Direct estimation of an equivalent shaliness parameter using resistivity and spectroscopy logs: a total illite/expansible clay approach

A method for estimating the degree of saturation in shaly sand formations based on an equivalent shaliness parameter has been developed. This method uses mineralogy to constrain the modelling of the nonlinear electrical properties of shaly sands. In shaly sands the measured electrical conductivity is influenced by the clay content because of the charge deficit associated with the clay particles. This charge deficit is offset by counter-ions located in the double layer that surrounds the clay and these counter-ions contribute to an increase in the electrical conductivity of a rock. The greater the illite/expansible clay content the higher the counter-ion contribution to the total charge transfer in a rock. A linear relationship between the cation exchange capacity (CEC) and the total illite/expansible clay (TIEC) content of a reservoir rock has been derived by combining mineralogy from quantitative X-ray diffraction (XRD) with wet chemistry CEC data to express the counter-ions in the double layer as a function of the total illite/expansible clays. The analytical expression obtained was then coupled with well-log data and compared to existing models. For determining surface conductivity, the new methodology eliminates the need for performing core measurements, subjective selection of CEC values from existing data, or the use of log based volume-porosity translations. In addition, it is shown that existing popular industry models, as well as effective medium and electrochemical models of rock electrical conductivity, can be adapted to use the new methodology. The results demonstrate a strong correlation between the estimated and measured bulk conductivity, and resolve the scatter in estimated bulk conductivity often observed under low pore water salinity conditions, where the ratio of surface to electrolyte conductivity approaches unity. The new methodology has been successfully applied to water saturation estimation in two Australian petroleum wells, Yolla-4 and Cliff Head-4. An investigation of the relationship of estimated water saturation based on the total illite/expansible clay model to independently determined rock properties confirm that water saturation estimates made from the total expansible clay model can be used to forward model measured elastic parameters such as primary and shear-wave sonic velocity (Vp, Vs)