Treasure chests at the bottom of the sea

Huge figures are involved in petroleum research. But the valuable liquid itself lies deep below the bed of the North Sea, which is where Roy Gabrielsen, Professor of Geology at the University of Bergen, does his research.

We are three or four kilometres below the seabed and anything between 50 and 250 million years back in time. This is where and when oil reservoirs were formed in sedimentary rocks such as sandstone, chalk or calcium, which are riddled with tiny holes in which oil and gas be stored.

Photograp of sandstone from the Troll field. The blue regions are poresk in which oil or gas can accumulate.

"Under a layer of impermeable rock we may find what we call traps of porous rock. Pressure and temperature are decisive factors that determine whether oil can form and be stored there. An oil trap is often formed in connection with a fault, a zone of weakness in the Earth's crust where one block of rock has slipped down relative to another. It is not unusual for such a migration to be several hundred metres in length, and it may even be up to a couple of kilometres long", says Gabrielsen.

Of course, faults of this sort are also found in bedrock on land, where knowledge of them is essential for mapping groundwater resources. The seabed harbours even more valuable drops of liquid. But how can we bring them up to the light of day?

"One of the most common methods of bringing up oil is to stimulate the reservoir. Water or chemicals are pumped into the porous rock in order to press the oil upwards. Our contribution to petroleum recovery is to provide descriptions of the reservoir and to estimate how oil and gas will migrate through the reservoir during production. To be able to do this, we need both an understading of geological conditions, physical measurements and numerical modelling techniques", says Gabrielsen.

Mutually dependent

Reservoir research at the University of Bergen has a long history. In 1983 came signals from the authorities that they wanted more research to be done on oil reservoirs. Six University of Bergen departments; Geology, Mathematics, Physics, Microbiology, Chemistry and Geophysics, Solid Earth Physics, realised the potential of joining forces to do research in this field. The Department of Informatics joined the team later.

"We saw that we had common interests, so we set up a scientific forum for reservoir research", explains Arnfinn Graue, professor of physics and for several years coordinator of the reservoir committee. Graue has recently been replaced by Professor Magne Espedal of the Department of Mathematics, who is currently on sabbatical leave in the USA.

"An interdisciplinary approach is essential for research of this sort, but this is not always easy to manage at a university. One reason for our ability to achieve this was the fact that as members of a medium-sized university we knew each other and were not hindered by departmental boundaries. Both the university leadership and the Faculty were also positive to the idea of cooperation, which was initiated by the researchers themselves; it was not forced on us. I think that this was a very good point of departure".

Graue illustrates how reservoir research is dependent on interdisciplinary research as follows:

"Oil is found inside various types of porous rock, which can be compared to a sponge. In order to discover an oil eservoir, expertise is needed in geology and geophysics. To produce oil and gas we need expertise in chemistry in order to visualise how the fluids in these rocks behave. Microbiologists look at how bacteria in the water behave, and study, for example, the possibility of using micro-organisms that grow in petroleum reservoirs to improve recovery rates.

"Expertise is also needed in the field of physical conditions in the reservoir; how the various forces affect each other. Water is heaviest and therefore lies at the bottom; the oil is in the middle and the gas at the top. If you pump in water, the oil is squeezed out. The physicists are trying to understand how these forces operate. The problem is to generalise the results of laboratory experiments to conditions in large reservoirs. This is where the mathematicians and informatics people come into the picture. With the aid of numerical analyses we can transfer the results from small-scale to large-scale conditions.

X-ray images

Graue is using a new method that consists of making a type of X-ray image of the rocks. This method is unique in global terms.

"This is an imaging technique that shows us the distribution of water, oil and gas in rock. The method is used in research on flow in heterogeneous media (rocks) and it can also be used to study other areas than oil recovery, e.g. groundwater flow or surveying chemical emissions to the environment", says Graue. He has long experience of research on fractured reservoirs, the field in which the University's reservoir group is currently concentrating most of its efforts.

"Most reservoirs in the North Sea are fractured. I have worked with Ekofisk, a reservoir which is both so dense and so fractured that few people believed that it would be possible to extract oil from it when it was discovered in 1969. Now, it produces around 300,000 barrels a day".

"This is a typical example of how research on a particular reservoir has made it possible to produce more and more from that reservoir. Studying reservoirs has enabled us to recommend the most suitable methods of production. Solving the Ekofisk mystery has also had consequences for petroleum recovery in several other reservoirs in the southern North Sea", says Graue.

Graue has received a total of NOK 18 million for his research during the past ten years, of which MNOK 6 have come from Phillips Petroleum. Altogether 13 oil companies have supported research in this field.