On a decades-old map of the southwestern Anadarko basin, the only sizable oil accumulation is the Elk City Hoxbar Sand Unit, discovered by Shell Oil Co. in Beckham and Washita counties, Okla., in the late 1940s.
The Anadarko was a natural gas basin, and its liquids production consisted almost entirely of condensate from gas wells.
These days, however, oil and condensate production is seeing enormous growth with horizontal drilling and multistage fracturing in the Cana Woodford play on the southeast side of the basin and the Granite Wash play farther west. The two plays support a considerable share of the nearly 200 rigs drilling in Oklahoma.
OGJ figures show Oklahoma's crude and condensate output at 194,000 b/d in late 2011, up from 176,000 b/d in early 2006.
Multicompanies, multiformations
The roster of Granite Wash operators includes Apache Corp., Chesapeake Energy Corp., Cimarex Energy Co., Forest Oil Corp., Linn Energy LLC, Newfield Exploration Co., Samson Investment Co., SM Energy Co., and QEP Resources Inc.
Apache has operated steadily in the basin since 1977. It discovered many Granite Wash fields while exploring for Upper Morrow gas in the 1970s. Today it has interests in 22 Granite Wash fields and thanks to horizontal drilling sees decades of drilling ahead.
Apache operated nine rigs in the Granite Wash in the quarter ended Sept. 30 and drilled 18 wells. For the year 2011, the company is on track to have drilled 34 Granite Wash wells and 14 Cherokee, 11 Hogshooter, and 9 Cleveland horizontal wells.
The company describes the Granite Wash as immature, dirty sandstones with up to nine 50-100 ft thick sand stringers over a 1,500 ft thick interval with permeabilities of 0.002-0.008 md.
Apache's progress
Early in the play, Apache drilled as many formations in as many geographical locations as possible to test concepts.
Its first horizontal well, in Washita County, had an initial potential of 17 MMcfd of gas and 800 b/d of oil from the Hogshooter wash in an area where a good vertical well might initial at 3-5 MMcfd.
Apache has 200,000 gross acres with Granite Wash potential in the Anadarko, where its holdings total close to 1 million acres. Horizontal drilling has helped the region triple its oil production each of the last 2 years.
For 2 decades the region has exploited its acreage base drilling infill wells in higher permeability sweet spots on legacy acreage. Horizontal drilling has made most of the acreage between the sweet spots prospective, and the company has leased and is drilling 3,000 to 4,000 acres in these areas, said Rob Johnston, vice-president and general manager of the central region.
"We're drilling horizontally on acreage we acquired 40 years ago, and it's maybe even more productive than in years past," Johnston said.
Since 2009 Apache has operated 60 wells in Texas and Oklahoma that targeted nine Granite Wash stringers. IP averages 5.5 MMcfd and 350 b/d, and estimated ultimate recovery is 50% oil, condensate, and natural gas liquids from wells that in most cases would be uneconomic if drilled vertically.
The typical lateral is 4,000 ft long at 12,000-15,000 ft.
Operational considerations
Granite Wash operators met and laid ground rules for the sharing of information because competition for acreage is relatively minimal.
For example, companies drilling horizontal wells are notified before offset wells are fractured in the same formation, especially along north-south lease lines where the reservoir fractures run east-west. The companies also share problems such as inefficient drill bit runs.
Horizontal completions have evolved. Apache cemented liners until mid-2011 and now favors packer-type completions that forgo intermediate casing. Spacing concepts are less certain, and geologists sit on the laterals for weeks, not days. Apache has drilled stacked laterals in one Oklahoma field.
At Stiles Ranch field in Wheeler County, Tex., 16-25 sq miles have three to five vertically separated formations with potential, and other zones are yet to be tested. Apache has drilled more than 20 wells at Stiles, which it calls a quasi-resource play.
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Alan Petzet | Chief Editor Exploration
Alan Petzet is Chief Editor-Exploration of Oil & Gas Journal in Houston. He is editor of the Weekly E&D Newsletter, emailed to OGJ subscribers, and a regular contributor to the OGJ Online subscriber website.
Petzet joined OGJ in 1981 after 13 years in the Tulsa World business-oil department. He was named OGJ Exploration Editor in 1990. A native of Tulsa, he has a BA in journalism from the University of Tulsa.