Determining the pressure of formation fluid in earth formations surrounding a borehole

• US 20050039527A1
• Filed: 08/20/2003
• Published: 02/24/2005
• Est. Priority Date: 08/20/2003
• Status: Active Grant

• Current Assignee: Schlumberger Technology Corporation (Schlumberger NV)
• Original Assignee: Schlumberger Technology Corporation (Schlumberger NV)
• Inventors: Shah, Jagdish, Jacobson, Aaron, Dhruva, Brindesh, Trittschuh, Jennifer, Supp, Michael G., Jenet, Fredrick A., Pierre, Stephane, Dussan, Elizabeth B. V
• Granted Patent: US 7,178,392 B2
• Time in Patent Office: Days
• US Class Current: 73/152.270
• CPC Class Codes: E21B 49/008 by injection test; by analysing pressure variations in an injection or production test, e.g. for estimating the skin factor measuring pressure E21B47/06
• CPC Class Codes: E21B 49/10 using side-wall fluid samplers or testers

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.

Claims

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 a the probe into contact with the mud cake;

expanding the volume of the cavity in sufficient amount to produce a break in the mud cake seal during a draw-down period;

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; and

setting formation fluid pressure equal to measured pressure.

2. A method according to claim 1, wherein detecting the break in the mud cake seal includes measuring cavity pressure and detecting an abrupt change associated with cavity pressure.

3. A method according to claim 2, wherein detecting the abrupt change includes using a finite moving average (FMA) algorithm on a function of cavity pressure.

4. A method according to claim 3, wherein the function of cavity pressure includes cavity pressure.

5. A method according to claim 3, wherein the function of cavity pressure includes a first derivative of cavity pressure.

6. A method according to claim 3, wherein the function of tool pressure includes a second derivative of cavity pressure.

7. A method according to claim 1, wherein detecting the a break in the mud cake seal includes detecting a difference between a measured cavity pressure and a corresponding cavity pressure from a reference cavity pressure profile.

8. A method according to claim 7, wherein the reference cavity pressure profile is measured in a previous drawdown with the cavity isolated from the formation.

9. A method according to claim 1, further comprising:

expanding the volume of the cavity during the draw-down period at a predetermined constant rate.

10. A method according to claim 9, wherein the predetermined constant rate is within the range of 3-160 cc/minute.

11. A method according to claim 10, wherein the predetermined constant rate is approximately 5 cc/minute.

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:

an elongated body adapted for downhole operation;

a probe, extendable from the elongated body, the probe defining an inflow aperture;

a pretest piston pump defining a variable-volume pretest cavity coupled to the inflow aperture;

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, and

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

a pressure sensor coupled to measure pressure in the pretest cavity.

13. A tool according to claim 12, wherein the control means includes an electromechanically driven roller screw planetary system.

14. A tool according to claim 13, wherein the control means further includes an electrically driven gearbox coupled to drive the roller screw planetary system.

15. A tool according to claim 12, wherein the control means includes downhole programmable control electronics coupled to control the electromechanical control means.

16. A tool according to claim 12, wherein the tool includes a constant-volume flow line.

17. A tool according to claim 16, wherein the constant-volume flow line includes a dedicated probe.

18. A tool according to claim 16, wherein the constant-volume flow line includes a flexible conduit.

19. A tool according to claim 16, wherein the constant-volume flow line has a volume in the range 20-120 cc.

20. A tool according to claim 12, wherein the probe is located between the pressure measuring means and the variable-volume pretest cavity.

21. A tool according to claim 12, further comprising a sample line coupled to the cavity, and an isolation valve located between the cavity and the sample line.

22. A tool according to claim 12, further comprising an isolation valve located between the cavity and the formation fluid inflow aperture.