Capillary Pressure Curve

Definition - What does Capillary Pressure Curve mean?

Capillary Pressure Curve is the means of describing the relationship between the capillary pressure which is necessary to obtain a given non-wetting saturation phase in a rock. Therefore, we can describe capillary pressure as the difference existing across the interface separating two immiscible fluids in capillaries (e.g., porous media) and can be given by the following equation:

Pc = pnwt - pwt


Pc = Capillary Pressure

Pnwt = Pressure in non wetting phase

pwt = Pressure in wetting phase

Petropedia explains Capillary Pressure Curve

Capillary Pressure Curves play an important role in understanding the saturation distribution of fluids in hydrocarbon reservoirs and affect the multiphase flow of fluids as well as imbibitions through the porous rocks. Every rock has a pore throat size distribution by which more pressure can be applied on the non wetted phase. All the openings of small pores are invaded because of pressure distribution.

Capillary Pressure Curve depends on the Drainage and Imbibition processes.

  • Drainage process – In this fluid flow process, the saturation of non wetting phase increases, which in turn increases the mobility of non wetting fluid phase.
  • Imbibition process – In this fluid flow process, the saturation of wetting phase increases which increases wetting fluid phase mobility.

Capillary Pressure Curve is shown in the figure below:


Si = Saturation of irreducible wetting phase

Sm = 1 - Residual saturation of non-wetting phase

Pd = Displacement pressure i.e., the pressure required to force non-wetting fluid into large pores of the rock.

= Pore size distribution index which determines shape of the Capillary Pressure Curve.
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