Contents lists available at ScienceDirect Journal of CO 2 Utilization journal homepage: www.elsevier.com/locate/jcou Insight investigation of miscible SC CO 2 Water Alternating Gas (WAG) injection performance in heterogeneous sandstone reservoirs Duraid Al-Bayati a,c, ⁎ , Ali Saeedi a, ⁎ , Matthew Myers b , Cameron White b , Quan Xie a , Ben Clennell b a Department of Petroleum Engineering, Curtin University, Kensington, WA, 6151, Australia b CSIRO Energy, Kensington, WA, 6151, Australia c Department of Petroleum Engineering, Kirkuk University, Kirkuk, Iraq ARTICLE INFO Keywords: Miscible WAG flooding Effect of crossflow Heterogeneous porous media Enhanced oil recovery ABSTRACT In this manuscript, we present the results of a systematic approach to investigate the impact of core scale heterogeneity on the efficiency of miscible CO 2 water-alternating-gas (WAG) flooding performance. Both vertical (by layering two axially-cut half plugs with differing permeability) and horizontal (stacking two smaller core samples with differing permeability in series) heterogeneities are explored. In the layered or vertically hetero- geneous sample, the permeability ratio (PR) defines the ratio between the permeability values of each half plug. Our special sample construction technique using either a thin impermeable Teflon sheet to prevent flow com- munication or a thin tissue to promote flow communication has enabled us to investigate the effect of crossflow between half plug on the performance of the WAG flood. For the stacked composite or the horizontally het- erogeneous core samples, short cylindrical core segments were used each with a different permeability value. We have also investigated the effect of the EOR injection mode (i.e. secondary vs. tertiary) on our results. For this study, core flooding experiments were performed using n-C 10 , brine and CO 2 at a temperature of 343 K and a pressure of 12.4 MPa. The results obtained for homogeneous, layered and composite samples indicate that CO 2 WAG flood performs better in all cases and achieves the highest recovery factor (RF) when conducted under the secondary mode (e.g. homogeneous: 93.4%, layered: 74.0%, and composite: 90.9%) compared with the tertiary mode (e.g. homo- geneous: 74.2%, layered: 64.1%, and composite: 71.3%). For the layered samples, it was found that the oil recovery decreases noticeably with an increase in the permeability ratio (PR). For instance, RFs of 93.4%, 90.1%, 78.8%, and 74.0% correspond to PRs of 1, 2.5, 5, and 12.5, respectively. In contrast to our previous findings with continuous CO 2 flooding which showed that crossflow enhances recovery in layered samples, for this study using WAG, crossflow was found to negatively affect the RF. Such an outcome may be attributed to the conformance control achieved by WAG flooding which would be more pronounced in the case of non-communication layers (i.e. no cross flow). In other words, the higher oil recovery of WAG flooding in a non-communicating system may be due to the dominance of viscous forces and, to a lesser extent, the vanishing effect of gravity forces that tend to reduce sweep efficiency. The effect of composite heterogeneity on the RF was also investigated with the results showing that the permeability sequence along the length of a composite sample has a noticeable but more subtle impact on RF. 1. Introduction and background Since the mid-20th century, many researchers have investigated the suitability of CO 2 as an EOR agent and the field applications also re- sulted in favourable outcomes [1–4]. With declining oil reserves worldwide, CO 2 flooding for EOR has great potential for more wide- spread use. Furthermore, in more recent years, it has been pointed out that CO 2 injection into oil reservoirs can also be an effective approach for mitigating the global warming and reducing greenhouse gas emis- sions [5–7]. In fact, the application of CO 2 for EOR (CO 2 -EOR) may be considered as an added advantage in helping to offset the cost asso- ciated with CO 2 geo-sequestration processes making them economically more attractive. Generally, CO 2 -injection can prolong a reservoir’s life by 15–20 years and may recover an additional 15–20% of the original oil in place [8]; this is mainly due to the high microscopic displacement efficiency https://doi.org/10.1016/j.jcou.2018.10.010 Received 3 July 2018; Received in revised form 8 October 2018; Accepted 11 October 2018 ⁎ Corresponding authors at: Department of Petroleum Engineering, Curtin University, Kensington, WA, 6151, Australia. E-mail addresses: duraid.al-bayati@curtin.edu.au (D. Al-Bayati), Ali.saeedi@curtin.edu.au (A. Saeedi). Journal of CO₂ Utilization 28 (2018) 255–263 2212-9820/ © 2018 Elsevier Ltd. All rights reserved. T