By Rie Jerichow
Long gone are the days when one drilled wells into an underground reservoir, and up came the oil. But there is much oil left. In some fields, it is possible to boost oil recovery by injecting water in strategic locations to push the oil towards the wells. At the Danish Hydrocarbon Research and Technology Centre’s core flooding laboratory, researchers now focus on identifying the most suitable advanced water flooding technique using modified seawater – and the addition of dimethyl ether is one of the options that shows significant technical potential.
The core flooding laboratory at DHRTC is a very sophisticated playground for oil scientists. You will find neither swings nor carousels, but a wide range of state-of-the-art equipment, oil samples from Danish fields in the North Sea and, of course, core flooding test rigs, the focal points of the core flooding lab.
“We have four of these core flooding rigs that we have built from scratch from bits and pieces,” says Sidsel Marie Nielsen, Team Leader and Senior Researcher at DHRTC. “ They are specifically designed to test the effect of injecting fluids into rock core samples to enhance oil recovery, a technique called core flooding. Here we can investigate whatever recovery process we want.”
Looking into the large glass cabinet in which the rig is located, you see a lot of steel cylinders, handles, pumps and thin metal tubes. For a non-expert, it is impossible to spot the sophisticated details.
Sidsel Marie Nielsen explains: “When you start a new test, you can virtually plug and play without changing too much. The glass cabinet is a kind of oven that keeps the desired temperature stable, and all the processes are monitored online – so if anything fails, e.g. if the pressure or the temperature drops, if the core breaks or if the density is wrong, you will know it immediately. All the tiny tubes are made from a special alloy, based on nickel that is corrosion-resistant and can survive in high temperature and high salinity conditions.”
Although the laboratory opened about a year ago, researchers have already achieved promising results, based on experiments carried out in the rig.
“One of our experiments shows that the use of dimethyl ether (DME) may have a favourable impact on oil recovery. When under high pressure we mix DME with seawater and inject the mixture into a core sample, it shows a significant increase in oil recovery,” Sidsel Marie Nielsen says.
In simple terms, she says, the explanation is this: “When the mixture is injected, it meets oil and formation water – water that occurs naturally within the pores of the rock in the underground. The salinity of the formation water is higher than in seawater and the DME has a preference to dissolve in the oil instead. The DME in the oil makes the oil more fluid and causes it to swell. Consequently, the mobility of the oil is improved and it can more easily be ‘pushed’ out and recovered.”
- In the virgin reservoir, the oil is plentiful.
- Oil recovery has been enhanced by waterflooding. Water has been injected into the reservoir to increase pressure and stimulate the production. However, the remaining oil cannot be recovered with this technique.
- A mixture of dimethyl ether (DME) and seawater is injected.
- The DME/seawater makes the oil more fluid and causes it to swell.
- Consequently, the mobility of the oil is improved and can more easily be “pushed” out and recovered.
- Finally, clean seawater is injected into the reservoir. On the platform, the produced water is cleaned and the DME is recycled.
The process might sound simple, but the experiments are tedious and require enormous attention.
“DME is a solvent,” she explains. “Although it has no effect on metal, it can dissolve the rubber material that we have in the system. The rubber in the rig may swell or break very easily and ruin the experiment. Therefore we must take extra precautions. Furthermore, DME is a gas at ambient conditions, so we must work with it above 5 bar to make it liquid. However, when the pressure is released, it turns back into gas vapour, and so we have to deal with that.”
The core samples were picked up maybe 30 years ago and have been stored in the open since then. It requires a thorough preparation of the cores before the experiment can start.
“It takes around two months,” says Sidsel Marie Nielsen. “First you must restore the core as it was initially. You must clean the core totally so absolutely no oil is left. You must measure the porosity and permeability, saturate the core with water and oil and recreate the precise mixture of oil and water that was in the core initially. Finally, you age the core for three weeks at the initial reservoir temperature so the rock can equilibrate with the fluids inside. Then you can start your experiment.”
Research in the laboratory is not a one-man show. Selection and planning of experiments, as well as performance and interpretation of test results, require collaboration between many interdisciplinary researchers. Along with the core flooding experiments, research is being carried out to understand the mechanisms behind the successful techniques in order to reduce the uncertainty of the recovery processes. It requires an understanding of the processes and what limitations they have.
“The integration of chemistry, rock-oil-water, and oil-water interaction with the core flooding experiments is important,” says Sidsel Marie Nielsen. “Senior Researcher Karen Louise Feilberg is responsible for projects where the chemistry of water, oil and rock is crucial – for planning and interpreting what happens in the lab. In general terms – what happens when the water floods through the core and how does it affect the oil? From there, Senior Researcher Hamid Nick and the researchers around him take over,” tells Sidsel Marie Nielsen and continues:
“They get the results from the lab and use them to tune and further develop models for the recovery processes. They scale the models up and so to speak take the results from core scale to sector or reservoir scale – in order to evaluate the potential of a given recovery process.”
Results of the core flooding experiments under reservoir conditions show that DME injection in chalk reservoirs can lead to a significant additional oil recovery. Evaluation of this method from a commercial point of view is ongoing.