Method for measuring formation properties with a time-limited formation test
An apparatus and method for determining at least one downhole formation property is disclosed. The apparatus includes a probe and a pretest piston positionable in fluid communication with the formation, and a series of flowlines pressure gauges, and valves configured to selectively draw into the apparatus for measurement of one of formation fluid and mud. The method includes performing a first pretest to determine an estimated formation parameter; using the first pretest to design a second pretest and generate refined formation parameters whereby formation properties may be estimated.
1. 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.