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Question about Casing Magnetic Interference
05 October 2018
We have a rare condition now while drilling.
I appreciate any help that you guys can give me.
We sidetrack from a old vertical well which had both casing 7in and 9 5/8in.
Window opened from 3075m using whipstock, @ 280deg Azimuth

The plan is to increase the inclination to 80 deg.
The problem is although we know that in theory nearby casing magnetic interference to MWD Survey sensor will minimize in at least 10m distance.
When we get MWD survey the azimuth is show much less
Gyro reading is 280deg, while at same time MWD surveys read 242~237
MWD total H is almost 1000nT more than the reference

Even when DDs slide most of the time using GTF 30/60 deg right The surveys always show increase in inclination while slightly decrease azimuth!

My question is is this lower reading of azimuth is because of nearby casing which is still present? (right now center-to center distance is 11m)
Will it be corrected if we drill further.

Finally we are sure that MWD works fine, toolface is not turning, there is no magnetic hotspot in collars, MWD to Motor toolface was ok, MWD Internal toolface was programmed correctly.

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