A new approach for determination of carbonate rock electrostatic double layer variation towards wettability alteration

Abstract

Focus on the molecular scale of low salinity brine effects seems to be necessary to clarify the possible mechanisms of carbonate rock wettability alteration. This study aimed at investigating the effects of different operational parameters such as pH, salinity and types of ions on wettability alteration (WA) during low salinity brine injection as a tertiary recovery process. In the present study, using the electrostatic double-layer concept and molecular behavior, along with a good view of the past studies, led to the justification of the obtained results. The studied reservoir is one of the Middle East carbonate reservoirs which is considered for low salinity brine injection plan. Although the concept of ζ potential is ill-defined in electrostatic or/and electrokinetic topics, nonetheless, to the extent possible, has been tried to use this concept to illustrate and justify underlying mechanism during low salinity process beneficially.Resultant findings have shown that pH value directly affects carbonate rock powder ζ potential and determine the type of surrounding ions that form Stern layer and ion distribution in diffuse layer as well. A close examination at two temperatures of 25 °C and 80 °C, showed that active ions in WA e.g. Ca2+ and Mg2+ and SO42− as an agent facilitator to access active ions to the rock powder surface, have profound impacts on the Stern layer. The obtained optimal ions concentrations, (i.e., 1000 ppm sulfate + 1000 ppm magnesium @ 80 °C) and (500 ppm calcium and 800 ppm sulfate @ 25 °C) have changed the ζ potential of oil wet rock powder in Seawater form −23.6 mV to −6.71 mW and −8.7 mV respectively, using a new designed procedure. In the present study, influence of ion valence on the electrostatic double layer variation of an oil-wet rock powder has been led to directly link this alteration to the wettability state changes. Therefore, this approach could be very helpful to monitor the wettability state of the rock powder in molecular scale.