Wettability Change

Wettability change in petroleum reservoir engineering refers to the alteration of the natural wetting preference of reservoir rock surfaces — from the original water-wet or mixed-wet condition toward oil-wet or from oil-wet toward water-wet — caused by adsorption of polar organic compounds from crude oil onto mineral surfaces, by drilling fluid contamination, by chemical injection, or by thermal alteration, with the resulting wettability state exerting profound control over relative permeability, capillary pressure, irreducible water saturation, residual oil saturation, and ultimately the recoverable hydrocarbon volume from the reservoir.

What Is Wettability Change?

Wettability describes which fluid — oil or water — preferentially contacts and spreads on a mineral surface in the presence of both fluids. A water-wet surface has a strong affinity for water, maintaining a thin water film between the oil and mineral grain even when the pore is oil-filled. An oil-wet surface adsorbs oil molecules preferentially, with oil contacting the grain directly and water occupying the center of the pore. Mixed-wet conditions, the most common state in aged reservoirs, see large continuous oil-filled pores become oil-wet while smaller water-filled pores remain water-wet due to the protection of the irreducible water film.

Wettability change occurs when the initial water-wet condition of freshly deposited sediment — a consequence of the original mineral surfaces being coated by water in the depositional environment — is altered by subsequent events. The most geologically significant wettability change is the natural aging process following oil migration into a trap: polar compounds in the crude oil (asphaltenes, naphthenic acids, nitrogen compounds) adsorb onto grain surfaces at the oil-water contact, replacing water films with organic coatings that render the surface oil-wet. This process takes geological time scales (millions of years) and produces the mixed-wet to oil-wet conditions observed in naturally aged reservoirs.

In production engineering, wettability change is both a challenge (drilling fluid contamination alters core wettability, complicating laboratory measurements) and an opportunity (deliberate wettability alteration toward water-wet through EOR chemistry can mobilize residual oil that waterflooding at native wettability cannot recover). Understanding the wettability state of a specific reservoir — and how it can be systematically altered — is central to designing effective EOR programs and correctly interpreting laboratory relative permeability measurements.

Wettability Change in Reservoir Management

Waterflood design is most directly impacted by wettability change. In water-wet reservoirs, water imbibes spontaneously into pores by capillary suction, displacing oil from the smaller pores and providing a piston-like flood front that sweeps efficiently. In oil-wet reservoirs, capillary forces resist water entry into small pores — water instead flows through the center of larger pores already wetted by oil, bypassing substantial oil volumes and arriving at the producing well at high water cut while much of the reservoir oil remains trapped. The shift in relative permeability endpoint values between water-wet and oil-wet conditions — particularly the increase in residual oil saturation from approximately 20% to 35% of pore volume — directly translates to a reduction in recoverable reserves of that magnitude.

Low-salinity waterflooding exploits wettability change deliberately. When injected brine salinity is reduced below approximately 5,000 ppm total dissolved solids (with specific reductions in divalent cations such as Ca²⁺ and Mg²⁺), the electric double layer at the mineral surface expands, and the multicomponent ion exchange mechanism desorbs polar organic compounds from the mineral surface, restoring water-wet conditions locally. The resulting spontaneous imbibition of low-salinity water into oil-wet pores that previously resisted waterflooding mobilizes residual oil, increasing recovery factor by 5 to 15 percentage points compared to conventional high-salinity waterflood in laboratory and field tests.

Core analysis programs must account for wettability change from drilling fluid contamination. Water-based mud filtrate invades the formation ahead of the core barrel, preferentially displacing oil from pores nearest the borehole and partly restoring water-wet conditions in the invaded zone. The resulting core sample has a wettability state shifted toward water-wet relative to the uninvaded reservoir. Relative permeability curves measured on such cores overestimate water-wet behavior and underestimate residual oil saturation, leading to over-optimistic waterflood recovery predictions. Wettability restoration protocols — cleaning the core with appropriate solvents to remove original oil, then re-saturating with representative crude oil and aging at reservoir temperature and pressure — attempt to recreate the original in-situ wettability state for more reliable flow property measurement.

Wettability Change Across International Jurisdictions

Canada (AER / WCSB): WCSB heavy oil and oil sands reservoirs have strongly oil-wet to mixed-wet conditions due to the high asphaltene content of bitumen and heavy oil, which extensively coats the quartz and clay surfaces of Athabasca, Cold Lake, and Peace River reservoir sands. AER reservoir characterization requirements for WCSB thermal recovery schemes (SAGD, CSS) include wettability measurement as part of the core analysis package supporting scheme design, since steam injection effectiveness in heavy oil reservoirs depends on thermally-induced wettability change as the bitumen viscosity drops and oil-wet surfaces are partly restored to water-wet conditions. Low-salinity waterflood pilots in Lloydminster heavy oil pools have demonstrated wettability-related incremental recovery consistent with laboratory measurements.

United States (API / BSEE): Permian Basin carbonate reservoirs (Wolfcamp, Bone Spring) have complex mixed-wet conditions driven by both the carbonate mineralogy (calcite surfaces more susceptible to organic adsorption than quartz) and the high asphaltene content of many Permian crude oils. SPE technical papers from Permian operators document wettability state characterization and its effect on primary and secondary recovery design. Gulf of Mexico deepwater turbidite sands with low-asphaltene light crude oil tend toward more water-wet conditions and show better waterflood sweep efficiency than comparable onshore oil-wet formations. BSEE requires wettability data in core analysis reports submitted with development plan supplemental environmental impact statements for offshore projects.

Norway (Sodir / NORSOK): North Sea chalk reservoirs (Ekofisk, Valhall, Eldfisk) are famously oil-wet to mixed-wet due to the adsorption of polar oil compounds onto the high-surface-area calcite microfossil fragments that make up chalk porosity. The discovery that seawater injection into Ekofisk chalk spontaneously improved oil recovery beyond simple pressure maintenance — later attributed to wettability alteration from seawater's elevated sulfate concentration interacting with the chalk surface to desorb organic compounds — was a landmark in North Sea EOR history, triggering the Sodir-supported FORCE (Facing Oil Recovery Challenges in Energy) research consortium studies on wettability alteration mechanisms in chalk. Norwegian petroleum research institutes (IRIS, now NORCE; IOR Centre at University of Stavanger) have published extensively on chalk wettability alteration mechanisms, providing the scientific foundation for Ekofisk and Valhall waterflood optimization.

Middle East (Saudi Aramco): Arab Formation carbonates in Saudi Arabia are mixed-wet to oil-wet in the oil column and water-wet below the oil-water contact, with the wettability transition zone coinciding with the transition zone in capillary pressure — a correlation that Saudi Aramco reservoir engineers use to calibrate wettability-dependent relative permeability models for Arab D and Arab C reservoir simulation. Aramco's research center (EXPEC ARC) has published research on low-salinity waterflooding and surfactant-enhanced wettability alteration for Arab Formation carbonates, with pilot tests in Ghawar field demonstrating wettability alteration EOR potential in the world's largest oil field. Diluted seawater injection programs in Aramco fields are partly designed to exploit wettability change as a mechanism complementary to pressure maintenance.

Synonyms and Related Terminology

Wettability change is also called wettability alteration, wettability modification, or wettability shift in different EOR and core analysis contexts. Related terms include wettability, relative permeability, capillary pressure, residual oil saturation, low-salinity waterflooding, Amott index, asphaltene, and enhanced oil recovery (EOR). The contact angle (measured through the water phase) is the fundamental pore-scale measurement of wettability: less than 90 degrees indicates water-wet, greater than 90 degrees indicates oil-wet, and intermediate angles describe mixed-wet conditions — though core-scale Amott and USBM indices are more practically meaningful for reservoir flow calculations than individual contact angle measurements.