Reservoir Sandstone Wettability in Relation to Injection Water Salinity and Reservoir Temperature
Authors: Neog Dhrubajyoti , Rahman A. S. , Borgohain Pradip 
Published: February 2024

The efficacy of low-salinity water flooding is attributed to various factors, including rock mineralogy, reservoir fluid composition, and the temperature and salinity of liquid-liquid and liquid-solid interactions. The objective of the proposed study is to examine the influence of injection water salinity on reservoir rock wettability as well as the functions fulfilled by monovalent and divalent cations derived from salt solutions in the water flooding procedure. The present study employs the sessile drop method to measure contact angle, enabling crude oil-reservoir rock interaction at varying formation water salinities. Contact angle measurements were taken at two temperatures representative of the reservoir temperatures of two wells in the upper Assam basin, India. The initial phase of the experiment involved the characterization of the porous medium responsible for crude oil production as well as the reservoir fluids. Subsequently, synthetic salt solutions with varying salinities were prepared. The alteration in wettability was then analyzed in relation to the salinity levels of the injected water at two distinct reservoir temperatures, namely 80°C and 100°C. The study was conducted in reference to an oil-saturated core located in the upper Assam basin. The results obtained were compared to draw conclusions regarding the effect of temperature and salinity on the wettability of reservoir rock. The results of the sessile drop method were further analyzed using the pendant drop method, with interfacial tension (IFT) estimation for liquid-liquid interaction.

The experimental study conducted on oil-saturated cores has yielded significant findings. It has been observed that, at a temperature of 80 °C and a salinity range of 500–7000 ppm in a sodium chloride (NaCl) solution, the contact angle increases with an increase in salinity. However, this trend deviates at 100 °C for salinities of 5000 ppm and 7000 ppm NaCl solutions. Similarly, the wettability measurement with contact angle estimation for calcium chloride (CaCl2) solutions at a low reservoir temperature of 80 °C also displayed an increasing trend of increasing contact angle with an increase in salinity. However, this trend deviates when the salinity of CaCl2 salt solutions is increased beyond 3500 ppm at an increasing temperature of 100 °C. The findings demonstrate that salinity and the effect of temperature on wetting properties are significant. Further analysis with interfacial tension (IFT) estimation infers that both contact angle and IFT are reduced with increasing temperature for interactions between the liquid and solid phases, as well as between liquid phases. Based on the results, it can be concluded that the wettability of sandstone rocks varies with salinity and temperature. Higher water-wetting properties are obtained when the temperature of interaction is increased for low-salinity brine solutions, irrespective of whether monovalent or divalent cationic brine solutions are used. Both the contact angle and IFT decrease within a certain range of temperature and salinity values, which facilitates higher oil recovery with increasing temperature.