Methods and apparatus to perform pressure testing of geological formations

1. A method for pressure testing a geological formation comprising:

positioning a testing tool in a wellbore formed in the geological formation;

sealing a sample interval around the testing tool;

sealing a first guard interval around the testing tool and adjacent to the sample interval;

reducing a first pressure in the sample interval;

reducing a second pressure in the first guard interval;

maintaining a volume of a first chamber fluidly coupled to the sample interval during a time interval; and

measuring a plurality of pressure data for a fluid captured in the first chamber during the time interval.

2. A method as defined in claim 1 , further comprising actuating a pump fluidly coupled to the sample interval to perform a cleanup operation and to reduce the first pressure to a drawdown pressure.

3. A method as defined in claim 1 , wherein sealing the sample interval comprises extending first and second packers around the testing tool, and wherein sealing the first guard interval comprises extending a third packer around the testing tool, the first guard interval formed by the second and third packers.

4. A method as defined in claim 1 , further comprising:

sealing a second guard interval around the testing tool and adjacent to the sample interval; and

reducing a third pressure in the second guard interval.

5. A method as defined in claim 1 , wherein the second pressure is reduced to substantially a formation pressure.

6. A method as defined in claim 1 , wherein the second pressure is less than a wellbore pressure and greater than a formation pressure to mechanically stabilize the sample interval.

7. A method as defined in claim 1 , further comprising maintaining the second pressure in the first guard interval during the time interval.

8. A method as defined in claim 1 , further comprising actuating a pump fluidly coupled to the first guard interval to reduce the second pressure.

9. A method as defined in claim 8 , wherein the pump comprises a variable-volume second chamber.

10. A method as defined in claim 1 , further comprising maintaining a pressure difference between the sample interval and the first guard interval during the time interval.

11. A method as defined in claim 10 , wherein the pressure difference is maintained at substantially zero.

12. A downhole tool for pressure testing a geological formation, the tool comprising:

first and second packers to form an inner interval around the testing tool;

a third packer to seal a first outer interval around the testing tool adjacent to the inner interval;

a first pump to reduce a first pressure in the inner interval;

a second pump to reduce a second pressure in the first outer interval; and

a pressure gauge to measure a plurality of pressure data for a fluid captured in the inner interval while the second pressure is reduced and a volume of the inner interval is maintained.

13. A downhole tool as defined in claim 12 , further comprising:

a fourth packer to seal a second outer interval around the testing tool adjacent to the inner interval, the second outer interval located on an opposite of the inner interval from the first outer interval;

a third pump to reduce a third pressure in the second outer interval.

14. A downhole tool as defined in claim 12 , wherein the first pump is to perform a cleanup operation and to reduce the first pressure to a drawdown pressure.

15. A downhole tool as defined in claim 12 , wherein the second pressure is reduced to substantially a formation pressure.

16. A downhole tool as defined in claim 12 , wherein the second pressure is reduced to less than a wellbore pressure and greater than a formation pressure to increase a mechanical stability of the first and second packers.

17. A downhole tool as defined in claim 12 , wherein the first pump comprises the second pump.

18. A downhole tool as defined in claim 12 , wherein the second pump comprises a variable-volume second chamber.

19. A downhole tool as defined in claim 18 , further comprising a pressure controller to maintain the second pressure in the first outer interval while the plurality of pressure data is measured.

20. A downhole tool as defined in claim 12 , further comprising a pressure controller to maintain a pressure difference between the inner interval and the first outer interval while the plurality of pressure data is measured.

21. A downhole tool as defined in claim 20 , wherein the pressure controller maintains the pressure difference at substantially zero.