Solving the evolution of carbonate porosity: a dynamic solution for enhanced oil recovery and carbon capture and storage

Fossil fuels, such as oil, natural gas and coal, provide approximately 80% of the world's energy supplies. During the burning of fossil fuels, carbon is released as carbon dioxide. Carbon dioxide is a potent greenhouse gas: when released into the atmosphere it traps outgoing longwave radiation, called the 'greenhouse effect'. Over the last 250 years, since the industrial revolution, carbon dioxide concentrations in the atmosphere have been increasing, enhancing the greenhouse effect and hence, global warming. An innovative solution to our continued reliance on carbon-based fuels is Carbon Capture and Storage, which seeks to reduce the amount of carbon dioxide released during the burning of fossil fuels by capturing some carbon dioxide and storing it deep underground. This carbon dioxide pumping, or flooding, in deep underground rocks is also used within the oil industry to aid in Enhanced Oil Recovery.

There are many challenges related to storing carbon underground, and one of the most important of these is understanding the chemical reactions it will have with the surrounding rock. When carbon dioxide is injected it dissolves in the pore fluid within the rock. Dissolved carbon dioxide is a weak acid, and can dissolve minerals that are soluble in weakly acidic conditions. This particularly impacts rocks made of limestone, due to the solubility of these carbonate minerals. The dissolution of these minerals creates more space within the reservoir, which allows for the storage of further carbon dioxide, but may lead to reservoir collapse if taken to extreme.

The addition of dissolved carbon dioxide to reservoir rocks can further stimulate microbial activity, which can create conditions favourable to the formation of carbonate minerals. These can be beneficial as they are chemically stable and therefore store the injected carbon dioxide over long time scales. However the formation of carbonate minerals can also lead to the destruction of the network of spaces within the rocks ("porosity"), which makes further injection impossible. The destruction of porosity also leads to the sealing of the reservoir, making the recovery of further fossils fuels impossible.

Our lack of understanding of what happens to carbonate rocks during carbon dioxide injection represents a major gap in our ability to store carbon underground. This fellowship seeks to understand how the addition of carbon dioxide to a carbonate reservoir will stimulate microbial activity, and how this will lead to the generation, or destruction, of porosity. This will be conducted by initially running multiple laboratory experiments to determine how the chemistry of the injected fluid affects the microbial processes within the reservoir rocks. I will use multiple high resolution geochemical tools to track the precipitation or dissolution of carbonate minerals within the reactors. The results from the laboratory experiments will be fed into numerical models, which can be used for predictive purposes, and will identify how changes in the chemistry of the injected fluid and reservoir properties interacts to impact the porosity evolution.

Samples will also be measured from multiple carbonate reservoirs in order to compare the laboratory data to current or potential sites for carbon capture and storage. The numerical models once trained with both the laboratory and field data will be used to create a predictive framework to predict the chemistry of the fluid that should be injected into any potential reservoir for carbon capture and storage, and how the particular fluid will lead to the creation or destruction of porosity, dependant upon the required outcome.