Improved Rxo Measurements Through Semi-Active Focusing

Abstract

A quantitative measurement of the flushed zone resistivity, Rxo, is one of the key parameters for complete log interpretation. When available at very high vertical resolution, evaluation of thin beds is rendered more accurate. To this end, a new device has been developed which not only provides good performance in thick mudcake/high resistivity contrast situations but has also proved reliable in the presence of thin mudcakes and/or low resistivity contrasts. The pad-mounted device uses dual current focusing — vertically unmonitored and azimuthally monitored. This combination provides fine vertical resolution and strong immunity to mudcake or standoff effects. Three measurement buttons, optimally positioned within the main electrode, provide resistivities at three different depths of investigation. The central button reads the flushed zone resistivity, while two others, located towards the pad edges, are predominantly sensitive to mudcake properties. Finite-element simulations were used for design optimization and for construction of the tool response data base. From this data base, the forward model of the tool was obtained by interpolation. The three parameters - Rxo, mudcake resistivity and mudcake thickness - are estimated in real time by a robust inversion algorithm based on the extended Kaiman filter method. This new tool provides superior quantitative measurements of Rxo at a vertical resolution of less than one inch. Log examples show significant improvements in vertical resolution and accuracy in the determination of Rxo. Coherent quantitative estimations of mudcake thickness have been obtained allowing identification of permeable zones.