Application of effective conductivity modeling in unconsolidated sandstone
Yingchun Tian, Qiang Chen, Hu Dong
To supplement scanty laboratory measurements, numerical methods are often used to analyze electrical properties of reservoir rocks using digital rock technology techniques. While some conventional numerical methods based their analyses on only the pore fluid conductivity, these properties however, are also influenced by other factors such as pore fluid, the types and distributions of clay, conductive minerals, etc.
In this study, 3D digital reservoir rock models with four phases - pore phase, quartz phase, clay phase and pyrite phase - are constructed from 3D X-ray CT and 2D quantitative mineral distribution images for two unconsolidated sandstone samples. The finite element method (FEM) that incorporates all the four phases conductivities is used to generate reliable electrical properties for the digital models. Estimation of the conductivities of the pore and clay phase at different water saturation values is an important step in this process.
The pore fluid conductivity at different water saturation is estimated with the Archie formula assuming that the oil and gas in the pore space are nonconductive and a holistic rock saturation index of 2. The Waxman and Smits (the W-S) model is used to estimate the clay conductivity. This model is based on the clay cation exchange capacity, clay porosity, water saturation and clay saturation index n. The value of n is a fitting parameter related to clay types and clay pore structure.
The effective resistivities of the two unconsolidated sandstone samples were simulated under 12 different water saturation values. Differences between the simulated and experimental resistivities vary between 0 – 17% with n values of 2.8 and 2.9 for sample #1 and #2 respectively. This method can be used as a guide to recognize low resistivity oil layer and analyze/identify the causes of low resistivity.
