Permanent cables increase oil recovery 

More frequent reservoir monitoring increases oil recovery opportunities. Optimal well positioning results in major value creation.

One of the focus areas in Statoil's technology strategy is to develop methods to improve reservoir monitoring and thus improve recovery.

The objective is to acquire even more extensive data to ensure more accurate drilling of both new and operated fields.

By acquiring new seismic data after a period of production, it is possible to observe the movement of gas and water in the reservoir and obtain an image of where the hydrocarbons were produced – and where there are any left.

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Ivar Sandø, senior adviser of Technology Excellence.

«Statoil has a large portfolio of such 4D seismic data.  This technology is an important contribution to reaching our ambition of increasing the oil recovery rate to 60 per cent on the Norwegian shelf," says Ivar Sandø, senior adviser of Technology Excellence.

Statoil is a world leader within increased oil recovery. Whereas operators on a world average recover 35 per cent of the oil, Statoil currently recovers 50 per cent of the oil from operated fields on the Norwegian shelf.

Every additional per cent the company recovers constitutes a value of NOK 200 billion with today's oil prices.

Seabed cables on their way

Statoil will, for the very first time, use a permanent seismic tool to further increase oil production. Starting in the summer of 2013, 700 kilometres of seismic cables will be installed on the seabed on the Snorre and Grane fields in the North Sea.

Both fields have large remaining reserves. Using permanent reservoir monitoring (PRM), Statoil can obtain more frequent and better seismic images of the reservoirs. This means that it will be easier to position and manage the wells to recover more oil and gas.

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Goro Elisabeth Müller, Petroleum Technology Manager of the Snorre field.

«We expect that PRM on Snorre and Grane will result in 30 million additional barrels of oil," says Goro Elisabeth Müller, Petroleum Technology Manager of the Snorre field.

More frequent and better seismic acquisition on the seabed

Seismic image gathering usually involves towing many kilometres of streamers side by side after a seismic vessel. When this operation is repeated over the same area, it is called 4D seismic acquisition.

Traditionally, this takes place every two or three years. Embedding the streamers into the seabed will make acquisition easier. Each acquisition operation will be quicker and can be performed more frequently, approximately every six months.

A vessel with an air gun will cross the area and transmit pressure waves down into the reservoir. These will be reflected back to the streamers which will register the information and transmit it to the platform they are connected to. The information will then be transmitted onshore via a fibre cable where the subsea teams will interpret it.

Reduces uncertainty

Statoil launched a PRM research pilot project on Snorre in 2008.

"Embedding cables into the seabed is complicated. Among other factors, it involves many different disciplines. Still, the pilot has proven that this technology has considerable benefits," says reservoir geophysics specialist Mona Andersen of Statoil.

Close to 75 per cent of Statoil's producing fields use 4D technology.

«This seismic method provides us with images that we would not be able to obtain otherwise. By embedding streamers on Snorre and Grane on a permanent basis, we bring the technology to the next level," says Andersen.

«Snorre has been in production since 1992. It is becoming increasingly difficult to avoid gas and water zones when drilling new wells," Múller says, explaining that:

«By collecting seismic data in the same area at different times, we can observe how oil, water and gas move in the reservoir over time. This makes it easier to see where the remaining oil is, where the wells should be drilled and how the reservoir can be drained to recover as much oil as possible," she concludes.

What is 4D seismic acquisition?

  • Detailed maps of geological structures on the seabed show the location of oil and gas

  • The maps are made by using an air gun which transmits pressure waves into the seabed. At each geological layer boundary the signal is reflected upwards

  • Geophysicists and geologists process the data and prepare three-dimensional seismic images of the subsurface

  • Seismic data collected over the same area during different time periods are called 4D seismic data. Time constitutes the fourth dimension.

     

    High ambitions:

  • Snorre has an ambition to improve the recovery rate from the current level of 46 per cent to 55 per cent.

  • Grane's ambition is to increase the recovery rate from 56 to 65 per cent.

  • Permanent reservoir monitoring will help achieve these ambitions.