In the span of a decade and a half, Wyoming's Powder River Basin has been transformed from an inconsequential gas-producing region to one of the nation's major supply provinces. And this gas is almost wholly derived from coalseams. Industry has drilled an astonishing 26,000 coalbed-methane (CBM) wells so far in the Wyoming portion of the Powder River Basin, and 17,400 wells currently produce 1.1 billion cubic feet (Bcf) of gas per day.

There's more on deck: some 6,800 wells have been drilled but are not yet producing, many because they are awaiting water-discharge permits. To date, just 2,000 wells have reached their economic limits and been plugged and abandoned.

Total gas-in-place resource in Wyoming's Powder River Basin coals is 37.5 trillion cubic feet (Tcf), excluding seams thinner than 20 feet and shallower than 200 feet. At an assumed recovery factor of 67.5%, the CBM play still holds around 25 Tcf of recoverable gas.

"That's technically recoverable gas," says Casper-based geologist Jimmy Goolsby, partner in Goolsby, Finley & Associates LLC. "The eventual volume will be greater or less-probably less-based primarily on regulatory and political issues."

Today, the original Wyodak play on the basin's eastern edge is at maturity. Current programs target Big George coal, an extra-thick seam that is related to the Wyodak. In the center of the basin, the Big George occurs above the Wyodak, but as the coals are traced updip, they merge together to form the shallow Wyodak interval. Close to 10,000 locations are available in the Big George fairway, at 80-acre spacing, says Goolsby.

Additionally, some operators are working the split coal region, north of the Wyodak fairway. Targets here are Anderson, Canyon, Cook and Wall/Pawnee coals, and multiple-seam completions are necessary to achieve economic rates. This area holds another 10,000 potential locations.

The shift from Wyodak to Big George is bringing a host of changes to the basin. Big George production mainly occurs on federal lands, and consequently production from federal leases has shot up rapidly during the past few years. In October 2006, federal gas production passed fee production for the first time, and current ratios are 53% federal, 40% fee and 7% state.

Federal lands are usually split estates, so surface owners don't receive royalties from wells on their properties.

Water production is also changing. Big George programs require massive dewatering efforts, and most produced water is disposed of on the surface, rather than injected into subsurface reservoirs.

"There aren't enough deep reservoirs in the basin that are capable of taking large volumes of water, and many of the shallow sands in the Fort Union are already at hydrostatic pressure," he says. "It's not as much a matter of cost as of physically being able to inject the produced water."

The corner is being turned however, as CBM water production has declined to 1.8 million barrels per day from an all-time high in October 2006 of 2.2 million per day.

Big George wells also have long lives. Early Wyodak wells on the basin's eastern side produced at economic rates for seven to 10 years, but production histories from the first Big George wells indicate they will be more robust.

As companies prepare their drilling programs to bring untapped CBM to market, they should keep in mind that an understanding of local geology is crucial to economic success, says Goolsby. The presence of such geologic features as tectonic anticlines or zones with no coal (wants) can make a material difference to production.

In a study done by Andrew Finley, partner in Goolsby, Finley & Associates, 54 Wyodak wells were drilled in a 2,021-acre area in 48n-72w in Campbell County. Of total gas produced, 70% came from 15 initial wells drilled on a structural high.

The 39 subsequent wells were sited in structural lows and only contributed 30% of total gas. Wells on the high produced an average of 353 million cubic feet apiece, and wells in the lows made just 59 million on average.

"The bottom line is that there is a place for geologists in these resource plays, and these plays can be high-graded," says Goolsby. In a world of horrible Rockies basis differentials and soft gas prices, that's a business necessity.

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