Carbonate reservoirs

Scientists are studying the chemical composition of modern limestones and how water interacts with them to help understand the development of porosity in ancient carbonate builds-up through time. The research is helping to develop a model that is being applied by industry to predict the quality of carbonate oil reservoirs.

Carbonates are sedimentary rocks formed by layers of carbonate sediment laid down by marine organisms such as corals. More than 60 per cent of the world’s oil and 40 per cent of the world’s gas reserves are held in carbonate rock, yet extraction from carbonate reservoirs can be very challenging and much of the oil and gas often remains unrecovered.

After the rock is formed, it undergoes a range of chemical and physical processes that can alter the rock’s structure – a set of processes known as diagenesis. For example, minerals can precipitate out of water flowing through the rock to form cements, while elsewhere waters can dissolve the rock. The resulting changes in the distribution and nature of porosity affect both the ability of the rock to store oil and gas, and the flow response on extraction - information that is very valuable to the oil industry.

University of Bristol researchers have been studying the hydrology and geochemistry of modern carbonate platforms, such as those found in Guam and the Bahamas, to improve our understanding of how conditions such as sea level change and climate can affect carbonate rock development. This information can then be used in conjunction with our best understanding of how these conditions have changed in order to predict how diagenesis has affected rock quality during the accumulation of carbonate build-ups.

Researchers at Bristol and Royal Holloway, University of London developed a 3-dimensional forward sediment model, CARB3D+, which is unique in its ability to predict where and which chemical and physical changes may have occurred in early sediments as they were converted to sedimentary rock. This makes CARB3D+ a valuable quantitative tool for exploring carbonate reservoirs and enables industry to reduce the uncertainty in prediction of reservoir quality.

“What we’ve been able to do is provide the oil industry with a tool that helps to quantify risk,” said Dr Fiona Whitaker, Senior Lecturer in the School of Earth Sciences and Principal Investigator for the CARB3D+ project. “It can be used to define the bounds of certainty around a best guess estimate of how a reservoir may behave and this helps industry to make business decisions based on what they are risking.”

Extracting oil from carbonate reservoirs is particularly challenging, and recovery rates from carbonates can be as low as 10 per cent compared with average recovery rates for all reservoir types of 35 per cent. Understanding early diagenesis, and using tools such as CARB3D+ can help improve the efficiency with which the oil is extracted.

“If you understand the connectivity of the porosity within the reservoir, you can design engineering approaches to get more of the oil out of the ground,” said Whitaker.

The oil and gas industry have been the primary beneficiaries and supporters of this research and in 2005 the Bristol Carbonates Consortium was established with the University of Bristol, industry partners and the Natural Environment Research Council (NERC) to provide financial support and expert advice on future research on carbonate diagenesis and to continue the development of CARB3D+.

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