Pressurizing rotating control devices

1. A rotating control device (RCD) for use in drilling a wellbore, the device comprising:

a RCD body;

a sealing element; and

a bearing assembly disposed within the RCD body and supporting an inner mandrel to rotate relative to the RCD body, the sealing element carried by the inner mandrel, the bearing assembly comprising a bearing sealed in an internal bearing fluid chamber, the internal bearing fluid chamber comprising a bearing fluid maintained at a first pressure greater than a second pressure of wellbore fluid in an interior of the RCD body by a pressure compensating piston between the bearing fluid contacting a first end of the piston and the wellbore fluid contacting a second end of the piston opposite to the first end of the piston, wherein the piston is disposed between the internal bearing fluid chamber and a wellbore fluid chamber in fluid communication with the wellbore fluid, the piston biased to pressurize the bearing fluid in the internal bearing fluid chamber to the first pressure greater than the second pressure of the wellbore fluid.

2. The rotating control device of claim 1 , wherein the wellbore fluid chamber comprises a flow port to the interior of the RCD body to allow wellbore fluid to flow into the wellbore fluid chamber.

3. The rotating control device of claim 1 , wherein the piston is housed in a piston housing integral with the bearing assembly.

4. The rotating control device of claim 1 , wherein the piston is housed in a piston housing apart from the bearing assembly.

5. The rotating control device of claim 4 , comprising a flow port connecting the bearing fluid in the internal bearing fluid chamber to a chamber in the piston housing about the first end of the piston to allow bearing fluid to flow about the first end of the piston.

6. The rotating control device of claim 1 , comprising a spring acting on the piston to bias the piston to pressurize the bearing fluid to the first pressure greater than the second pressure of the wellbore fluid.

7. The rotating control device of claim 6 , wherein the spring comprises a compression spring to push against the second, opposite end of the piston.

8. The rotating control device of claim 6 , wherein the spring comprises a tension spring acting to pull on the first end of the piston.

9. The rotating control device of claim 1 , the piston further comprising an annular piston member extending into a sealed annular chamber comprising a fluid at a third pressure, the fluid at the third pressure configured to bias the piston to pressurize the bearing fluid to the first pressure greater than the second pressure of the wellbore fluid.

10. The rotating control device of claim 9 , wherein the annular piston member disposed in the sealed annular chamber contacts the fluid at the third pressure at a third end of the annular piston member and contacts a fluid at a fourth pressure at a fourth, opposite end of the annular piston member, the third pressure being greater than the fourth pressure.

11. The rotating control device of claim 1 , wherein a first surface area of the first end of the piston contacting the bearing fluid is greater than a second surface area of the second, opposite end of the piston contacting the wellbore fluid.

12. The rotating control device of claim 1 , wherein the rotating control device is free from control lines exterior to the rotating control device to maintain the bearing fluid at the first pressure.

13. A method comprising:

receiving a pressure from a wellbore fluid in a wellbore on a bearing system of a rotating control device (RCD), the rotating control device comprising a RCD body, a sealing element, and a bearing assembly disposed within the RCD body and supporting an inner mandrel to rotate relative to the RCD body, the sealing element carried by the inner mandrel, the bearing assembly comprising a bearing sealed in an internal bearing fluid chamber; and

maintaining a pressure of the bearing fluid at a first pressure within the internal bearing fluid chamber greater than a second pressure of the wellbore fluid in an interior of the RCD body by a pressure compensating piston between the bearing fluid contacting a first end of the piston and the wellbore fluid contacting a second end of the piston opposite the first end of the position, wherein the piston is disposed between the internal bearing fluid chamber and a wellbore fluid chamber in fluid communication with the wellbore fluid, the piston biased to pressurize the bearing fluid in the internal bearing fluid chamber to the first pressure greater than the second pressure of the wellbore fluid.

14. The method of claim 13 , wherein pressurizing the bearing fluid in the internal bearing fluid with the piston comprises biasing the piston with at least one of a spring acting on the piston, a fluid in a sealed pressure chamber acting on the piston, or a differential area between the first end and the second, opposite end of the piston.

15. The method of claim 13 , wherein maintaining a pressure within the internal bearing fluid chamber greater than a pressure of the wellbore fluid in an interior of the RCD body comprises pressurizing the bearing fluid in the internal bearing fluid chamber with bearing fluid in a sealed pressure chamber in fluid communication with the internal bearing fluid chamber, and wherein the sealed pressure chamber is disposed closer to the RCD body than a top surface of the wellbore.