Time-Resolved Pore-Scale Imaging of Multiphase Dissolution during CO2-Saturated Brine Injection into a Carbonate: Competition between Hydrocarbon Mobilisation and Swelling

TL;DR

This study uses time-resolved pore-scale imaging to investigate how CO2-saturated brine dissolves carbonate rocks containing residual hydrocarbons, revealing a non-monotonic dissolution rate driven by hydrocarbon swelling and ganglion mobilisation.

Contribution

It provides the first detailed pore-scale analysis of the dynamic interplay between hydrocarbon swelling and ganglion mobilisation affecting carbonate dissolution under reservoir conditions.

Findings

Dissolution rate exhibits three regimes: initial advection, inhibition, and acceleration.

Swollen hydrocarbon ganglia persistently block large pores during the inhibited regime.

Flow reorganization and loss of advective access control dissolution dynamics.

Abstract

We present time-resolved pore-scale experiments in which CO2-saturated brine was injected into a water-wet Ketton limestone sample containing residual hydrocarbon under reservoir conditions (8 MPa, 50 {\deg}C) and monitored by 4D X-ray microtomography. Equivalent pore-network models were extracted at each scan time to track pore geometry, topology, and fluid occupancy, while fluid-fluid and fluid-rock interfacial areas and the effective reaction rate were determined from segmented images. The dissolution rate is non-monotonic in time and proceeds through three regimes, consistent with a shifting balance between hydrocarbon swelling and ganglion mobilisation, which control advective access to reactive surfaces. In the initial advection-dominated regime, pore-throat widening leads to ganglia mobilisation and efficient acidic brine delivery to reactive surfaces. The second, dissolution-inhibited regime is marked by up to two orders of magnitude reduction in effective reaction rate. Pore-network analysis shows that swollen hydrocarbon ganglia persistently occupy the largest throats throughout this regime. This occupancy is associated with a reorganisation of the advective flow field into preferential flow paths and stagnant zones. We interpret the rate suppression as primarily reflecting a path-dependent loss of advective access to reactive surfaces, with subordinate contributions from localised H+ depletion near ganglia and reduced near-wall mass transfer in widened flow paths. The inhibited state persists until hydrocarbon is displaced from the largest throats, after which, in the third stage, advective access improves and rock dissolution accelerates. These results show that the effective dissolution rate in residual-hydrocarbon-bearing carbonate depends dynamically on the competition between hydrocarbon swelling and ganglion mobilisation, governing advective access to surfaces.