Research Project CX8

Mass-transfer processes at the liquid-gas interface — such as dissolution, exsolution, and evaporation — strongly influence multiphase flow in porous media and can lead to salt precipitation. In this project, the impact of these coupled mass-transfer and salt-precipitation processes will be experimentally characterized for heterogeneous porous media. The results will be relevant to many engineering and environmental applications. However, the primary focus will be on subsurface gas storage, which is expected to play a crucial role in the transition toward a sustainable energy future.

Subsurface gas storage will involve next to periods of injection and production, periods of no-flow. During these no-flow periods redistribution of the liquid and gas phases can occur, driven by dissolution and exsolution processes. To better understand this redistribution, we have carried out microfluidic drainage and imbibition experiments, using pre-equilibrated CO₂ and water, after which the system was isolated. Experiments were conducted under two configurations: 1. Fully isolated (undrained) system, where the chip was sealed at the inlet and outlet. 2. Connected outlet system, where the chip was sealed at the inlet, but the outlet was connected to a small reservoir filled with CO₂, mimicking a connection to a highly porous rock or fracture. A pH indicator was used to monitor the amount of dissolved gas in the water (Figure 1). In the fully isolated (undrained) system, redistribution was observed due to the dissolution-driven process of Ostwald ripening. In the connected outlet system, gradual gas exsolution occurred over time. Furthermore, we repeated the experiment with the connected outlet system using NaCl brine, where we observed that the exsolution process led to salt precipitation in the thin water film covering the solid surface of the gas-saturated regions. In this project, the observed thin film salt precipitation will be investigated in addition to evaporation-driven and pH-induced salt precipitation.

Rock heterogeneity, from grain shapes and pore structures to laminae and bedding layers, significantly impacts mass-transfer processes such as dissolution, exsolution, and evaporation, which in turn affect multiphase flow and trapping behavior during subsurface gas storage in porous reservoirs. Furthermore, evaporation and exsolution can induce salt precipitation in brine, further altering flow and trapping dynamics. This project aims to investigate these processes in depth across different gas-liquid-solid systems at multiple scales — from the micro-scale to the pore-scale to the Darcy scale. This experimental study will be conducted using the HPHT cell setup (project CX7) and the microfluidic setup of the Porous Media Lab (Z02), in close collaboration with research project D01 for visualization, and the modelling studies of A01, A02, C01, and C02, as well as our internal collaborator Theresa Schollenberger and external partner Carina Bringedal. The overarching objective is to develop meaningful Darcy-scale descriptions of these coupled mass-transfer and salt-precipitation processes.