CAPILLARY TRAPPING OF CO2 IN RESERVOIRS WITH FLUVIAL ARCHITECTURE

The idea of reducing the Earth’s greenhouse effect by sequestration of CO₂ into the Earth’s crust has been discussed and evaluated for more than two decades. Deep saline aquifers are primary candidates for CO₂ reservoirs. Evaluation of reservoir capacity and the risk of CO₂ leakage require a detailed modeling of the post-injection migration and distribution of CO₂. Capillary trapping may be very important to storage efficacy, and is the focus of our investigation. A number of important candidate CO₂ reservoirs exhibit sedimentary architecture reflecting fluvial deposition. Recent studies have led to new conceptual and quantitative models for sedimentary architecture in fluvial deposits over a range of scales that are relevant to CO₂ injection and storage. These, in turn, have led to new geocellular modelling approaches for representing this architecture, and to new computational studies of CO₂ plume dynamics during and after injection. The processes of CO₂ trapping depend upon a complex system of non-linear and hysteretic characteristic relationships including how relative permeability and capillary pressure vary with brine and CO₂ saturation. New computational studies of capillary trapping in conglomeratic reservoirs strongly suggest that representing small-scale (decimeter to meter) textural facies among different rock types, including their organization within a hierarchy of larger-scale stratification, representing differences in characteristic relationships between rock types, and representing hysteresis in characteristic curves can all be critical to understanding trapping processes. In this context, CO₂ trapping was evaluated in conglomeratic reservoirs with fluvial architecture including different rock types with different and hysteretic characteristic curves, and with capillary pressure defined for each rock type. The results show that in these reservoirs the capillary trapping rates are quite sensitive to differences between the approaches, and that heterogeneity and hysteresis in characteristic relationships must both be represented.