The effect of wetting on the relative permeability behavior and oil recovery

Abstract

Oil is one of the major contributors to energy consumption. Oil reserves are expressed as the total amount of economically and technically producible oil. Total oil consumption is increasing (per capita it remains more or less the same) but it has no unambiguous influence on the remaining reserves due to new discoveries. However, increasingly sophisticated recovery methods are used to produce oil i.e. enhanced oil recovery methods. A recently proposed enhanced oil recovery method is by injection of low-salinity water in completely oil-wet reservoirs, which leads to more water-wet behavior and “consequently” to improved oil recovery.
This report will focus on the effect of wettability on the recovery efficiency. Based on the papers by Lomeland, Ebeltoft and Thomas we derive so-called LET relative permeability curves that only depend on irreducible water saturation. This is possible by using the irreducible water saturation dependence of the residual oil saturation, end point relative water permeability and the sketched behavior (Lomeland, Ebeltoft and Thomas) of the other six parameters. Admittedly this is a gross simplification, but it grasps the essence of the relative permeability behavior and makes it possible to study the recovery in terms of two parameters viz. the irreducible water saturation and viscosity ratio as opposed to eight relative permeability parameters and the viscosity ratio. High irreducible water saturation is both indicative of pore size heterogeneity and water-wet behavior. We use the theory of Buckley-Leverett to construct recovery curves for 1D and 2D displacement and various mobility (M = displacing fluid mobility / displaced fluid mobility) ratios. We solve the 1-D equations both analytically (using fractional flow theory) and numerically. For the numerical simulations in 1D and 2D we use COMSOL 5.2©. The simulations show that water-wet behavior is conducive to stable displacement, however, low recovery at breakthrough, whereas intermediate oil-wet behavior is more unstable but conducive to high ultimate recoveries. Completely oil-wet behavior leads to less stable displacement and low ultimate recoveries.