Carbonate reservoir challenges
Storing Carbon Dioxide | Recovering Oil and Gas
More than half of the world's conventional oil resources are contained in carbonate reservoirs that are typically extensively fractured. Many of these are in the Middle East and recovery factors are often low: while the fracture network carries most of the flow, the matrix retains the vast majority of original oil in place. These reservoirs are poorly understood in comparison to classic (sandstone) reservoirs found, for example, in Europe and the Americas.
Oil and gas recovery
An essential precursor to optimising oil and gas recovery from these carbonate reservoirs, or using them to store carbon dioxide (CO2) to mitigate catastrophic climate change, is the understanding of their evolution combined with appropriate and detailed characterisation of the reservoir and the in situ fluids. This can be used in conjunction with reservoir simulators to optimise production and recovery factors or to design CO2 injection processes to ensure safe and effective long-term storage.
Characterisation procedures, simulators and enhanced oil recovery (EOR) processes have been much studied and applied to good effect for sandstone reservoirs over several decades, but their development is much less refined for carbonate reservoirs, where the structures and physical chemistry are quite different. Enhancing this capability is at the heart of QCCSRC's activities.
Capturing carbon dioxide
Another key issue to be addressed as we move towards cleaner production and use of oil and gas is the need to prevent release of CO2 produced by power generation and various industrial sources (e.g., hydrocarbon processing). Carbon Capture and Storage (CCS) involves collecting CO2 from these large point sources and injecting it underground into depleted hydrocarbon reservoirs or saline aquifers. It is an essential technology for reducing CO2 emissions to the atmosphere and hence to mitigate climate change. QCCSRC is the largest CCS research-driven programme in the UK, along with many other CCS research projects you can find out more through the Imperial College Centre for Carbon Catpture and Storage - IC4S.
Storing carbon dioxide
Currently there are two main options for CO2 storage: saline aquifers and hydrocarbon reserves (especially depleted oil/gas reservoirs or small coal beds). Saline aquifers are typically an order of magnitude more saline than seawater and therefore utilising these resources for long term storage does not compete with underground potable water resources. Saline aquifers are also an order of magnitude greater in terms of storage capacity than hydrocarbon reservoir capacities. A limiting issues with their current use is that we typically have far more data on specific hydrocarbon reservoir characterisation and performances than we normal would have for a saline aquifer.
Concerns with CCS
The principal concern with CCS is to ensure that the injected CO2 does not subsequently leak back into the oceans or atmosphere. Again, most studies to date have involved sandstone reservoirs. Exploring the feasibility of using carbonate reservoirs and aquifers for carbon sequestration, based on the same characterisation and reservoir modelling approaches developed to optimise oil and gas recovery, is another critical element of QCCSRC.
Optimising CCS and EOR
New procedures for optimising CCS and EOR in fractured carbonate reservoirs and integrating them into field practice are important elements of QCCSRC's programme. They build on the Energy Future Lab's existing Clean Fossil Fuels Grand Challenge programme in collaboration with Shell on CO2 properties and sandstone reservoir injection and storage processes. This programme involves the fundamental study of the thermophysical and multiphase flow properties of CO2/brine/hydrocarbon mixtures, in the context of enhanced oil and gas recovery and CO2 storage applications. QCCSRC aims to harness this research to address specific issues with Qatar's gas and oil fields, including carbon storage, enhanced hydrocarbon recovery and their possible combination.