Abstract
Improved resolution in estimating formation resistivities can provide more accurate values of residual and movable hydrocarbon saturations. Routinely, formation resistivity information derived from galvanic and induction logging measurements are interpreted independently. As a result, several models of formation resistivity can emerge that are typically not consistent. Here, we illustrate how simultaneous inversion of data from galvanic and induction tools can yield more consistent and better resolved parameters.
When measuring formation resistivities, different References and illustrations at end of paper physical principles govern galvanic and induction tools. In galvanic tools, the voltage differences measured are related to the surface charges at the interfaces between adjacent media of different resistivities, whereas in induction tools, the voltage induced in a measuring coil is caused by magnetic fields of alternating currents flowing in the surrounding formation.
A joint inversion of galvanic and induction data is proposed, in which a single earth model, consisting of a distribution of resistivities, is simultaneously applied to the two types of measurements. Combining measurements based on different physical principles also improves resolution. A complete concept of resolution is proposed that involves earth model, measuring device, data noise, and interpretation technique as components.
A number of earth models and tool combinations (Dual Laterolog, Dual Induction, Thin Bed Resistivity Tool, Microlaterolog) are analyzed and discussed under the framework of the proposed resolution concept. The analysis clearly shows the advantages of combining data from different instruments to improve interpreted results.
