Volumetric Sweep Efficiency — A New Way to Reach Bypassed Oil

Volumetric Sweep Efficiency — A New Way to Reach Bypassed Oil

Mature waterfloods can leave significant oil behind due to reservoir heterogeneity and high-permeability “thief zones.”
Overall recovery is:
Eᵣ = Eᴰ × Eⱽ
Where:
• Eᴰ = Microscopic displacement efficiency
• Eⱽ = Volumetric sweep efficiency
The challenge is improving Eⱽ by redirecting flow into unswept zones.

The Mechnism
The method uses two dilute surfactant slugs separated by a brine spacer.
Because the surfactants travel at different velocities due to adsorption, the second slug can catch the first.
When they mix in-situ → phase separation occurs → a solid precipitate or high-viscosity coacervate forms → the thief zone is selectively plugged.
Key advantages:
• ppm-level chemical concentration
• Near-brine viscosity → high injectivity
• Programmable plug placement

The Math Behind It
Vᴳᴿᴼᴺᵀ = Vᴮᵁᴸᴷ / (1 + Γᴄᴹᶜ/Cᴵᴺᴶ)
Vᵀᴬᴵᴸ = Vᴮᵁᴸᴷ / [1 + (dΓ/dC)₍C₌₀₎]
By controlling:
• Injection concentration
• Slug volume
• Brine spacer
Engineers can target where the plug forms.

Lab Results
Three experiments demonstrated the concept.
Single Column
→ 34% flow-rate reduction
Parallel Columns
→ High-perm flow reduced from 69% → 30%
Tertiary Recovery
→ Low-perm zone increased from 0% → 35% additional oil recovery
The process successfully diverted flow from the thief zone into previously bypassed rock.

Summary
In-situ surfactant phase separation offers a promising method to improve volumetric sweep efficiency.
The key concept is simple:
Control surfactant transport → control phase separation → control flow diversion → recover bypassed oil.
Low chemical concentration + high injectivity + selective plugging could provide an attractive approach for heterogeneous reservoirs.

Reference:

https://onepetro.org/SPEATCE/proceedings-abstract/85SPE/85SPE/SPE-14291-MS/61568