Pressure-transient analysis of CO 2 flooding based on a compositional method Longlong Li a , Jun Yao a, * , Yang Li a, b , Minglu Wu a , Lei Zhang a a School of Petroleum Engineering, China University of Petroleum, Qingdao, 266580, China b Department of Oilfield Exploration and Development, Sinopel, Beijing, 100029, China article info Article history: Received 23 December 2015 Received in revised form 27 April 2016 Accepted 29 April 2016 Available online 3 May 2016 Keywords: CO 2 flooding Compositional model Monitor Transient flow abstract To find a reliable and inexpensive method to estimate miscibility and other reservoir parameters, as well as to monitor the CO 2 flood progress, this paper proposed a transient flow model based on a composi- tional model that considers the wellbore storage effect, skin factor and multiple-contact processes. The model solution is obtained with the finite volume method and by performing pressure transient analysis. The results demonstrate that the pressure derivative curve of the swept area radial flow rises first and later declines slightly; the pressure derivative curve of the unswept area radial flow rises first and later becomes flat. The CO 2 flood front can be recognized when the derivative curve begins to rise after it declines slightly in the swept area radial flow regime. A numerical well test interpretation method can be established based on the model used in this paper to estimate the permeability, miscibility, and for- mation damage and to monitor the CO 2 flood progress. © 2016 Elsevier B.V. All rights reserved. 1. Introduction CO 2 Enhanced Oil Recovery (EOR) is the green way to produce oil as it can recover the stranded or trapped oil by injecting the carbon emissions (CO 2 ) from fossil-fueled power plants into the reservoir which can mitigate the greenhouse effect simultaneously (Gozalpour et al., 2005; NETL, 2010, 2006; Malik and Islam, 2000; Holtz et al., 2001; Jaramillo et al., 2009). This proven method has promising prospects as a result of technological advances, eco- nomic improvements, and environmental needs, and a study (NETL, 2011) conducted by DOE found that Next Generation CO 2 EOR can provide 137 billion barrels of additional technically recoverable domestic oil. As the breakthrough of gas usually occurs, which leads to lower oil recovery and failure of the CO 2 storage, useful methods should be found to monitor the CO 2 flood progress and estimate the miscibility. The most common method is seismic monitoring (Araman et al., 2008; Kendall et al., 2003; Terrell et al., 2002), as Time-Lapse and Multicomponent seismic data analysis is an effective tool for monitoring CO 2 injection through the detection of changes in reservoir properties, such as porosity and fluid distri- bution. Although this method has been demonstrated to be accu- rate, it is expensive and not convenient enough to be used frequently. Another method is the Material Balance Equations (MBE) model (Tian and Zhao, 2008), which is convenient to use and inexpensive, but it is hard to obtain accurate results with this method, as it has many simplifications and assumptions. Well testing technology is commonly (Lee, 1982; Ozkan, 2001; Stratton, 2005, 2006; Zheng and Corbett, 2005; Yao and Wu, 2009; Fan et al., 2015) used to estimate the reservoir parameters and is a promising method to solve the above problem. MacAllister (1987) presented a procedure based on a three region composite model to analyze the CO 2 and enriched-gas injection and produc- tion wells with emphasis on the real gas pseudo approach. Tang and Ambastha (1988) presented a three region analytical radial com- posite model to analyze the CO 2 pressure transient; Su et al. (2015) established a three-region composite transient pressure analysis model for CO 2 flooding, which considered the skin and storage effects of the well. Although the analytical composite model can describe the trend of the pressure change, it used too many sim- plifications and assumptions, which led to imprecise estimation of the parameters. In order to solve the above problem, an accurate model should be built and subsequently used to analyze the pressure transient of CO 2 flooding. This paper describes a transient flow model that is * Corresponding author. E-mail addresses: bzlilonglong@163.com (L. Li), RCOGFR_UPC@126.com (J. Yao), liyang@sinopec.com (Y. Li), w97612@163.com (M. Wu), zhlei84@163.com (L. Zhang). Contents lists available at ScienceDirect Journal of Natural Gas Science and Engineering journal homepage: www.elsevier.com/locate/jngse http://dx.doi.org/10.1016/j.jngse.2016.04.062 1875-5100/© 2016 Elsevier B.V. All rights reserved. Journal of Natural Gas Science and Engineering 33 (2016) 30e36