Loss of annular integrity in a well can create loss of zonal isolation, which can significantly impair the productive capability and efficiency of the well. Effective remedial action must be based on a clear understanding of the underlying causes. Cement evaluation wireline tools provide static information on cement sheaf placement but do not provide dynamic fluid movement measurements. Hence it has often been difficult or near-impossible to locate and explain the undesired fluid movements symptomatic of loss of annular integrity. Without this information, repair or control is at best a matter of informed guesswork or trial-and-error.
An oil well in offshore Indonesia was logged with the conventional Cement Bond Log (CBL) and production logging suite (PLT) to identify the water source causing very high water-cut from a reservoir believed to be low in water-cut oil production. A wide range of data including mud logs, petrophysical and pressure-test data during drilling gave us high confidence the reservoir should produce dry oil. However, the CBL and PLT survey failed to provide an answer to the unwanted water source. The CBL data suggested the cement placement behind casing is good-to-fair quality and the PLT data indicated the water was entering the wellbore at the perforated interval. This, however, is contradictory to the reservoir model that predicts the oil-water contact is far below the perforated zone.
To resolve this problem a further dataset was acquired from Acoustic Spectral Noise Analyzer Tool combined with fast response temperature were run in order to identify the true water source. The results showed water was coming from an unperforated interval flowing via channeling behind casing to the perforated zone. This water was flowing preferentially as the oil is of lower mobility preventing efficient flow from the reservoir into the wellbore.
The development of an ultra-sensitive Acoustic Spectral Noise Analyzer Tool combined with temperature analysis techniques can significantly improve zonal isolation surveillance.