Estimation of saturation height function using capillary pressure by different approaches

The saturation-height function greatly impacts reserve calculations and is used by geologists or reservoir engineers to predict the saturation in the reservoir for a given height above the free water level. If cores are preserved, the water saturation information can be obtained directly. However, cores are often subject to changes from their original state before brought to laboratories. Hence, capillary pressure (Pc) curves measured on different core samples of a heterogeneous formation are usually correlated to porosity, permeability, and/or rock type using various techniques to generate field wide saturation height function.

This paper evaluates performance of six saturation-height methods (i.e., Leverret-J[1], Cap-Log[2], Johnson[3], Cuddy et al. [4], Skelt-Harrison[5] and Sodena[6] methods) employed in the oil industry.

Measured capillary pressure data and core properties for a well in one of the North Sea reservoirs were used with rock permeability varying from less than 1 mD to over 1000 mD. We have reviewed advantages and disadvantages of each method while comparing the output of each method with saturation obtained from the log data. The results indicated that Skelt-Harson method with coefficient independent of core properties, a simple average Pc function, could be the best option and complicating the functional form by including the porosity and/or permeability data might inversely impact the predictions. Furthermore, modifying Cap-Log Cuddy et al. and kelt-Harison by including the core permeability data in the mutli-regressione exercise resulted in higher errors confirming that increasing the degree of freedom does not necessarily mean a better performance.