Patent ID: 8230947

Claim:
A method of selecting a drilling motor for rotating a drill bit relative to a drill string made up of well casing, comprising: (a) determining a delivery torque (TDe) provided by a first candidate motor; (b) based on a cross-sectional area of the casing, determining a change in weight on the drill bit (DWOB) occurring in response to a change in drilling fluid pressure (DP) within the casing, the change in weight on the drill bit (DWOB) occurring as a result of a tendency for the casing to elongate in response to an increase in the drilling fluid pressure: (c) calculating a torque demand (TDm) for the casing based on the type of drill bit and the effect of DP on the change in weight on the bit (DWOB) determined in step (b); and (d) comparing the result of step (a) to the result of step (c) and if the result of step (a) does not exceed the result of step (c) by a selected amount, selecting a second candidate motor and repeating steps (a)-(d).