Ultralow Interfacial Tension

Ultralow interfacial tension (ultralow IFT) in enhanced oil recovery refers to the reduction of the oil-water interfacial tension from its natural value of 15 to 30 millinewtons per meter (mN/m) to values below 0.01 mN/m — achieved by injecting carefully formulated surfactant solutions into the reservoir — with this orders-of-magnitude reduction in IFT being the primary mechanism by which surfactant flooding mobilizes residual oil that waterflooding cannot recover, because the extremely low capillary forces at ultralow IFT allow oil ganglia trapped in pore throats by capillary pressure to be displaced by the viscous forces of the injected flood.

Key Takeaways

The capillary number (Nc = μv/σ, where μ is flood viscosity, v is flood velocity, and σ is interfacial tension) is the fundamental dimensionless group governing residual oil mobilization: at the natural IFT (σ ≈ 20 mN/m), the capillary number is too small (approximately 10⁻⁷ to 10⁻⁵) for viscous forces to overcome capillary trapping forces; reducing IFT to ultralow values (σ ≈ 0.001 mN/m) increases the capillary number by four to five orders of magnitude to values above the critical capillary number for mobilization (approximately 10⁻³ to 10⁻²), enabling displacement of residual oil from pore throats that conventional waterflooding cannot access.
Achieving ultralow IFT requires precise matching of the surfactant formulation to the oil and brine chemistry — the optimal salinity concept describes the specific brine salinity at which a microemulsion system exhibits minimum IFT between the aqueous and oleic phases, and deviations from this optimal salinity by as little as 20% can increase IFT by two to three orders of magnitude, making surfactant performance highly sensitive to the formation brine composition and temperature that the flood encounters in the reservoir.
Surfactant types used to achieve ultralow IFT include anionic surfactants (petroleum sulfonates, alpha-olefin sulfonates, alkyl benzene sulfonates, internal olefin sulfonates) and extended surfactants (molecules with a propylene oxide or ethylene oxide spacer group between the hydrophilic head and hydrophobic tail) — extended surfactants are preferred for carbonate reservoirs and high-salinity brines because their molecular geometry matches the interfacial structure needed for ultralow IFT with heavier crude oils and high-divalent-cation brines.
Microemulsion formation is the thermodynamic state associated with ultralow IFT: at optimal salinity conditions, a middle-phase microemulsion (Winsor Type III system) forms spontaneously between the aqueous and oleic phases, representing the minimum free energy configuration at that composition — the IFT between the microemulsion and both excess phases (water and oil) is ultralow, and the microemulsion contains both oil and water in a bicontinuous or droplet structure at the nanometer scale, enabling spontaneous mobilization of residual oil into the microemulsion phase.
Surfactant-polymer flooding (SP flooding) combines ultralow IFT surfactant with polymer viscosification to simultaneously mobilize residual oil (through ultralow IFT) and improve mobility control (through the higher viscosity of the polymer solution), preventing the mobilized oil bank from fingering through the injected fluid and ensuring efficient displacement across the sweep volume — polymer addition without ultralow IFT only improves sweep efficiency on bypassed oil, not the residual oil in the swept zone.

Fast Facts

The theoretical framework for ultralow interfacial tension and chemical flooding was developed primarily in the 1960s and 1970s by researchers at the University of Texas at Austin (Prausnitz, Pope, Schechter) and the Shell Development Company (Reed, Healy, Lake), with the concept of optimal salinity and microemulsion phase behavior being formalized in the classic papers by Winsor (1954) and the comprehensive EOR studies of the 1970s oil crisis era. Commercial surfactant EOR has been implemented at field scale in a number of projects including Shell's Minas field in Indonesia, the Daqing field in China (world's largest ASP flood), and more recently North Sea Brent and chalk reservoirs, though the high cost of surfactant and the precision required to maintain optimal salinity conditions in heterogeneous reservoirs have limited adoption relative to polymer flooding alone. Current surfactant EOR research focuses on surfactant retention reduction (surfactant adsorbs onto mineral surfaces and is lost from solution, increasing chemical cost), in-situ generation of surfactant from alkaline-enhanced natural surfactants in crude oil, and low-cost biosurfactant alternatives to synthetic petroleum sulfonates.

What Is Ultralow Interfacial Tension?

Interfacial tension is the energy required to create a unit area of interface between two immiscible liquids — oil and water in a petroleum reservoir. The natural IFT between crude oil and formation brine ranges from 15 to 35 mN/m depending on the crude oil composition (asphaltene and resin content, acid number, base number) and brine chemistry. This IFT creates capillary pressure that traps residual oil ganglia in pore throats during conventional waterflooding — the capillary pressure (Pc = 2σcosθ/r, where r is pore throat radius and θ is contact angle) at natural IFT is high enough to prevent the viscous pressure gradient of a typical waterflood from squeezing residual oil droplets through the smallest pore throats, leaving them permanently trapped.

Ultralow IFT reduces this capillary trapping to near-zero. When IFT falls below approximately 0.01 mN/m, the capillary pressure retaining oil in pore throats becomes negligible compared to the viscous pressure gradient created by the flowing injection fluid — oil drops that were immovable at natural IFT can now be pushed through pore throats by the hydraulic pressure gradient alone. The mobilized oil coalesces into a connected oil bank that grows as the surfactant flood advances, and this oil bank can be produced at the production wells if the flood is properly designed to maintain the low IFT conditions across the reservoir.

Achieving ultralow IFT is challenging because it requires creating a specific thermodynamic condition — the Type III microemulsion phase behavior — that exists only within a narrow window of surfactant concentration, brine salinity, temperature, and oil composition. The surfactant formulation must be designed to match the specific oil and brine chemistry of the target reservoir, and the injection conditions must maintain optimal salinity throughout the reservoir volume being flooded, including through dilution by formation brine and adsorption losses on mineral surfaces.

Ultralow IFT in Chemical EOR

The optimal salinity concept governs surfactant flood design for ultralow IFT. Petroleum sulfonate surfactants (and other anionic surfactants used for EOR) exhibit phase behavior that depends critically on the brine salinity in contact with the surfactant solution. At low salinity (underdoped), the surfactant partitions into the water phase and produces a Type I microemulsion (oil-in-water); at high salinity (overdoped), the surfactant partitions into the oil phase and produces a Type II microemulsion (water-in-oil). At an intermediate optimal salinity, the surfactant partitions equally between oil and water phases and a middle-phase (Type III) microemulsion forms with ultralow IFT to both excess phases. The optimal salinity for a given crude oil-surfactant system is determined in laboratory phase behavior experiments (phase salinity scans) before surfactant flood design.

Surfactant retention is the most significant technical and economic obstacle to commercial surfactant flooding. As the surfactant slug moves through the reservoir, surfactant molecules adsorb onto mineral grain surfaces — especially onto clay minerals and carbonate surfaces — reducing the surfactant concentration in solution below the critical concentration needed for ultralow IFT. Heavy surfactant losses to adsorption require injecting large (expensive) surfactant slugs to maintain effective concentration in the flood front, or adding sacrificial agents (sodium carbonate, polysilicates) that competitively adsorb on mineral surfaces and reduce surfactant adsorption. Carbonate reservoirs are particularly challenging because of the strong interaction between anionic surfactants and the positively charged calcite surface, making surfactant flooding in carbonates an active area of EOR research focused on new surfactant structures with lower carbonate adsorption.

Alkaline-surfactant-polymer (ASP) flooding combines three chemical mechanisms: alkali (sodium carbonate, sodium hydroxide) reacts with naturally occurring acidic components of crude oil (naphthenic acids) to generate in-situ soaps that contribute to ultralow IFT and reduce synthetic surfactant adsorption; surfactant achieves ultralow IFT for residual oil mobilization; and polymer improves mobility control for efficient displacement. ASP flooding has achieved recovery increments of 15 to 20% above waterflood at the Daqing field in China and in several North American pilots, representing one of the most successful commercial chemical EOR implementations at scale.

Ultralow Interfacial Tension Across International Jurisdictions

Canada (AER / WCSB): Chemical EOR (surfactant and ASP flooding) has been piloted in WCSB heavy oil pools (Lloydminster, Provost, Pelican Lake) where waterflood recovery is limited by unfavorable mobility ratios between the high-viscosity oil and injected water. AER Energy In Situ Recovery (EISc) scheme approvals for chemical EOR pilots in Alberta require detailed documentation of surfactant formulation, optimal salinity design, and environmental risk assessment for surfactant-containing injection water. The Pelican Lake polymer flood (which approaches the ultralow IFT mechanism through viscoelastic oil displacement) has demonstrated significant incremental recovery over waterflood, providing Canadian field validation of polymer-based chemical EOR concepts that are being extended to surfactant systems.

United States (API / BSEE): The Permian Basin and Midcontinent regions of the US have hosted numerous surfactant EOR pilots and field applications over the past decades, with mixed commercial results due to surfactant adsorption and the precision required to maintain optimal salinity in heterogeneous carbonate and sandstone reservoirs. The US Department of Energy through the National Energy Technology Laboratory (NETL) has funded multiple surfactant and ASP EOR demonstrations in domestic fields, generating public-domain technical data on ultralow IFT chemical flooding performance. API RP 40 references for core analysis EOR floods provide testing methodology for laboratory surfactant formulation evaluation including IFT measurement, phase behavior characterization, and core flood experiments.

Norway (Sodir / NORSOK): The Norwegian IOR centre (NORCE, formerly IRIS) at the University of Stavanger has conducted extensive research on surfactant flooding for North Sea chalk reservoirs (Ekofisk, Valhall, Eldfisk), where the combination of natural fractured chalk matrix and water-wet to mixed-wet wettability creates an environment where low-IFT surfactant injection could recover significant incremental oil above the seawater waterflood baseline. Sodir supports IOR research through the Norwegian Petroleum Directorate's research program, with ultralow IFT surfactant EOR among the priority technologies for mature North Sea field life extension. Equinor's Heidrun field and Statfjord field have been the subjects of laboratory-scale ASP flooding research targeting realistic NCS crude oil-brine-rock systems.

Middle East (Saudi Aramco): Saudi Aramco has actively developed surfactant and ASP flooding technology for Arab Formation carbonate reservoirs as a next-generation EOR strategy beyond the current seawater and low-salinity waterflooding programs. Aramco's EXPEC ARC research center has published studies on extended surfactant formulations for Arab D limestone, where the high-salinity formation water (over 200,000 ppm TDS) and the calcite surface chemistry create challenging conditions for conventional anionic surfactants. Aramco is conducting laboratory-scale ASP flood experiments targeting the specific Arab Formation crude oil (Arab Extra Light) and brine chemistry to identify surfactant formulations that can achieve ultralow IFT in the presence of high divalent cation concentrations — a technically difficult but potentially high-value application given the scale of Arab Formation reserves.