Water Alternating CO2 Injection Process - Does Modifying the Salinity of Injected Brine Improve Oil Recovery?

Abstract

In the recent past, IOR/EOR technologies have been modified to include the effects of salinity of injected brine on oil recovery. Water Alternating Gas (WAG) is one such tertiary oil recovery process in the field. It greatly improves the synergy of various individual mechanisms underlining EOR processes, such as ion exchange, wettability alteration, and mobility control. This process has matured over the years, but the alteration of the salinity of injected brine during WAG has not been experimented extensively. This paper investigates the effect of injected brine salinity during water-alternating-CO2 injection and compares its performance with standalone brine injection, using 20 in. length outcrop Grey Berea sandstone cores. The water alternating CO2 process was done under immiscible conditions. It also studies the effect of aging on the oil recovery during waterflooding process.In the present work, six coreflood experiments were performed: four experiments before aging the cores and two experiments after aging the cores. All of the experiments were done at 149°F. The evaluation of oil recovery and pressure drop were done for each coreflood. The effect of salinity on the rock wettability during waterflooding and water alternating CO2 processes was studied using axisymmetric contact angle measurements.The effect of salinity on the waterflooding process as well as water alternating CO2 injection process was observed through the coreflood experiments. Low-salinity brine gave the highest recovery during the waterflooding process in aged cores, and seawater brine gave the highest recovery during water alternating CO2 injection process. Wettability alteration towards a more water-wet state was the cause of improved oil recovery by low-salinity brine during the waterflooding process. The lower solubility of CO2 in seawater brine compared to the solubility of CO2 in low-salinity brine was the reason for the higher oil recovery by seawater brine during water alternating CO2 injection process. Fines migration was observed during low-salinity brine alternating CO2 injection process. Aging played an important role in the recovery of oil during the waterflooding process. There was greater oil recovery in aged cores compared to unaged cores. The salinity of brine affected the contact angle measurements of the Grey Berea sandstone rock. The rock was strongly water-wet in the presence of low-salinity brine. Seawater brine made the rock more oil-wet due to the ion binding nature of the divalent ions in seawater brine. The aging of the cores increased the contact angle of the rock, thereby making the rock more oil-wet.