Use of Borehole Gravimetry for Reservoir Characterisation and Fluid Saturation Monitoring

Abstract

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^

GRAVILOG: An update on the development and use of Borehole Gravity for Mining Exploration

“…Borehole gravity does not require active sources, and extends the density information beyond the borehole. This unique feature of borehole gravity data has been successfully used by the petroleum industry for reservoir characterization and fluid saturation monitoring (Popta et al, 1990) and for locating off-hole reef formations (Rasmussen, 1975). When the geology is more complex and dipping, these calculations yield apparent densities and require gravity modeling and inversion techniques to interpret.…”

“…A tool for acquiring borehole gravity measurements was built by LaCoste & Romberg in the early 1960s, and survey results have been reported in several papers between 1966and 1990(see, for example, McCulloh, 1966, and Popta et al, 1990. The deployment of these first generation borehole gravimeters was limited to large-diameter, near vertical boreholes, and their use was almost exclusively for hydrocarbons.…”

Co2 Capture Project - An Integrated, Collaborative Technology Development Project for Next Generation Co2 Separation, Capture and Geologic Sequestration

“…The reservoir is between 1325 and 1350 m outlined in blue. Popta et al (1990) showed that a geological structure with a sufficient density contrast can be detected by borehole gravity measurements if the observation well is not further away than one or two times the thickness of the anomalous zone. This means that borehole gravity could be used to detect CO2-water saturation changes up to 60m away from the borehole in this case.…”

West Coast Regional Carbon Sequestration Partnership - Report on Geophysical Techniques for Monitoring Co2 Movement During Sequestration

“…The change in the response is about 0.1 EU, which is not measurable with current technology. Popta et al (1990) showed that a geological structure with a sufficient density contrast can be detected by borehole gravity measurements if the observation well is not further away than one or two times the thickness of the zone of density contrast. Figure 13 shows a CO 2 wedge of 250 m radius and density of 2,260 kg/m 3 (representing 20%…”