Active Acoustic Ranging to Locate Two Nearby Wellbores in Deepwater Gulf of Mexico

Abstract

Salt drilling in Deepwater Gulf of Mexico (GOM) presents unique challenges. One of these challenges is the effect salt has on ranging technologies used in contingency relief well designs. A new technique called Active Acoustic Ranging (AAR) addresses the challenge of locating and tracking the target wellbore to the interception phase. This case study details the degree of precision that this new technique provided while locating two nearby wellbores within salt. This study is intended to improve industry awareness and understanding of relief well ranging options available to the industry, specifically wellbore ranging activities conducted within a salt formation. Sonic logging started in the early 1930s to determine rock characteristics by measuring the refracted signals from a combination of transmitters and receivers. The technique evolved by recording acoustic signals beyond the refracted zone, by positioning the transmitters and receivers downhole in the logging tool. AAR utilizes surface seismic processing methods to determine azimuthal direction and distance of compressional and shear acoustic signals, reflected from around the borehole. After processing the reflected signals, the distance and direction of nearby wellbores can be determined. This can be effective in salt formations, where resistivity inhibits use of active electromagnetic ranging tools. This case study presents test results conducted in a GOM Deepwater operation to locate two nearby wellbores, a cased hole and an open hole, using AAR. It shows that AAR signals can be successful in locating offset wellbores within salt formations. The acoustic signals, both compressional and shear, were recorded using a stack of 13 receivers. Each stack had 8 sector azimuthal receivers to determine the distance and direction of the corresponding target wellbores. By utilizing compressional and shear signals generated from the various distances of monopole and dipole transmitters, a redundant process was provided to determine the location of the target wellbores with a high degree of accuracy. In addition, the acoustic images provided estimates of the salt quality, which can be used to select the interception location for hydraulic kills. The maximum ranging distance is dependent on the velocity and attenuation of the transmitted acoustic signals in the traversed formations. Salt typically has higher velocities that will enable ranging at greater distances than other formations. One of the primary benefits of this technique is the ability to provide two different measurements using monopole and dipole sources to locate nearby wellbores. This reduces uncertainties that may arise when ranging with a system using only a single measurement. AAR benefits from the salt formation, whereas other ranging technologies, i.e. electromagnetic, were not optimized for salt. AAR increases the ranging options in salt formations.