Cedar Hills Field lies on the southern plunge of the Cedar Creek Anticline, a well-known, 100-mile-long structure that has been a locus of Williston Basin production since the 1950s. In 1994, Continental Resources Inc., based in Enid, Oklahoma, and Burlington Resources Inc., Houston, began developing the Middle Ordovician Red River B zone at Cedar Hills, in Slope and Bowman counties, North Dakota, and Fallon County, Montana. Today, Cedar Hills Field covers 180 square miles and contains more than 600 million barrels of estimated original oil in place. The Red River B is a classic resource play, in which the chance of finding hydrocarbons is high and the challenge lies in fine-tuning drilling, completion and production technologies to achieve commercial volumes. The thin, tight Red River B dolomite is found at 9,000 feet. It is remarkably continuous, extending over a sizeable chunk of the basin. Prior to the early 1990s, drilling for the reservoir was rare in the townships southeast of Fallon County's Pennel Field. Although the Red River B was present and oil-saturated in the Cedar Hills area, only a few scattered vertical wells had tested the area. Over a long period, some of these had produced a fair amount of oil and not much water. "We felt like the Red River B play could be economic if we could increase the rates by three or four times," says Harold Hamm, president of Continental. "We wanted to try horizontal wells." Continental owned a drilling company in Oklahoma, and had a good bit of experience with highly deviated and horizontal wells. It had drilled numerous such wells under Oklahoma towns and cities, including Enid, Chickasha and Oklahoma City, and in the Mississippian, Tonkawa and Austin Chalk formations. Precision horizontal drilling turned out to be the answer to economically developing the 10-foot-thick dolomite, although the solution was not as obvious then as it might seem today. "At that time, we were using some outdated technology, and people looked at horizontal drilling much as a novelty," says Hamm. "We had to convince people that horizontal drilling was here to stay and that we needed better near-bit technology." Development at Cedar Hills featured horizontal wells on 640-acre spacing, with single, 5,600-foot laterals oriented in a northeast-southwest direction. Surprisingly, the field was much larger and extended much farther downdip than was first anticipated, with 1,100 feet of relief between the crest of the Anticline and the oil/water contact on its eastern flank. By 1998, Continental had concluded its first round of development drilling in the field. Primary recovery from the wells had peaked in late 1997 at 9,000 barrels of oil per day. The company projected it could produce 23 million barrels of oil under primary recovery, out of the 277 million barrels of oil in place. "The primary recovery was quite low, and we knew that secondary recovery was going to play a huge role," he says. Indeed, a number of very successful secondary-recovery projects had been under way for decades in such nearby fields as Pennel, Pine and Cabin Creek. A period of study and debate ensued, and the two Cedar Hills operators decided to try different secondary technologies. In March 2001, three units were formed in the huge accumulation. Burlington Resources operates the 54,799-acre Cedar Hills South Unit and Continental operates the 49,680-acre Cedar Hills North and the 7,774-acre Cedar Hills West units. Together, its two units cover almost 90 square miles. It holds a 97.5% working interest in the North and a 100% interest in the West. Burlington Resources started a horizontal line-drive waterflood in Cedar Hills South. (For details, see "New Life in Old Basins," Oil and Gas Investor, October 2002.) For its properties, Continental decided to try a high-pressure air injection (HPAI) technique. HPAI is a thermal process in which air is compressed to approximately 4,500 psi and injected down tubing into an injection well. The air is heated by the Earth to 215 degrees Fahrenheit, and spontaneously combusts with a small portion of the residual oil. Near the burn front, reservoir temperatures are in the 600- to 900-degree range. The combustion produces nitrogen and carbon dioxide, and these gases repressurize the formation and mobilize incremental oil. In addition, the heat reduces the viscosity of the oil in the reservoir and vaporizes the lighter hydrocarbons, results that aid oil production. The pioneer for the technology in the area was Koch Oil Co., which had started using HPAI in 1978 in a small pilot project in nearby Buffalo Field. The pilot-in vertical wells-was successful and HPAI was eventually expanded over the entire field. Continental was intrigued with the potential of HPAI, bought Koch's production and acquired the technology in 1996. "The bulk of their manpower also came to us with the acquisition," says Hamm. The Buffalo units are still producing today, and have another 15 to 20 years of life left. To date, the units have produced 6.5 million barrels of oil on primary recovery and 13.5 million barrels on secondary. Continental decided to push the technique further at Cedar Hills by drilling new horizontal wells and using them as injectors. "We estimated that the sweep efficiency from the vertical wells in the Buffalo units was about 48%, and we think we are getting 65% to 70% sweep efficiency with the horizontal laterals at Cedar Hills," says Hamm. The company also built an automated air-compression plant and laid hundreds of miles of steel pipe to deliver air to the injection wells. Currently, Continental is injecting some 29 million cubic feet of air per day at Cedar Hills. It also injects 5,500 barrels of water into the boundary wells, which are jointly operated, between its properties and Burlington Resources' waterflood project. "This is a good area for HPAI, because the characteristics of the oil and the reservoir support spontaneous combustion," says Hamm. "The technique fits certain applications, and right here it works very well." There are several advantages to the HPAI technique. The Red River B is tight, and its permeabilities are so low that it is hard to put energy into the reservoir. The gases produced during combustion will go where water won't, and Continental believes that HPAI delivers considerably more energy to the reservoir than a traditional waterflood can. In addition, core studies indicate that residual oil saturation will be significantly lower with air than with water. The response is rapid as well: In November 2003, Continental's primary oil production had declined to 2,619 barrels per day. Since the field began to respond to the HPAI efforts, its production has headed steeply up to its current rate of 6,500 barrels per day. "We are very gratified to see the response coming just as predicted," says Hamm. The company anticipates daily rates will peak at around 8,200 barrels in 2006 under the current development configuration. With its present program, it expects to recover 52 million barrels of oil during secondary recovery. Continental has finished drilling the injectors, in which horizontal wells were infilled on 320-acre spacing between the existing producing wells. Each well, whether an injector or a producer, cost about $1.1 million to drill and complete. The field now has 57 air-injection wells, 14 water injectors, and 86 oil-producing wells. This spring, Continental plans to drill some extensions to the 5,600-foot producing laterals to reach the unswept ends of the sections. Cedar Hills ranks as the largest single asset of the company, accounting for almost 50% of Continental's proved reserves and 25% of its daily production. Since unitization, it has invested $125 million in the project. "Pretty conservatively, we are looking at a life of 25 to 35 years on this field. We've accelerated the time of recovery with the horizontal wells, and by gaining additional sweep efficiency we have extended the life of the field."