Patent ID: 7765862

Claim:
A method for determining true formation pressure while drilling in an earth formation, using recirculating fluid that flows downward through a drill string and returns to a surface in a void between the drill string and the earth formation that has a portion of mudcake thereon, the method comprising: a) providing a measurement device with a probe on the drill string; b) controlling a mud flow rate to obtain a first measured borehole pressure and measuring a corresponding first probe pressure with the probe wherein the corresponding first probe pressure is the pressure in the earth formation adjacent the mudcake; c) controlling the mud flow rate to obtain a second measured borehole pressure and measuring a corresponding second probe pressure with the probe wherein the corresponding second probe pressure is the pressure in the earth formation adjacent the mudcake; d) deriving the true formation pressure from calculating a first ratio from the first and second measured borehole pressures and the first and second probe pressures.