Sweep efficiency
- Also known as
- volumetric sweep efficiency, areal sweep efficiency
Sweep efficiency is the fraction of a reservoir that an injected fluid, such as water, actually contacts as it moves from injection wells to producing wells. Oil in parts of the reservoir the fluid bypasses is not displaced, however efficient the fluid is where it does flow.
What is sweep efficiency?
When water or another fluid is injected to push oil toward producing wells, how much oil it recovers depends on two things: how much of the reservoir it reaches, and how much oil it removes from the rock it does reach. Sweep efficiency is the first of these. The second is displacement efficiency, which is limited by residual oil saturation. Overall recovery from a flood is roughly their product.
Sweep efficiency is itself made up of:
- Areal sweep: the share of the reservoir’s area, seen from above, that the fluid contacts.
- Vertical sweep: the share of the reservoir’s thickness that it contacts.
What reduces sweep
- Unfavorable mobility ratio. When the injected fluid flows much more easily than the oil, for example water displacing viscous oil, it fingers through the oil instead of pushing it evenly.
- Heterogeneity. Layers and streaks of higher permeability carry most of the fluid, leaving tighter zones unswept.
- Fractures and channels that connect injectors directly to producers.
- Gravity, which makes injected water slump beneath oil and gas rise above it.
- Well pattern and spacing.
How sweep is improved
- Polymer flooding thickens injected water to improve the mobility ratio.
- Conformance control, using gels or other treatments to block high-permeability paths and divert fluid into unswept zones.
- Foams and water-alternating-gas injection in gas floods.
- Infill drilling and pattern changes.
Improving sweep recovers bypassed oil, which is a different target from the trapped residual oil addressed by surfactant flooding.
Frequently asked questions
Sweep efficiency is how much of the reservoir the injected fluid contacts. Displacement efficiency is how much oil it removes from the rock it contacts. Both limit total recovery.
Because water flows far more easily than viscous oil, it tends to finger through the oil along the easiest paths and reach producing wells early, leaving much of the reservoir unswept.
Sources
- sweep efficiency, SLB Energy Glossary
- Enhanced Oil Recovery, L. W. Lake, Prentice Hall, 1989; reprinted by the Society of Petroleum Engineers