Why use the Shuttle Sonde?

The practical vertical resolution of Borehole Gravity Deep Density Logging has been limited in the past to intervals greater than 3 meters. This is due to the rapid increase in density error caused by increasing depth measurement errors in small depth intervals. This uncertainty results from measuring the depth change at the surface (either electronic odometers or paint marks), not at the tool itself. Logging cables stretch under tool weight, cable weight, and hole friction, so depth changes measured at the surface do not necessarily indicate downhole tool movements accurately.

What is the Shuttle Sonde?

The Shuttle Sonde moves the gravity sensor over a 2.5 meter interval inside the tool housing. A miniaturized stepper motor winch moves the gravity sensor up and down with a small cable. Small wheels attached to optical encoders push on the inside of the tool housing and can measure depth changes to an accuracy of +/- 1 mm.

Applications of the Shuttle Sonde:

1. Monitoring fluid saturations and fluid movements in thin beds/short intervals.

This method is useful in certain gas reservoirs and gas drive oil reservoirs. Multiple casing strings, large cement filled washouts, deep invasion, low formation water salinity, and other types of formation damage can restrict the use of neutron and other nuclear measurements. BHGM density is relatively unaffected by these types of problems.

Shuttle BHGM surveys can be done in either single survey mode or time-lapse mode. Time-lapse surveys can give more accurate saturation changes because some error sources (esp. porosity and terrain corrections) remain constant. However a means of relocating the tool in the well to within 1 cm is required. The EDCON high resolution CCL tool can do this if there are closely spaced casing collars. In one case special gaps between casing joints were used to physically latch the tool to the same location (Alixant & Mann,1995).

2. Locating bypassed production in thin beds.

In many older wells potentially productive zones have been bypassed in favor of the main production target. In shaley sand sequences the bypassed zones sometimes can be located using the gamma ray or SP logs originally run in the open hole, otherwise a neutron thermal decay time log could be run to locate zones for further investigation with the BHGM tool.

More Information:

The Shuttle Sonde was originally developed by EDCON, Inc. with support from the Gas Research Institute.