Overcoming limitations in locating relief well targets
Relief wells are an important contingency response in the event of a loss of well control. The basic concept involves intercepting the target well at a particular depth and then pumping kill fluid from the relief well. To achieve this, operators need a reliable and robust method of locating, tracking, and intercepting the target.
Current directional survey technologies present a high cumulative uncertainty in the absolute location of the target well, posing the need for a direct and accurate locating technique. For example, magnetic ranging technologies are helpful but not optimal for this purpose, especially in salt formations, where active systems are sensitive to the salt resistivity and passive systems are limited by target wellbore residual magnetism.
For a contingency application in the Gulf of Mexico, BP was concerned about these limitations and wanted to test another solution to locate two target wellbores—one open hole and one cased hole—from an openhole wellbore.
Interpreting acoustic signals to enable ranging in salt
Schlumberger recommended 3D far-field sonic service, which uses AAR technology to identify formation features. The service employs reflected compressional and shear waveforms from the target wellbore as well as the acoustic properties of the formation rock to detect nearby wellbores and formation features and determine their distance, dip, and azimuthal direction.
Initially, the workflow used for 3D far-field sonic service manually identified the reflection from the wellbore that was ranged on eight azimuthal images, each covering a 45° sector, obtained at each of the Sonic Scanner acoustic scanning platform’s 13 receiver stations. As processing techniques have evolved, the dataset has been reprocessed using an automated sonic imaging technique that enables a more accurate and consistent azimuth and range estimate. Automation also reduces turnaround time and the bias that can be introduced by manual interpretation.