as these deep coal seams arepenetrated, sometimes causing hole stability problems, and indicating that thecoals have adsorbed large quantities of methane. This study was supported by the Asian Development Bank and Indonesia'sDepartment of Energy and Mineral Resources (MIGAS). It builds on earlier datacompilation efforts supported by PT Caltex Pacific Indonesia and Pertamina[2,3] and laboratory work performed by the Indonesian Ministry of Energy andMineral Resources . A separate component of the project, not discussed here, involved assisting MIGAS in formulating regulations for commercial CBMinvestment and development. Data Control Extensive surface and subsurface data are available in Indonesia for basicCBM parameters such as coal thickness, depth, rank and other coal properties.We assembled a GIS data base of these coal properties acquired from coalexploration coreholes, deep petroleum exploration well logs, measured coaloutcrop sections, and laboratory data such as vitrinite reflectance andvolatile matter analyses. Recently, adsorption isotherms have been run on ahandful of coal samples, which remain confidential. Other data on CBM-specificreservoir properties remain scarce or non-existent. Coal seam permeability hasnot yet been tested in situ using well testing. No CBM production wells havebeen tested to date, nor has hydraulic fracturing of coal seams been attempted.Despite this paucity of data, it is still possible to make rough estimates ofin-place CBM resource distribution and potential producibility. Basincharacteristics are summarized in Table 1.