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Unconventional Petroleum Systems |
1 Harold Vance Department of Petroleum Engineering, Texas A&M University, TAMU 3116, College Station, Texas, 77843-3116; ayers{at}spindletop.tamu.edu
Walter B. Ayers Jr. is visiting professor in the Harold Vance Department of Petroleum Engineering and adjunct professor in the Department of Geology and Geophysics at Texas A&M University. His teaching and research interests include petroleum geology, integrated reservoir studies, clastic depositional systems, and unconventional hydrocarbon reservoirs. He holds B.S. and M.S. degrees in geology from West Virginia University and a Ph.D. in geology from the University of Texas at Austin.
Coalbed gas has been produced commercially from the northern Appalachian basin since the 1930s and from the San Juan basin since the early 1950s. However, the magnitude and economic significance of coalbed gas resources were realized only in the 1970s and early 1980s when the U.S. Bureau of Mines, U.S. Department of Energy, the Gas Research Institute, and oil and gas operators made a concerted effort to demonstrate commercial production of coalbed gas from vertical wells. Exploration and development expanded in the late 1980s and early 1990s, due partly to an unconventional fuels tax credit. By 2000, coalbed gas accounted for 8.8% of the reserves (15.7 tcf [0.44 Tm3]) and 9.2% of the annual production (1.38 tcf [40 Gm3]) of dry gas in the United States. From 1989 through 2000, cumulative United States coalbed gas production was 9.63 tcf (272 Gm3). Today, coalbed gas development has spread to about a dozen basins in the United States, and exploration is progressing worldwide.
Coal beds are self-sourcing reservoirs that can contain thermogenic, migrated thermogenic, biogenic, or mixed gas. Coalbed gas is stored primarily within micropores of the coal matrix in an adsorbed state and secondarily in micropores and fractures as free gas or solution gas in water. The key parameters that control gas resources and producibility are thermal maturity, maceral composition, gas content, coal thickness, fracture density, in-situ stress, permeability, burial history, and hydrologic setting. These parameters vary greatly in the producing fields of the United States and the world.
In 2000, the San Juan basin accounted for more than 80% of the United States coalbed gas production. This basin contains a giant coalbed gas play, the Fruitland fairway, which has produced more than 7 tcf (0.2 Tm3) of gas. The Fruitland coalbed gas system and its key elements contrast with the Fort Union coalbed gas play in the Powder River basin. The Fort Union coalbed play is one of the fastest developing gas plays in the United States. Its production escalated from 14 bcf (0.4 Gm3) in 1997 to 147.3 bcf (4.1 Gm3) in 2000, when it accounted for 10.7% of the United States coalbed gas production. By 2001, annual production was 244.7 bcf (6.9 Gm3).
Differences between the Fruitland and Fort Union petroleum systems make them ideal for elucidating the key elements of contrasting coalbed gas petroleum systems.
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