Wettability Studies Using Low-Salinity Water in Sandstone Reservoirs
Authors: Alotaibi Mohammed B. , Azmy Ramez Masoud , Nasr-El-Din Hisham A. 
Published: July 2010

Abstract
It has been reported that the ionic strength of injection water can have a major impact on the recovery of hydrocarbons during waterfloods, with increased recovery resulting from the use of low salinity brines. Understanding how the water and oil chemistry affects the final recovery from a physicochemical point of view is necessary in order to optimize low salinity water flooding process. It is clear from the literature over the last two decades that wettability is considered a key factor in achieving the low salinity effect. Optimum ionic strength and conditions for low salinity flood with respect to wettability is still uncertain.
In this study, we studied oil/water/rock interactions at different salinity level and elevated temperature conditions. Wettability is determined by a HTHP contact angle method and zeta potential technique. Outcrop rocks and stock-tank crude oil sample are utilized in all experiments. Synthetic formation brines, aquifer, and seawater are evaluated under high pressure conditions. We determined zeta potential of sandstone rocks and selected clays minerlas as a function of ionic strength. Wettability of oil-brine-sandstone systems depends on the brines salinity, temperature, rock nature and minerlogy. Aquifer water decreased the Berea sandstone wettability toward strong water wet condition. But, different sandstone rock showed opposite result completely. In Scioto sandstone, aquifer water enhanced the wettability to neutral state. The optimim salinity for this rock was seawater. We observed direct relationship between the zeta potential and ionic strength. High salt ions in the seawater showed excellent stability for sandstone particles and others clay minerals. The zeta potential moved too close to zero point of charge after using only the seawater.

Introduction
Wettability is considered a key factor that affects fluids distribution in a porous medium. Reservoir wettability depends strongly on oil composition, surface chemistry of the rock, and presence or absence of the other aqueous phase. In addition, the composition of the aqueous phase, temperature, pressure, and contact time can be considered important parameters. Any wettability adjustment will affect capillary pressure, relative permeability, and waterflooding behavior. Recently, the ionic strength of the injected water proved to significantly improve oil recovery in many sandstone fields (Morrow et al., 1998; Zhang et al., 2007; Alotaibi and Nasr-El-Din, 2009a; Agbalaka et al., 2009). However, the reaction mechanism is still not well understood. Therefore, wettability alteration is believed to be a main factor behind this mechansim. For that reason, we decided to study the effect of low salinity water on the contact angle, and ultimately on oil recovery. In all contact angle experiments, reservoir temperature and pressure conditions were simulated.
Sandstone rock has negative surface charges; therefore, monovalent and divalent cations will interact with rock surface and fluids in the reservois. Hence, this complex interaction will disturb the charges stability. We measure zeta potential to examine the effect of ionic strength on the surface charge of various clays. Sandstone rocks and several clays minerals (illite, kaolinite, chlorite, and montmorillonite) were tested at ambient temperature (77ºF).

Literature Review

Contact angle.
Researchers are using contact angle methods to evaluate the wetting characteristics of solid surfaces. This method is usually applied by using a small piece of rock and two immiscible fluids. To avoid any hystereies issue, the rock surface should be smoothed and well polished.