New Insights into the Low Salinity Water Injection Effect on Oil Recovery from Carbonate Reservoirs
Authors: Al Shalabi Emad W. , Sepehrnoori Kamy , Delshad Mojdeh 
Published: May 2016

Low salinity water injection (LSWI) is gaining popularity as an improved oil recovery technique on account of being cost effective compared to other water based enhanced oil recovery methods such as chemical and steam flooding. In this paper, the wettability alteration option in our in-house simulator is used to history match and provide some insights in different seawater dilution cycles based on recently published corefloods. Two newly proposed methodologies to model dilution cycles are employed. We successfully modeled the experiments enhancing the wettability alteration model in the simulator using two different scaling factors. The study also revealed that the process is more sensitive to oil relative permeability compared to that of the water phase. A linear interpolation model for residual oil saturation (Sor) was proposed.

Introduction

Wettability alteration has the potential to produce oil from highly fractured carbonate reservoirs with mixed-wet to oil-wet and low permeability matrix and high fracture density. Most of the experimental work shows that the wettability alteration mechanism is believed to be the main reason for incremental oil recovery by low salinity water injection. The LSWI technology is believed to be cost effective compared to chemical and thermal methods. The low salinity waterflood requires a low salinity source of water or desalination plant compared to conventional waterflood projects.

Several research studies have been performed in the LSWI area at laboratory-scale and to a limited extent at field-scale on both sandstone and carbonate rocks. Studies have confirmed a positive response of LSWI, which is expressed in additional oil recovery in both secondary and tertiary modes. Experimental and simulation works are still progressing to understand the chemical interactions between crude oil/brine/rock (COBR) in the rock system. A review of LSWI in carbonate and sandstone formations highlights the differences between these two rocks.

Almost all producing sandstone rocks contain clays as a coating on individual sand grains and/or discrete particles mixed with the sand. Carbonates may contain clays as well, but these clays are usually encapsulated in the rock matrix and not significantly affected by the invading fluids (Alotaibi, et al., 2009). Low salinity water injection disturbs the initially established thermodynamic equilibrium between rock/oil/formation brine, which leads to a new equilibrium between the different phases, resulting in favorable wettability alteration and increase in oil recovery during production period. In this process of wettability alteration, the activation energy is important as it controls the rate of chemical reactions between the mineral surface and injected water. If the reaction rate is too slow, there will be no improvement in wettability and oil recovery during the water injection period. Reservoir temperature plays a catalytic role in increasing the rate of chemical reactions as there is a strong relation between activation energy and temperature (Puntervold et al., 2007).