Enhanced oil recovery from high-temperature, high-salinity naturally fractured carbonate reservoirs by surfactant flood Jun Lu n , Ali Goudarzi, Peila Chen, Do Hoon Kim, Mojdeh Delshad, Kishore K. Mohanty, Kamy Sepehrnoori, Upali P. Weerasooriya, Gary A. Pope Department of Petroleum and Geosystems Engineering, The University of Texas at Austin, Austin, TX 78712-0228, USA article info Article history: Received 23 July 2014 Accepted 17 October 2014 Available online 28 October 2014 Keywords: chemical EOR surfactant flooding naturally fractured carbonate reservoirs abstract Water floods are often very inefficient in naturally fractured carbonate oil reservoirs because many of these reservoirs are mixed-wet or oil-wet as well as extremely heterogeneous. Naturally fractured reservoirs are challenging targets for chemical flooding because they typically have a high permeability contrast between the fractures and the matrix with a low matrix permeability. Some of the world's largest oil reservoirs are fractured carbonates with a high reservoir temperature and a high salinity formation brine. Some of them also have low API gravity oils, which also increases the difficulty of recovering the oil. A surfactant formulation has been developed that shows promising results for such difficult reservoirs. Ultra-low interfacial tension (IFT) and good aqueous stability were achieved with this new carboxylate surfactant in a hard brine at a high reservoir temperature of 100 1C. Both static and dynamic imbibition experiments were conducted using a fractured carbonate core. 65.9% Oil recovery was obtained in fractured coreflood compared to 33.3% oil recovery in static imbibition test. The surfactant retention was low at 0.086 mg/g of rock. The oil recovery is excellent taking into account that the temperature and salinity conditions were harsh, the core was extremely vuggy and fractured, no mobility control was used, and only a small surfactant slug was injected. The coreflood results were interpreted using a mechanistic chemical reservoir simulator. It showed that both the mechanisms of IFT reduction and wettability alteration were important for oil recovery. Neither IFT reduction nor wettability alteration alone recovered oil as high as the combined contributions from both. & 2014 Elsevier B.V. All rights reserved. 1. Introduction Carbonate reservoirs hold approximately 60% of the world's oil reserves (Akbar et al., 2000). A large number of carbonate reservoirs are naturally fractured and are mixed-wet to oil-wet (Roehl and Choquette, 1985; Chillenger and Yen, 1983). Most of carbonate reservoirs have a high degree of heterogeneity and complex pore structure. Naturally fractured carbonate reservoirs typically have high permeability fractures and a low permeability matrix. This high contrast of permeability between matrix and fractures leads to poor water flood efficiency. The oil recovery from naturally fractured carbonate reservoirs is typically much less than one-third. Wettability has been long recognized as an important factor strongly affecting oil recovery using EOR methods (Zhou et al., 2000; Morrow and Mason, 2001; Tong et al., 2002; Hirasaki and Zhang, 2004). Water floods are often inefficient because many of these reservoirs are mixed-wet or oil-wet as well as extremely heterogeneous. Changing the wettability of the fractured reservoirs from oil or mixed-wet toward water-wet improves oil recovery efficiency. A lot of research has been conducted on wettability alteration by surfactants (Austad et al., 1998; Zhang et al., 2004; Seethepalli et al., 2004; Xie et al., 2004; Chen and Mohanty, 2013; Sharma and Mohanty, 2013; Sagi et al., 2013; Wang and Mohanty, 2014; Bourbiaux et al., 2014). Static imbibition experiments have been widely used to eval- uate different EOR surfactants. The recovery from fractured carbo- nate reservoirs is frequently considered to be dominated by gravity and capillary forces. However, the role of viscous forces may also be important and should be investigated (Delshad et al., 2009). The Marangoni effect (Austad and Milter, 1997) and spontaneous emulsification (Zhang et al., 2008) might also promote imbibition in some static imbibition experiments. Goudarzi et al. (2012) have suggested that changing the matrix block size affects the oil recovery from static imbibition experiments. Imbibition experiments using surfactants that produce low IFT have been conducted by several investigators (Hirasaki and Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/petrol Journal of Petroleum Science and Engineering http://dx.doi.org/10.1016/j.petrol.2014.10.016 0920-4105/& 2014 Elsevier B.V. All rights reserved. n Corresponding author. E-mail address: junlu@utexas.edu (J. Lu). Journal of Petroleum Science and Engineering 124 (2014) 122–131