METHOD AND APPARATUS FOR FAST PORE PRESSURE MEASUREMENT DURING DRILLING OPERATIONS

• US 20040144533A1
• Filed: 01/27/2003
• Published: 07/29/2004
• Est. Priority Date: 01/27/2003
• Status: Active Grant

• Current Assignee: Schlumberger Technology Corporation (Schlumberger NV)
• Original Assignee: Schlumberger Technology Corporation (Schlumberger NV)
• Inventors: Zazovsky, Alexander
• Granted Patent: US 7,331,223 B2
• Time in Patent Office: Days
• US Class Current: 166/250.02
• CPC Class Codes: E21B 47/06 Measuring temperature or pressure
• CPC Class Codes: E21B 49/10 using side-wall fluid samplers or testers

Abstract

A method and apparatus for measuring the pressure of a formation penetrated by a wellbore is provided. A downhole tool is positioned in the wellbore and a probe is extended therefrom into sealing engagement with the formation. A piston in the probe is retracted therein where by a cavity is defined for receiving a fluid from the formation. A pressure underbalance in the cavity draws fluid from the formation into the cavity. An oscillator may be used to fluctuate the flow of fluid into the cavity. A pressure gauge is used to measure the pressure of fluid in the cavity.

Claims

1. An apparatus for measuring the pressure of an underground formation penetrated by a wellbore, comprising:

a housing positionable in the wellbore;

a probe operatively connected to said housing, said probe positionable in sealing engagement with a sidewall of the wellbore;

a piston contained in said probe and axially movable therein, said piston having an end positionable adjacent the sidewall of the wellbore and retractable therefrom whereby a cavity is defined for receiving fluid from the formation;

an oscillator operatively connected to the piston for fluctuating the flow of fluid into the cavity; and

a gauge for measuring the pressure of fluid in the cavity.

2. The apparatus as described in claim 1 wherein said probe has a packer adapted to seal with the sidewall of the wellbore.

3. The apparatus of claim 1 further comprising a seal between the piston and the probe.

4. The apparatus of claim 1 wherein the probe is extendable and retractable from the housing.

5. The apparatus of claim 1 wherein the piston has a chamber therein adapted to receive the oscillator.

6. The apparatus of claim 1 wherein the oscillator is adapted to axially extend and retract from the piston.

7. The apparatus of claim 1 wherein movement of the oscillator vibrates fluid in the cavity.

8. The apparatus of claim 1 wherein the gauge is operatively connected to the cavity via a conduit.

9. The apparatus of claim 1 wherein the conduit extends through the piston.

10. The apparatus of claim 1 wherein said apparatus is a drill string.

11. The apparatus of claim 1 wherein said apparatus is a wireline tool.

12. A method for measuring the pressure of an underground formation penetrated by a wellbore comprising the steps of:

placing a probe of a downhole tool in sealing engagement with a sidewall of the wellbore, the probe having a retractable piston therein defining a cavity;

retracting the piston within the probe such that an underbalance is created to draw fluid into the cavity;

equalizing the pressure in the cavity to the pressure of the formation;

oscillating the flow of fluid between the cavity and the formation and measuring the pressure in the cavity.

13. The method of claim 12 wherein the downhole tool is a wireline tool.

14. The method of claim 12 wherein the step of oscillating comprises extending and retracting an oscillator in the piston to vibrate fluid in the cavity.

15. The method as described in claim 12 wherein the step of placing comprises extending a probe with a packer thereon against the mudcake to create a seal therewith, the probe having a retractable piston therein defining a cavity.

16. The method of claim 12 wherein the cavity is less than about 1 cu. cm.

17. The method of claim 12 wherein the step of retracting comprises decompressing formation fluid in the cavity by quickly displacing the piston a short distance into the probe and away from the formation interface.

18. The method of claim 17 wherein said piston is retracted such that the fluid continuity is broken and the pressure of the fluid drops to zero.

19. The method of claim 18 wherein the piston displacement is determined by estimating the initial cavity volume and taking into account fluid compressibility while neglecting fluid flow into the cavity.

20. The method of claim 19 wherein the piston displacement is determined from the following equation:

Δ

δ

≧

δ

Δ

p/K

where Δ

p=p_w is the pressure variation and K is the bulk modulus of fluid or the mixture of fluid and mud cake.

21. The method as described in claim 12 further comprising the steps of loosening external mudcake and mobilizing internal mudcake.

22. The method as described in claim 21 wherein the oscillator creates periodic pressure variations in the cavity, the pressure variations having a pressure amplitude.

23. The method as described in claim 22 wherein said pressure amplitude is determined from the following equation:

Δ

p=ρ

_fC_fω

where ρ

_f is the density of fluid (suspension) inside the volume, ω

is the velocity of fluid displacement by the oscillator and C_f is the sound velocity.

24. The method as described in claim 12 further comprising estimating the oscillation frequency of the pressure inside the confined fluid volume.

25. The method as described in claim 24 wherein an optimum pressure oscillation frequency is determined by the characteristic time of particle travel within the neck of a pore in a formation, where time is estimated by the following equation:

t_b≈

r_n/u

where r_n is the neck radius and u is the Darcy flow velocity.

26. The method as described in claim 25 wherein said oscillating frequency estimation is obtained by the following equation:

27. The method as described in claim 26 wherein pressure gradient

|∇

p|

depends on the pressure amplitude created by the oscillator in the cavity inside the probe and also on the distance from the rock surface into the formation.

28. The method of claim 12 further comprising analyzing the pressure measurements to determine downhole parameters.

29. The method of claim 12 wherein the downhole tool is a drilling tool.