A novel triple-porosity model for fractured-vuggy reservoirs based on Maxwell-Garnett mixing rule

Abstract

Under the new energy pattern, the petrophysical properties of carbonate reservoirs are still the research focus in the petroleum industry. Compared with conventional reservoirs, the calculation of water saturation is more complicated for carbonate reservoirs. One of the main challenges is that the accurate calculation of the porosity exponent of the reservoirs is influenced by the development of fractures and vugs. The existing triple-porosity model provides a way to calculate the porosity exponents; however, the existing model still has some problems: (1) The conduction theory adopted to establish the model varies from the actual conductivity process of the reservoir; (2) The model is deduced from the effective medium theory and regional empirical formula without considering the fracture shapes, thus it is not applicable for the carbonate reservoirs with fractures developed. Therefore, this paper proposed a novel triple-porosity model to calculate the porosity exponents by applying the Maxwell-Garnett mixing rule to the fractured-vuggy reservoir. This novel model can calculate the porosity exponents of reservoirs with the joint development of the vugs (non-connected vugs), fractures (fractures and connected vugs), and matrix pores. From the results of the novel model, the more developed the vugs, the greater the porosity exponents, and the more developed the fractures, the smaller the porosity exponent. Therefore, the porosity exponents of the reservoir can reflect the combined influence of the vugs, fractures and matrix pores. In order to effectively calculate the water saturation through this model, a method based on the binary processing of the Full-bore Formation Micro-Imager (FMI) logs was further proposed to calculate the vug porosity and fracture porosity. The field application demonstrated that the novel triple-porosity model can accurately calculate the water saturation of the fractured-vuggy reservoirs and provide guidance to the actual production.