Pore-scale Visualisation and Geochemical Modelling of Low Salinity Water Flooding as an Enhanced Oil Recovery Method

Abstract

Low salinity water (LSW) injection is a promising technique for improving oil recovery from reservoirs. The salinity ranges from 1000 to 2000 ppm which is significantly less than the salinity of the formation brine. However, the mechanisms underlying the Low Salinity Effect (LSE) are not well understood. To investigate the dynamics of displacement, clean and clay coated micromodels were fabricated and used where the wettability was set to both water-wet and oil-wet. This allows the visualisation of pore-scale displacement mechanisms in the presence and absence of kaolinite and illite. It is observed that in water-wet systems, in the absence and presence of clays, LSW hinders “snap-off,” perhaps due to the development of a viscoelastic water/oil interface. There was a transition from snap-off to piston-like displacement in much of the volume of each pore and the amount of trapped oil was decreased by almost 10%. The wettability alteration toward water-wetness is also visualised for oil-wet systems which resulted in 15% improved recovery in oil-wet porous medium. Throughout LSW injection microfluidic experiments after the long-term aging period with oil, fines migration was insignificant. In another set of experiments, when the systems were flushed with LS brine without any delay, fines migration was significant. These observations suggest that the aging time and crude oil composition are contributing factors to fines release. Also analysed is the impact of type of clay mineral and nature of cations on LSE, using geochemical modelling complemented with visualisation experiments. The models are developed to provide the charge density and electrostatic potential profiles (the ζ-potential) at the brine/mineral interface of illite as a basal-charged clay against an edge-charge dominated kaolinite. Based on the observations and simulation results, it was concluded that since kaolinite has higher density of acid/base reaction sites on its surface (edges) than does illite, the charge development under differing ionic strength and pH was significant. As a result, kaolinite responded well to LSW injection, whereas illite did not. This could elucidate the unlike sensitivities of clays to LSW, such as their extent of being prone to wettability alteration. The SCM results agree well with the microfluidic results when DLE is considered the mechanism behind LSE. The extent of the ζ-potential calculated by SCMs shows the degree of electrostatic repulsion between adjacent, similarly charged surfaces, that is, the brine− oil and brine−clay interfaces, so the enhanced repulsion forces weakened the oil and clay pinning points. As a result, wettability altered toward more water-wetness, which is confirmed by contact angle measurements. Once the attraction forces were low enough, the oil was released from the clay surface and replaced by water, as observed in microfluidic experiments. If the chemical composition of connate water and the content of clay minerals in a particular reservoir can be determined to a degree, the findings of this study will determine the concentration of ions and pH of the injecting water that maximise LSE (wettability alteration) and the resulting oil recovery.