Ann: Quarterly Report & Cashflow, page-70

A few comments on LWD versus wireline depth control.

LWD is run using driller's depth....ie a geolograph or similar depth sensor with corrections done to data from same by referencing the drillers pipe tally.

Wireline depth control is done differently. A reference down log is usually done while running in hole with the first logging run. This is corrected for cable stretch and all subsequent logging runs are tied into this reference log. Companies such as Baker sometimes also use magnetic depth marks on their cable as a further check on depth.

When you lay out an LWD log next to a wireline log you'll notice the traces are off depth with the difference in depth increasing with depth.

If you're running a wireline formation tester using points picked off an LWD log then you'd usually do a GR pass with the wireline tool over the zone of interest and then depth correct the points you've picked on LWD back to wireline depth. Its a pain in the butt procedure and even after applying depth corrections there will be errors....large enough to be an issue if youre trying to test/sample thin sands.

Ive been on a number of jobs where the client has decided to run a formation tester with test points picked on LWD but with not much thought given to depth control procedures and errors associated with same. They've been some of the most nightmarish wireline jobs Ive ever run.