It has been proposed that increased oil recovery in carbonates by modification of ionic composition or altering salinity occurs mainly at a temperature exceeding 70–80 °C. The argument was that elevated temperatures enhance adsorption of the potential determining ions which then modifies wettability to a less-oil-wetting state. According to this rationale, it becomes questionable if diluted brines or brines without these ions can be still applicable. Therefore, the aim of this paper is to investigate if the wettability alteration truly depends on temperature and if so how the trend with temperature can be explained. We followed a combined experimental and theoretical modeling approach. The effect of brine composition and temperature on carbonate wettability was probed by monitoring contact angle change of sessile oil droplets upon switching from high salinity to lower salinity brines. IFT measurements as a function of salinity and temperature along with extensive ζ-potential measurements as a function of salinity, pH, temperature, and rock type were conducted. Interaction potentials between oil and carbonate surfaces were estimated based on DLVO theory, and its consistency with oil-droplet data was checked to draw conclusions on plausible mechanisms. Three carbonate rocks (two limestones and one dolomite) were used along with two reservoir crude oils, high salinity formation water (FW), seawater (SW), and 25 times diluted seawater (25dSW) as low salinity (LS) brine. It was observed that (i) wettability alteration to a less-oil-wetting state can occur at ambient temperature for specific rock types and brines, and (ii) there is no univocal increase in response to SW and LS brine at elevated temperature. The largest improvement in wettability was observed for dolomite, while, among the limestones, only one rock type showed noticeable wettability improvement at elevated temperature with SW. The difference in behavior between limestones and dolomite indicates that the response to brine composition change depends on rock type and mineralogy of the sample. These observations are consistent with the ζ-potential trends with salinity at a given temperature. Dolomite generally shows more positive ζ-potential than limestones. However, even the two limestones react differently to lowering salinity and exhibit different magnitude of ζ-potential. Moreover, it is observed that, at a specific salinity, an increase in temperature leads to reduction of ζ-potential magnitude on both rock/brine and oil/brine interfaces toward zero potential. This can affect positively or negatively the degree of wettability alteration (to a less-oil-wetting state) at elevated temperature depending on the sign of oil/brine and rock/brine ζ-potential in SW/LS. The observed trends are reflected in the DLVO calculations which show consistency with contact angle trends with temperature and salinity. According to the DLVO calculation, the lack of response to SW/LS in some of the systems above can be explained by stronger electrostatic attractive forces und...

Summary Low-salinity waterflooding (LSF) is receiving increased interest as a promising method to improve oil-recovery efficiency. Most of the literature agrees that, on the Darcy scale, LSF can be regarded as a wettability-modification process, leading to a more-water-wet state, although no consensus on the microscopic mechanisms has been reached. To establish a link between the pore-scale and the Darcy-scale description, the flow dynamic at an intermediate scale—i.e., networks of multiple pores—should be investigated. One of the main challenges in addressing phenomena on this scale is to design a model system representative of natural rock. The model system should allow for a systematic investigation of influencing parameters with pore-scale resolution while simultaneously being large enough to capture larger-length-scale effects such as saturation changes and the mobilization and connection of oil ganglia. In this paper, we use micromodels functionalized with active clay minerals as a model system to study the low-salinity effect (LSE) on the pore scale. A new method was devised to deposit clays in the micromodel. Clay suspensions were made by mixing natural clays (montmorillonite) with isopropyl alcohol (IPA) and were injected into optically transparent 2D glass micromodels. After drying the models, the clay particles were deposited and stick naturally to the glass surfaces. The micromodel was then used to investigate the dependence of the LSE on the type of oil (crude oil vs. n-decane), the presence of clay particles, and aging. Our results show that the system is responsive to low-salinity brine as the effective contact angle of crude oil shifts toward a more-water-wetting state when brine salinity is reduced. When using n-decane as a reference case of inert oil, no change in contact angle occurred after a reduction in brine salinity. This responsiveness in terms of contact angle does not necessarily mean that more oil is recovered. Only in the cases where the contact-angle change (because of low-salinity exposure) led to release of oil and reconnection with oil of adjacent pore bodies did the oil become mobile and the oil saturation effectively reduce. This makes contact-angle changes a necessary but not sufficient requirement for incremental recovery by LSF. Interestingly, the wettability modification was observed in the absence of clay. Osmosis and interfacial tension (IFT) change were found not to be the primary driving mechanisms of the low-salinity response.

Low salinity waterflooding (LSF) is receiving increased interest as a promising method to improve oil recovery efficiency. Most of the literature agrees that on the Darcy scale, LSF can be regarded as a wettability modification process leading to a more water-wet state, although no general consensus on the microscopic mechanisms has been reached. While wettability alteration may be a valid causal mechanism also on the pore scale, it is currently unclear how oil that detaches from mineral surfaces within individual pores connects to an oil bank or finds its way to a producer. In order to establish a link between the pore scale and the Darcy scale description, the flow dynamic at the scale of (networks of) multiple pores should be investigated. One of the main challenges in addressing phenomena on this intermediate "pore network" scale is to design a model system representative for natural rock. The model system should allow for a systematic investigation of influencing parameters with pore-scale resolution whilst simultaneously being large enough to capture larger length scale effects like saturation changes and the mobilization and connection of oil ganglia. In this paper, we use micro-models functionalized with active clay minerals as model system to study the low salinity effect (LSE) on the pore scale. A new method was devised to deposit clays in the micro-model. Clay suspensions were made by mixing natural clays (Montmorillonite) with isopropyl alcohol (IPA) and injected into optically transparent 2D glass micro-models. By drying the micro-model, the clay particles are deposited and stick naturally to the glass surfaces and remain attached even under flow of high salinity (HS) and low salinity (LS) brines. In a parametric study the dependence of the LSE on the type of oil (crude oil versus n-decane), the presence of clay particles and ageing was investigated. Our results show that the system is responsive to LS brine as the effective contact angle of crude oil shifts towards a more water-wetting state when brine salinity is reduced. When using n-decane as a reference case of inert oil, no change in contact angle occurred after a reduction in brine salinity. This responsiveness in terms of contact angle does not necessarily mean that more oil is recovered. Only in the cases where the contact angle change (due to low salinity exposure) led to release of oil and re-connection with oil of adjacent pore bodies, the oil became mobile and oil saturation was effectively reduced. This makes contact angle changes a necessary but not sufficient requirement for incremental recovery by LSF. Interestingly, the wettability modification was observed in absence of clay. Osmosis and IFT were found to be not the primary driving mechanisms of the low salinity response.