The invasion zone structure and its radial depth depend on formation properties and fluid parameters. Previous research described a methodology for invasion zone reconstruction established from logging data and mud filtration simulation. Fluid mobility in porous media is affected by viscosity; therefore, it is important to understand the effects of oil viscosity on mud filtration. The study presents how oil viscosity and its dependence on formation conditions affect formation resistivity measurements.
Buckley-Leverett equations are used to simulate the invasion process established from drilling and formation parameters pertinent to true reservoir conditions. At the end of the invasion process, the resistivity radial profile (from borehole wall to undisturbed formation) strongly depends on oil viscosity: for example, increasing oil viscosity from 1 cP to 25 cP increases the resistivity of the invaded zone by three times. Resistivity is computed via the Archie equation by taking into account water/oil saturation and salinity. The resistivity profiles are used as input data for forward modeling of the induction logging tool (IL). High-frequency probes (2 MHz) are very sensitive to variations in oil viscosity. The responses of short-spaced (23 inches) probes vary from each other by approximately 21%, with signals of long-spaced (35 inches) IL probes varying by approximately 7%.
Detailed understanding of formation oil properties reduces the ambiguity of resistivity models in the inversion of electromagnetic data. Particularly, knowledge of oil viscosity influence on a resistivity profile is useful for improving the quality of formation evaluations using resistivity logs.