Electrical resistivity data provided by borehole logging are valuable for assessing the properties of reservoir rock. However, it is generally recognized that the resistivity data can be extremely sensitive to various factors such as porosity, clay mineral content, ionic concentration of water and degree of water saturation. In this study, the factors contributing to resistivity changing were examined in more detail and our understanding of the measured resistivity has been improved in terms of rock property.
7 sandstones and 2 carbonate rocks were prepared for this study and the resistivity was measured with different brine concentrations, 10,000, 30,000and 60,000 mg/L-NaCl. From the measured resistivity data, the FRF (Formation resistivity factor) values were calculated to eliminate the influence of brine ionic concentrations.
2 carbonate samples showed the stable and low FRF values, which indicates that they contain no clay minerals and have high porosities. The sandstone samples with a small amount of clay mineral showed the variation in FRF values at the brine injection of low ionic concentration, 10,000 mg/L-NaCl. On the other hand, these sandstones showed the similar values of FRF at high ionic concentration, 60,000 mg/L-NaCl, where a clay mineral impact on resistivity is considered to be small.
The cementation factor ‘m’ is calculated from Archie's equation. The distribution of ‘m’ has been widely studied by many researchers and usually ranged from 1.3 to 2.5 especially for sandstones. In our study, 4 sandstone samples showed the appropriate values of ‘m’, about 2.3. However, 3sandstone samples showed the high FRF and high ‘m’ values ranging from 2.6 to 3.8. To investigate the factors which have some impacts on the high FRF, rock properties such as permeability and pore size distribution were analyzed. 3 samples with high FRF values have low brine permeabilities and one of 3 samples showed the heterogeneity in its pore geometry.
We considered that the resistivity measurement could be conducted effectively when a brine ionic concentration is taken into consideration and the rock resistivity is highly affected by the pore geometry even in sandstones. Comparison various rock properties in parallel with the rock resistivity could allow us a multiple analysis including pore throat characteristics.
