Dual gradient drilling system and method

1. A dual gradient drilling system comprising:

a subsea blowout preventer disposed above a wellhead, the subsea blowout preventer comprising a central lumen configured to provide access to a wellbore;

a lower section of a marine riser fluidly connected to the subsea blowout preventer;

a closed-hydraulic positive displacement subsea pump system fluidly connected to the lower section of the marine riser and disposed at a subsea depth from shallow to mid-riser depth or equivalent;

an independent mud return line fluidly connecting one or more pump heads of the closed-hydraulic positive displacement subsea pump system to a floating platform of a rig without use of an additional pump system; and

an annular sealing system disposed above the closed-hydraulic positive displacement subsea pump system,

wherein a pump speed of the closed-hydraulic positive displacement subsea pump system is adjusted to achieve a target amount of fluid mass in a fluidly connected system upstream of the closed-hydraulic positive displacement subsea pump to achieve a target inlet pressure of the closed-hydraulic positive displacement subsea pump.

2. The dual gradient drilling system of claim 1 , further comprising:

a bypass riser injection system configured to bypass the closed-hydraulic positive displacement subsea pump system for injection of fluids into the lower section of the marine riser disposed below the closed-hydraulic positive displacement subsea pump system in total loss drilling conditions.

3. The dual gradient drilling system of claim 1 , further comprising:

a pressure relief valve that fluidly connects the lower section of the marine riser disposed below the closed-hydraulic positive displacement subsea pump system to an upper section of the marine riser disposed above the annular sealing system.

4. The dual gradient drilling system of claim 1 , further comprising:

a pressure relief valve that fluidly connects the independent mud return line to a top section of the marine riser disposed above the annular sealing system.

5. The dual gradient drilling system of claim 1 , further comprising:

an anti-u-tubing flow stop valve disposed on a booster line.

6. The dual gradient drilling system of claim 1 , further comprising:

an anti-u-tubing flow stop valve disposed on the drill string downhole.

7. The dual gradient drilling system of claim 1 , further comprising:

an annular packer or sealing device disposed below the closed-hydraulic positive displacement subsea pump system.

8. The dual gradient drilling system of claim 1 , wherein the independent mud return line is configured to divert returning fluids from the lower section of the marine riser disposed below the closed-hydraulic positive displacement subsea pump system to a choke manifold disposed on the floating platform of the rig.

9. The dual gradient drilling system of claim 1 , wherein the closed-hydraulic positive displacement subsea pump system comprises a first pump head, an independent linear drive motor, and a second pump head.

10. The dual gradient drilling system of claim 9 , wherein each of the first pump head and the second pump head comprise an inlet port, a bottom check valve assembly, a fluid cavity disposed between pressure balanced liners, a top check valve assembly, and an outlet port.

11. The dual gradient drilling system of claim 9 , wherein the independent linear drive motor comprises a reciprocating piston having a first piston face and a second piston face that is electronically actuated to compress or uncompress a hydraulic drive fluid in a closed-hydraulic system.

12. The dual gradient drilling system of claim 10 , wherein the pressure balanced liners isolate drilling fluids from hydraulic drive fluid.

13. The dual gradient drilling system of claim 1 , wherein the closed-hydraulic positive displacement subsea pump system comprises a hydraulic drive fluid that is wholly contained by the pump system and is not vented into a sea.

14. The dual gradient drilling system of claim 1 , wherein the closed-hydraulic positive displacement subsea pump system does not include dynamic seals exposed to drilling fluids.

15. The dual gradient drilling system of claim 1 , wherein a second linear drive motor is used to drive a third pump head and a fourth pump head, wherein the third pump head and fourth pump head are synchronized with the first pump head and the second pump head to provide a smooth flow rate.

16. The dual gradient drilling system of claim 1 , wherein the annular sealing system comprises an active control device, a rotating control device, or an annular seal configured to seal an annulus surrounding a drill string disposed therethrough.

17. The dual gradient drilling system of claim 1 , wherein the annular sealing system comprises one or more sealing elements.

18. The dual gradient drilling system of claim 1 , wherein dual gradient drilling operations are conducted with continuous circulation.

19. The dual gradient drilling system of claim 1 , wherein gas in the marine riser is controlled by the annular sealing system and diversion of riser fluids through the independent mud return line to a choke manifold and a mud-gas-separator disposed on the floating platform of the rig.

20. The dual gradient drilling system of claim 1 , wherein an upper section of the marine riser disposed above the annular sealing system is voided during dual gradient drilling operations.

21. The dual gradient drilling system of claim 1 , wherein an upper section of the marine riser disposed above the annular sealing system is filled with fluids sufficient to reduce or eliminate a pressure differential across the annular sealing device.

22. The dual gradient drilling system of claim 1 , wherein the predetermined depth is a subsea depth in a range between 3500 feet and 5500 feet.