Casing head seals lock casing strings, using S-seals (slip-type) or mandrel hangers with elastomeric packoffs. S-seals compress via bowl weight, sealing 5,000 psi with NBR or HNBR. Weatherford’s CasingSeal uses FKM packoff with PEEK backups, rated 10,000 psi. Metal-to-metal bowl seals add redundancy. In shale wells, these seals support 9-5/8 inch casing at 7,500 psi, with https://globalsealstech.com/product-category/oil-gas-seals/api-6a-wellhead-equipment-seals/