This paper describes the results of a field application of borehole gravimetry to measure secondary gas saturations in a fractured limestone reservoir. Owing to its deep-reading capability and insensitivity to near-wellbore effects, the borehole gravimeter succeeded where conventional cased-hole logging methods had failed. Borehole-fluid pressure data, recorded together with the gravity data, proved useful in ensuring that the density data had the necessary high accuracy. This paper additionally presents modelling results that indicate the potential usefulness of time-lapse borehole gravity data for monitoring flood fronts remote from a borehole. This application would benefit from the development of a sensor with a very reliable absolute calibration and low drift. Introduction The potential of borehole gravimetry In hydrocarbon exploration and production was recognised already in 1950 by Smith, and the results of such measurements have been reported since 1966. Several applications of the technique have become established, and additional applications have been suggested on the basis of modelling studies (see, for example, Ref. 4). Borehole gravimetry has attractive characteristics. A borehole gravimeter (BHGM) has a large radius of investigation, and the formation bulk density derived from its gravity data is hardly influenced by the borehole fluid, casing and near-borehole features, such as mud-filtrate invasion and drilling-induced formation disturbance. Therefore, the BHGM tool is suitable for use in a cased hole. In fact, it provides the only method of obtaining the important formation bulk density measurement through casing. Its deep density measurement can sometimes be turned to good advantage in the evaluation of exploration or appraisal wells. The benefits of BHGM density measurements could be even larger if the tool were used more widely in the area of reservoir management. The tool could be applied, for example, to monitor fluid saturations averaged over a large volume or the position of flood fronts remote from a borehole. Modelling studies of these two applications have been reported, but to our knowledge no field applications have been documented. P. 151^
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