Characterizing Rayleigh Taylor Instability and Convection in a Porous Medium with Geoelectric Monitoring
Résumé
Geoelectric imaging of the subsurface is widely used to monitor flow and transport in the subsurface (e.g. saline tracer, two-phase flow). However, relating the measured geoelectrical signals to the monitored transport processes is not straightforward and often requires numerical simulations. In the context of CO2 sequestration, monitoring by geoelectrical methods is attracting increasing interest. This work focuses on the effective electrical conductivity of a formation saturated with a brine into which supercritical CO2 (sCO2), positioned above, dissolves. The CO2-rich brine at the interface with sCO2 is denser than the brine below, so it destabilizes and gives rise to natural convection into the fresh brine. This so-called Rayleigh Taylor (RT) instability, featuring the coupling between buoyant flow and solute transport, is studied here with 2D Darcy-scale numerical simulations, under different convection strengths (quantified by the Rayleigh number, controlled by the initial density contrast). We examine if monitoring the effective electrical conductivity can provide quantitative information on the convection. The conductivity is computed by injecting current with a virtual pair of electrodes and measuring the potential difference between the two others. The computation is validated by comparing its outcome to known conductivities in several complex domains. The conductivity anisotropy is the ratio of the longitudinal to the transverse effective conductivities. Initially, it is very high, and as the convection develops and solute mixing within the brine increases, it drops off to around 1, thus showing that the convection-mediated fluid fingering has a clear geoelectrical signature. Since that signature depends on the Rayleigh number, the convection’s strength may be monitored from electrical measurements. These first results open up a range of opportunities to use electrical signals for the in-situ monitoring of CO2 sequestration in geological reservoirs.
