Low Salinity Watertlooding: Effect of Fines Migration and Ion Type on Oil Recovery

Abstract

This is a PhD thesis by publication that includes six published papers, five of which are journal papers and one is a full manuscript conference paper. The goal of this thesis is to investigate the effect of low salinity waterflooding (LSW) and salt ion type on enhancing oil recovery, which is implemented to select LSW reservoir candidates for a Wintershall Holding project. In addition, another aim of this thesis is to design a set of criteria to plan two-phase corefloods to accurately determine relative permeability by using Welge-JBN method. Low salinity water injection in oil fields has gained wide interest in the literature over the last two decades due to the fact that it is a cost-effective enhanced oil recovery technology. However, not only the mechanisms of LSW are not clearly understood, but there is some controversy around some LSW phenomena, especially fines migration, which is deemed to have the detrimental impact of formation damage in oil and gas reservoirs. This thesis focuses on the fines-migration mechanism of LSW and shows that it can be utilised to produce incremental oil using the induced formation damage in the reservoir. This study shows that micro-scale sweep efficiency is improved during low salinity waterflooding by flux diversion, that is caused by fines detachment and migration. Initially, clay particles are attached to the rock surface by electrostatic forces caused by the initial high-salinity formation water that saturates the rock. In this work, it is shown that injecting low salinity brine into the rock causes clay particles to be detached due to the weakening of electrostatic forces. As a result, fines migration results in blockage of high permeability water channels during high salinity water injection and diversion of the water flux to thin pores where residual oil is trapped. The results indicate that residual oil saturation was decreased by 5-18% in multiple low salinity coreflooding experiments with different salinity concentrations. Another part of this study investigates the effect of brine ion type on fines migration and oil recovery during LSW. It is demonstrated that having divalent ions such as calcium in the initial formation water, and the water injected into the porous media (including LSW), aids to stabilise fines due to the strong affinity and adsorption of such ions on the clay and rock surface. Deionised water injection confirms this, as hydrogen ions cannot exchange with calcium ions. This is confirmed by the fact that there are no clay particles in the effluent solution, no rise in pressure drop across the samples, and no detection of desorbed calcium ions in the Ion Chromatography results. Injection of low salinity sodium chloride solution, followed by deionised water flooding, induced desorption of the calcium ions, which then enabled clay particles to detach as a result of the weak electrostatic forces between clay and rock surface, that is caused by the sodium. This is important as it can be applied in controlling formation damage programs and preventing injectivity/production issues in oil and gas wells. Furthermore, enhanced oil recovery can also be achieved as proven by the incremental oil production observed when fines migration takes place in the two-phase flow tests due to the improved micro-scale sweep efficiency as explained above in the first part of the thesis. Moreover, a new set of criteria for coreflooding parameters to model relative permeability, for the experimental tests performed in this study, is introduced in this thesis and can, also, be applied in any two-phase flow experiments. These criteria are essential as they are needed for valid determination of relative permeability by the Welge-JBN method. They fulfil the assumption of low capillary-viscous ratio to achieve a large-scale approximation by optimising the core length and displacement rate. The numerical simulation results demonstrate that this ratio should not exceed 0.5 for the model to have valid relative permeability calculations by the Welge-JBN technique. The criteria include capillary number, precision of water-cut measurement, sampling period, and pressure measurement accuracy, which are critical to plan any coreflood tests to achieve accurate results.