Development of practical 2-D induction log inversion

Abstract

Formation resistivity is one of the key parameters in hydrocarbon reserve evaluation. However, resistivity logs are often influenced by both the shoulder beds and invasion zone and the log values may differ significantly from the true formation properties. A 2-D inversion method for inverting induction logs is investigated here to simultaneously correct for these undesirable effects. The developed software is based on the familiar Marquardt method. However, we have introduced a few efficiency enhancements to improve the calculation speed and to stabilize the solution. In particular, we developed a hybrid approach to calculate the Jacobian matrix. We also separated the bed boundary determination from the inversion algorithm to reduce the number of unknowns. The prototype software demonstrates the feasibility and usefulness of 2-D inversion. Several invasion profiles are considered in the numerical experiments, covering conductive invasion, resistive invasion, and low resistivity annulus invasion profiles. Inversion successfully corrects the undesirable environmental effects on resistivity logs and thus enables more accurate hydrocarbon reserve computation.