Fast anisotropic resistivities inversion of logging-while-drilling resistivity measurements in high-angle and horizontal wells

Abstract

Information about anisotropic resistivity is essential in real-time correlation, updating of formation model and making more confident geosteering decisions in logging-while-drilling (LWD) application. However, abnormal responses such as curve separations and apparent resistivity “horns” often exist in the LWD resistivity measurements due to the influences of complex downhole environments. Thus, accurate formation resistivity is not readily available. In this paper, we present an efficient inversion scheme for the rapid estimation of anisotropic resistivity from LWD resistivity measurements acquired in high-angle and horizontal wells. Several strategies are adopted in the inversion: (1) a one-dimensional (1D) simulator with a simplified three-layered model guarantees the forward speed and keeps the number of inverted parameters as few as possible; (2) combined with geological and petrophysical bounds, the tool constraints derived from a detection capability analysis of LWD resistivity measurements are applied to scale down the inverted parameters’ searching scope, which avoids meaningless solutions and accelerates the inversion significantly; (3) multiple-initial guesses are used in the inversion to ensure a global solution. Inversion results over synthetic examples demonstrate that the proposed 1D inversion algorithm is well suited for complex formation structures. It is also robust and fast in extracting anisotropic resistivities from LWD resistivity measurements.