HORIZONTAL DRILLING BOOSTS DEVONIAN GAS FLOW

A successful horizontal well in eastern Kentucky's Big Sandy field underscores that technology's potential for economically exploiting the huge gas resource of U.S. eastern Devonian shales. Columbia Natural Resources Inc. (CNR) and Columbia Gas System Service Corp. (CGSS) 21747 Pocahontas Development Corp. in Martin County, Ky., flowed a combined 3.1 MMcfd of gas from six zones in Devonian Lower Huron shale. Its reserves are estimated at 1.5 bcf. That compares with productive capacity

June 10, 1991

3 min read

A successful horizontal well in eastern Kentucky's Big Sandy field underscores that technology's potential for economically exploiting the huge gas resource of U.S. eastern Devonian shales.

Columbia Natural Resources Inc. (CNR) and Columbia Gas System Service Corp. (CGSS) 21747 Pocahontas Development Corp. in Martin County, Ky., flowed a combined 3.1 MMcfd of gas from six zones in Devonian Lower Huron shale. Its reserves are estimated at 1.5 bcf. That compares with productive capacity and reserves of nearby Devonian shale wells typically pegged at 10-50 Mcfd and 290 MMcf, respectively.

The well was drilled to 6,263 ft measured depth (MD) with 2,872 ft of displacement. True vertical depth (TVD) is 3,754 ft.

Results of the well, part of a U.S. Department of Energy three well program focusing on horizontal drilling and hydraulic fracturing in Devonian shales (OGJ, Nov. 6, 1989, p. 20), were covered in a paper by CGSS's Gregory Koziar and CNR's Mian M. Ahmad and Larry L. Friend given at the annual Appalachian petroleum geology symposium in Morgantown, W.Va. in March. The authors note the resource base of U.S. eastern Devonian shales is estimated at as much as 2.5 quadrillion cu ft of gas. However, depending on gas prices, only about 20-60 tcf may be recoverable with conventional technology.

WELL DETAILS

Columbia drilled an 8 3/4 in. vertical hole to 3,035 ft before kicking off to build angle in 21747 Pocahontas.

The angle build section was designed to include a tangent section to provide for corrections in build rate. From kickoff to tangent, the build rate averaged 10.4/100 ft.

At 3,426 ft TVD-about 45-Columbia drilled a tangent section of 117 ft. The second build rate averaged 13.7/100 ft to an angle of about 88.

Angle build continued to a maximum 96 at 4,700 ft MD-3,649 TVD-before the build could be reversed to drop the well through the target zone.

Three completion techniques were used to boost recovery.

The bottom 725 ft produced through open hole and a slotted liner. Most of the horizontal section was cased with 5 1/2 in. casing and isolated by external packers with formation access provided by sliding sleeve completion tools. Uphole, the remaining two zones were cemented.

The open hole/slotted liner section and a portion of the lateral near the angle build section were completed natural, while the other four zones were stimulated with straight nitrogen.

ECONOMICS

Total cost for 21747 Pocahontas was about $1.4 million, with part of the funding coming from DOE.

The authors note that about $700,000 was directly related to research and learning curve experience,and say the same well could be drilled for $700,000. If advances are forthcoming in measurement while drilling systems for air drilling, wells of this type could be drilled routinely for $500,000, the authors said.

"it appears that application of horizontal drilling will result in at least acceleration of gas production and the addition of recoverable reserves from the Devonian shale," they wrote. The Pocahontas well is awaiting hookup to a pipeline being laid in the area. Columbia plans another horizontal well in the area this year.