The wellhead serves as a crucial hub in oil and gas field development, playing a pivotal role in controlling downhole fluids, ensuring extraction safety, and optimizing production. Its core structure comprises three main components: the casing head, tubing head, and Christmas tree, which are connected via flanges or threads to form a comprehensive pressure control system. Core Components and Functions
- Casing Head: Secures the surface casing and technical casing, seals the annulus between casings, and provides a mounting foundation for wellhead equipment. It can withstand pressures ranging from 35 to 140 MPa.
- Tubing Head: Suspends the production tubing, seals the annulus between tubing and casing, and channels oil and gas into the Christmas tree. Some models integrate downhole pressure monitoring interfaces.
- Christmas Tree: Composed of gate valves, choke valves, tees, and other components, it controls the flow rate of oil and gas production and facilitates operations such as wellhead sampling, paraffin removal, and well killing. The valves typically adhere to API 6A design standards.
Technical Characteristics and Classification
Wellheads are categorized by extraction stages:
- Exploration Wellheads: Temporary installations prioritizing blowout prevention and formation testing, often equipped with annular blowout preventers.
- Production Wellheads: Permanently installed to accommodate long-term high-yield operations, featuring double-wing Christmas tree designs for live well interventions.
- Injection Wellheads: Used for water or gas injection to enhance production. Their valve bodies are made from corrosion-resistant materials (e.g., 13Cr stainless steel) and can withstand pressures exceeding 70 MPa.
Typical Application Scenarios
- Onshore Oilfields: Wellheads in cluster well groups are integrated through manifolds for centralized multi-well control. For instance, digital wellheads in the Changqing Oilfield enable remote choke diameter adjustment.
- Offshore Platforms: Deepwater wellheads employ subsea Christmas trees operated by ROVs (Remotely Operated Vehicles). These systems must resist seawater corrosion and high-pressure environments, as demonstrated by the Nanhai Liwan Gas Field, where wellheads operate at depths exceeding 1,500 meters.
- Shale Gas Development: Horizontal well wellheads are equipped with fracturing sleeves to support staged fracturing processes. Their Christmas trees can withstand pressures up to 105 MPa to accommodate intensive operations.
Safety and Intelligent Upgrades
Modern wellheads commonly integrate:
- Blowout Prevention Systems: Annular and ram blowout preventers work in tandem, achieving well closure in under 15 seconds.
- Smart Monitoring: Pressure and temperature sensors transmit real-time data to SCADA systems. For example, intelligent wellheads in the Tarim Oilfield can detect casing leaks in advance.
- Emergency Shutdown Devices: Subsea wellheads are fitted with fusible plugs that automatically trigger closure when temperatures exceed 70°C, preventing blowout accidents.
As the “throat” for oil and gas flow from subsurface to surface, wellhead technology is evolving towards higher pressure tolerance, intelligence, and maintenance-free operation. Its reliability directly impacts the efficiency of oil and gas field development as well as safety and environmental protection standards.
