An electrical conduction model for calcium-bearing shaly sandstone based on the effective medium Hanai-Bruggeman theory

Abstract

The research on the effect of calcareous cementation on rock resistivity has been limited to qualitative analysis, while quantitative research has been lacking. Therefore, a quantitative study on the effect of calcium on rock resistivity is carried out based on the effective medium H-B theory; then, a new electrical conduction model suitable for the evaluation on oil–gas potential of calcium-bearing shaly formations is established accordingly. First, the repeatability of two parallel artificial cores is analyzed with petrophysical parameters to insure the accuracy of the artificial core technology. Then, 51 artificial rock samples with different calcium and clay content but basically with consistent quartz particle size distribution are made using artificial core technology, and the resistivity of these artificial rock samples saturated with different salinity water is measured experimentally. Next, considering the salt exclusion effect of clay pore water and its dispersion distribution, the effective medium Hanai-Bruggeman resistivity model is improved by treating the clay pore water as a dispersed phase. The improved model is more accurate and reasonable when compared with the oil testing results and the sealed coring results. Then, based on the improved effective medium Hanai-Bruggeman resistivity model, the relationship between the cementation exponent and calcium content and that between the cementation exponent with clay content was established. Meanwhile, the relationship among the cementation exponent, calcium content and clay content was established. These were done using the resistivity experimental data of the artificial cores with different clay and calcium contents, while being saturated with water of different salinities. Finally, treating the calcareous particles as a dispersed phase, the established relationship among the cementation exponent, calcium content and clay content was added to the improved model, after which a new electrical conduction model suitable for calcium-bearing shaly sandstone formation was formed.