EFFECTS OF PORE TYPES ON ELECTRICAL PROPERTIES OF CARBONATE ROCKS AND DETERMINATION OF ELECTRICAL PARAMETERS FROM NMR LOGGING DATA

摘要

Water saturation is a key reservoir property, which is usually calculated from resistivity logs with Archie’s equations. Due to a variety of pore types and complex pore spaces, understanding the electrical properties of carbonate formations is of great challenging. Conventionally, the electrical parameters (m and n in Archie’s equations) are assumed to be fixed values along the whole interval in carbonate evaluations. However, the values of m and n can vary at different depths due to the heterogeneity of carbonate formations.In this paper, we discuss the effect of pore types on electrical properties of carbonate rocks based on the rock experiments and pore-scale numerical simulations. Three types of carbonate rocks from the Ordovician formation are selected: type I without vugs and fractures, type II with developed secondary vugs, and type III with fractures. For all the cores, high-resolution micro-CT test, NMR measurement both at 100% water saturation and at bound water saturation are first conducted. Then the resistivities at various water saturations are measured by gas displacement with porous plates. The microporosity, vug porosity, fracture porosity and bounded water saturation of all abovementioned rocks are quantitatively calculated based on the micro-CT and NMR T2 spectrum, and the electrical properties are examined. The results indicate that both fractures and micro pores have significant effects on the resistivity of carbonate rocks, and the resistivities decline with the existence of more fractures and micro pores. On the other hand, the effect of vugs on the resistivity response is negligible. This observation is consistent with our experimental results by using the synthetic vuggy porous media with different vug distributions. Moreover, the dual-medium models including matrix-vug model and matrix-fracture model are established. The electrical characteristics of these two models are simulated with the invasion percolation algorithm. The simulation results indicate that the vugs have weak influences on the resistivity response if they are not connected by fractures; the fracture shows strong effect on the resistivity while its influence on the saturation is negligible.Meanwhile, it is found that the electrical parameters have good correlations with bound water saturation: the values of m linearly decrease with the increasing of bound water saturation in all three types of carbonate rocks; the exponent n in the cores with m<2.0 linearly decreases with the increasing of bound water saturation while the exponent n in the cores with m>2.0 linearly increases with the increasing of bound water saturation. Then we propose an approach for calculating m and n values that can vary continuously with depths in carbonate reservoirs with NMR bound water saturation. Finally, the method is verified by the applications in carbonate reservoirs in Ordos basin.