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A hybrid wellhead system for a hydrocarbon well, having a wellhead supported by a conductor; an intermediate head spool secured to the wellhead by a hammer union; and a tubing head spool secured to the intermediate head spool by a hammer union.

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1. A hybrid wellhead system for a hydrocarbon well, comprising: a wellhead supported by a conductor;an intermediate head spool secured to the wellhead by a hammer union; anda tubing head spool secured to the intermediate head spool by a hammer union. 2. The hybrid wellhead system as claimed in claim

1. A hybrid wellhead system for a hydrocarbon well, comprising: a wellhead supported by a conductor;an intermediate head spool secured to the wellhead by a hammer union; anda tubing head spool secured to the intermediate head spool by a hammer union. 2. The hybrid wellhead system as claimed in claim 1 wherein the wellhead is threadedly connected to a surface casing of the hydrocarbon well. 3. The hybrid wellhead system as claimed in claim 1 further comprising an intermediate casing mandrel supported in a tubing bowl of the wellhead. 4. The hybrid wellhead system as claimed in claim 3 wherein the intermediate casing mandrel comprises a frusta-conical bottom end having outward-facing annular grooves that respectively receive a seal that contacts the tubing bowl of the wellhead. 5. The hybrid wellhead system as claimed in claim 1 wherein the tubing head spool comprises a flanged top end that supports a flow-control stack to control a production of hydrocarbons through a production tubing string suspended by the tubing head spool. 6. A hybrid wellhead system for a hydrocarbon well, comprising: a wellhead supported by a conductor and connected to a surface casing of a well;an intermediate head spool secured to the wellhead by a hammer union; anda tubing head spool secured to the intermediate head spool by a hammer union. 7. The hybrid wellhead system as claimed in claim 6 further comprising intermediate casing string slips supported in a tubing bowl of the wellhead. 8. The hybrid wellhead system as claimed in claim 7 further comprising an annular seal plate above a top end of the intermediate casing string slips. 9. The hybrid wellhead system as claimed in claim 8 further comprising a packing nut that locks down the annular seal plate. 10. The hybrid wellhead system as claimed in claim 9 further comprising a drop sleeve above a top side of the annular seal plate. 11. The hybrid wellhead system as claimed in claim 10 wherein the drop sleeve comprises an inner wall with annular grooves that receive elastomeric seals. 12. The hybrid wellhead system as claimed in claim 6 further comprising a tubing hanger supported in a tubing bowl of the tubing head spool. 13. The hybrid wellhead system as claimed in claim 12 further comprising lock screws that radially pierce a top flange of the tubing head spool to lock the tubing hanger in the tubing bowl of the tubing head spool. 14. A hybrid wellhead system for a hydrocarbon well, comprising: a wellhead connected to a surface casing of the hydrocarbon well;an intermediate head spool secured to the wellhead by a hammer union; anda tubing head spool secured to the intermediate head spool by a hammer union. 15. The hybrid wellhead system as claimed in claim 14 wherein the wellhead is supported by a conductor. 16. The hybrid wellhead system as claimed in claim 14 further comprising an intermediate casing mandrel supported in a tubing bowl of the wellhead. 17. The hybrid wellhead system as claimed in claim 14 further comprising intermediate casing slips supported in a tubing bowl of the wellhead. 18. The hybrid wellhead system as claimed in claim 14 further comprising a production casing mandrel supported in a tubing bowl of the intermediate head spool. 19. The hybrid wellhead system as claimed in claim 14 further comprising production casing slips supported in a tubing bowl of the intermediate head spool. 20. The hybrid wellhead system as claimed in claim 14 further comprising a production tubing string hanger that is locked in a tubing bowl of the tubing head spool.