Modeling the Combined Effect of Injecting Low Salinity Water and Carbon Dioxide on Oil Recovery from Carbonate Cores
Authors: Al-Shalabi Emad Waleed , Sepehrnoori Kamy , Pope Gary Arnold 
Published: May 2016

Abstract
This paper investigates the combined effect of injecting low salinity water (LSWI) and carbon dioxide (CO2) on oil recovery from carbonate cores. The combined effect of LSWI and CO2 injection on oil recovery was predicted by performing several 1D simulations using measured reservoir rock and fluid data. These simulations included the effect of salinity on both miscible and immiscible continuous gas injection (CGI), simultaneous water-alternating-gas (SWAG), constant water-alternating-gas (WAG), and tapered (WAG). For SWAG and constant and tapered WAG, both seawater and its dilutions were simulated. CO2 was injected above its minimum miscibility pressure. Baker's three-phase relative permeability model was modified to account for the effect of salinity on the water/oil relative permeability.
The results show that SWAG, whether using seawater or its dilutions, outperformed all other tertiary injection modes in terms of oil recovery. Moreover, the SWAG process has both the highest tertiary recovery factor (TRF) and the lowest utilization factor (UF). This study highlights the advantage of using low salinity water along with miscible CO2. The miscible CO2 displaces the residual oil saturation whereas the low salinity water boosts the production rate by increasing the oil relative permeability through wettability alteration towards more a water-wet state. The latter finding was supported by comparing our simulations with the two corefloods reported by Chandrasekhar and Mohanty (2014). These corefloods were conducted in SWAG tertiary mode using seawater and its dilutions. Fractional flow analysis shows that SWAG with low salinity water requires less injected solvent compared to SWAG with seawater and miscible CGI.

Introduction
One of the emerging improved oil recovery (IOR) techniques for wettability alteration in carbonate reservoirs is low salinity water injection (LSWI). The popularity of this technique is due to its high efficiency in displacing light to medium gravity crude oils, ease of injection into oil-bearing formations, availability and affordability of water, and lower capital and operating costs. CO2 miscible flooding is a well established commercial enhanced oil recovery (EOR) method to recover light crude oils from carbonate formations. The purpose of this study was to explore the combined benefits of these two processes.

Low Salinity Water Injection (LSWI).
The low salinity water injection IOR technique is also known in the literature as LoSalTM by BP, Smart WaterFlood by Saudi Aramco, Designer Waterflood by Shell, and Advanced Ion Management (AIMSM) by ExxonMobil. Several laboratory studies have been performed using low salinity water injection in carbonates (Hognesen et al., 2005; Webb et al., 2005; Zhang et al., 2007; Gupta et al., 2011; Yousef et al., 2011, 2012a; Zhang and Sarma, 2012; Chandrasekhar and Mohanty, 2013). Most studies have confirmed a positive response to low salinity injection, which is translated into additional oil recovery in both secondary and tertiary injection modes. The first ever LSWI field application in carbonate reservoirs was reported by Yousef et al. (2012b). Two single well chemical tracer tests (SWCTT) were applied in an Upper Jurassic carbonate reservoir using a diluted version of Qurayyah seawater. The tests resulted in about 7 saturation units reduction in the residual oil beyond conventional seawater injection. The results obtained were in match with their previous experimental work which is encouraging to plan a multi-well demonstration pilot.