Mature fields While big discoveries and major new developments deservedly grab the headlines, mature fields are the backbone of global oil and gas production. Revitalizing these fields extends their productive lives and offers significant opportunity to expand worldwide reserves. A 2011 report, “Mature Oil Fields—Unleashing the Potential,” by IHS Cambridge Energy Research Associates, indicated that approximately two-thirds of global daily average oil production comes from mature fields and that the percentage is increasing over time. The report considered fields mature if they had produced more than 50% of their established proved plus probable resource estimates or had produced for more than 25 years. The term mature field does not have a single definition. Many engineers consider a field mature when production has declined to less than 50% of its plateau rate. Individual companies may apply their own definitions. “We consider the subsurface and the surface,” said Olivier Heugas, a member of the mature field team at Total’s headquarters near Paris. “For the subsurface, we consider a field mature when the cumulative production has reached 50% of the initial 2P (proved plus probable) reserves. And for the surface, given the aging facilities, we consider a field mature after 10 years of production. So when one of the two criteria is reached, we consider it a mature field. “There are other criteria we might consider, such as BSW (basic sediment and water) levels or a sharp increase in gas/oil ratio or depletion rate. It is on a case-by-case basis. There have been young fields with these subsurface or surface problems, and other fields that have produced for 10 years without these problems.” A Huge Global Resource Regardless of the definition, mature fields are a huge global resource. With reserves categorized as proved or probable, attempts to expand reserve levels come at a relatively low risk. Modest additions to a base of this size can be very substantial.
Horizontal drilling and continuing advances in hydraulic fracturing have made the Barnett Shale formation of north Texas one of the great recent success stories in gas production and a showcase for tight-reservoir development technologies. Yearly production from the north-Texas Barnett Shale, officially called the Newark East field by the Texas Railroad Commission, grew to 1.1 Tcf of gas equivalent in 2007, making it second in size only to the Panhandle-Hugoton field among US producing gas fields. Cumulative Barnett production from 2000 onward now exceeds 4 Tcf. The north-Texas Barnett Shale extends over 5,000 square miles and at least 17 counties, with the core areas lying within Denton, Tarrant, and Wise counties. While the formation can be found at depths as shallow as 3,000 ft in some areas, it primarily appears between 7,000- and 9,000-ft depths. Pay-zone thickness ranges from 100 to 1,000 ft and averages 300–500 ft. Notably, shales like the Barnett once were seen mainly for their role as barriers that trapped hydrocarbons in other rock or were useful for containing secondary-recovery repressurization or fracturing operations. They were seldom considered producible formations because of shale's low permeability, making it difficult for fluids to move within the rock and, thus, for hydrocarbons to flow to the wellbore. Matrix permeability in the Barnett is extremely low, ranging generally between 10–7 and 10–9 darcies. Partially improving the permeability is the presence of interbedded silt- and sand-sized particles. The Barnett Shale was believed to be hydrocarbon-rich even before a discovery well was drilled by Mitchell Energy in 1981. Nonetheless, development activity had to wait another decade or more and did not attain any momentum until the late 1990s. With the extremely tight formation, the use of fracturing to break up as much reservoir rock as possible was a necessity. At first, Mitchell's development strategy consisted of drilling vertical wells completed with massive hydraulic fracturing treatments, using crosslinked gels to transport from 1 to 1.5 million lbm of sand proppant in the formation. In 1997, with the success that another operator had experienced with slickwater fracturing (water-frac) technology in the east-Texas Cotton Valley formation, Mitchell began to apply slickwater fracs in the Barnett, with impressive results.
The testing flare burns brightly during a methane hydrate production test of the Ignik Sikumi No. 1 well on the Alaskan North Slope. A production method that could unlock large reserves of methane hydrate in sand-dominated reservoirs was tested successfully from a scientific and operational standpoint in a recent research experiment on the Alaskan North Slope (ANS). The experiment was conducted by the National Energy Technology Laboratory (NETL) of the United States Department of Energy (DOE) in partnership with ConocoPhillips and Japan Oil, Gas, and Metals National Corporation. A proof-of-concept test was conducted between 15 February and 10 April at the Ignik Sikumi No. 1 well in the Prudhoe Bay field operated by ConocoPhillips. The production technique featured the injection of carbon dioxide (CO2) to exchange and release methane (CH4) from the hydrate, a method developed through laboratory collaboration between the University of Bergen in Norway and ConocoPhillips. The released gas was then produced by means of reservoir depressurization. “The test objective was to perform injection and flow-back from a single well to validate that the CO2/CH4 exchange mechanism demonstrated in laboratory tests will occur in a reservoir of natural methane hydrates,” said Ray Boswell, technology manager for gas hydrates at the NETL. It was the first field-level trial of a production method involving the exchange of CO2 with the methane molecules contained in a methane hydrate structure. “The focus of the test, including the design of the well, was on the technical feasibility of this new technology, rather than an attempt to produce gas at commercial rates,” Boswell said. CO2 Mixture Injected in Reservoir The Ignik Sikumi well test was equipped with downhole fiber-optic distributed temperature and acoustic sensing, three downhole pressure gauges, and full surface instrumentation, including high-resolution in-line gas chromatography. Over a 13-day period, a carbon dioxide/nitrogen mixture was successfully injected into the 30-ft-thick reservoir interval, saturated with methane hydrate, without loss of injectivity. This was followed by a production stage in which the pressure was held above the stability pressure of the in-situ methane hydrate. CH4 was produced during this stage, and initial data analyses indicated that CO2 exchange was achieved. Ongoing analyses of the extensive datasets acquired at the field site are under way to determine the overall efficiency of simultaneous CO2 storage/CH4 production from the reservoir. As part of the demonstration, the depressurization phase of the test extended for 30 days. The longest previous field test of depressurization to extract gas from hydrate lasted 6 days as part of a Japanese-Canadian testing program at the Mallik well in Canada’s Northwest Territories during 2007 to 2008.
Interest is growing in drilling the Austin Chalk formation, with oil and gas companies hopeful that applying the latest unconventional resource development technologies can open a new chapter of expansion in a historically prolific play that dates to the 1920s. Much of the new drilling is in areas of the Chalk that overlie some of the most active parts of the Eagle Ford Shale play in south Texas. Drillers there are taking advantage of the additional stacked play opportunities that can often be drilled with the same rigs and crews they are using in the Eagle Ford and sometimes in the same well. Some operators may have also drilled in the Chalk during the depths of the industry downturn to hold leases and temporarily defer deeper Eagle Ford wells. The Austin Chalk extends from Mexico across south and east Texas and a large portion of Louisiana to Mississippi. The history of the Chalk play has seen several booms, the last one coming in the 1990s with the introduction of horizontal drilling. Cumulatively, the formation has produced 1.7 billion BOE with approximately 9,500 wells having been drilled there. Abundant Resources There is good reason to believe that abundant oil and gas resources remain in the Austin Chalk. The United States Geological Survey released a study of four Austin Chalk-area assessment units (AUs) in 2010 that estimated mean undiscovered resources for the Austin Pearsall-Giddings AU of 879 million bbl of oil, 1.3 Tcf of gas, and 106 million bbl of natural gas liquids (NGLs). Three other AUs were estimated to hold a combined mean 78 million bbl of oil, 2.3 Tcf of gas, and 257 million bbl of NGLs. Among the most active participants in the latest Austin Chalk play have been EnerVest, EOG Resources, Encana, and Murphy. Other companies active in the Chalk include GulfTex Energy, ConocoPhillips, Devon Energy, Marathon, Abraxas Petroleum, and Chesapeake. EnerVest, which acquires, develops, and operates oil and gas fields on behalf of its institutional investors, is the largest producer in the Austin Chalk. The company built a substantial position there beginning with its 2007 acquisition of Anadarko’s holdings in Texas’ Giddings field, which has been producing since the 1930s. EnerVest’s move into the Chalk came years before most other current players. In the Giddings field, as in certain other parts of the Austin Chalk, the formation lies above the Eagle Ford. But Eagle Ford development has been minimal there, compared with core Eagle Ford activity taking place from 20 to 80 miles southwest of the field.
As oil and gas development projects tackle some of the world's most remote locations and extreme environments, pushing out into 8,000 ft water depths or beyond and going several hundred miles offshore, innovations in subsea technology will play a vitally important role in enabling the execution of these projects and will form the basis of future, even more-ambitious plans. Subsea technology comprises wellhead and related production infrastructure placed on the seafloor, rather than on a conventional surface platform, a spar, or a semisubmersible or other floating production facility. Subsea wells and infrastructure are tied back by flowlines, risers, and umbilicals to surface producing facilities that may be directly overhead or many miles away, possibly even onshore. While deploying subsea technology can be very expensive, as water depths increase, conditions grow harsher, and locations become more remote, the alternative of building a larger, heavier surface facility to hold the wellheads, pumps, separation equipment, and other infrastructure is often more expensive, sometimes prohibitively. "There are two things that drive much of the move toward subsea," said David Morgan, Director, Subsea Processing, Cameron. "One is cost, the other is the reservoir. Typically, you must have at least one surface facility for drilling, processing, or storage. But surface facilities are a huge expense, and reservoirs sometimes are very spread out, making it hard to drill from one surface facility. That starts to drive multiple drill centers or the move to subsea development. By minimizing the size and number of surface facilities, you can save your project a lot of money." On the one hand, when equipment is placed on a surface facility, maintenance and interventions typically are conducted from that facility, and costs are easier to project and control—compared with contracting costs for a rig or vessel to service subsea equipment. On the other hand, space is at a premium on any kind of floating facility, and the need for additional equipment (e.g., separation, water-handling, and compression facilities) as a project matures is difficult to accommodate by retrofitting late in field life. By installing the additional equipment subsea, the need for platform changes can be minimized. "How effective we are at developing subsea technologies and evolving existing technologies is going to be an indication of how successful we are in recoverable reserves and maintaining our increasing production globally," said Neil Holder, Vice President, Sales and Marketing, Aker Solutions. "More and more companies are looking at stranded gas, offshore oil, and resources in the Arctic with long tieback distances that without subsea technology can't be accessed at all. So it becomes a facilitator."
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.