The goal of this work is to develop an improved model of CO2 bubble rise through porous media in the deep subsurface. Under the geologic carbon sequestration (GCS) conditions of interest, a rising parcel of CO2 will be subject to at least three dynamic forces:
buoyant forces;
surface tension forces; and
shear drag forces.
To fully characterize these, this work involved several experimental measurements focused on the second and third forces in particular. To better understand the effect of shear drag forces, the viscosity of brines was explored under bubbly flow scenarios to understand the rheological conditions that might impact leakage. To better understand the role of surface tension forces on influencing flow, contact angle measurements were carried out for a range of relevant mineral, brine, CO2 combinations. Predicting leakage from geologic carbon sequestration sites is difficult because of the large length scales that are involved and because of the complex geophysics and geochemistry that a rising parcel of CO2 will be subject to as it travels to the surface. To better understand how quickly and where a parcel of CO2 is likely to escape, better modeling tools are needed. These tools must be based on experimental results collected for GCS-relevant conditions. The results of the brine viscosity work suggest that under vapor liquid equilibrium (VLE) conditions CO2-brine mixtures will exhibit complex viscoelastic behavior. This is because CO2 bubbles in the matrix will respond to the varying levels of shear that will exist in the porous media to resist flow. Similarly, the contact angle measurements suggest that CO2 is less wetting of some common minerals and clays that prevail near GCS sites. The experimental results described here will be used to describe an enhanced model of CO2 vertical flow through the subsurface. At smaller scales, this enhanced model could help explain preferential flow pathways and potential hysteresis that could influence leakage from GCS sites. At larger scales, the results of this work could contribute to more accurate prediction tools for managing the risk associated with GCS.
Geologic carbon sequestration (GCS) has been discussed as a scalable and economically viable approach for keeping large volumes of anthropogenic carbon dioxide out of the atmosphere (Chen, Gingras et al. 2003; Eccles, Pratson et al. 2009). In GCS, the flue gas from power plants and other point sources is captured, separated, compressed, transported and injected into porous geologic formations several kilometers under the surface. Candidate formations are bound by impermeable layers that prevent the buoyant rise of the injected CO2 and are generally filled with brines that would have little other economic value (Widjajakusuma, Biswal et al. 1999; Kneafsey and Pruess 2010). At these depths, hydrostatic pressures and geothermal temperatures are large and the CO2 exists in the liquid or supercritical phase where it would intermingle with the endogenous brines (Chen and Zhang 2010). The densities of CO2 under all states are lower than that of the native brines and so the parcel of CO2 will be subject to buoyant forces. The CO2 could escape under several scenarios including leakage through abandoned well bores, heterogeneities in the bounding formation, groundwater flow to shallower unconfined aquifers and other pathways (Figure 1 (left)) (Nordbotten, Celia et al. 2004; Zhang, Oldenburg et al. 2009; Wollenweber, Alles et al. 2010).
