Enhanced Oil Recovery Methods targeting Danish North Sea Chalk Reservoirs

In this thesis, the impacts of injecting water composition on the wetting tendency of the carbonate rock surface and the role of reactive transportation of ionic species on the displacement of two phases of oil and water in homogenous and heterogeneous porous media are investigated. In chapter 1, a short introduction about the past endeavours on the context of modified salinity water flooding (MSW) is presented. In chapter 2, the effect of water salinity and ionic composition on the wettability of the carbonate rock surface is studied. The wetting preference of carbonate rock surface is estimated using the extended DLVO theory through the calculation of surface forces and energies
induced by structural, van der Waals and electric double layer forces. For the estimation of structural and van der Waals forces, the empirical equations are employed. Also, for the electrical double layer force, the Poisson-Boltzmann equation for dissimilar surface polarity surrounded by a general electrolyte solution is solved numerically. These analyses are performed for different individual salts to elucidate the impact of each salt on the wetting behaviour of the rock surface. Through estimation of disjoining pressure, the wettability of carbonate rock is predicted by the calculation of contact angle for an ideal solid surface. The results suggest that while increasing the concentration of salts such as MgCl₂, MgSO₄, and CaCl₂ results in more water-wet conditions, other salts like NaCl, KCl, and Na₂SO₄ leads to less water-wet conditions.

In Chapter 3, the behaviour of modified salinity water injection during secondary and tertiary modes in forced flooding tests are simulated. Previous investigations for mathematically modelling of forced MSW flooding include complex reactive transport models and an indicator for the mobility alteration of phases. Because of several fitting parameters, almost all proposed models can reasonably fit a limited set of core-flooding recovery data, which makes the choice of physical mechanisms for the development of a mechanistic model irrelevant. To address this issue, the transport of the two phases (e.g. oil and water) in the porous medium is analysed through the engagement of Buckley-Leverett frontal advance theory combined with the reactive transport of ionic species in carbonate porous medium. Instead of matching only the recovery factor and pressure drop history, a higher priority for matching the different ion concentrations and oil breakthrough times is given. The obtained results show that the oil breakthrough time can be correctly predicted by considering the wettability alteration due to the adsorption of potential determining ions on the carbonate surface.

The success of MSW flooding in the improvement of oil recovery both in core and oil reservoir scales depends on the activation of the physicochemical interactions at the locations where the electrochemical equilibrium condition has not been disturbed before the injection of MSW. Usually, the rock surface is covered with the residual oil that is left after the conventional water flooding. Therefore, the potential determining ions need to diffuse into the thin water film between the mineral surface and the residual oil to change the wettability of the rock surface. To study the contribution of wettability change in the area covered by the residual oil, the wettability modification at thin water
film domain was linked to the mobility of the phases in Darcy domain in Chapter 4. The results show that the combination of ionic adsorption on the rock surface and ionic diffusion process in the water films control the onset and acceleration of extra oil production during MSW flooding.

Chapter 5 describes the impact of heterogeneity on the performance of MSW flooding in the low permeable reservoir. The analysis of oil breakthrough time in a homogenous system that is discussed in previous chapters shows that the oil production in secondary and tertiary mode occurs with a delay due to the transport retardation of ionic species. Obviously, this effect is economically unfavourable. However, the geological and petrophysical properties of oil filed reservoirs are considerably different from the homogenous system. For this chapter, the size of the delay, caused by ionic adsorption, in oil breakthrough time for various heterogeneous systems are examined using commercial reservoir simulator Eclipse 100. The obtained results confirm that as the level of heterogeneity increases, the adverse effect of ionic adsorption on oil production decreases.