Mobility Buffer
A mobility buffer in enhanced oil recovery (EOR) is a fluid slug injected between the driving fluid (water, gas, or CO₂) and the resident crude oil during a chemical or miscible displacement process to create a favorable mobility ratio between the displacing and displaced fluid fronts — typically a viscosified polymer solution, foam, or gel system that has higher viscosity than the injected drive fluid but lower viscosity than the crude oil, acting as a transition zone that prevents the low-viscosity drive fluid from channeling (fingering) through the crude oil and bypassing large portions of the reservoir; by providing an intermediate-viscosity fluid bank between the high-viscosity oil and the low-viscosity drive fluid, the mobility buffer promotes stable, piston-like displacement of oil toward the producing wells.
Key Takeaways
- Mobility ratio (M) is the fundamental dimensionless parameter that determines displacement stability in EOR processes — defined as M = (mobility of displacing fluid) / (mobility of displaced fluid) = (krD/µD) / (kro/µo) where kr is relative permeability and µ is viscosity; when M is greater than 1, the displacing fluid is more mobile than the oil, causing viscous fingering instability where fingers of displacing fluid penetrate through the oil bank and leave large volumes of unswept oil behind; when M is less than or equal to 1, displacement is stable and the displacement front advances as a relatively sharp, uniform front that sweeps the majority of the pore space; mobility buffers reduce M toward or below 1 by increasing the effective viscosity of the displacement front.
- Polymer flooding uses high-molecular-weight water-soluble polymers (typically partially hydrolyzed polyacrylamide, HPAM, at 200 to 2,000 ppm concentration) to increase the viscosity of injected water from 1 cP to 3 to 30 cP, creating a polymer drive bank that serves as the mobility buffer between fresh injection water and the crude oil — the polymer bank's mobility ratio with the oil depends on the polymer concentration and molecular weight (higher concentration and molecular weight = higher viscosity = lower mobility ratio = better stability); the polymer is injected as a specific-volume slug (typically 0.3 to 0.7 pore volumes) rather than continuously throughout the flood because the full polymer slug cost is prohibitive for most reservoirs; tapered polymer concentration (declining polymer concentration toward the end of the slug) creates a gradual mobility transition between the polymer slug and the following chase water, preventing the chase water from fingering through the polymer slug.
- Foam as a mobility buffer uses a stable dispersion of gas (nitrogen, CO₂, or air) bubbles in a surfactant-stabilized aqueous continuous phase to create a high-apparent-viscosity fluid in the porous medium — foam's apparent viscosity in porous rock (typically 10 to 1,000 times the base gas viscosity) arises from the lamella drainage mechanism where gas bubbles must deform to flow through pore throats, creating additional flow resistance; foam is particularly effective as a mobility buffer in gas EOR projects (CO₂ flooding, nitrogen flooding) where the injected gas alone would have very high mobility relative to oil, causing severe channeling; the foam selectively reduces gas mobility more in high-permeability streaks than in lower-permeability rock, providing conformance improvement by diverting the gas drive into lower-permeability zones that would otherwise be bypassed.
- Surfactant-polymer flooding uses a combined surfactant slug (to reduce oil-water interfacial tension below 0.01 mN/m, mobilizing residual oil by ultralow IFT) followed by a polymer mobility buffer (to displace the surfactant-mobilized oil bank toward producers without bypassing it) — the polymer mobility buffer in a surfactant-polymer (SP) flood is critical because the mobilized oil forms a low-viscosity microemulsion bank that would be severely channeled by the following injection water without the polymer viscosity buffer maintaining favorable mobility ratio between the microemulsion bank and the polymer drive; the polymer concentration must be high enough to have lower mobility than the microemulsion bank but not so high that it causes injectivity loss in low-permeability formations.
- Mobility buffer placement and slug sizing in field chemical EOR requires reservoir heterogeneity characterization to determine the volume of mobility buffer needed to achieve acceptable conformance improvement — in highly layered reservoirs with permeability ratios exceeding 10:1 between high and low-perm layers, the polymer mobility buffer must be thick enough (high enough pore volume fraction) to divert the drive fluid into lower-permeability zones before the high-permeability streaks are fully swept; permeability heterogeneity quantified by the Dykstra-Parsons coefficient is the primary reservoir characterization input to mobility buffer slug size design, with more heterogeneous reservoirs (higher Dykstra-Parsons coefficient) requiring larger polymer slugs to achieve equivalent sweep improvement.
Fast Facts
The concept of mobility buffer was formalized as part of the EOR methodology developed during the 1960s and 1970s oil price shocks, when the US National Petroleum Council and major oil companies invested heavily in chemical EOR research. The Daqing field in northeast China has produced more oil by polymer flooding than any other single field in the world — cumulative polymer flood incremental recovery at Daqing exceeds 3 billion barrels since the program began in the late 1990s, establishing polymer flooding as a proven, commercial EOR method at gigascale. The mobility buffer concept is also fundamental to miscible CO₂ flooding in the Permian Basin, where the CO₂ slug must be followed by either a water-alternating-gas (WAG) process or a foam mobility buffer to prevent the highly mobile CO₂ from channeling through the oil reservoir and producing at wells without providing effective drive to the oil.
What Is a Mobility Buffer?
When a thin fluid is pushed against a thick fluid in a porous medium, the thin fluid tends to finger through the thick fluid rather than pushing it uniformly forward. This viscous fingering instability is controlled by the mobility ratio — the ratio of how easily the injected fluid flows relative to how easily the displaced fluid flows. A mobility ratio greater than 1 means the injected fluid is more mobile and will finger; less than 1 means it is less mobile and displacement will be stable.
The challenge in most EOR processes is that the driving fluid (water, CO₂, gas) is far less viscous than the crude oil being displaced. Raw injection water at 1 cP injected into 100 cP oil creates a mobility ratio of 100 — a severe fingering condition that results in the injection water bypassing most of the oil and breaking through quickly at production wells, sweeping only a fraction of the reservoir. The same problem afflicts CO₂ injection, where the supercritical CO₂ viscosity of 0.05 to 0.1 cP against even moderate-viscosity oil creates catastrophic fingering without a mobility buffer.
A mobility buffer interposes a fluid of intermediate viscosity between the drive fluid and the oil, creating two successive displacement fronts each with a more favorable mobility ratio than the direct drive fluid-to-oil interface. The polymer slug, for example, might be 10 cP against 100 cP oil (mobility ratio 10, much more favorable than 100) while the chase water behind it faces the 10 cP polymer (mobility ratio 0.1, very stable). The result is significantly better sweep efficiency and higher oil recovery than is achieved by the drive fluid alone.
Mobility Buffer Design and Field Applications
Polymer selection for mobility buffer design requires matching the polymer's in-situ viscosity (which depends on concentration, molecular weight, shear rate, and salinity) to the target mobility ratio for the specific reservoir oil viscosity and permeability contrast — HPAM polymers provide high viscosity at low concentration in fresh or low-salinity water but degrade rapidly at temperatures above 75 to 80°C and lose viscosity in high-salinity brines through ionic shielding of the polyanionic backbone; temperature-stable synthetic polymers (sulfonated HPAM, acrylamide terpolymers) are required for polymer flooding in high-temperature, high-salinity reservoirs (above 80°C or above 100,000 ppm TDS) where HPAM would quickly degrade and lose its mobility control function.
WAG (water alternating gas) processes provide an alternative mobility buffer mechanism for CO₂ flooding — alternating slugs of CO₂ and water create a pseudo-fluid bank with effective mobility between pure CO₂ and water, improving conformance over continuous CO₂ injection; the WAG ratio (barrels of water per barrel of CO₂) is optimized to balance the CO₂ mobility reduction (too much water) against the CO₂ flooding efficiency (insufficient CO₂ contact with residual oil); WAG projects in the Permian Basin, the North Sea Ekofisk Chalk (water-CO₂ WAG), and Gulf of Mexico reservoirs represent the largest commercial applications of CO₂ mobility buffer concepts outside pure CO₂ and polymer flooding.
Mobility Buffer Across International Jurisdictions
Canada (AER / WCSB): WCSB heavy oil polymer flooding in Pelican Lake and other Lloydminster area polymer flood projects uses polymer mobility buffers for heavy oil (50 to 1,000 cP) displacement — even polymer at 10 to 50 cP is insufficient to achieve favorable mobility ratio with these oils, but the partial improvement in mobility ratio from polymer versus water improves sweep efficiency enough to justify the polymer cost at typical heavy oil prices; AER Directive 065 (Well Spacing) and Alberta EOR regulations require documentation of the EOR flood design including mobility buffer polymer concentration and slug size as part of the production scheme approval submitted to AER. Canadian Natural Resources, Husky, and Cenovus operate the largest polymer flood operations in WCSB, with total polymer EOR production contributing several tens of thousands of barrels per day to Canadian production.
United States (API / BSEE): Permian Basin CO₂ enhanced oil recovery using WAG mobility buffer processes represents the largest CO₂ EOR program in the world — operators including Occidental Petroleum, Kinder Morgan CO₂ Company, and Denbury Resources inject CO₂ at continuous or WAG mode into Permian carbonate and sandstone reservoirs, with the WAG mobility buffer essential for managing CO₂ conformance in the heterogeneous Permian formation; DOE's EOR programs and the Office of Fossil Energy have funded mobility buffer research and demonstration projects throughout the history of the US EOR industry, with results published through the National Energy Technology Laboratory (NETL) EOR database.
Norway (Sodir / NORSOK): NCS polymer flooding pilot projects in North Sea sandstone reservoirs (Snorre, Heidrun, Gullfaks) have evaluated high-temperature, high-salinity polymer systems as mobility buffers for offshore EOR — the NCS presents particularly challenging conditions for polymer flooding because reservoir temperatures (60 to 100°C) and salinities (50,000 to 200,000 ppm) near the limits of HPAM stability, requiring synthetic polymer alternatives; Equinor (formerly Statoil) operates the Snorre polymer flood pilot and has collaborated with polymer manufacturers and universities to develop heat-stable, salt-tolerant polymers suitable for NCS conditions.
Middle East (Saudi Aramco): Saudi Aramco operates large-scale polymer EOR pilots in the Arab Formation, evaluating polymer mobility buffers for improving waterflood conformance in the mature Arab D reservoir at Ghawar — the Arab D's high-salinity formation brine (over 200,000 ppm TDS) and moderate reservoir temperature (80 to 100°C) challenge conventional HPAM, driving Aramco's research into synthetic polymer systems and associative polymers that maintain viscosity in these conditions; Aramco's EOR programs are among the most ambitious in the world in terms of the potential reservoir volume and incremental recovery that could be accessed through improved conformance achieved by polymer mobility buffering in the Arab Formation.