Capillary Pressure
Capillary pressure is the pressure difference between two fluid phases in a porous medium that arises due to the interfacial tension between the fluid phases and the surface tension between fluids and the porous medium.
Reservoir capillary pressure is the pressure difference between two immiscible fluids (such as oil and water or gas and water) within the pore spaces of a reservoir rock. This pressure difference is a result of the interaction between the fluids and the solid surface of the rock, influenced by surface tension and the curvature of the fluid interface.
Aspects of Reservoir Capillary Pressure
Fluid Distribution  Capillary pressure determines the distribution of fluids within the reservoir. In a waterwet reservoir, water will preferentially occupy the smaller pores, while oil or gas will occupy the larger pores.Reservoir Saturation  It influences the saturation levels of different fluids within the reservoir. Higher capillary pressures typically lead to higher nonwetting phase saturations (oil or gas) in the reservoir.
Reservoir Performance  Understanding capillary pressure is crucial for predicting reservoir behavior, estimating recoverable reserves, and planning efficient extraction methods.
Capillary Pressure Curves  These curves are generated from laboratory measurements and show the relationship between capillary pressure and fluid saturation. They help in understanding how fluids will move through the reservoir during production.
Height Above Free Water Level  Capillary pressure is also related to the height of the reservoir above the free water level. The pressure difference can be used to calculate the height of the oil or gas column.
Applications in Reservoir Engineering
Enhanced Oil Recovery (EOR)  Understanding capillary pressure helps in designing EOR methods, such as water flooding or gas injection, to improve oil recovery.Reservoir Characterization  Capillary pressure data is used to characterize the reservoir, including porosity, permeability, and fluid distribution.
Simulation and Modeling  Capillary pressure is a key input parameter for reservoir simulation models that predict fluid flow and recovery over time.
Capillary Pressure formula


\( P_c \;=\; 2 \; \sigma \; cos(\theta) \;/\; r \) (Capillary Pressure) \( \sigma \;=\; P_c \; r \;/\; 2 \; cos(\theta) \) \( cos(\theta) \;=\; P_c \; r \;/\; 2 \; \sigma \) \( r \;=\; 2 \; \sigma \; cos(\theta) \;/\; P_c \) 

Symbol  English  Metric 
\( P_c \) = Capillary Pressure  \(lbf \;/\; in\)  \(dyn \;/\; cm\) 
\( \sigma \) (Greek symbol sigma) = Fluid Interfacial Tension  \(lbf \;/\; in\)  \(dyn \;/\; cm\) 
\( \theta \) = Angle of Wettability  \(deg\)  \(rad\) 
\( r \) = Radius of Capillary  \(in\)  \(cm\) 