Extension of Vertical Equilibrium Model for Two-Phase Displacement in Layer-Cake Reservoirs: Accounting for Water-CO2 Partial Miscibility and Fines Migration

TL;DR

This paper extends vertical equilibrium models for two-phase CO2 displacement in layered reservoirs by including water vaporization, CO2 dissolution, and fines migration, providing analytical solutions and insights into injectivity and sweep efficiency.

Contribution

It introduces an extended VE model that captures coupled physical and geochemical mechanisms, offering analytical solutions for layer-cake aquifers and insights into displacement fronts and saturation profiles.

Findings

Fines migration reduces injectivity but can improve sweep efficiency.

Two displacement fronts identified: dissolution front and evaporation front.

Saturation profiles depend on permeability variation with depth.

Abstract

Numerical simulations of geological CO2 storage in deep saline aquifers have demonstrated that vertical equilibrium (VE) models offer a robust and computationally efficient framework for reservoir optimization and upscaling. These studies emphasize the influence of fines migration and partial miscibility between water and CO2 on the evolving storage behaviour. Specifically, capillary-driven mobilization of clay and silica fines can impair injectivity, while water evaporation into the CO2 phase leads to near-wellbore drying, salt precipitation, and additional injectivity loss. However, conventional VE models do not account for these coupled physical and geochemical mechanisms. This work introduces an extended VE model that incorporates water vaporization into CO2, CO2 dissolution into the displaced brine, and fines migration leading to permeability reduction. For layer-cake aquifers, the model admits an exact analytical solution, providing closed-form expressions for both injectivity decline and sweep efficiency. The analysis identifies two distinct fronts during displacement: a leading displacement-dissolution front and a trailing full-evaporation front. Vertical model downscaling further reveals that gas saturation varies with the depth-dependent permeability profile. In formations where permeability decreases with depth, saturation declines monotonically; in contrast, in systems with increasing permeability, the competition between viscous and gravitational forces can generate non-monotonic saturation profiles. The results also indicate that while fines migration reduces injectivity, it may simultaneously enhance sweep efficiency; highlighting a complex trade-off that must be considered in the design and optimization of CO2 storage operations.