Modeling of Chemical Tracers for Two-Phase Flow in Capillary-Dominated Porous Media

Abstract

Transport of chemical tracers in porous media is an important physical mechanism for applications in oil reservoir characterization, aquifer remediation, and CO2 storage. Although numerous analytical solutions exist to model the flow of solutes under single-phase flow, the modeling of solute transport in two-phase media driven by imbibition has been insufficiently examined. In tight porous media and the matrix of fractured reservoirs, spontaneous imbibition (SI) represents a key driving mechanism for fluid infiltration because the low permeability in these systems results in negligible transport by advection. Here, we present a new semi-analytical solution to model the transport of chemical tracers under one-dimensional countercurrent SI in oil–water porous media.The model presented is derived from the analysis of fluid imbibition driven by capillarity and numerically solved as a function of water distribution. We model ideal and partitioning tracers to examine the contacted region and estimate the average oil saturation in capillary-dominated media under countercurrent SI. The concentration profiles obtained with the derived model show an excellent agreement against numerical simulation results, verifying that the semi-analytical solution accurately models the mechanisms of partitioning, hydrodynamic dispersion, and adsorption. The concentration profiles exhibit a significant delay in displacement behind the imbibition front when hydrodynamic dispersion is ignored and for high partitioning coefficients, demonstrating the importance of determining these properties under two-phase flow conditions. We consider that our solution derived represents an accurate alternative to time-consuming simulations that can be extended for the analysis of tracers in fractured reservoirs to estimate oil saturation in the matrix medium.