The Benson lab was formed in Spring 2007, when Sally Benson joined the Stanford faculty. Our laboratory studies fundamental aspects of geologic carbon dioxide sequestration in saline aquifers. We are able to replicate reservoir salinity, temperature, and pressure conditions upon cores of rock and flow mixtures of CO2 and brine. A medical CT-scanner is used to image the distribution of CO2 and brine inside the rock in real time. Our main interest is in the methods by which CO2 is immobilized in rock.
Better understanding of the relative permeability of CO2 and brine is essential to this goal. Currently, our focus is to investigate the sensitivity of relative permeability to injection flow-rate and various fluid properties such as viscosity, pressure, temperature, and interfacial tension.
The Importance of Relative Permeability
Injection Rate and Wells: The limit on CO2 injection rate is the overpressure limit of the geologic formation. Therefore, because pressure is fixed, the injection rate is determined by relative permeability. Relative permeability can essentially determine the number of injection wells needed, something critically important when determining capture and storage costs and feasibility.
Monitoring: Predicting CO2 leakage volumes and the spatial extent of the plume are critical steps in designing an optimal monitoring scheme. Relative permeability of the CO2-brine system is an essential input to these calculations as it governs the mobility of each phase in the storage reservoir, as well as in pathways out of the injection zone. For example, periodic seismic surveys may be the ideal means to track a CO2 plume that is spreading over a large area, whereas observation of pressure transients within the storage zone (or an overlying zone) may be adequate to monitor a localized plume. Understanding the relative permeability of the system is, therefore, crucial in making the appropriate choice.
Extent of the Plume: The size of the plume is controlled by relative permeability: the lower the relative permeability, the larger the plume can be. If relative permeability is very low, CO2 will not sweep across the reservoir as efficiently and will instead move in isolated channels, leading to low storage capacity and large plume size, which determines the geographic area that must be monitored.
Leakage Potential: Identifying likely leakage pathways is a significant component of the overall assessment of a CO2 storage reservoir. Furthermore, a particular site may have multiple potential pathways including abandoned wells, nearby faults, or a fractured cap rock, each of which must be evaluated quantitatively from a risk standpoint. In order to accomplish this, the relative permeability of the CO2-brine system must be known since the ease of flow through any of the potential paths is dictated by this fundamental mechanism.