Conductivity of Double-Porosity Carbonate Formations Saturated with Water – Oil Mixtures

摘要

The determination of water saturation from resistivity logs is an important problem in formation evaluation required for the correct assessment of oil-resources. In carbonate formations this problem is especially complicated because of the complex structure of pore space composed of the small-scale matrix porosity and large-scale secondary pores of different shapes. In this paper we introduce a new approach for calculating the conductivity of carbonate formations with different wettability types. This approach is based on the double porosity model and computing the conductivity of pore saturating fluid treated as a mixture of conductive water and non-conductive oil (gas) inclusions. The non-conductive component is represented by spheroids embedded in the water (host). The variation of inclusion shapes allows us to model the dependence of conductivity on the formation wettability. The calculations have shown that the effective conductivity of the double porosity formations significantly depends on the type of the secondary pores, rock wettability, and water saturation of the matrix and secondary pore-systems. This result indicates that the assessment of the water saturation in carbonate formations, using traditional approaches without considering the secondary-porosity and wettability types, may lead to erroneous oil reserve estimation.