Influences of pore geometry, porosity and permeability on initial water saturation — An empirical method for estimating initial water saturation by image analysis

Abstract

Investigation of the relationships between two-dimensional pore geometrical data, from image analysis of thin sections, and initial water saturation, measured by conventional laboratory techniques on core plugs, reveals that the volumes of small pores within the pore space have strong influences on initial water saturation. A North American and a North Sea reservoir were investigated and empirical models specific to each reservoir show that 72% and 73% of the variance in initial water saturation can be explained by image data. The average amount that estimated initial water saturation differs from the experimentally measured initial water saturation is ± 3% and ± 3.5%, respectively, for the two reservoirs. It is also shown that the negative relationship between porosity or permeability and initial water saturation, which commonly have been used for prediction of saturation, are not universal but depend on the degree of pore size variation. Significant negative relationships between permeability and initial water saturation occur for rocks with relatively uniform pore systems but different pore sizes. For rocks with varying degrees of non-uniform pore systems, quantitative determination of pore structure is essential in order to estimate initial water saturation. The porosity classification technique used in this study is shown to be useful for determining the pore geometrical parameters which are significant for the prediction of initial water saturation. This technique could be used to evaluate initial water saturations of larger numbers of samples including samples too small for conventional testing.