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An experimental study on the complex resistivity of fractured rock under different saturation conditions

Pan Baozhi, Ruhan A., Guo Yuhang, Zhang Lihua, Wei Boyang, Zhou Weiyi

Acta Geophysica

2022

Vol. 70, no 3

1061--1081

The fracture can be a good channel for oil and gas migration, which has a great influence on the permeability of the reservoir. Therefore, it is of major significance to identify fractures and determine the characterization parameters and physical proper ties of fractured reservoirs. In this study, homogeneous sandstone was used to simulate different artificially fractured rocks. The fractured rock samples had different fracture widths, fracture numbers, and fracture dip angles. In addition, the complex impedance and weights of the rock samples were measured during the process of natural evaporation, and the relationships between the water saturation and the complex resistivity values at different frequencies were examined. The frequency range is 100 Hz–10 kHz. It was found that the influence effects of frequency on the resistivity, dielectric constant, and loss factor had differed among the homogeneous samples and the fractured rock sample. The fracturing had led to the resistivity index and the water saturation curves separating under the different frequencies, and the degree of the dielectric constant index and water saturation curve separation became larger. Furthermore, the influencing effects of the fracture widths, fracture numbers, and fracture dip angles mainly occurred in terms of three aspects. The first was the slope of the resistivity index and water saturation curves (IR–Sw). The second was the slope of the dielectric constant index and water saturation curves (Iε–Sw), and the third was the loss tangent D and water saturation Sw curves.

Study on the C0–Cw relationship of clay bearing sandstones based on digital cores

Fan Yufei, Pan Baozhi, Mo Xiuwen, Guo Yuhang, Lei Jian

Acta Geophysica

2020

Vol. 68, no. 3

641--649

Based on the three-dimensional digital core of Berea sandstone, three-phase (matrix, wet clay and free water) digital cores of clay-bearing sandstone are constructed. We divide clay into structural clay and dispersed clay according to the location where clay growth occurs. The fnite-element method is used to simulate the electrical characteristics of digital cores in order to study the relationship between the conductivity of core saturated with brine (C0) and the brine conductivity (Cw). The infuence of clay mineral type, content and porosity on core electrical characteristics is taken into account. The results show that the additional conductivity is related to the clay minerals, and montmorillonite has the highest cation exchange capacity, resulting in the largest additional conductivity. The increase in clay content in cores increases the conductivity of core C0. At the same time, clay that flls pores decreases the porosity and causes the decrease in C0. These are two opposing factors of conductivity that coexist in clay-bearing sandstone.