A team of researchers at the Massachusetts Institute of Technology (MIT) has developed a modelling method to determine the capacity and to assess the leakage risks of potential underground CO2 reservoirs.
One strategy for mitigating greenhouse gases is to inject compressed CO2 into natural aquifers made of permeable rock soaked with brackish salt water. CO2 is less viscous and less dense than the water and, once injected, it rises to the top of the aquifer. The permeable rock usually lies underneath a dense, impermeable ‘cap rock’ that traps the gas deep underground for long periods of time.
Cap rocks are often tilted, however, and as the CO2 rises through the aquifer, it can slip out, eventually making its way back into the atmosphere. Engineers seek to avoid leakage by mapping potential reservoirs and using theoretical tools to predict CO2 flow.
Now doctoral students Christopher MacMinn and Michael Szulczewski and Prof Ruben Juanes of the MIT have developed the modelling methodology for determining the capacity of potential reservoirs and for assessing the risks of leakage.
The tool takes into account key aspects of the underlying physics to predict the shape and pattern of flow when CO2 is injected into a deep underground aquifer.
‘Our new modelling tool is analytical rather than numerical, which means it incorporates the three primary physical mechanisms by which CO2 is trapped in briny substrate – structural, capillary and dissolution trapping – into a single, comprehensive mathematical expression that can be solved quickly,’ said MacMinn.
‘This makes it possible for us to alter key parameters, such as the aquifer permeability, the fluid viscosities or the tilt of the cap rock and, within seconds, predict how the plume of CO2 will migrate through the subsurface,’ he added.
Before, each parameter change in a numerical model added hours or days to the time it took a computer to model discrete sections of the substrate and pull all these together into a prediction of CO2 behaviour under those limited circumstances.
Engineers would have needed to run dozens – if not hundreds – of these to incorporate all the likely parameter permutations, making this an infeasible means of assessment.
The hope now is that engineers and geologists may be able to use this new modelling tool to quickly and inexpensively determine whether CO2 would escape from a geological reservoir.