24/04/2026
Capillary Pressure, BVW & Swirr – Practical
Saturation-Height Modeling
Understanding saturation distribution in reservoirs requires integrating capillary pressure physics, pressure gradients, and rock properties.
Pressure Gradients Reflect the Mobile Phase
Formation testers (MDT/RFT) measure the pressure of the mobile phase only, not the total fluid mixture.
* Above FWL 'n Oil is the mobile 'n Oil pressure gradient
* Below FWL 'n Water is the mobile 'n Water pressure gradient. Capillary-bound water above FWL and residual oil below FWL do not appear in the pressure trend.
Swirr is a Laboratory Concept
Irreducible water saturation (Swirr):
* Depends on capillary pressure, wettability, and pore geometry
* Changes between drainage and imbibition
* Is controlled by experimental conditions
In real reservoirs, water saturation decreases continuously with height above FWL – there is no universal minimum value
BVW Can Be Useful - With Caution
Bulk Volume Water (BVW = φ × SW) works well when:
Reservoir pore systems are uniform
Facies distribution is consistent
Capillary behavior follows a single trend
But BVW becomes less reliable in:
* Laminated sands
* Heterogeneous clastics
* Multiple pore systems
* Strong facies variation
* OBM wells with uncertain SW
Variation in BVW may reflect rock quality changes rather than height alone.
Zero Water Cut = No Water in the Rock
Even reservoirs producing dry oil contain water:
* Capillary-bound water remains in pores
* The water phase is immobile
Zero water cut simply means no mobile water flow.
Practical Saturation-Height Workflow
Step 1: Test multiple SHF models
Use Brooks-Corey, Leverett J-Function, Thomeer, etc.
The reservoir behavior should determine the best model
Step 1 Test multiple SHF models. Use Brooks-Corey, Leverett J-Function, Thomeer, etc. The reservoir behavior should determine the best model.
Step 2 Calibrate using Dean-Stark core measurements. In OBM wells producing zero water, Dean-Stark core measurements often provide the best estimate of in-situ core.
Step 3 Honor pressure gradients
Your SHF must match:
* Oil gradient above FWL
* Water gradient below FWL
* Correct FWL intersection
Step 4 Validate with geology. Capillary behavior is controlled by:
* Pore throat size
* Rock fabric
* Diagenesis
* Lithofacies