Hydrophilic

Key Takeaways

• Formation rock wettability and its deviation from hydrophilic (water-wet) toward mixed-wet or oil-wet conditions fundamentally controls the shape of the relative permeability curves and the residual oil saturation achievable by waterflooding, with water-wet formations having relative permeability curves that favor lower residual oil saturation (higher ultimate oil recovery) and more favorable mobility ratios during waterflooding than oil-wet formations: in a strongly water-wet formation, the capillary pressure holds water preferentially in the small pore throats and oil in the larger pore bodies, and when water is injected for flooding the water spontaneously imbibes into the smaller pores and displaces oil from the larger pores in a piston-like fashion that achieves high microscopic sweep efficiency; in an oil-wet formation (where the asphaltene and resin compounds in the crude oil have adsorbed onto the rock surface and reversed the wettability from water-wet to oil-wet), the oil clings to the rock surface and the water occupies the center of the pores, producing relative permeability curves with higher residual oil saturations, lower oil mobility, and less favorable recovery efficiency by waterflooding; the Amott wettability test (measuring the spontaneous and forced imbibition and drainage volumes of water and oil into a core sample) and the USBM wettability index (measuring the area under the capillary pressure curve for water drainage and oil drainage) provide quantitative measures of wettability that correlate with the relative permeability curves and flood recovery efficiency.
• Surfactant hydrophilic-lipophilic balance (HLB) quantifies the relative affinity of a surfactant molecule for water versus oil phases, determined by the ratio of the hydrophilic (water-loving) to the lipophilic (oil-loving) molecular segments, with the HLB number guiding the selection of surfactants for specific emulsification, demulsification, and wettability alteration applications in oil and gas operations: a surfactant with a high HLB value (above 10 to 12) has predominantly hydrophilic character, adsorbs preferentially at the water-oil interface with its hydrophilic head in the water and its lipophilic tail in the oil, and tends to form oil-in-water emulsions where the oil droplets are dispersed in a water continuous phase; a surfactant with a low HLB value (below 6 to 8) has predominantly lipophilic character and tends to form water-in-oil emulsions where the water droplets are dispersed in an oil continuous phase (as in the invert emulsion OBM); demulsifiers for breaking production emulsions are selected to have an HLB value that preferentially displaces the natural emulsifiers (asphaltenes and resins) from the water-oil interface and promotes coalescence of the dispersed phase; wettability alteration chemicals for EOR applications are cationic surfactants that adsorb onto the negatively charged clay mineral surface (reversing the adsorbed asphaltene coating) with their hydrophilic cationic head group anchored to the rock surface and their lipophilic tail exposed to the pore fluid, creating a hydrophilic surface coating that restores water-wet conditions and improves oil recovery.
• Hydrophilic solids in drilling fluids must be maintained in a water-wet state for proper dispersion and suspension in the water-based mud system, with the wetting agent chemistry of the mud ensuring that newly added barite, bentonite, and drill solids are coated with water-wetting surfactants rather than being converted to oil-wet by contamination: bentonite clay is inherently hydrophilic (the clay mineral surface has a strong affinity for water, which it absorbs between the clay platelets through cation exchange and hydration of exchangeable sodium or calcium cations), and this hydrophilicity is the basis for bentonite's exceptional viscosity-building and fluid-loss-control effectiveness in water-based muds; in oil-based mud systems, the required behavior is reversed: barite, clays, and drill solid cuttings must be oil-wet rather than water-wet to ensure they disperse and remain suspended in the oil continuous phase, and the wetting agents added to OBM (typically fatty acid derivatives or oil-wetting surfactants) coat the solid surfaces to convert them from their natural hydrophilic state to an oil-wet state; contamination of OBM by water-wet solids (formation cuttings that have not been fully coated by the OBM wetting agent) creates clumping of the water-wet cuttings in the water internal phase of the emulsion, increasing mud weight non-uniformly and causing rheology problems that indicate inadequate wetting agent treatment.
• Hydrophilic polymers used in water-based drilling fluids and completion fluids provide viscosity, fluid-loss control, and shale inhibition through their interaction with water, with their performance depending on the degree of hydration, molecular weight, and the specific hydrophilic functional groups that give them their rheological and filtration properties: hydroxyethyl cellulose (HEC) and carboxymethyl cellulose (CMC) are nonionic and anionic hydrophilic polymers, respectively, that hydrate in water to form viscous solutions by entangling their long polymer chains, providing fluid viscosity for gravel pack carrier fluids and completion brines without requiring clay minerals; xanthan gum (a high-molecular-weight biopolymer with strongly hydrophilic side chains) provides pseudoplastic viscosity and excellent suspension of barite and drill solids in water-based mud at very low concentrations compared to conventional synthetic polymers; polyacrylamide copolymers with hydrophilic amide groups adsorb onto shale clay surfaces and reduce water swelling by occupying the cation exchange sites that would otherwise allow water hydration of the clay interlayer, providing shale stabilization in WBM applications; the performance of these hydrophilic polymers is sensitive to the ionic composition of the water phase (high salt concentrations suppress hydration of some polymers through osmotic effects), temperature (higher temperatures accelerate polymer degradation and reduce viscosity), and contamination by calcium or divalent ions (which can cause cross-linking and gelation of anionic polymers like CMC or xanthan at high concentrations).
• Wettability alteration for enhanced oil recovery uses chemical or thermal treatments to convert oil-wet or mixed-wet formation surfaces back to water-wet conditions, reducing residual oil saturation and improving waterflood sweep efficiency in reservoirs that have been aged to oil-wet or mixed-wet conditions by adsorption of crude oil polar components: low-salinity waterflooding (LSWF) is the most widely tested wettability alteration EOR method, which reduces the salinity of the injected water from formation water salinity (typically 50,000 to 200,000 ppm TDS) to a low-salinity level (1,000 to 5,000 ppm TDS), with the resulting changes in ionic composition near the mineral surface thought to desorb the adsorbed asphaltene and resin compounds and restore hydrophilic (water-wet) conditions that reduce residual oil; the mechanism of LSWF wettability alteration is actively debated but appears to involve the expansion of the electrical double layer at the mineral surface as salinity decreases (reducing the screening of surface charge and allowing the surface to re-establish its hydrophilic character), combined with multi-component ionic exchange that displaces the crude oil polar compounds from the mineral surface; surfactant-based wettability alteration injects a specifically designed surfactant that adsorbs onto the oil-wet surface with its hydrophilic head group anchored to the rock and its lipophilic tail in the pore fluid, converting the surface from oil-wet to hydrophilic and improving oil recovery from the treated zone beyond what continued waterflooding alone can achieve.