Patent ID: 7290443

Claim:
A method for determining an estimated formation pressure from a drawdown operation using a formation tester in a formation penetrated by a borehole, comprising: setting the formation tester against a wall of the borehole so that a pretest chamber of the formation tester is in fluid communication with the formation, a drilling fluid in the pretest chamber having a higher pressure than the formation pressure; decompressing the drilling fluid in the pretest chamber by withdrawing a pretest piston in the formation tester at a constant drawdown rate; collecting data points representing fluid pressures in the pretest chamber as a function of time; identifying a range of consecutive data points that fit a line of pressure versus time with a fixed slope, the fixed slope being based on a compressibility of the drilling fluid, the constant drawdown rate, and a volume of the pretest chamber; and determining the estimated formation pressure from a first data point after the range of the consecutive data points.