G. Alan Petzet
Exploration Editor
A recent acquisition will give impetus to an active play for oil in 8-10 ft thick Ordovician Red River B pay at about 9,000 ft. The B zone has been marginal to uneconomic in vertical wells.
Acquisition
Continental Resources Inc., Enid, Okla., acquired the Harding County, S.D., and Bowman County, N.D., operated assets of Koch Exploration Co., Wichita. Both are private companies. Price is not disclosed.
The Dec. 1 acquisition, retroactive to Nov. 1, includes four unitized properties with in situ combustion in progress, about 11,000 undeveloped acres, 30 miles of gas pipeline, and a 4 MMcfd gas processing plant.
Continental plans to introduce horizontal injection and production wells in hopes of boosting ultimate recovery to as much as 50% of original oil in place (OOIP) from the present 20-30%. Air injection has been under way since 1981. Continental believes this will be the world's first combination of the two technologies.
Meridian Oil Inc., Houston, Flying J Oil & Gas, North Salt Lake City, Utah, and Wyoming Resources Corp., Casper, have drilled Red River B wells in the fairway this year.
Flying J participates on properties acquired earlier this year from Cenex, Billings, Mont. Citation Oil & Gas, Denver, likely will be active in the area through its acquisition of Apache Corp.'s Rocky Mountain properties.
Red River C and D and Mississippian Lodgepole are also prospective in the general area.
Koch properties
Continental's acquisition from Koch covers 154 active wells, including 144 wells and nearly full interest in four high pressure air injection enhanced oil recovery units: Medicine Pole Hills Unit in 130n-104w in North Dakota, and Buffalo, West Buffalo, and South Buffalo units mostly in 20n-4e, 21n-4e and 5e in South Dakota.
Considerable development potential exists in the units, said Mike Purdum, Continental's acquisitions manager. The acquirer believes future net reserves will exceed 8 million bbl of oil. Gravity of the Red River B oil is 40-42 in Medicine Pole Hills Unit and 31-33 in the Buffalo units.
Production from the three Buffalo field units in October 1995 averaged a combined 2,069 b/d of oil, 17.5 MMcfd of low BTU gas, and 3,631 b/d of water, state figures show.
Red River B primary recovery is usually 8-15% of OOIP in traditional vertical wellbores, Continental said. Low injection rates have limited waterflood effectiveness.
The B zone consists of about 8-10 ft of clean, micro-sucrosic dolomite generally capped by 3-4 ft of interbedded anhydrite. Present recovery factors in the fireflood units are considered to be 20-30% but may be as high as 35-40% depending on actual pay thickness. Previous studies on a 12 sq mile Buffalo field pilot project indicated 50% recovery to be achievable under lab conditions.
Continental injects more than 50 MMcfd of air from compressors fueled by a 30 mile, 6 in. gas line. The 450 BTU return gas from the units contains carbon dioxide and nitrogen. Continental extracts about 6,000 gal/day of natural gas liquids and vents the 100 BTU residue gas.
The acquisition lifts Continental to the area's largest leaseholder, with more than 300,000 acres in Bowman and Harding counties. Continental considers the Red River B play one of the largest in the Lower 48 states the past 10 years in terms of remaining ultimate recovery potential.
Play evolution
Continental entered the Williston basin with a 1990 property acquisition from Exxon Co. U.S.A. in far northeastern Montana.
It bought 8,000 line miles of existing seismic data and began searching for Red River C and D anomalies. Then Meridian began horizontal development of the B zone.
Continental noticed the B was present and had shows in nearly every existing well, Purdum said. It began leasing what it considered to be the most prospective areas for horizontal drilling based on its geologic and geophysical mapping.
A geological/engineering team is dedicated to keeping the bit in the thin pay zone.
Continental began developing the B zone using medium radius horizontal drilling techniques. It has consistently achieved 3,500-5,200 ft laterals in the horizontal wells. It has drilled 18 horizontal wells since April 1995 in Cedar Hills pool of Bowman County, N.D. As of early December, 12 were producing, one was waiting on completion, and one was dry. Four others were still drilling.
Continental's first well in North Dakota's Cedar Hills pool, opened by Meridian in late 1994, was the 1 Peterson, in 26-132n-106w. It flowed 497 b/d of oil with 174 Mcfd of gas from open hole at 9,405-13,145 ft measured depth. It produced more than 37,000 bbl of oil and 1,400 bbl of water in its first five months on production.
Under high pressure air injection, a typical Red River B well in the South Dakota units produces 150-200 b/d once influenced by air injection, then levels off at 40-50 b/d. Gas-oil ratio determines the economic limit. Once gas breaks through in a ratio of 60 Mcf/bbl, a well becomes a candidate for recompletion as an injector, Purdum said.
Continental plans to expand the limits and enhance the existing EOR units with strategically positioned horizontal wells and increase air injection. It will utilize horizontal wellbores for injection and production in the unitized areas. Development of these two technologies will allow Continental to commence in situ operations in its developing fields.
Horizontal drilling is new to South Dakota. Meridian is drilling its second well, and Continental so far has scheduled four horizontal wells in the South Dakota units.
South Dakota has approved Continental and Meridian applications for 640 acre spacing on more than 50 sq miles in Harding County with as many as two horizontal wells allowed per unit.
Copyright 1996 Oil & Gas Journal. All Rights Reserved.
