PNNL

Abstract

Motivated by CO2 capture and sequestration (CCS) design considerations, we consider the coupled effects of permeability heterogeneity and background flow on the dissolution of a supercritical CO2 lens into an underlying deep, confined aquifer. We present results of a large-scale Monte Carlo simulation study examining the interaction of background flow rate and three parameters describing multi-Gaussian log-permeability fields: mean, variance, and correlation length. Hundreds of simulations were performed at high resolution using the PFLOTRAN finite volume software to model CO2 dissolution in a kilometer scale aquifer over 1000 y. Predictive dimensionless scaling relationships relating CO2 dissolution rate to heterogeneity statistics, Rayleigh (Ra) and Peclet (Pe) numbers were developed for both gravitationally dominated free convection to background flow-dominated forced convection regimes. An empirical criterion, Pe = Ra^{3/4}, was discovered for regime transition. All simulations were found to quickly converge to a quasi-steady, approximately linear dissolution rate, though this rate displayed profound variability between permeability field realizations sharing the same heterogeneity statistics, even under mild permeability heterogeneity. In general, increased heterogeneity was associated with lower mean and higher variance of dissolution rate, undesirable from a CCS design perspective. The relationship between dissolution rate and background flow was found to be complex and nonlinear. Dimensionless scaling relationships were uncovered for a number of special cases. Results call into question the validity of the Boussinesq approximation in the context of modest-to-high background flow rates, as well as the general applicability of numerical simulations without background flow.