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

Abstract

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.