Fundamentals of Fluid Flow in Porous Media
Laboratory Measurements of Relative Permeability: Determining Fluid Saturations
The accuracy of relative permeability measurements (regardless of measurement technique) depends largely on accurate determination of fluid saturation. There are generally two methods to determine the fluid saturation in the core: external and internal techniques. External techniques involve inferring saturation from fluid production. A material balance calculation is performed (fluid in – fluid out = fluid retained in core). The saturation obtained from material balance is an average value for the core. Another external technique is the gravimetric method, in which the weight of a saturated core is compared with the weight of a dry core, and the saturation is estimated from the difference in the weights. Problems associated with external techniques that can affect the material balance are the possibility of dead volume, evaporation in the core, or difficulties separating the fluid at the outlet.
Internal techniques involve measuring the quantity of fluids in the core directly. This method offers greater reliability than external techniques. A primary advantage is that it allows for different saturation levels to be measured along the length of the core, rather than just returning an average value. The basic premise of internal techniques is that a stimulus is applied to the core, and is then compared against the response from a completely dry and a completely saturated core. Saturation is then inferred from this measurement. The most common forms of in-situ fluid saturation determination include x-ray absorption and electrical resistivity.