Improved recovery without side effects

reservoar
21.03.2011
New measures to increase recovery of oil from fields could lead to a more energy-efficient recovery, while emitting less chemicals and carbon dioxide than with current practices.

This is the assertion of principal engineer Reidar Kristensen at the Norwegian Petroleum Directorate (NPD), a view which is at odds with the prevailing view of  improved recovery – specifically, that it leads to increased energy consumption and increased emissions to the environment.

This view assumes that the recovery strategies of the future will remain the same as today, an assumption that Kristensen does not believe to be true.

The most common method of increasing recovery from oil fields today – injecting seawater into the reservoirs to extract more oil – requires a lot of energy, e.g. for running pumps and process facilities. As the oil fields mature, water production increases while oil production declines. The produced water is either re-injected or cleaned before being discharged at sea.

”If we continue circulating seawater, and just pour in more and more water, the energy-efficiency is reduced – i.e. energy consumption per barrel of oil produced,” he says.

Multiple measures must be combined to ensure the recovery is as efficient as possible. These include drilling more and better wells, chemically blocking water-bearing zones in the reservoirs and increasing the mobile oil volume by adding chemicals to the injection water.


More than half remains

The average recovery rate from fields on the Norwegian shelf is currently 46 per cent. Based on current approved plans, this means that more than half of the oil may be left behind when production ceases. Some of the remaining oil can be drained using current injection methods, while about half remains trapped in the pores of the reservoir. This type of immobile oil cannot be released using seawater, but requires the use of chemicals or CO2 in order to be produced.

Polymers, or ”water thickening agents”, are among the chemicals that are most promising in recovering more of the mobile oil. For the immobile oil, surfactants can be used – a type of ”soap” that reduces the interfacial tension between oil and water so the oil is released from the pores. Several laboratory experiments have been executed with these substances, where 80-90 per cent of the oil was produced. In practice, it is not possible to achieve such a high recovery rate, but the laboratory experiments still hold promise.

As of today, none of these chemicals have been tested on fields in the North Sea. According to Kristensen, this is partially due to some of them being classified as environmentally harmful.

To avoid discharging chemicals at sea, water with a lower salt content could be a contribution. The less salty water increases the chemicals’ efficiency and can be re-injected in the reservoir without increasing bacteria growth.


Investments pay off
These new strategies require relatively large investments both in the testing and implementation phase – something which, according to Kristensen, could explain why they have not been used yet.

In order for the operators on the shelf to be able to see the rewards of investing in a changed recovery strategy, it is necessary for them to consider larger areas when making plans for the future.

”They cannot consider each reservoir individually, but must look at one or more fields simultaneously, preferably also in connection with neighbouring licences,” says the principal engineer, who has 27 years of experience from Statoil as a reservoir engineer before starting at the NPD in 2008.

Kristensen is the project leader in Force – a co-operating forum between oil and gas companies and authorities in Norway for improved oil and gas recovery and exploration – and is thus a driving force for using new recovery methods.

”Many companies don’t want to make investments and take risks on producing fields, but improved recovery strategies can yield increased recovery with improved energy-efficiency and lower CO2 emissions per produced unit – without discharging more chemicals,” Kristensen believes.