A newly developed multi-laterolog device, the Rt eXplorer™, has been introduced in the Mediterranean region. The device is based on the dual laterolog (DLL) principle, but is designed with four independent, focused measurements having different depths of investigation (ranging from 10 to 50 inches), with a common enhanced vertical resolution of one foot, providing a detailed high-resolution radial resistivity profile.
A new hardware design, with symmetrical configuration, using a feedback loop ensures the required focusing, eliminating the need for a bridle or long isolation subs. This configuration greatly reduces Groningen effect, significantly improving resistivity measurements below evaporitic sequences or thick tight carbonate beds, which are typical conditions in several Mediterranean fields.
These measurements, particularly the shallow ones, are subject to large borehole and eccentricity effects in very conductive muds and large boreholes and conventional borehole corrections using an assumed tool position become ineffective. To better remove borehole effect, an adaptive borehole correction has been designed, which accounts for the variable tool position. The technique includes a 1-D radial inversion that provides improved borehole corrections and Rt (true formation resistivity), Rxo (flushed zone resistivity), and Lxo (length of mud filtrate invasion). The inverted Rt may be significantly higher than the deepest laterolog measurement in situations of deeper invasion.
In most cases a micro-laterolog (MLL) will be run in combination with this device to complement the four focused laterolog resistivities and used in the inversion process as flushed zone resistivity (Rxo), reducing uncertainty during the 1 D-Inversion.
The advantages of this multi-laterolog and its processed data are illustrated with field data examples.
