Application of Array Resistivity Measurements in Horizontal Wells

When interpreting array resistivity measurements in horizontal wells, the objectives are quite different from traditional data interpretation in vertical or near vertical boreholes. While the emphasis for the latter is put on resolving the invasion profile and true formation resistivities, the former measurements are aimed at the identification of any nearby geometry, e.g., cap rock or oil-water contacts in the vicinity of the wellbore. In order to accomplish this, new algorithms have been developed ranging from real-time geosteering applications to sophisticated inversion-based processing. Early resistivity tools were complex devices that utilized bucking coils and guard electrodes to focus the measurement onto a particular region in the formation, for example, shallow and/or deep. These analog focusing methods were designed for and worked well in quasi-horizontally layered formations penetrated by a near vertical well. In the last decade array tools offering a multitude of electrode or coil arrangements have been introduced. By combining data from different electrodes or receiver coils and transmitters, it is possible to mathematically focus in on a particular formation area, such as illuminating the invasion profile and obtaining a better estimate of the formation resistivity. In horizontal and highly deviated wells, the array data still can be used to characterize the invaded zone, but, more importantly, the array data can be used to probe the layered structure above and below the well. Interpreting the response of today's array resistivity tools in these situations is critical to the drilling and formation evaluation process. In horizontal wells, the response of resistivity tools in even a simple layered formation is complicated and may differ significantly from our vertical well-based intuition. However, forward modeling of the expected formation earth model and inversion techniques, from the simplest to the most sophisticated approach, can assist both the driller and the petrophysicist in extracting required information. In this paper we demonstrate the use and effectiveness of the array resistivity tools in horizontal and near-horizontal wells. We focus on the use of the array data collected by the logging-while-drilling (LWD) Multiple Propagation Resistivity (MPR) tool as well as the wireline or pipe-conveyed driven High-Definition Induction Log (HDIL) and High-Definition Lateral Log (HDLL). For each tool the interpretation process is illustrated with field examples.