Abstract
One main mechanism for Low Salinity Water Flooding (LSWF) is the change of reservoir wettability from oil or mixed wet to more and more water wet, resulting in higher oil recovery, in the process. If, however, the injected water is switched back to sea water due to economic or operational issues such as combining LSWF with other processes such as miscible flooding, polymer flooding, or foam injection, will a wettability reversal occur thereby halting the improvement in the oil recovery? This paper is based on the belief that the wettability change is permanent. It accordingly presents a novel technique to modelling irreversible wettability during LSWF and demonstrates its effect on the oil recovery.
Wettability alteration is modelled by interpolation between multiple sets of relative permeability (Kr) curves corresponding to different wettability states of the reservoir. The interpolating parameter used is dependent on the reservoir lithology and the mechanism modelled. In carbonate reservoirs, the mechanism for wettability alteration is still under investigation, but one theory is that anion exchange with decreasing SO4−− concentration in the injected brine changes the wettability to increased water wetness and improves the oil recovery. Chemical reactions could be used to represent this ion exchange and the resulting wettability alteration. These reactions are reversible, in nature, and any subsequent increase in the salinity of the injected water leads to wettability reversal.
To model irreversible wettability, a hypothetical aqueous component (WAT_INT) is introduced. This component, having the same properties as water, is generated from water by a reaction catalyzed by a salinity component (Sulphate) and serves as a tracer that tracks the salinity.
This reaction is modeled as a partial equilibrium reaction that deviates from equilibrium. This causes the reaction rate to be directly proportional to the difference between the sulphate concentration and its specified equilibrium composition. The reaction rate increases when the sulphate concentration decreases below this equilibrium concentration, and it does not occur when the sulfate concentration is higher than this equilibrium concentration. Setting the equilibrium sulphate concentration to be very close to, but slightly higher than that in the low salinity water ensures that the reaction occurs only when the low salinity slug passes through. When the injected fluid is switched to the sea water having higher salinity, thisreaction ceases to occur. This results in the maximum concentration of this component when the low salinity slug is injected.
An irreversible adsorption isotherm is modeled for the new WAT_INT component, and its adsorbed phase concentration is used as the interpolating parameter. This parameter reaches its maximum value when the low salinity slug passes through. The irreversible adsorption isotherm ensures that it remains at its maximum value when the injection water is switched to sea water/formation water. It also ensures that the wettability alteration is irreversible and relative permeability does not switch back to a more mixed/oil wet value.
Operators typically inject LSW continuously to prevent a wettability reversal from occurring if the composition of the injected water is altered. This study, however, presents the idea that reversal of wettability may not occur, and potentially allowing for the injection of formation brine/sea water after a slug of LSW. In addition, this paper presents a novel technique to model the concept of irreversible wettability during LSWF.
