Borehole gravity in Horizontal Wells

Gravity can now be measured practically anywhere a drill hole can go. This extends our ability to perform 3D and 4D gravity mapping of the sub-surface with applications in the energy and mineral sectors. New tools have been developed to perform these measurements. The tools address issues of size, measurement accuracy, stability and ruggedness. The data handling has also changed to address the 3 dimensional nature of the measurements. Whereas previous borehole gravity measurements in near vertical wells could ignore latitude corrections and use simpler terrain correction, surveys in highly deviated wells involve calculating and using 3 dimensional coordinates for each gravity station in the correction process. This all needs to be done preserving the survey accuracy requirements that are often only a few microGals. Tool location is extremely important, more in the vertical direction due to the vertical free air and Bouguer gradients. Tool location control tends to be more difficult in more horizontal holes. A new high sampling resolution CCL tool has proven very useful for repeat location of the tool for drift calculations and time lapse measurements. For oil and gas, most applications in highly deviated or horizontal wells are expected to be of a time lapse mode such as monitoring of traditional and SAGD type steam floods. For mining the tool significantly extends the volume of investigation for deep exploration and development holes.