Patrick Draw Field was discovered in 1959 in Sweetwater County, Wyoming, in the Green River Basin. The sizeable, oil-filled stratigraphic trap created quite a stir in the Rockies when it was found. The reservoir was an Almond sandstone-the upper unit of the Lower Cretaceous Mesaverde Formation-that pinched out against the southern flank of the Wamsutter Arch. Patrick Draw has produced a total of 60 million barrels of oil and 130 billion cubic feet of gas from its two units, Arch and Monell. The Monell Unit, which encompasses the southern portion of the field, was formed in 1964 as a secondary recovery unit after primary production by solution-gas drive had begun to decline. Originally developed on 80-acre spacing, Monell was infilled to 40-acre spacing for waterflooding. Altogether, 150 wells were drilled for both the primary and secondary recovery phases, and Monell produced 24 million barrels of oil on primary and an additional 16 million barrels via waterflooding. When Monell was acquired in 2000 by Anadarko Petroleum Corp., as part of its merger with Union Pacific Resources, the old field's glory days seemed long past. It was producing just 10 barrels of oil per day from a handful of stripper wells. "Most of the wells had already been abandoned, and there wasn't much salvage value either," says Frank Lim, a reservoir engineer with Houston-based Anadarko. A lot of companies would have walked away from Monell. After all, in the merger Anadarko had just acquired the famed Union Pacific Land Grant, some 8 million acres of mineral rights that lay in a checkerboard pattern along the transcontinental railroad in Wyoming, Utah and Colorado. The company had plenty of lucrative opportunities on its new swath of lands from which to choose. However, Anadarko looked hard at Monell. The unit, in which it held a 100% interest, had original oil in place of 110 million barrels and only 35% of that had been recovered. Monell's reservoir, found at a depth of about 4,500 feet, was an oil-charged sandstone covering 7,000 acres that was deposited in a barrier island and tidal channel environment. The Almond UA-5 sand was 25 feet thick and remarkably homogenous, with uniform porosities of 20% and permeabilities of 30 millidarcies. Additionally, the reservoir was not faulted. Anadarko wasn't ready to walk away from the considerable oil in place that remained. Anadarko evaluated various tertiary recovery options for the field, using screening methods supported by laboratory testing. CO2 enhanced oil recovery seemed a good match, particularly because Monell contained 43?gravity oil and had a reservoir temperature of 120? Fahrenheit. The combination of high-gravity oil and low reservoir temperatures meant the CO2 would be highly miscible and should result in very high oil-displacement efficiency. "All our work conclusively indicated that a miscible CO2 process could potentially recover much more oil than thermal or chemical methods," says Lim. Additionally, ExxonMobil's Shute Creek gas-sweetening plant could supply its waste CO2 to the project. Since the mid-1980s, Shute Creek, some 50 miles northwest of Rock Springs on the Moxa Arch, had been supplying CO2 to Chevron's Rangely Field in northwest Colorado and the Lost Soldier/Wertz fields in the Red Desert Basin. Enhanced oil-recovery operations are common in the Permian Basin, where some 170,000 barrels of oil per day are produced using CO2. In a typical flood, CO2 is pumped into a reservoir, causing changes that improve oil-production rates: viscosity of the crude oil is reduced, the crude swells and pressure is increased. The production wells make back CO2 and water in addition to the oil. Water and CO2 are separated and recycled, and natural gas and oil are sold. CO2 floods require substantial up-front investments, and operating costs are high, but they can recover significant volumes of oil that would otherwise remain unproduceable. Pilot project Anadarko kicked off its fieldwork at Monell with a pilot project. The company had high hopes for the efficiency of CO2 flooding in the unit: according to its simulations, the combination of the uniform reservoir and highly miscible oil could achieve recoveries of 25% of the original oil in place. "This is well beyond what is normally expected," says Lim. "A typical recovery factor in the Permian Basin might be 15% of original oil in place." Anadarko knew that it would need new wells at Monell, since the original wells were largely abandoned. A key issue that the pilot addressed was determining the correct balance between well spacing and response time. "The wider the well spacing, the lower the well cost, but the longer the response time between injection and production," he says. Conversely, tighter spacing means higher well costs but quicker response time. "From past waterflood experience, and by using simulation studies, we determined that five-spot patterns on 40-acre well spacing would yield optimum results." The pilot operated from 2001 to 2003, consisting of one new injection well, two recompleted production wells and two recompleted water-injection wells. It was set up on a typical five-spot pattern, with the water-injection wells used to maintain reservoir pressure and confine the CO2 flood. For the next phase of the project, Anadarko decided it would drill new 40-acre locations on the 20-acre well positions. The fresh locations would help with reservoir sweep, and allow for excellent new well completions. At the same time it was working out the technical details of the project, the company secured the CO2 source and obtained the rights of way for an eight-inch, 33-mile line to move the gas from the existing pipeline to Monell. The pilot flood was highlighted by the use of state-of-the-art technology, in particular time-lapse vertical seismic profiling. The technique was able to detect the movement of the CO2 in the reservoir and to determine the areal sweep of the flood. The pilot test worked very well and confirmed the simulation models. The company was confident that the major technical risks had been reduced. Project development The target at Monell was considerable: Anadarko believed that it could add 26- to 28 million barrels of incremental oil reserves by enhanced oil recovery using CO2. In early 2003, the company approved a full-scale expansion of the CO2 flood. It started constructing the CO2 pipeline, drilling wells and finalizing facilities designs for procurement and installation. By August 2003, the pipeline to Monell was completed, and in September, the company began to inject CO2. The initial phase of development was finished in November 2004, consisting of 67 wells, associated flow lines and all new related facilities. The tertiary oil response was obtained in just a few months, and production has increased steadily. Total rates have risen from 200 barrels of oil equivalent per day on January 1, 2004, to 2,500 per day at present. The entire project consists of three phases, which will yield a peak oil production rate of 10,000 barrels per day by 2010. The cost, including purchase of the CO2, is expected to total $244 million. All of the wells and facilities at Monell are fully automated, says Gordon Goebel, production engineer. Monell is in the U.P. Land Grant, so the fee acreage is checkerboarded with federal minerals. "We wanted to automate the project to limit the travel of the field personnel on the federal land, to reduce our staffing needs, and to give our people better control." Anadarko built a state-of-the-art CO2 extraction plant and water-treating and oil-treating facilities, including a recycling and recompression station. "Our people can see all of the pressures and readings from the facilities at their offices or homes at all times." It also built test facilities to enable it to run a minimum of three 24-hour tests per well per month to track the CO2 response and recovery. Because CO2 reacts with water to form carbonic acid, the equipment must be resistant to corrosion. At Monell, Anadarko used high-density polyethylene pipe lined with fiberglass for gathering and injection lines; plastic-coated vessels; and stainless steel piping and valves. Monell is engineered as a water-alternating-gas (WAG) project, with dual lines for water and CO2 injection, but it hasn't needed to inject water for mobility control. "Even in the pilot, we were able to continuously inject CO2 without any premature breakthrough." However, it has drilled horizontal injection wells along Monell's gas cap to inject water to maintain pressures and prevent encroachment of the CO2. It also sells gas from the gas cap. Last year, Anadarko completed the first phase of the project, drilling 67 wells. This year, it started the second phase and has drilled 12 additional wells. During 2006, it will finish that phase with another 27 wells. The final phase will require 81 wells. At present, the company is conducting an environmental assessment for the Bureau of Land Management. "We project that we will receive approval in early 2006," says Goebel. CO2 sequestration A strong environmental plus of the Monell project is its program to sequester CO2. It is set up as a full-stream production recycle project. "All of the gas that we produce is high in CO2, so we dehydrate, compress and reinject all of it," says Lim. At present, the company is injecting an average of 5 million cubic feet of CO2 per day of recycled gas and 25 million per day of purchased CO2. The peak injection rate will be 50 million cubic feet per day. Over the life of the Monell flood, the company is expecting to sequester a total of 123 billion cubic feet of CO2. "That's 7.5 million tons that would otherwise be vented into the atmosphere," he says. Using the Environmental Protection Agency's figures for average automobile emissions, that's equivalent to the CO2 emissions of 1.2 million cars. Additionally, the facilities are almost entirely run on electrical power, which eliminates air emissions at the site. The wellheads were redesigned, to prevent CO2 from leaking into the air. "We've changed the technology to help with the sequestration efforts," says Goebel. The state of Wyoming is encouraging CO2 flooding by providing severance tax relief for the projects. The state benefits not only environmentally from the sequestration of the CO2, but also economically from the increased oil production. These projects bring royalties, income taxes, economic growth and jobs. And it's particularly heartening that these benefits are flowing from Monell, a field that five years ago was just a step away from total abandonment.