Molecular dynamics simulation on volume swelling of CO 2 –alkane system Bing Liu a , Junqin Shi a , Baojiang Sun b , Yue Shen a , Jun Zhang a,⇑ , Xu Chen a , Muhan Wang a a School of Science, China University of Petroleum, Qingdao 266580, Shandong, China b School of Petroleum Engineering, China University of Petroleum, Qingdao 266580, Shandong, China highlights  Molecular dynamics simulation was first used to address CO 2 –alkane swelling.  Volume swelling coefficients from simulation are in well agreement with experiments.  Swelling mechanism was investigated by process, structure and interaction energy.  Effects of pressure and temperature on volume swelling were interpreted.  Simplification of alkane molecular structure promotes volume swelling. article info Article history: Received 17 August 2014 Received in revised form 20 September 2014 Accepted 16 November 2014 Available online 26 November 2014 Keywords: Volume swelling CO 2 –alkane system Dispersion interaction Molecular simulation abstract The microscopic mechanism of the volume swelling of CO 2 –alkane (decane, octane, hexane and cyclohex- ane) systems and the effects of temperature, pressure and alkane structure on the volume swelling of CO 2 –alkane systems are investigated by performing molecular dynamics simulation. It is shown that the increase in pressure, the reduction in temperature and the straight-chain structure of the alkane are of benefit to the volume swelling of CO 2 –alkane systems by calculating the volume swelling coeffi- cient; CO 2 in supercritical state plays a dominant role in the volume swelling of CO 2 –alkane systems. The microscopic process of the volume swelling of CO 2 –decane system shows that the increase in the average separation distance between decane molecules and the stretch of decane molecules result in the volume swelling of decane as CO 2 dissolve into decane. The calculations of interaction energies in CO 2 –decane system indicate that the interaction between CO 2 and decane molecules is responsible for the volume swelling of CO 2 –alkane system. Further study on the interaction between CO 2 and decane molecules shows that the dispersion interaction, resulting in the different solubility of CO 2 in alkanes, is the essence of the volume swelling for CO 2 –alkane system. This work is a good start on understanding the mechanism of alkane swelling influenced by CO 2 at the molecular level and provides useful informa- tion for guiding CO 2 enhancing oil recovery. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Combining geological CO 2 storage and CO 2 flooding to enhance oil recovery (EOR) has been attracting both industrial and scientific interest because of the growing concern about global warming and shortage of energy supply [1–4]. When CO 2 is injected into oil-bearing reservoirs and dissolved in crude oil, there appear favorable characteristics of CO 2 including that the oil volume swells, the oil viscosity is reduced, and CO 2 and oil could be miscible [1,5–9]. Even under immiscible conditions CO 2 can also considerably lower the oil viscosity and expand the oil volume, leading to a significant improvement of oil recovery [1,5–9]. Mean- while, as parts of CO 2 injected into the reservoirs could be seques- trated to effectively perform the geological storage of CO 2 in the reservoirs, it is also an attractive solution for the fighting with glo- bal warming aiming at mitigating CO 2 emission and accumulation [5,10–17]. In the petroleum industry, experimental studies and reservoir simulations on binary systems composed of CO 2 and hydrocarbon were implemented for investigation of improving hydrocarbon recovery [18–26]. Most of the researches focused on the oil swell- ing effect mainly caused by CO 2 preferably dissolved in the light fractions of oil and accurately quantified at reservoir conditions. David et al. [20] and Erdogan et al. [21] studied the change in the http://dx.doi.org/10.1016/j.fuel.2014.11.046 0016-2361/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +86 053286983366. E-mail address: zhangjun.upc@gmail.com (J. Zhang). Fuel 143 (2015) 194–201 Contents lists available at ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel