Patent ID: 6076268
Filing Date: 2000-06-20
Classification: E21B

Abstract:
A method for sub-surface placement of a directional tool in a bore hole and for determining the orientation of said directional tool within said bore hole comprising the steps of:measuring an electromagnetic field profile within said bore hole prior to a placement of ferromagnetic casing strings within said bore hole;measuring an electromagnetic field profile within said bore hole after said ferromagnetic casing strings have been placed within said bore hole;calculating a magnetic field bias profile brought about by a combination of modifications to the earth's magnetic field caused by ferrous anamolies in a formation around said bore hole and interferences caused by said ferromagnetic casing strings in order to establish a field bias for said bore hole;measuring an azimuthal orientation at a specific depth location in said bore hole;applying a field bias for said specific depth location within said bore hole; andcalculating a corrected azimuthal orientation based upon said applied field bias for said specific depth location.