By Rie Jerichow
The Valdemar Field produces first class oil but “tricky” chalk makes oil recovery a challenging exercise. Now a research group at DHRTC hopes to find new ways to recover the oil.
If you go 245 kilometres west of Esbjerg, dive 38 metres into the sea and a further 2.5 kilometres into the rock, you will find the Lower Cretaceous Reservoir in the Valdemar Field – a true nightmare for petroleum engineers.
“The quality of the oil is very good – as good as it gets. However, the rock is ‘tricky’,” says Alexander Shapiro, Associate Professor from CERE – Center for Energy Resources Engineering, DTU. “It is very tight, which in general causes the oil to flow very slowly. It is fragile, it contains clay, and it is difficult to inject water to push the oil out. We encounter all possible problems connected to the rock. It is a nightmare – but an interesting challenge.”
He is part of a research group with colleagues from DTU Chemical Engineering, DTU Chemistry, GEUS and DHRTC R&D. They strive to uncover why some of the wells in Valdemar produce fine and other wells badly and whether – or how – gas injections can improve the oil recovery.
Several aspects of the system are little understood.
“It is almost a mystery,” continues Alexander Shapiro. “If we look at two wells, one well produces significantly better than the other though they are very close – and there is no obvious reason why. If only we could understand why, we could find a way to increase the production of the other well.”
In the research project, he studies a hypothesis that could explain the difference between the two wells.
“Maybe the fine producing well has a gas cap above the horizontal part of the well. The gas cap expands as the oil is being sucked up and so to speak presses the oil into the well,” he explains.
And as for the well that produces poorly: “A reason for the limited oil production here could be tiny gas bubbles in the rock. They develop when the pressure decreases as the oil is being sucked up. You see a similar effect when you remove the pressure of a soft drink by lifting the cap.”
A newly started PhD project will look further into the matter. In the lab, the researchers will restore the oil close to what it was originally, put it back into rock samples and slowly reduce the pressure to find out what happens. X-ray computer tomography will make it possible to look inside the rock samples. “Will the gas accumulate in a gas cap or will small bubbles develop? Or both? Or neither of the two?” Alexander Shapiro asks.
Another part of the project looks at the potential of injecting gas into the rock. Gas injection can increase the mobility of the oil and make it flow more easily – maybe instead of forming bubbles – and also increase its miscibility (ability to mix with another fluid). Simply speaking, injected gas will exchange components with the reservoir oil during the flow. The process will make the gas heavier and oil lighter. At a certain point, the gas and oil are no longer distinguishable, which means the gas can displace out oil almost completely.
To understand and describe how components will be exchanged or redistributed, a study of the phase behaviour during the gas injection processes is required.
“Oil is composed by thousands of different compounds,” says Senior Researcher, Chemist Karen Louise Feilberg from the Enhanced Oil and Gas Recovery group at DHRTC. “From the lightest ones like methane, going up in molecular size and weight to the heaviest ones like asphaltenes. But to understand the behaviour of the oil, you need to understand the molecules and the properties that determine the phase behaviour of the oil – why certain oil compositions will be liquid and others will be in gaseous form.”
When you inject gas into the reservoir, you change the total phase behaviour of the oil.
Senior Researcher Wei Yan from the Department of Chemistry, DTU, explains: ”In the lab we will study the impact of different gasses like hydrocarbon gas, nitrogen, and flue gas. For each injection we will perform tests to provide the basis for developing a model for gas injection. We hope and believe that the method can be used to economically recover additional oil in Lower Cretaceous reservoirs in the Danish North Sea.”
Another part of the project is being carried out in collaboration with researchers at GEUS. When the researchers have generated knowledge about how gas is liberated, it is incorporated into a large geological model – a reservoir simulator – together with many other parameters.
The researchers have also planned flooding experiments at DHRTC and GEUS to get first hand data on the effects of gas injection in real reservoir rocks.
“It will, for example, help us predict how a gas cap will expand, whether one oil well will produce more than another one, and maybe give us an idea where the next well should be drilled,” says Alexander Shapiro.
…is a geologic period and a system that spans 79 million years from the end of the Jurassic Period 145 million years ago to the beginning of the Paleogene Period 66 million years ago. The Lower Cretaceous Period ended 99 million years ago.