Electrical Resistivity an Aid in Core-Analysis Interpretation

A correlation of permeable rock characteristics is important to advance our understanding of rock structure in general as well as help in a practical way to detect oil reservoirs exposed in a bore hole and produce the reservoirs efficiently. Although rocks are heterogeneous and relations between their characteristics cannot be expressed by true mathematical equations, these relations do follow definite trends. The electrical resistivity which is found useful in outlining different formations exposed in a bore hole, is found to be closely related to total porosity. The type of rock structure, provided it is well consolidated, that is, fine-grained sandstone, coarse-grained sandstone, oolitic limestone, et cetera, has remarkably little effect, notwithstanding the heterogeneity and marked variation between different types. This is fortunate because the resistivity log can be used in a more quantitative way. A comparison of connate water in situ, determined by resistivity curves, with residual fluid in contaminated or flushed cores resulted in an empirical relation between the two. The residual water in flushed cores at the surface is approximately 20 per cent higher than the connate water in situ. Residual oil (gravity, gas-oil ratio, and viscosity being nearly constant) is also related to connate water.