Retrievable frac mandrel and well control stack to facilitate well completion, re-completion or workover and method of use

1. A retrievable frac mandrel and well control stack for facilitating completion, re-completion or workover of a cased well having a wellhead with a tubing head spool, comprising:

the frac mandrel having a top end and a bottom end that are separate pieces interconnected by a threaded connection with a fluid seal to prevent a migration of stimulation fluids between the separate pieces, the bottom end being contoured to be received in the tubing head spool and to be locked in the tubing head spool by tubing head spool lock down screws that engage an annular groove in the bottom end, and to further provide a high-pressure fluid seal with a seal bore located above a bit guide of the tubing head spool;

a well control adapter that mounts to a top flange of the tubing head spool, the well control adapter including an axial seal bore that receives the top end of the frac mandrel to provide a high-pressure fluid seal around a periphery of the top end, and an axial passage that has a diameter smaller than an outer diameter of the top end of the frac mandrel but at least as large as an internal diameter of the frac mandrel; and

flow control equipment mounted to a top flange of the well control adapter.

2. The retrievable frac mandrel and well control stack as claimed in claim 1 wherein the high-pressure fluid seal with the axial seal bore located above the bit guide of the tubing head spool comprises a plurality of O-ring grooves in a periphery of the bottom end of the frac mandrel, each of the O-ring grooves receiving a high-pressure O-ring that seals against the seal bore.

3. The retrievable frac mandrel and well control stack as claimed in claim 2 wherein the bottom end of the frac mandrel further comprises a bevel that matches a bevel angle of the bit guide, and the bevel includes an O-ring groove that receives an O-ring which seals against the bit guide.

4. The retrievable frac mandrel and well control stack as claimed in claim 1 wherein the bottom end of the frac mandrel further comprises an O-ring groove located below the annular groove, and an O-ring received in the O-ring groove that seals against a secondary seal bore in the tubing head spool.

5. The retrievable frac mandrel and well control stack as claimed in claim 1 wherein the axial seal bore comprises O-ring grooves that receive O-rings to provide the high-pressure fluid seal around the periphery of the top end of the frac mandrel.

6. The retrievable frac mandrel and well control stack as claimed in claim 1 wherein O-ring seals provide the fluid seal between the separate pieces.

7. The retrievable frac mandrel and well control stack as claimed in claim 1 wherein the flow control equipment comprises a cross-flow tee and a high pressure valve.

8. The retrievable frac mandrel and well control stack as claimed in claim 1 wherein the frac mandrel further comprises one selected from the group consisting of: backpressure plug threads for the connection of a backpressure plug to provide a high-pressure seal in a central passage through the frac mandrel; an annular groove engaged by a hydraulic shear-off tubing plug to provide the high-pressure seal in the central passage through the frac mandrel; and, a smooth internal passage that is gripped by a through-tubing plug to provide the high-pressure seal in the central passage through the frac mandrel.

9. A method of performing a completion, re-completion or workover of a cased well having a wellhead with a tubing head spool, comprising:

inserting a bottom end of a frac mandrel into the tubing head spool and locking the bottom end of the frac mandrel in the tubing head spool, the fac mandrel having a top connected to the bottom end by a threaded connection with a fluid seal to prevent a migration of stimulation fluids between the bottom end and the top end;

mounting a well control stack to a top flange of the tubing head spool, the well control stack including a well control adapter with an axial seal bore that receives the top end of the frac mandrel to provide a high-pressure fluid seal around a periphery of the top end, and an axial passage that has a diameter smaller than an outer diameter of the top end of the frac mandrel but at least as large as an internal diameter of the frac mandrel;

mounting well completion, re-completion or workover equipment to a top of the well control stack; and

performing a well completion, re-completion or workover operation using the equipment.

10. The method as claimed in claim 9 further comprising:

closing a fluid path through the well control stack;

removing the well completion, re-completion or workover equipment from the top of the well control stack;

mounting a frac head to the well control stack;

opening the fluid path through the well control stack; and

pumping high-pressure well stimulation fluids through the well control stack and the frac mandrel into the cased well.

11. The method as claimed in claim 10 further comprising:

flowing back the high-pressure well stimulation fluids;

closing the fluid path through the well control stack;

bleeding off fluid pressure from the well control stack; and

removing the frac head from the well control stack.

12. The method as claimed in claim 11 further comprising;

mounting a backpressure plug insertion tool to the top of the well control stack;

pressure balancing well pressure across the closed fluid path through the well control stack;

opening the fluid path through the well control stack;

stroking a plug through the well control stack using the backpressure plug insertion tool;

inserting the plug into the frac mandrel to provide a high pressure fluid seal in a central passage through the frac mandrel;

releasing fluid pressure from the well control stack;

removing the backpressure plug insertion tool from the well control stack; and

removing the well control stack from the tubing head spool.

13. The method as claimed in claim 12 further comprising:

mounting a lubricator tube and a master valve to a top of the tubing head spool;

mounting the backpressure plug insertion tool to a top of the lubricator tube;

stroking a plug adapter down through the lubricator tube and master valve and connecting the plug adapter to the plug in the frac mandrel;

releasing the frac mandrel from the tubing head spool; and

pulling the frac mandrel up into the lubricator tube.

14. The method as claimed in claim 13 further comprising:

closing the master valve;

releasing fluid pressure from the lubricator tube; and

removing the lubricator tube, the backpressure plug insertion tool and the frac mandrel from the master valve.

15. A retrievable frac mandrel for facilitating well completion, re-completion or workover of a cased well equipped with a wellhead with a tubing head spool, comprising:

a bottom end contoured to be received in a central passage of the tubing head spool, the bottom end including a plurality of O-ring grooves with high pressure O-rings that seal off against a seal bore above a bit guide of the tubing head spool, and an annular groove engaged by lockdown screws of the tubing head spool to lock the frac mandrel in the tubing head spool;

a top end threadedly connected to the bottom end with a fluid seal to prevent a migration of stimulation fluids between the top end and the bottom end; and

a well control adapter that mounts to a top flange of the tubing head spool, the well control adapter including an axial seal bore that receives the top end of the frac mandrel to provide a high-pressure fluid seal around a periphery of the top end, and an axial passage that has a diameter smaller than an outer diameter of the top end of the frac mandrel but at least as large as an internal diameter of the frac mandrel.

16. The frac mandrel as claimed in claim 15 wherein the top end further comprises an internal passage with backpressure plug threads for receiving a backpressure plug for providing a high-pressure fluid seal for blocking the internal passage, and for providing an attachment point for retrieving the frac mandrel from the tubing head spool after the well completion, re-completion or workover is completed.

17. The frac mandrel as claimed in claim 15 wherein the top end further comprises an internal passage with an annular groove engaged by a hydraulic shear-off plug for providing a high-pressure fluid seal for blocking the internal passage, and for providing an attachment point for retrieving the frac mandrel from the tubing head spool after the well completion, re-completion or workover is completed.

18. The frac mandrel as claimed in claim 15 further comprising O-ring grooves with high-pressure O-rings located adjacent the threaded connection between the top and bottom ends to provide the fluid seal between the top and bottom ends.

19. The frac mandrel as claimed in claim 15 wherein the bottom end further comprises a bevel that matches a bevel angle of the bit guide, the bevel including an O-ring groove with a high-pressure O-ring that seals against the bit guide of the tubing head spool.