API 6A wellhead seals for HPHT reservoirs (above 350°F and 15,000 psi) require special elastomers such as Kalrez® or metal seal technology. These seals maintain low compression set and high modulus at temperature, preventing leakage that could lead to loss of well control. https://globalsealstech.com/product-category/oil-gas-seals/api-6a-wellhead-equipment-seals/