Patent ID: 7350597

Claim:
A drilling system for drilling a bore hole into an earth formation, the bore hole having an inside wall, and the system comprising: a drill string reaching into the bore hole from a surface, leaving a drilling fluid return passage between the drill string and the bore hole inside wall; a bottom hole assembly supported by the drill string; a drilling fluid discharge conduit in fluid communication with the drilling fluid return passage; pump means for pumping a drilling fluid through the drill string into the bore hole and to the drilling fluid discharge conduit via the drilling fluid return passage; means for obtaining information on the existing down hole pressure of the drilling fluid in the vicinity of the bottom hole assembly; back pressure means for controlling the drilling fluid back pressure; back pressure control means for controlling the back pressure means, wherein the back pressure control means comprises a programmable pressure monitoring and control system arranged to receive the information on the existing down hole pressure, calculate a predicted down hole pressure using a model, compare the predicted down hole pressure to a desired down hole pressure, and to utilize the differential between the calculated and desired pressures to control said fluid back pressure means.