Carbonated waterflooding in carbonate reservoirs: Experimental evaluation and geochemical interpretation Ahmad Sari a, ⁎, Yongqiang Chen a , Matt B. Myers b , Mojtaba Seyyedi b , Mohsen Ghasemi a , Ali Saeedi a , Quan Xie a, ⁎ a Discipline of Petroleum Engineering, WA School of Mines: Minerals, Energy and Chemical Engineering, Curtin University, Western Australia, Australia. b CSIRO - Energy, 26 Dick Perry Ave., Kensington, WA 6151, Australia. abstract article info Article history: Received 3 February 2020 Received in revised form 25 March 2020 Accepted 1 April 2020 Available online 3 April 2020 Carbonated water flooding (CWF) appears to be an important means in enhanced oil recovery (EOR) in carbonate reservoirs. While a few CWF coreflooding experiments have been done to reveal the controlling factor(s) behind incremental oil recovery, few has examined the impact of calcite dissolution on the contribution of the proposed mechanisms, and fewer have looked beyond the impact of calcite dissolution on different length scale (from core to reservoir). We thus conducted a series of core flooding experiments to investigate the residual oil saturation and recovery factor during waterflooding with and without carbonation. We also imaged the core plugs using a computed tomography scanner to examine the evolution of calcite dissolution along the core plug. Further- more, we performed 1D reactive transport modelling at core- and reservoir-length-scale to delineate the impact of calcite dissolution process during carbonated waterflooding. Coreflooding experiments confirm that lowering salinity increases oil recovery from 53% to 64.5% without car- bonation. However, low salinity carbonated water at secondary mode yielded 47.6% and 52% oil recovery, be- tween 1 and 5.4% less recovery compared to formation brine flooding without carbonation, lower than the formation brine flooding without carbonation. Carbonated waterflooding also resulted in a significantly increases of permeability. CT images clearly show the generation of wormholes along the core, accounting for the low re- covery and increased rock permeability. 1D reaction transport modelling at core-scale reveals the calcite dissolu- tion taking place throughout the core plugs, supporting the wormholes evolution from CT images. One- dimensional reactive transport modelling at reservoir-scale shows the calcite dissolution distance from wellbore increases from 13 to 45 m with increasing flow rate from 0.05 to 1 m/day. Taken together, our results imply that calcite dissolution may deteriorate heterogeneity of reservoirs particularly near the wellbore. This may signifi- cantly undermine the contribution of oil-swelling, viscosity reduction, IFT reduction and wettability alteration on incremental oil recovery, as well as wellbore stability. However, the negative impact of calcite dissolution may not prevail at in-depth of reservoirs because the calcite dissolution would reach equilibrium at a certain dis- tance, which is also associated with injection rates. © 2020 Elsevier B.V. All rights reserved. 1. Introduction Carbonate reservoirs host most of the global oil reserve, yet they yield b40% oil recovery Downs et al. [1] due to their strong oil-wet char- acteristics in nature [2,3]. Wettability of oil-brine‑carbonate systems is a key petrophysical parameter, which regulates the relative permeabil- ities, residual oil saturation thereby oil recovery. In this context, finding practical, yet feasible solutions to alter carbonate wettability towards more water-wet has been the perceived the centre point of shifting rel- ative permeability whilst reducing the residual oil saturations. Published works have demonstrated that lowering salinity or engi- neering injected water chemistry would disturb in-situ geochemical equilibrium between oil and carbonate pore surfaces. This physico- chemical process would enable oil film lifting off from pore surfaces and thus yield incremental oil recovery. Although the mechanism (s) of wettability alteration remains incomplete, brine pH is shown to play a key role in regulating wettability of oil-brine-carbonate system particularly at low pH (pH b 7). For examples, surface complexation modelling supported by contact angle measurements confirm carbon- ate rocks become strongly hydrophilic in acidic brines regardless of brine salinity and ion types [4]. Similar results were also reported for carbonated brine, showing that CO 2 uptake by water (i.e., carbonation) increases hydrophilicity of oil-brine‑carbonate sys- tem [5]. Chen et al. [6] reported that acidic and carbonated brines give Journal of Molecular Liquids 308 (2020) 113055 ⁎ Corresponding authors. E-mail addresses: ahmad.sari@postgrad.curtin.edu.au (A. Sari), quan.xie@curtin.edu.au (Q. Xie). https://doi.org/10.1016/j.molliq.2020.113055 0167-7322/© 2020 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq