The recently introduced electromagnetic look-ahead (EMLA) tool applies deep-directional resistivity logging-while-drilling technology for geostopping and other applications in vertical and deviated wells. The EMLA tool shares some of the same basic architecture used in the standard deep-directional resistivity tool. The difference is that the transmitter and shallow-resistivity sensor are nearer to the bit, enabling it to sense tens of feet ahead of the bit. In addition to the sensor technology, the inversion-based interpretation workflow is essential for obtaining successful results from this service. The measurements are processed using a different strategy for look-ahead applications than the one used for reservoir mapping and geosteering high-angle and horizontal wells.
This paper presents the details of the two-step 1D inversion-based workflow, which maximizes the sensitivity of look-ahead-of-the-bit measurements. A look-around inversion initially estimates the anisotropic resistivity profile and dip of the crossed layers using both conventional shallow-resistivity data and deep-directional resistivity measurements. In the second step of the workflow, the look-ahead inversion takes the anisotropic resistivity profile and uses the deep-directional resistivity data to determine the formation resistivity profile ahead of the sensor. To make the algorithm robust, both look-around and look-ahead workflows are carefully designed, processing the most sensitive measurements within specific intervals for each inversion. In real-time applications, previous look-around and look-ahead resistivity profile estimates are used to aid the current look-ahead inversion. Furthermore, the original EMLA look-around-look-ahead (LALA) workflow is adapted to process the deep-directional resistivity and conventional resistivity data, enabling improved geostopping in deviated wells compared with the conventional interpretation workflow designed for geosteering horizontal wells.
Case studies show the performance of the LALA workflow for detecting changes in resistivity ahead of the bit. An example well drilled in Australia at a 59° inclination is presented to illustrate how the LALA workflow enables detection of the reservoir top 20 m earlier compared with the conventional inversions. Applications using both EMLA and deep-directional resistivity from different parts of the world illustrate successful use of the workflow in detecting reservoir tops at various well inclinations.