Low Salinity Flooding (LSF) in Sandstones at Pore Scale: Micro-Model Development and Investigation

Abstract

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.

Novel Insights into the Pore-Scale Mechanism of Low Salinity Water Injection and the Improvements on Oil Recovery

“…The possible role of crude oil and fluid–fluid interactions during LSWI in carbonate and sandstone reservoirs had been underestimated until microdispersion formation was introduced as the main responsible mechanism of LSWI . The microdispersion theory could support the extra oil recovery during LSWI regardless of rock type, which was a remarkable discovery for the oil and gas industry. , It was shown that microdispersions were water clusters surrounded with oil surface-active materials leading to wettability alteration through the detachment of surface-active materials from the rock surface; − this work was also supported by other researchers. , Several studies have been published in the literature reporting the formation of water microdispersion as the main mechanism of LSWI irrespective of different terminologies. ,− Other mechanisms were also introduced to attribute the additional oil recovery during LSWI to the fluid–fluid interaction, such as interfacial viscoelasticity variations and osmotic effects . Alvarado et al suggested that either increasing the sulfate content or decreasing the salinity of injection water would lead to incremental oil recovery through increasing the interfacial viscoelasticity.…”

“…45,46 It was shown that microdispersions were water clusters surrounded with oil surface-active materials leading to wettability alteration through the detachment of surface-active materials from the rock surface; 47−49 this work was also supported by other researchers. 50,51 Several studies have been published in the literature reporting the formation of water microdispersion as the main mechanism of LSWI irrespective of different terminologies. 45,52−65 Other mechanisms were also introduced to attribute the additional oil recovery during LSWI to the fluid−fluid interaction, such as interfacial viscoelasticity variations 66 and osmotic effects.…”

Experimental Investigation of Osmosis as a Mechanism for Low-Salinity EOR

“…However, it is still uncertain whether the mechanism has any quantitative significance compared to other low-salinity effects. Bartels et al (2010) reported from their micromodel experiments that osmosis was not a primary low-salinity mechanism [35].…”

Microfluidics for Porous Systems: Fabrication, Microscopy and Applications

“…Low-salinity brine was found to result in the preferential development of dynamic interfacial viscoelasticity, thereby suppressing pore-level snap-off events. Bartels et al (2016) and Bartels et al (2017) used clay-coated micromodels to investigate the low-salinity effect and its dependence on crude oil properties, presence of clay particles and aging. Amirian et al (2017) utilized the approach, proposed by Song and Kovscek (2015) to deposit clay particles into micromodels and visualized the micro-mechanism of displacement under LSW injection.…”