Large quantities of oil from gas injection

Since 1975, gas has been injected to increase recovery on the Norwegian shelf. The result is 320-360 million standard cubic metres more oil and condensate from the fields.

That is equivalent to between 2 and 2.3 billion barrels.

“We want to be a driving force for the best possible utilisation of resources, to enhance the value for society. By using gas injection we can recover more oil”, says principal engineer Leif Hinderaker in the Norwegian Petroleum Directorate (NPD).

“Originally, gas was injected back into the reservoirs because there were no pipelines to transport it away from the fields. The Norwegian authorities also prohibited burning or flaring the gas,” he said. Several gas injection projects have gradually been approved because they provide higher oil recovery than water injection, and a greater economic return from the fields.

High recovery rate on the Norwegian shelf
Ekofisk was the first field on the Norwegian shelf to inject gas at the top of the reservoir. This was done from 1975 to 1997. During this period, one-fourth of the gas from Ekofisk and surrounding satellite fields was re-injected, while the rest of the gas was exported. The objective of the gas injection on Ekofisk was that the gas needed to be stored due to operational problems caused by construction of the gas pipeline to Emden in Germany, as well as low demand for gas in the summer months. It is difficult to prove that this injection improved recovery. However, simultaneous simulation of reservoir models indicates that gas injection may have improved oil recovery somewhat and contributed to maintaining production at a higher level. However, after water injection started in 1987, this is still the method, combined with compaction of the reservoir rocks, which contributed most to the increase from 17 per cent to 50 per cent.

The next field was Statfjord. When production started in 1979, there were no pipelines to export the gas. It was therefore injected back into the Statfjord formation, which is the next largest formation on the field. Eventually gas was also injected into the largest formation, Brent, as a supplement to water injection. In addition to good reservoir properties and the drilling of many wells, the combination of gas and water injection has ensured that the recovery rate for oil on Statfjord is expected to reach 66 per cent, according to approved plans.

Oseberg was the first field where gas injection was approved as the main method to recover oil, both injection of its own gas and imported gas from the Troll field, where the subsea template Troll Oseberg Gas Injection (TOGI) was put into use. The authorities made an active contribution to ensuring that this project was approved, and, according to Hinderaker, the project has proved very successful. With the current approved plans the oil recovery rate on Oseberg is estimated at 63 per cent.

On the Norwegian shelf, the expected average recovery rate from oil fields is at 46 per cent – a high number compared with oil provinces in other parts of the world.

An important tool
Since the oil and gas activity started on the Norwegian shelf in 1971, over 2000 billion standard cubic metres of gas have been produced. Most has been exported to the European market, but more than one-fourth of the produced gas has been injected back into the reservoirs.

So far, the number of fields on the Norwegian shelf utilising gas injection has reached 28. Around 60 per cent of the injected gas has been used on the Oseberg, Statfjord and Åsgard fields.

For some fields, gas injection depends on the demand for gas, which often fluctuates throughout the year. When the demand is lowest, more can be injected. The method can therefore also be an important commercial tool, according to Hinderaker.

He points out that some of the gas can be injected several times, and most of it can be produced and sold at a later point in time.

Various ways of injecting gas
When the gas is injected down into the well and comes in contact with the oil, the gas can behave in two different ways: Depending on pressure and temperature, the gas can mix with the oil in a so-called miscible injection, which is what takes place in the Statfjord formation on Statfjord and in parts of Åsgard. Alternatively, the gas will not mix with the oil, but forms a separate phase in so-called immiscible injection. This is. the case on, among others, the Oseberg and Grane fields.

To increase production of condensate on gas/condensate fields, some fields have utilised gas injection. Among these are Sleipner Øst and parts of Åsgard.

Alternating water and gas injection (WAG) is used as a supplement to water injection, which is the main method on most of the fields on the Norwegian shelf. Alternating water and gas are then injected into the same well. Since WAG does not require large amounts of gas, the method is used by several Norwegian fields such as, Snorre, Gullfaks, Statfjord and Ula. It only accounts for 10-12 per cent of the total amount of gas injected on the shelf annually.