A New Microresistivity Logging Device

The Micro-Triple-Resistivity (MTR), is a new microresistivity logging device, which measures the resistivities of the invaded zone (Rxo), the mudcake zone (Rmc), and the mudcake thickness (hmc). These three parameters are inferred from three resistivity measurements with three different depths of investigation, each with one inch vertical resolution. The measurements are obtained from the currents emitted from three electrodes located on the surface of a rugged conducting pad raised to some potential above the ground. The electrodes are isolated by apportioning the pad by means of insulation into a center electrode and two electrodes displaced from the center near the edge of the pad. The focusing is achieved without the use of active circuits thereby providing stable measurements. The electrode located at the pad center gives rise to an Rxo resistivity measurement with less mudcake effect than conventional microresistivity tools. The combined measurements of the two additional electrodes located near the edge of the pad simulate a Microlog with improved separation over the standard Microlog in permeable formations. Response charts are developed which enable the log analyst to correct the Rxo measurement for mudcake effect as well as provide Rmc and hmc values. The basic design was based upon an exact mathematical model which was confirmed via a series of experiments conducted in a test tank. Additionally, a series of field tests corroborated the results predicted by the computer modeling.