Downhole sampling tool with check valve piston

1. A sampling tool, comprising:

a tubular member;

a sampling piston positioned within the tubular member, wherein the sampling piston has a bore formed axially-therethrough;

a secondary piston positioned within the bore of the sampling piston; and

a check valve assembly, wherein the secondary piston and the check valve assembly are configured to move together, simultaneously and in the same direction, with respect to the tubular member.

2. The sampling tool of claim 1 , wherein the sampling piston is configured to move axially within the tubular member, and wherein the secondary piston is configured to move axially within the bore of the sampling piston.

3. The sampling tool of claim 2 , wherein the secondary piston is prevented from exiting the bore of the sampling piston by first and second barriers that are axially-offset from one another.

4. The sampling tool of claim 1 , wherein at least a portion of the check valve assembly is configured to be inserted into the bore of the sampling piston.

5. The sampling tool of claim 1 , wherein the check valve assembly comprises:

a housing;

a seat positioned within the housing;

an impediment positioned within the housing and configured to be received within the seat; and

a biasing member positioned within the housing that exerts an axial force on the impediment toward the seat.

6. The sampling tool of claim 5 , wherein the check valve assembly further comprises a check valve piston positioned within the housing, wherein the biasing member is positioned at least partially between the check valve piston and the impediment.

7. The sampling tool of claim 6 , wherein an outer surface of the check valve piston comprises an axial groove that provides an axial flow path between the check valve piston and the housing.

8. The sampling tool of claim 7 , wherein the outer surface of the check valve piston also comprises a protrusion that is circumferentially-offset from the axial groove, and wherein an inner surface of the housing comprises a protrusion.

9. The sampling tool of claim 8 , wherein the check valve piston is configured to be inserted into the housing when the axial groove of the check valve piston is rotationally-aligned with the protrusion of the housing, and wherein the check valve piston is secured within the housing when the protrusion of the check valve piston is rotationally-aligned with the protrusion of the housing.

10. The sampling tool of claim 1 , wherein the sampling tool is configured to capture a fluid sample in a wellbore, and wherein a ratio of the fluid sample to water in the sampling tool is greater than about 50:1.

11. A system, comprising:

a sampling piston configured to move axially within a tubular member, wherein the sampling piston has a bore formed axially-therethrough;

a secondary piston positioned within the bore of the sampling piston, wherein the secondary piston is configured to move axially within the bore of the sampling piston; and

axially-offset barriers within the bore of the sampling piston configured to prevent the secondary piston from exiting the sampling piston, wherein at least one of the axially-offset barriers is removable from the sampling piston.

12. The system of claim 11 further comprising a check valve assembly, wherein at least a portion of the check valve assembly is configured to be inserted into the bore of the sampling piston, and wherein the secondary piston and the check valve assembly are configured to move together with respect to the tubular member.

13. The system of claim 12 , further comprising a fixing head positioned at least partially within the tubular member, wherein the check valve assembly is positioned at least partially within the fixing head.

14. The system of claim 13 , wherein the check valve assembly is configured to move axially within the fixing head to substantially equalize a pressure across the check valve assembly.

15. The sampling tool of claim 14 , wherein the fixing head defines a port, and wherein water is configured to flow out of the sampling tool through the port when the sampling piston moves toward the check valve assembly.

16. The sampling tool of claim 12 , wherein the check valve assembly comprises:

a housing;

a seat positioned within the housing;

an impediment positioned within the housing and configured to be received within the seat; and

a biasing member positioned within the housing that exerts an axial force on the impediment toward the seat.

17. A method for capturing a fluid sample in a wellbore, comprising:

running a sampling tool into the wellbore, wherein the sampling tool comprises:

a tubular member;

a sampling piston positioned within the tubular member, wherein the sampling piston has a bore formed axially-therethrough;

a secondary piston positioned within the bore of the sampling piston; and

a check valve assembly; and

increasing a pressure of a fluid in the tubular member on a first side of the sampling piston and the check valve assembly, causing the secondary piston and the check valve assembly to move together, simultaneously and in the same direction, with respect to the tubular member.

18. The method of claim 17 , further comprising capturing the fluid sample within the wellbore using the sampling tool after the secondary piston and the check valve assembly move together.

19. The method of claim 18 , wherein capturing the fluid sample comprises opening a valve on a second side of the sampling piston and the check valve assembly.

20. The method of claim 17 , further comprising:

pumping water into the sampling tool in a first direction; and

pumping additional water into the sampling tool in a second, opposing direction after the water is pumped into the sampling tool in the first direction and before the sampling tool is run into the wellbore.

21. The method of claim 20 , wherein at least a portion of the check valve assembly is inserted into the bore of the sampling piston in response to pumping the additional water into the sampling tool in the second, opposing direction.