Water Flood Monitoring in an Oman Carbonate Reservoir Using a Downhole Permanent Electrode Array

Abstract

Abstract In 1998, Schlumberger and Shell jointly launched a project to prove the concept and economic viability of Dynamic Reservoir Drainage Imaging. The goal was to provide a timelapse monitoring of water saturation, allowing an evaluation of drainage efficiency in oil and gas reservoirs. To achieve this objective, the resistivity approach was used: in this technique, an array of electrodes is cemented at reservoir level, providing a continuous reading of the formation resistivity. Applications of this technology include real-time monitoring of water cone development and water table rise, provided there is a sufficiently large resistivity contrast between hydrocarbon and water zones. A field test program was set up in 1999, with Petroleum Development Oman, to demonstrate the technology and to evaluate data to be acquired in Oman. The well that was selected for the electrode array deployment is located in the Fahud field. By the end of August 1999, an observation well was drilled in the Natih-E carbonate reservoir, at the center of a water-flooding cell formed by dual horizontal producers and injectors. Two 16-electrode resistivity arrays were deployed behind the casing, across the zone of interest, to measure the variation in formation conductivity related to water injection in the nearby injector wells. The measurement period was anticipated to last approximately 18 months before the monitoring cell would be flooded. After the array deployment was successfully performed, the initial acquisition period indicated that the array reading was affected by nearby casing effects. This resulted from current leaks to the casing that reduced the depth of investigation of the system considerably. Nevertheless, the data recorded versus time through successive acquisitions made in late 1999 and early 2000 has clearly shown variations of the array response across the reservoir. Those variations have been interpreted as water saturation increase at particular depths, stressing the fact that the oil had been unevenly swept, and that early water breakthrough was to occur earlier than expected at the producers. This interpretation was backed-up by production data and saturation logging, which indicated respectively water cut and localized saturation increases.