Investigating a subterranean annulus using a radiation detector to identify a substance

摘要

A method of investigating a wellbore 402 by directing radiation from source 428 within a tubular 412. Radiation is directed along a path oblique to an axis of the tubular, so that radiation passes through the tubular into an annulus 422 around the tubular and scatters back into the tubular. The radiation is detected by detectors 426 and a material in the annulus is identified based on the rate of detection of radiation, or count rate. There may be a second set of radiation directed along a second path, and the thickness of the annulus may vary around its circumference. A fluid may be identified when a ratio of the rate of detection of radiation reduces with reducing thickness. A second invention relates to a gamma ray source in a logging instrument where scattered rays are classified. A third invention relates to downhole imaging using a baseline image and an updated image.