Patent ID: 7178392
Filing Date: 2007-02-20
Classification: E21B

Abstract:
1. A method for determining formation fluid pressure in earth formation surrounding a borehole, the borehole defining a borehole wall, the borehole wall covered with mud cake forming a mud cake seal, the method comprising: providing a tool defining a probe and a variable-volume pretest cavity fluid-coupled to the probe; pressing the probe into contact with the mud cake; expanding the volume of the cavity to draw fluid from the formation in sufficient amount to produce a break in the mud cake seal during a draw-down period; detecting an occurrence of a break in the mud cake seal by detecting an abrupt change in cavity pressure; holding constant the volume of the cavity immediately after detecting the occurrence of the break in the mud cake seal, for a sufficient build-up period to establish pressure equilibrium between cavity fluid and formation fluid; measuring pressure in the cavity; setting formation fluid pressure equal to measured pressure; and minimizing the volume of fluid drawn, thereby preventing excessive overshoot; such that formation pressure is determined more quickly and the risk of the tool sticking in the borehole is reduced.