Core retrieving tool

1. A core retrieving tool for retrieving a core sample of an underground formation, the core retrieving tool comprising:

an outer housing;

a coring bit connected to a first end of the outer housing;

a core-receiving chamber having a first end and a second end enclosed by the outer housing;

a core barrel positioned within the core-receiving chamber that receives the core sample entering the core-receiving chamber;

an expandable fluid chamber positioned within the outer housing in fluid communication with the second end of the core-receiving chamber, the expandable fluid chamber being expandable in response to a pressure differential between a pressure of the core-receiving chamber and an external pressure;

an inner housing positioned between the outer housing and the core barrel, the inner housing defining the core-receiving chamber and the expandable fluid chamber, the inner housing being removable from the outer housing; and

a valve carried by the inner housing, the valve having an open state that permits the core sample to enter the core-receiving chamber, and a closed state that seals the first end of the core-receiving chamber, wherein, with the valve in the closed state, the expandable fluid chamber is in open fluid communication with the core-receiving chamber such that a pressure of the expandable fluid chamber is equalized with the pressure of the core-receiving chamber.

2. The core retrieving tool of claim 1 , wherein the expandable fluid chamber is in open fluid communication with the core-receiving chamber when the valve is in the open state and in the closed state.

3. The core retrieving tool of claim 1 , the expandable fluid chamber comprising a piston that is movable away from the core receiving chamber in response to the pressure differential.

4. The core retrieving tool of claim 3 , wherein the piston separates the expandable fluid chamber into a first volume and a second volume such that, as the piston moves toward an expanded position, the second volume contracts and the first volume expands and receives fluids from the core-receiving chamber.

5. The core retrieving tool of claim 4 , wherein the second volume is filled with hydraulic fluid that is expelled as the second volume contracts.

6. The core retrieving tool of claim 5 , further comprising a check valve in fluid communication with the second volume that permits the hydraulic fluid to be expelled from the second volume.

7. The coring retrieving tool of claim 1 , further comprising a transfer valve in fluid communication with the expandable fluid chamber for selectively removing fluid from the inner housing, the fluid being stored within the expandable fluid chamber.

8. A method of retrieving a core sample of an underground formation, the method comprising the steps of:

inserting a core retrieving tool in a wellbore, the core retrieving tool comprising an outer housing carrying a coring bit, and a core barrel moveably positioned within a core-receiving chamber within the outer housing, the outer housing further defining an expandable fluid chamber in fluid communication with the core-receiving chamber;

drilling the core sample with the coring bit and causing the core sample to enter the core-receiving chamber such that the core sample is received by the core barrel;

sealing the core sample within the core-receiving chamber;

withdrawing the core retrieving tool from the wellbore and permitting the expandable fluid chamber to expand in response to a pressure differential between a pressure of the core-receiving chamber and an external pressure, the expandable fluid chamber being in open, fluid communication with the core-receiving chamber such that a pressure of the expandable fluid chamber is equalized with the pressure of the core-receiving chamber; and

transferring fluid from the expandable fluid chamber or the core-receiving chamber to a storage container.

9. The method of claim 8 , wherein the expandable fluid chamber is in open fluid communication with the core-receiving chamber before and after a valve seals the core sample within the core-receiving chamber.

10. The method of claim 9 , wherein permitting the expandable fluid chamber to expand comprises moving a piston within the expandable fluid chamber from a first position toward the core receiving chamber towards a second position away from the core receiving chamber.

11. The method of claim 10 , wherein the expandable fluid chamber comprises the piston that moves to expand a first volume in response to the pressure differential to receive fluids from the core-receiving chamber and contract a second volume.

12. The method of claim 11 , wherein second volume is filled with hydraulic fluid that is expelled through a check valve as the second volume contracts.

13. The method of claim 8 , wherein the core retrieving tool further comprises an inner housing between the outer housing and the core barrel, wherein the inner housing defines the core-receiving chamber and the expandable fluid chamber, the inner housing carries a valve, and the core barrel is moveable within the inner housing.

14. The method of claim 13 , further comprising the step of removing the inner housing from the outer housing within the core sample within the inner housing.

15. A core retrieving tool for retrieving a core sample of an underground formation, the core retrieving tool comprising:

an outer housing;

a coring bit connected to a first end of the outer housing;

a core-receiving chamber having a first end and a second end enclosed by the outer housing;

a core barrel positioned within the core-receiving chamber that receives the core sample entering the core-receiving chamber;

an expandable fluid chamber positioned within the outer housing in fluid communication with the second end of the core-receiving chamber, the expandable fluid chamber being expandable in response to a pressure differential between a pressure of the core-receiving chamber and an external pressure;

a piston that separates the expandable fluid chamber into a first volume and a second volume filled with hydraulic fluid;

a check valve in fluid communication with the second volume that permits the hydraulic fluid to be expelled from the second volume such that, as the piston moves toward an expanded position, the first volume expands and receives fluids from the core-receiving chamber and the second volume contracts and expels hydraulic fluid via the check valve; and

a valve having an open state that permits the core sample to enter the core-receiving chamber, and a closed state that seals the first end of the core-receiving chamber, wherein, with the valve in the closed state, the expandable fluid chamber is in open fluid communication with the core-receiving chamber such that a pressure of the expandable fluid chamber is equalized with the pressure of the core-receiving chamber.

16. The core retrieving tool of claim 15 , further comprising an inner housing positioned between the outer housing and the core barrel, the inner housing defining the core-receiving chamber and the expandable fluid chamber, the inner housing being removable from the outer housing, the inner housing carrying the valve.

17. The core retrieving tool of claim 15 , wherein the expandable fluid chamber is in open fluid communication with the core-receiving chamber when the valve is in the open state and in the closed state.

18. The core retrieving tool of claim 15 , the expandable fluid chamber comprising a piston that is movable away from the core receiving chamber in response to the pressure differential.

19. The coring retrieving tool of claim 15 , further comprising a transfer valve in fluid communication with the expandable fluid chamber for selectively removing fluid from the inner housing, the fluid being stored within the expandable fluid chamber.