Simulation of Oil Displacement from Oil-Wet Cores by Interfacial Tension Reduction and Wettability Alteration

Abstract

Waterflooding in oil-wet naturally fractured reservoirs is not successful because the ability of matrix blocks to imbibe the injected water and displace the oil into the fracture system is poor. Chemical enhanced oil recovery methods such as surfactant flooding are used in oil-wet naturally fractured reservoirs to enhance oil recovery. This method cannot be successfully implemented in the field unless all of the mechanisms involved in this process are fully understood. Surfactant can act in several ways to enhance the oil production in oil-wet systems. Lowering interfacial tension between oil trapped in small capillary pores and the water surrounding those pores may allow the oil to be mobilized via buoyancy forces. Altering the matrix wettability toward water-wet may increase spontaneous imbibition of water. This change in rock wettability leads to positive capillary pressure and results in higher brine counter-current imbibition and therefore a higher oil production rate. A three-dimensional, two-phase numerical simulator was developed which models the process of surfactant and/or brine imbibition in different rock wettability states. The simulator incorporates all mechanisms involved in the surfactant/brine imbibition, changing interfacial tension between oil and water and also altering rock wettability. The simulator considers that alterations in IFT and wettability lead to changes in relative permeability of phases, capillary pressure and residual saturations. The simulator was used to model pure brine imbibition into water-wet rocks. First, the results from the simulation showed consistency between the simulated results and those obtained from other modeling data in the literature. Second, the results demonstrated that both gravity and capillary forces can be important when the IFT is moderately low, and that there is a transition from capillary-dominated flow to gravity dominated flow as the IFT is reduced. In the limit of very low values of interfacial tension, the flow is completely dominated by gravity. Next, the numerical results also showed an excellent agreement with experimental data reported in the literature in modeling cases with different boundary conditions, which demonstrates the capability of the simulator. Also, the core orientation, either vertical or horizontal, plays an important role for oil recovery. A current available model in the literature (UTCHEM) for wettability alteration in oil-wet cores uses two extreme wetting conditions (water-wet and oil-wet) and interpolates between them to consider the wettability alteration mechanism in surfactant flooding of the reservoirs. We propose an alternative model with the capability to simulate oil recovery using chemicals to mechanistically alter wettability and lower interfacial tension. The proposed model of wettability alteration was tested against a number of experimental and simulation results. Excellent agreements between the simulation outcomes and experimental data from the literature were shown. The simulation of surfactant solution imbibition in laboratory scale cores showed that both interfacial tension reduction and wettability alteration play important roles in oil production from oil-wet systems. Even if the surfactants are not able to change the rock wettability toward a less oil-wet condition, gravity force can enhance oil production depending on the magnitude of the interfacial tension reduction between oil and water. The contributions of the present work will provide better guidelines in designing and improving waterflooding performance in oil-wet naturally fractured reservoirs.