More general relationship between capillary pressure and resistivity data in gas-water system

Capillary pressure data, with fundamental significance in reservoir engineering, can be determined in laboratory through different methods. However, these methods are expensive, complex and time consuming. Scarce literature has been published to describe the relationship between capillary pressure and resistivity data. In this study, a more general model inferring dimensionless capillary pressure directly from resistivity index data was derived from correlating the modified Kr-RI (relative permeability and resistivity index) model with the widely-used Kr-Pc (relative permeability and capillary pressure) model based on the same function of Kr. This model demonstrated a linear relationship between capillary pressure and (I⋅Sw)1/2/Sw* (I is the resistivity index, Sw is wetting phase saturation, and Sw* is the normalized wetting phase saturation). The feasibility of this model was verified by experimental data from different literatures and ours. The results demonstrated that the model works satisfactorily in most cases except for cores with extra-low permeability (generally, permeability less than 10 mD). In addition, this proposed model could also match the experimental data both at ambient and reservoir conditions, indicating that it reveals a more general relationship applicable for determining capillary pressure from resistivity data both in laboratories and reservoirs.