Brent, the oil field that gave its name to one of the best known traded crudes in the world, is enjoying a second lease on life. It also is witnessing one of busiest reservoir management and associated drilling and well work programs experienced anywhere offshore.

John Gallagher, Shell's development manger in the northern North Sea, talks with passion about a project he has lived with for the past 4 years: the largest offshore oilfield reservoir depressurization project in the world.
Shell operates the North Sea's huge Brent field, which has oil and gas reserves of 2 billion bbl and 6 Tcf, respectively. Shell UK Exploration and Production (Shell Expro) discovered the field in 1971, and the operator was responsible for design and construction of the four huge offshore production platforms. These are in a water depth of 450 ft (137 m) and built to withstand wind speeds in excess of 100 mph and 100-ft (31-m) waves. Three of the production platforms are concrete structures, and one is a steel jacket. The field has been in production from 1976 and underwent full-field water flood from each of the four platforms until the end of 1997.
From 1994 to 1997, Shell additionally invested US $2 billion in platform redevelopment, which covered refurbishment of the offshore facilities to handle low-pressure operation, as well as safety and process reliability upgrades. Since 1998 the field has been successfully undergoing the largest offshore field depressurization ever attempted. The Brent field depressurization is at the forefront of hydrocarbon recovery process design, extracting gas from the residual oil in the swept zone of the reservoir following water flood.
On Dec. 31, 1997, Gallagher said, the instruction went offshore to shut down every water-injection pump across the field to trigger the reservoir depressurization and Brent's new lease on life. Since 1998, Shell has invested heavily in well drilling and reservoir management of the field (a further $800 million during 5 years) throughout the massive depressurization project, meticulously managing gas supply from 36 individual reservoirs and using state-of-the-art reservoir management tools and systems.
Project performance
Gallagher said not only did the $2 billion platform redevelopment project (the world's largest ever offshore brownfield engineering project) deliver on time and within budget, but the subsequent massive reservoir depressurization is running amazingly close to, if not slightly ahead of, the original field redevelopment plan laid out in 1992.
The offshore production and processing equipment designed and built in the 1970s was replaced with 1990s thinking and technology. A much safer working environment has been created for offshore personnel. The Brent platform redevelopment was completed in the post-Cullen era, and emergency shutdown, venting, fire and gas systems were upgraded. Low-pressure separation and processing facilities were installed to deal with the lower reservoir and producing well pressure. And even the living quarters were replaced; all two-person cabins feature in-suite facilities plus satellite television.
Wet gas production is at a life-of-field high of 900 MMscf/d, some 10% better than expected, and gas reserves also are higher than expected in the 1992 plan. The additional gas reserves generated by the project are in excess of 1.5 Tcf. Gross liquid production (oil plus water) is also a record high of 700,0000 b/d, in order to void the reservoir, lower the pressure and force the gas reserves out of the residual oil. Consequently, Brent supplies some 8% to 10% of the national gas demand and is one of the most reliable sources of gas in the United Kingdom. The reliability of Brent's redeveloped infrastructure and "the commitment of its people," Gallagher said, has resulted in no daily interruptions to gas supply since the start of the project.
At the same time, Shell Expro has realized the improved operational efficiencies targeted with the huge redevelopment expenditure. Unit operating cost is $1.80/bbl, more than half the unit cost prior to redevelopment. State-of-the-art operating systems coupled with innovative work processes have facilitated a platform and drill crew complement of 120 people, compared to 180 prior to redevelopment.
Field reservoir management
Since January 1998, the 36 individually managed reservoirs of the Brent field in the Upper Jurassic Brent Group and Lower Jurassic Statfjord Formation have been rapidly depressurized from close to their initial pressure of 6,000 psi to just above 3,500 psi. Gallagher's team manages total reservoir withdrawal of more than 1 million reservoir barrels per day and meticulously balances the relative area and subreservoir offtakes of gas, oil and water to optimize the massive fieldwide depressurization process. Reservoir pressure is dropping by some 40 psi a month on a reservoir pore volume of 5 billion bbl that spans 23 sq miles (60 sq km). By 2010, reservoir pressure is expected have dropped to about 1,000 psi.
Depressurization has allowed gas to come out of solution in the residual oil, which then migrates to individual gas gaps within the reservoir and causes movement of the associated oil rims. Gas production from these changing gas cap and oil rim positions requires a continuous drilling program on all four Brent platforms and a large number of well entries - principally through-tubing workovers to recomplete a well from one zone to another (Tables 1, 2). "We have continual evolution of the gas from the residual oil zone, and chasing the gas caps and rapidly moving oil rims is a 365-day-a-year job," Gallagher said.
"We are drilling some fairly elaborate wells," Gallagher said. One well on the Brent Alpha platform is aiming for a total distance of more than 23,000 ft (7,015 m). Areal target tolerances are small, on the order of 60 ft (18 m), as there are so many subreservoir layers, and the well delivery teams need to make sure they hit the targeted gas caps and oil columns at the correct level. With such a heavy drilling program, Gallagher said they take every opportunity to apply new technology. "We drill multilateral wells and also sidetrack wells through tubing without removing the completion," Gallagher said. "Plus we just shot ocean-bottom cable seismic, which gave us an improved reservoir image to assist our full-field geological and dynamic modeling and lead to better targeted infill wells." Shell manages the field with the assistance of a full-field reservoir simulation model containing 80,000 reservoir grid blocks. It also models the field's surface network.
Managing field information
Because chasing hydrocarbons is the name of the game due to the rapidly changing positions of gas caps and oil rims, up-to-date information is required on the state of the reservoir. A mountain of downhole pressure measurements is recorded, and Brent boasts possibly the largest offshore cased-hole logging program in the world. Shell Expro developed a customized, Web-based information management system that can handle the huge amount of well surveillance and production data necessary to optimize the ongoing field depressurization. Project personnel, subsurface engineers, petrophysicists and drillers, as well as production and operations staff, onshore and offshore, have the same access to one high-integrity data management and processing system. "This is an essential requirement if everyone on the project is to remain on the same wavelength, whether it be Wednesday afternoon in the office or Sunday morning offshore," Gallagher said. "We effectively track the oil and gas distribution within the field in real time and completely remap the oil and gas distribution in each of the 36 subreservoirs every 6 months."
The spin-off of such visible transparency is that all employees have an overview of the bigger picture, fueling "the strong ownership and teamwork that is a hallmark in Brent," Gallagher said. "We put a lot of effort into that part of the business in the first year of the depressurization. Our original data management system was simply not coping with the huge activity level and the massive amount of data gathering we had to do."
Springboard to the future
The redevelopment and depressurization project extended the life of the field some 7 years, and the newly installed facilities with the latest technology position Brent well for the future. "An additional benefit of Brent redevelopment is the ready availability of a modern, well-equipped hub for gas business from elsewhere," Gallagher said.
Brent's infrastructure allows gas export to Shell Expro's St. Fergus terminal on Scotland's east coast, where natural gas liquids are recovered and shipped to Shell Expro's Mossmoran complex in Fife for ethane extraction and onward to the chemicals industry, thus extracting maximum benefit from any wet gas stream. Brent oil is exported to the Sullom Voe terminal in the Shetland Islands. That means the field is well-placed to handle new business from satellite fields. One possibility lies in a cluster of five fields, called Penguins, about 40 miles (65 km) north of Brent. The Penguins could be tied back to Brent in what would be the North Sea's longest multiphase subsea tieback development. "We are at an advanced stage in evaluating development options for the Penguins fields," Gallagher said. "Hopefully, we will be able to bring those plans to fruition in the very short term."
Best practice
Gallagher cited "the structure of workforce involvement" as the successful key to delivering the $1.8 billion (£1.3 billion) brownfield engineering project on time and within budget in what is one of the harshest offshore environments in the world. "Another significant learning of which we can be proud is the interactive and flexible approach we have employed to handle the complexity of reservoir management against the breakneck speed and high-activity environment dictated by depressurization.
In particular, the parallel approach of deploying modern field modeling techniques together with traditional analytical evaluation methods based on direct observation, mapping and trending."
He also cited "the incredible improvements we have realized in operational and cost efficiency, stemming from the same structure of workforce involvement as well as attention to detail."
There has been no silver bullet in this area but rather an overriding philosophy among the Brent organization to live cost management every day, using a complete array of tools and techniques.
Teamwork and flexibility
Sharing up-to-date information is routine. While Gallagher sits in his office at Shell Expro's headquarters in Aberdeen, he and his onshore colleagues are in close contact with those on the four Brent platforms. "The dialogue between onshore and offshore is fairly constant," he said. "We make sure everybody understands the big picture, instilling ownership and ensuring team work."
Because this large bank of emerging data is readily available, Shell Expro is quickly able to respond to new reservoir situations rapidly. Decisions about drilling new infill wells or recompletions are made "within weeks" - subject to regulatory approval from the UK Department of Trade and Industry. "We have learned to be extremely flexible," Gallagher said. "It is not unusual for us to get a new well plan from conception to spud within weeks as we respond to the rapidly changing nature of the field." Equally, the Brent team has been flexible in optimizing compression expenditure vs. well expenditure by redistributing gas in the field. The opportunities of two major compression projects have been seized in the past 2 years, both easing demands on the drilling program, and both online within 9 months of first observing the opportunity.

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