What are criteria for using gyro surveys near other wells?

I'd like to get some feedback on standard practices for the requirement for gyro surveys when drilling near other wells, where there is the likely possibility of external (non-drillstring) magnetic interference. At one end there is Sperry Alaska, which plans for gyro when there is another well within 40 feet (Angus Jamieson recommends 50 feet). On the other end are some operators who do not routinely use gyro surveys even when pad drilling on 10 foot centers.

I recently completed a multi-well pad project on 17 foot centers. We did not use gyro surveys for the top hole. There was significant magnetic interference, but this was planned and allowed for by using an IPM that generated larger ellipses of uncertainty to account for the additional error. I'll be giving a talk on this at the ISCWSA meeting in Denver on November 3; the presentation will eventually be available on the iscwsa.org website.

I believe this methodology could be used even with much closer C-C distances, but there is reluctance by the service companies to rely on MWD surveys that don't pass their standard QC measures.

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