Determining the pressure of formation fluid in earth formations surrounding a borehole
First Claim
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.
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Accused Products
Abstract
A method for determining formation fluid pressure in earth formation surrounding a borehole wall uses a downhole probe coupled to a variable-volume cavity. The probe is driven into contact with formation at the borehole wall. The method includes expanding the volume of the cavity during a first period of time to establish fluid communication between tool fluid and formation fluid, by withdrawing a minimal amount of fluid from the formation. During a second period of time the tool pressure is allowed to equilibrate to formation pressure. When pressure equilibrium is established, formation fluid pressure is set equal to tool pressure. A preferred embodiment includes terminating expanding the volume of the cavity on detecting a break in the mud cake seal. An associated formation pressure tester tool includes an elongated body; a probe defining a formation fluid inflow aperture, an electromechanical assembly defining a variable-volume cavity, a pretest flow line coupling the aperture to the cavity, a pressure sensor coupled to the cavity; and downhole electronic means for controlling the expansion of the volume of the cavity.
82 Citations
23 Claims
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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:
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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. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11)
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12. A tool 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 tool comprising:
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an elongated body adapted for downhole operation; a probe, extendable from the elongated body, the probe defining an inflow aperture and a low-volume flow line; a pretest piston pump defining a variable-volume pretest cavity coupled to the inflow aperture via the low-volume flow line; a) means for expanding the volume of the pretest cavity in sufficient amount to produce a break in the mud cake seal, b) means for detecting an occurrence of a break in the mud cake seal, c) means for 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 pretest cavity fluid and formation fluid; and d) means for minimizing the volume of fluid drawn, thereby preventing excesive overshoot, such that formation pressure is determined more quickly and the risk of the tool sticking in the borehole is reduced, and a pressure sensor coupled to measure pressure in the pretest cavity. - View Dependent Claims (13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23)
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Specification