Residual Oil: Residual Oil Saturation (Sor), Capillary Trapping, and Waterflood Recovery Limits
Residual oil is the crude that stays locked inside the pore space of a reservoir rock after a given displacement process has run its course, the fraction that simply will not flow when water, gas, or another fluid is pushed through the formation under ordinary primary and secondary recovery conditions. Engineers quantify it as residual oil saturation, written Sor or Sorw, the volume of trapped oil divided by the total pore volume and expressed as a fraction or percentage once the oil phase has stopped moving and its relative permeability has effectively fallen to zero. It is the floor that bounds how much oil any conventional recovery scheme can ever produce, because the difference between the original oil in place and the residual saturation defines the maximum movable oil. The mechanism behind it is capillary trapping. As an invading water front sweeps through a water-wet sandstone or carbonate, interfacial tension between oil and water snaps oil into disconnected ganglia and isolated droplets that lodge in pore throats and dead-end pores, held in place by capillary forces that the viscous pressure gradient of the flood cannot overcome. The balance between those viscous and capillary forces is captured by the capillary number, and only by raising it through orders of magnitude, by adding surfactant to cut interfacial tension or polymer to raise sweep efficiency, can residual oil be remobilized. Residual saturation is not a single universal number. It varies with lithology, pore size distribution, permeability, wettability, fluid properties, flow rate, and the recovery method itself, and laboratory waterflood values commonly land between 20 and 40 percent of pore volume in clastic reservoirs. In the Western Canadian Sedimentary Basin, where waterflood has been the dominant secondary scheme across Cardium, Viking, and Pembina pools for decades, the residual oil left behind in swept zones is precisely the target volume that enhanced oil recovery and miscible flood projects are designed to chase. Distinguishing laboratory Sorw from field remaining oil saturation matters: remaining oil at the field scale includes bypassed and unswept oil and is almost always larger than the relative-permeability residual measured on a clean core plug.
Key Takeaways
- Defined as immobile pore-scale oil: Residual oil saturation (Sor) is the oil saturation below which the oil phase becomes discontinuous and immobile, its relative permeability reaching zero. Typical waterflood Sorw in WCSB sandstones such as the Cardium and Viking runs 20 to 40 percent of pore volume, setting the practical limit on secondary recovery before any tertiary scheme is applied.
- Capillary trapping is the cause: Interfacial tension between oil and water, roughly 20 to 30 dynes per centimetre, snaps oil into isolated ganglia in pore throats. The viscous pressure gradient of a normal flood cannot dislodge them, so residual oil persists until the capillary number is raised by surfactant injection or miscible displacement that lowers interfacial tension toward zero.
- Lab Sor differs from field ROS: Laboratory residual oil saturation from a core flood measures the cleanest achievable displacement on one plug, while field remaining oil saturation includes bypassed oil in unswept layers, low-permeability streaks, and attic or basal zones. Field ROS is therefore larger and is what infill drilling and conformance work target.
- Wettability shifts the value: Strongly water-wet rock traps oil in the centres of large pores, giving moderate Sor, while oil-wet or mixed-wet carbonate films oil onto grain surfaces and can produce higher residual saturation. Wettability alteration is one lever EOR chemistry uses to lower the residual saturation and free additional oil.
- It governs EOR economics: The volume between waterflood Sor and a lower miscible or chemical residual is the prize for tertiary recovery. A CO2 or hydrocarbon miscible flood that drops residual saturation from 35 to 10 percent of pore volume can add substantial recoverable reserves, which is why accurate Sor measurement underpins every WCSB EOR economic model.
Measuring Residual Oil Saturation in Core and Log Analysis
Residual oil saturation is determined by laboratory core flooding, by single-well chemical tracer tests, and by pulsed-neutron and sponge-core logging. In a Special Core Analysis program, a representative WCSB plug is restored to native wettability, flooded to irreducible water, then waterflooded at reservoir-equivalent rate until oil production ceases, with the trapped fraction read by mass balance or Dean-Stark extraction. A typical Cardium SCAL contract runs 40,000 to 90,000 CAD depending on plug count. Single-well chemical tracer tests give an in-situ Sor across a near-wellbore drainage radius and avoid the flow-rate and capillary end effects that bias plug-scale results, which makes them a favoured calibration point for EOR pilots in mature Pembina pools.
Lowering Residual Saturation Through Enhanced Recovery
To produce oil below the waterflood residual, an operator must change the force balance at the pore throat. Miscible flooding with CO2 or enriched hydrocarbon gas develops zero interfacial tension at the displacement front, collapsing capillary trapping and driving residual saturation toward single digits. Surfactant-polymer chemical floods cut interfacial tension by three to four orders of magnitude while polymer improves mobility ratio and sweep. In the WCSB, hydrocarbon miscible floods in the Pembina Cardium and CO2 schemes elsewhere have demonstrated incremental recovery factors of 5 to 15 percent of original oil in place by attacking exactly this trapped, immobile oil that secondary waterflood leaves behind.
Fast Facts
The capillary number that controls residual oil trapping spans an enormous range. At ordinary waterflood conditions it sits near 0.000001, and laboratory work shows residual saturation barely moves until the capillary number climbs above roughly 0.0001, after which trapped oil falls steeply. Reaching that threshold requires reducing oil-water interfacial tension from about 25 dynes per centimetre down to less than 0.01, a thousand-fold change. This single relationship, the capillary desaturation curve, explains why simply pumping water harder never recovers residual oil and why chemistry, not pressure, is the key to tertiary recovery.
Related Terms
Residual oil saturation is inseparable from relative permeability, since the residual point is precisely where the oil relative permeability curve reaches zero. It sets the endpoint for any waterflood, defining the swept-zone saturation that secondary recovery approaches but cannot beat. The concept underpins enhanced oil recovery, because every tertiary method exists to mobilize oil below the waterflood residual. It also connects to wettability, the rock-fluid surface property that decides how and where oil is trapped at the pore scale.
WCSB Field Scenario: Pembina Cardium Residual Oil Target
A producer operating a mature Pembina Cardium waterflood unit in west-central Alberta logs an average waterflood residual oil saturation of 32 percent of pore volume from a 75,000 CAD SCAL program on four cored wells. With original oil in place across the unit at roughly 40 million barrels and a recovery factor stalled near 30 percent after thirty years of injection, the operator commissions a single-well chemical tracer test at 120,000 CAD to confirm in-situ Sor before sanctioning a hydrocarbon miscible pilot. The tracer returns an in-situ residual of 29 percent, close enough to the SCAL value to validate the economics.
The miscible flood pilot, designed to drop residual saturation toward 12 percent, projects an incremental 8 percent of original oil in place, about 3.2 million barrels. At a conservative netback the trapped residual oil that thirty years of waterflood could never touch becomes the single largest reserves addition available on the asset, justifying the solvent injection facility capital.