text: In addition to measuring the capillary pressure in a laboratory setting to model that of an oil/natural gas reservoir, there exist several relationships to describe the capillary pressure given specific rock and extraction conditions. For example, R. H. Brooks and A. T. Corey developed a relationship for capillary pressure during the drainage of oil from an oil-saturated porous medium experiencing a gas invasion:
  where:
  : is the capillary pressure between oil and gas phases 
  : is the oil saturation
  : is the residual oil saturation that remains trapped in the pore at high capillary pressure
  : is the threshold pressure (the pressure at which the gas phase is allowed to flow)
  : is a parameter that is related to the distribution of pore sizes
  : for narrow distributions
  : for wide distributions
  Additionally, R. G. Bentsen and J. Anli developed a correlation for the capillary pressure during the drainage from a porous rock sample in which an oil phase displaces saturated water:
  where:
  : is the capillary pressure between oil and water phases
  : is a parameter that controls the shape of the capillary pressure function
  : is the normalized wetting-phase saturation
  : is the saturation of the wetting phase
  : is the irreducible wetting-phase saturation