Design of a sliding-weight borehole gravimeterto measure rock density in highly deviated and narrow bore holes

摘要

Conventional borehole density instruments measureup to 30 cm deep, while borehole gravimeters lookup to 100 m away from the borehole. Gravimetersmatch vertical seismic profiling both in resolutionand reservoir coverage. Borehole gravimeters usingLacoste-Romberg spring sensors are currentlyavailable, but use is limited due to their largediameters and near vertical hole requirement. Somesurface gravimeters use a falling weight in a vacuumtube. The tube has to be positioned perfectly vertical,which prohibits its use in boreholes. This paperdescribes the design of a tilted sliding-weightinstrument that circumvents the verticalitylimitations. All measurements including tilt, traveltime and travel distances of the weight should havean accuracy of 1 in 10~8. This is achieved by usingaccurate tilt-meters for the angle, atomic clocks fortime, and a laser interferometer to measure thedistances. All these techniques are readily available.By subtracting the up and down going paths of thesliding weight, zeroed at the top of its trajectory, theeffect of friction is eliminated. A logging tool basedon this design is expected to resolve reservoirmanagement questions such as whether gas came outof solution, or from a secondary gas cap. Moreover,combining gravimeter and through casing resistivitydata, would allow saturations to be monitored overmuch larger reservoir volumes.