Effect of Water Salinity and pH on the Wettability of a Model Substrate

While the wettability of oil reservoirs has been the focus of many studies, little is known as to whether, and to what extent, the wettability evolves during oil recovery by waterflooding. To this end, a model silicate substrate, namely glass, was treated by oil drainage of the surrounding salt solution and aging (representing the initial state), followed by oil displacement by a second salt solution (flooded state). The two states were analyzed by scanning electron microscopy of the oil components attached to the substrate and measurement of their influence on macroscopic contact angles. Initial-state wettability took the form of an incomplete asphaltenic film interrupted by nanoscale channels and pockets of trapped salt solution. The film was observed to remain fluidic and, on flooding, could retract and detach to leave a more incomplete coverage, usually of oil nanodroplets. The influence of pH of the initial and flooding solutions on these two states was generally opposite; high pH, at which oil–substrate repulsion is prevalent, tended to reduce film coverage in the initial state but aid its retention by the substrate on flooding. Contact angles on flooded substrates depended on this residual adhering nanoscale oil and on the ability of bulk oil to adhere by reconnecting to it. Again, the pH dependence of these two factors was opposite. The results suggested a possible supplementary mechanism for enhanced recovery by low salinity flooding

Micro-CT and Wettability Analysis of Oil Recovery from Sand Packs and the Effect of Waterflood Salinity and Kaolinite

An image-based approach was developed by combining microtomography with electron microscopy and contact angle goniometry to determine the pore-scale distribution of crude oil in plugs after waterflooding and shed light on the molecular-scale mechanisms responsible. The approach was applied to a model rock comprising a pack of quartz sand grains without or with a preapplied lining of kaolinite, imaged prior to and after capillary-driven oil recovery by flooding with a model brine of high or low salinity. The presence of kaolinite increased residual oil and reversed its brine dependence, with high-salinity flooding giving greatest recovery from the clean sand and least recovery from kaolinite-coated sand. These two extremes tended to exhibit the most connected residual oil clusters, while low salinity gave smaller blobs, to the detriment or advantage of oil recovery. Low-salinity flooding in secondary or tertiary recovery mode resulted in comparable oil residuals in kaolinite-coated sand. Surface analysis of the grains and model substrate analogs without or with this coating showed that recovery was correlated to the advancing contact angle. In particular, kaolinite was far more resistant than quartz to wettability alteration by this particular crude oil, resulting in a more water-wet state prone to oil trapping via bypassing and snap-off mechanisms

Pore-Scale Distribution of Crude Oil Wettability in Carbonate Rocks

Carbonate reservoir rocks can exhibit highly variable and intricate pore systems at multiple scales, for which the distribution of wettability is largely unknown. To improve understanding of pore-scale wettability, a set of outcrop and reservoir carbonate plugs was treated by partial drainage of brine by crude oil and aging, for a variety of brine–oil combinations and conditions. Wettability alteration was imaged by high resolution scanning electron microscopy of the oil footprint remaining on internal rock surfaces after removal of oil and brine with mild solvents. The wettability distribution on the calcite microparticles, which comprised microporous regions and lined vugular macropores, showed a characteristic, but unconventional, mixed-wet pattern of distinct, coexisting oil-wet and water-wet subareas. Oil deposition was limited to the less crystalline (anhedral) faces of these particles, while neighboring crystalline (euhedral) facets remained water-wet. Supporting measurements of ζ-potential, contact angle, and initial brine saturation demonstrated that this face-selective alteration was formed by spontaneous drainage during aging, which appeared to favor oil deposition on facet edges and surrounding anhedral faces, thus preventing brine drainage from euhedral facets. This unifying pattern may simplify the integration of realistic wettability distributions into pore models of carbonate cores to predict oil recovery. Spontaneous imbibition of brine was sometimes observed to cause retraction of oil deposits on anhedral faces. The visualization of such changes can aid in designing the ionic composition of the flood brine to induce a shift toward water-wetting and enhance recovery from carbonates

Pore-Scale Analysis of Residual Oil in a Reservoir Sandstone and Its Dependence on Water Flood Salinity, Oil Composition, and Local Mineralogy

Core-flooding of clay-containing reservoir sandstones can yield substantial tertiary recovery by reducing the flood brine salinity, associated with a shift toward water-wetting. Spontaneous imbibition experiments, in which this salinity-induced shift is the main driver for additional recovery, can provide insight into the extent and source of the wettability change, especially when combined with pore-scale imaging of changes in residual oil configurations using micro-CT. Spontaneous imbibition experiments were performed on two sister mini-plugs of a reservoir sandstone. To explore the influence of crude oil composition, each mini-plug used a different oil, of similar density and viscosity and of low asphaltene content, but primarily distinguished by their total acid number (TAN) of 3 and <0.1 mg KOH/g oil. Mini-plugs were imaged by micro-CT after each experiment. Additional insight into the origins of the apparent shift to more water-wetting were obtained by preparing polished embedded sections of the mini-plugs for higher resolution SEM imaging and SEM-EDS mineral mapping. These 2D images were registered into the corresponding cross-section of the 3D tomograms of each mini-plug. In this way the salinity-induced release of oil could be overlain and directly compared to the mineral originally contacting it at each location. Results show that the low TAN oil exhibited much less tertiary recovery compared to the high TAN oil. The local mineralogical analysis revealed that oil removal by low salinity brine was less favored from kaolinite than from silicate grains (quartz, K-feldspar, and Na-plagioclase). Nanoscale imaging of grain surfaces also showed that asphaltene films from the high TAN oil were less prevalent on the rock surfaces than for the low TAN oil

Mobilization of Fine Particles during Flooding of Sandstones and Possible Relations to Enhanced Oil Recovery

The mounting evidence that waterflooding of clay-containing sandstone reservoirs using floodwater with reduced salinity can enhance oil recovery, but with unpredictably large variation in responses, demands improved understanding of the underlying mechanisms. Mobilization of clays and other fines is one candidate mechanism. Flow experiments in Berea sandstone plugs were designed such that the change in their fines distribution from before to after the oil and water injections could be imaged in exactly the same pores using scanning electron microscopy. This technique also allowed imaging of the wettability distribution on pore surfaces and was coupled to spectroscopic analysis of the adsorbed asphaltene amounts. One-phase flows switching from high- to low-salinity water led to only a low level of fines mobilization, compared to two-phase experiments in which high- or low-salinity water displaced crude oil from mixed-wet prepared plugs. The images reveal that loosely bound, partially oil-wet fines lining sandstone grains are stripped by the adhering oil during its recovery and redeposited on grains further downstream. Reduced salinity increases the fraction of fines thus mobilized by weakening their bonds to grains and strengthening their bonds to oil. Evidence suggests that these more oil-wet fines stabilize the water-in-oil curved menisci, which can aid in maintaining the connectivity of the oil phase and thus enhance oil recovery