energies Article CO 2 Convective Dissolution in Oil-Saturated Unconsolidated Porous Media at Reservoir Conditions Widuramina Amarasinghe 1,2, *, Ingebret Fjelde 1 , Nils Giske 1 and Ying Guo 1,2   Citation: Amarasinghe, W.; Fjelde, I.; Giske, N.; Guo, Y. CO 2 Convective Dissolution in Oil-Saturated Unconsolidated Porous Media at Reservoir Conditions. Energies 2021, 14, 233. https://doi.org/10.3390/ en14010233 Received: 9 December 2020 Accepted: 30 December 2020 Published: 4 January 2021 Publisher’s Note: MDPI stays neu- tral with regard to jurisdictional clai- ms in published maps and institutio- nal affiliations. Copyright: © 2021 by the authors. Li- censee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and con- ditions of the Creative Commons At- tribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 NORCE Norwegian Research Center AS, P.O. Box 8046, 4068 Stavanger, Norway; infj@norceresearch.no (I.F.); nigi@norceresearch.no (N.G.); yigu@norceresearch.no (Y.G.) 2 Department of Energy Resources, University of Stavanger, P.O. Box 8600, 4036 Stavanger, Norway * Correspondence: widuramina@norceresearch.no Abstract: During CO 2 storage, CO 2 plume mixes with the water and oil present at the reservoir, initiated by diffusion followed by a density gradient that leads to a convective flow. Studies are available where CO 2 convective mixing have been studied in water phase but limited in oil phase. This study was conducted to reach this gap, and experiments were conducted in a vertically packed 3-dimensional column with oil-saturated unconsolidated porous media at 100 bar and 50 ◦ C (rep- resentative of reservoir pressure and temperature conditions). N-Decane and crude oil were used as oils, and glass beads as porous media. A bromothymol blue water solution-filled sapphire cell connected at the bottom of the column was used to monitor the CO 2 breakthrough. With the increase of the Rayleigh number, the CO 2 transport rate in n-decane was found to increase as a function of a second order polynomial. Ra number vs. dimensionless time τ had a power relationship in the form of Ra = c × τ −n . The overall pressure decay was faster in n-decane compared to crude oil for similar permeability (4 D), and the crude oil had a breakthrough time three times slower than in n-decane. The results were compared with similar experiments that have been carried out using water. Keywords: convection; porous media; reservoir conditions; oil; CO 2 dissolution; 3-dimensional column 1. Introduction CO 2 storage is a commonly considered topic when it comes to climate change miti- gation. Injection of CO 2 to active and abandoned oil and gas fields is a well-discovered solution for a viable utilization of CO 2 due to its commercial benefits of enhancing the oil recovery (EOR) as well as achieving permanent CO 2 storage [1,2]. During CO 2 injection into existing oil fields for EOR, the added CO 2 will swell and reduce the viscosity and will lead to an increase of the oil recovery percentage [3,4]. EOR for CO 2 utilization can also reduce a significant cost of the whole CCS value chain [5–7]. When CO 2 is injected into the oil fields, a CO 2 plume will usually develop above the fluid phases inside the porous media due to the low density of CO 2 compared to the density of the reservoir fluids, as shown in Figure 1 [8]. Initially, this CO 2 plume mixes with the oil and water phases present in the reservoir mainly due to diffusion. The mixing process creates a density gradient (e.g., increase the density of oil). This phenomenon leads to a convective flow, which will accelerate the CO 2 mixing and mass transfer and will significantly enhance the underground CO 2 storage rate as well as the oil production [7,9–11]. It is essential to know the behavior of the CO 2 plume in the reservoir along with how CO 2 will dissolve convectively into the oil phase. This helps to understand how CO 2 will be transported during long-term storage after injection for storage and EOR. The convectively driven dissolution has been extensively studied for accelerated CO 2 dissolution in saline water for CO 2 storage in 2-dimensional (2-dim) Hele-Shaw experimental setups [12–18] and using 3-dimensional (3-dim) confined experimental setups [19–24]. An extensive review Energies 2021, 14, 233. https://doi.org/10.3390/en14010233 https://www.mdpi.com/journal/energies