Sjur Talstad (left), senior vice president for subsea technology at Statoil, with chief geophysicist Nils Erik Bakke in the background.
A proactive approach to squeezing as much oil and gas as possible out of the geological structures which contain hydrocarbons has proved to be profitable. Our experience shows that astute reservoir management opens the way to recovering much greater volumes of oil and gas from its fields.
But this calls for careful assessment of how many wells are to be drilled and in what locations, whether and where water is to be injected to assist production, and so forth. There is no standard solution for getting the best return from a reservoir. Each formation is different, and needs to be treated individually to optimise recovery.
The IOR toolbox consists primarily of technology development in five key areas, says Sjur Talstad, senior vice president for sub-surface technologies in StatoilHydro’s Technology & Projects business area.
These are four-dimensional seismic surveys, reservoir modelling, advanced wells and cost-efficient infill drilling, water and gas injection, and managing produced water.
“The first step in managing a reservoir is to build a static picture of what it looks like – the geological layering and the location of the oil, gas and water,” says Talstad.
“You then have to create a dynamic understanding, which involves monitoring the movement of the fluids when you start production.”
Once the mechanisms concerned, and the way they fields better interact, are grasped well enough, the operations team can use this knowledge to decide where to place production wells and locate water injection in order to optimise production. Statoil has been one of the leaders in developing expertise in this discipline, according to Talstad.
Daunting challenge
Simply understanding the reservoir is a daunting challenge, however, since oil and gas are usually found at depths of 1,000-5,000 metres. In addition, offshore fields typically lie in waters from 50 to more than 2,000 metres deep.
A key source of information is seismic survey data, gathered by directing sound waves into the sub-surface and collecting the echoes reflected back from the depths. These signals make it possible to identify different geological strata, and thus provide the basis for constructing a three-dimensional map of the reservoir.
Wells drilled into the reservoir provide further information. Readings, known as logs, taken of the area surrounding the well provide a good indication of where hydrocarbons lie.
Although they cover a relatively small area, the data gathered from logs are very precise. They resemble needle points of clarity in the reservoir, says Monika Bertheussen, project manager for Statoil’s corporate initiative on continued IOR improvement.
That contrasts with seismic data, which cover the whole reservoir but at a much greater level of uncertainty. “We need to reduce uncertainty and improve accuracy,” says Bertheussen.
More data come from sensors installed in the wells to measure parameters such as temperature and pressure. Production data on the volume of oil, gas and water flowing from each well provide further input.
Time to monitor. Fluid movement is monitored by taking the same 3D seismic picture at different dates. This is known as time-lapse or four-dimensional seismic, with time as the fourth dimension. Since Statoil ran its first 4D seismic survey on Gullfaks in 1996, it has found the technique invaluable, Talstad confirms.
“We’re probably the leading user of 4D seismics. We apply this method on more than 70% of the fields we operate, and have over 20 such projects under way.”
The 4D approach makes it possible to monitor closely the effects of another important reservoir management technique – water injection.
This has two main aims, Talstad explains. One is to maintain pressure in the reservoir, so that oil and gas are pushed into and up the production wells where the pressure is lower.
The second is to inject water at particular locations, where it will have the maximum effect in driving hydrocarbons towards the production wells.
Gas may also be injected. Recycling gas helps to improve oil recovery, and it can be recovered later. In some reservoirs, cycles of water injection followed by gas have been found to give good results. This technique is known as water-alternating-gas (WAG).
The illustration shows a modern, horizontal well construction where the production comes from two branches and are gathered in a mother well.
Advanced wells
Implementing a successful reservoir management strategy also depends on the technology for advanced wells and cost-efficient infill drilling. Accurate reservoir models and drilling operations enable wells five-six kilometres long to be drilled with an accuracy of a couple of dozen metres into predetermined locations within an oil pocket with an areal spread of a couple of hundred metres, Talstad reports.
This applies particularly to older fields operated by StatoilHydro, such as Statfjord and Gullfaks in the North Sea where production has been under way since the 1970s and 1980s.
Ever more sophisticated equipment is being installed in wells to enable close monitoring and control of production. Both drilling and production activities are increasingly being implemented through integrated operations, where land-based teams provide support and expertise for offshore activities.
Such an approach, in which operations are monitored as far as possible in real time, has greatly increased the efficiency of reservoir management and has the potential to do so even more. Despite the speed with which reservoir management has developed, Bertheussen is keen to sound a note of caution.
“A reservoir still produces surprises, and even though you think you know what it looks like, you can still turn out to be wrong,” she says.
Nevertheless, reservoir management has contributed to a significant increase in recovery from Statoil’s fields, particularly the older ones.
Nine hundred million barrels have been added to the recovery target on Statfjord since 1982, and 700 million on Gullfaks. At USD 50 a barrel, the gross value of that additional output is USD 80 billion (NOK 500 billion).
“We can manage our reservoirs so much better today than 10 years ago,” says Talstad. “But we’ve still got a long way to go.”
