Technological advances for recovering natural gas from coalbeds continue to expand the role this gas plays in meeting the US energy balance, as well as make it more likely that other countries will develop their coalbed gas resources.
Coalbed natural gas (CBNG) may contain heavier hydrocarbons such as ethane and propane, and therefore some prefer this term to coalbed methane (CBM), which implies that coalbeds produce only methane.
The CBNG industry has had its ups and downs, according to Charles M. Boyer, principal consultant, Schlumberger Oilfield Services, speaking Oct. 7 at a press conference at the Society of Petroleum Engineers' annual conference and exhibition in Denver.
Boyer said that in 1992, after the US federal (Section 29) tax credit expired, it was feared that coalbed gas production would peak in 1994 and then steeply decline, but new technology and the industry's efforts to look at other basins have continued to increase the importance of these resources.
Coal resources
Currently, the US has more than 16,000 CBNG wells, producing more than 1.6 tcf/year, with proved reserves having increased to 17 tcf from an estimated 4 tcf in 1984, according to Boyer.
He said that the US has potential CBNG resources of 89 tcf in the East, 40 tcf in the Midcontinent, 620 tcf in the West, and 1,045 tcf in Alaska. Outside the US, Canada may contain 538 tcf, with the rest of the world containing 4,000-9,000 tcf.
Boyer explained that although 35 countries besides the US have had some CBNG activity, only Canada, starting in 2002, and Australia, starting in 1998, produce commercial CBNG.
Tom Brown
Tom Brown Inc., Denver, is one company that has expanded its CBNG production by applying new technology in its White River Dome field in the Piceance basin of northwestern Colorado. The field has the world's deepest economic coalbeds for producing CBNG—some at 8,500 ft, according to Adam Sayers, Tom Brown senior operations engineer.
Because of the complications entailed in White River Dome wells producing gas from multiple coalbeds and sandstones, Tom Brown had to change its casing cement program. It now cements wells with a lightweight cement and has improved its cementing success in 15 wells drilled in 2003 to 80% from 40% in wells cemented in prior years with other methods, Sayers explained.
This lightweight cement exerts a lower pressure on the coalbeds, preventing breakdown of the seams, and also it prevents flow into fractures and cleats because it contains fibers that bridge and prevent slurry loss, according to Sayers.
Charles Hager, Tom Brown reservoir engineering manager, explained the importance of improvements in reservoir models that have allowed Tom Brown to obtain better reservoir characterization and a better understanding of the interplay of the fast-depleting sandstones and the longer-term production from the coalbeds that produce substantial amounts of ethane and propane along with the methane and carbon dioxide.
Marathon
Marathon Oil Co. also has encountered problems in completing wells with multiple stacked coalbeds in the Powder River basin of Wyoming, as it has moved its development efforts north, according to Denise M. Cox, Marathon senior geologist.
Marathon's wells in the southern part of the basin were relatively shallow at 1,000 ft and contained one or two coalbeds. It completed these openhole. But as the company moved north, Cox said, the resource has becomes a technology play because of the complications caused by wells encountering up to 11 coalbeds stacked to a 2,000-ft depth.
Cox said Marathon is looking at various technologies for optimizing production from all the beds at once. For instance, she said, Marathon runs a full suite of logs, including imaging logs, to understand better the flow capacity of the wells and coalbed permeability.
It will require technologies such as lightweight cement, better completion techniques, and new water-handling approaches for successfully producing CBNG in the northern part of the basin, Cox said.
