Patent ID: 7617049

Claim:
A method for determining a distance between a twin well and a target well, the method comprising: (a) deploying a drill string in the twin well, the drill string including a magnetic sensor in sensory range of magnetic flux emanating from the target well, the target well being magnetized such that it includes a substantially periodic pattern of opposing north-north (NN) magnetic poles and opposing south-south (SS) magnetic poles spaced apart along a longitudinal axis thereof; (b) measuring a magnetic field with the magnetic sensor; (c) processing the magnetic field measured in (b) to determine a magnitude of an interference magnetic field attributable to the target well; (d) processing the magnitude of the interference magnetic field to determine a preliminary distance to the target well; (e) estimating an axial position of the magnetic sensor relative to at least one of the opposing magnetic poles imparted to the target well; and (f) processing the preliminary distance determined in (d) and the axial position estimated in (e) to determine a corrected distance to the target well.