Well-control equipment refers to a complete set of special equipment, instruments and tools installed to implement pressure-control technology for oil, gas and water wells. It is the key equipment for preventing, monitoring, controlling and handling blowout accidents. With well-control equipment, controlled operation can be achieved: damage to oil and gas formations can be reduced, casing can be protected, blowout and uncontrolled blowout can be prevented, and safe operation can be realized.
From the process point of view, well-control equipment mainly solves three basic problems. The first is sealing: when the well is shut in because of kick or blowout during downhole operations, the tubing and the tubing-casing annulus must be sealed without external leakage. The second is pressure holding: during running and pulling of the pipe string, when the upward force applied by well pressure to the tubing exceeds the tubing's own weight, upward movement of the tubing must be controlled. The third is choking and killing: during downhole operations, successful kill operations must be possible under different operating conditions.
Well-control equipment is mainly composed of three parts: detection equipment, control equipment and handling equipment. Detection equipment includes gas measuring equipment, liquid-level measuring instruments, density meters and viscometers. Control equipment includes blowout preventers, internal blowout-prevention tools and oil or gas Christmas trees. Handling equipment includes Choke Manifold, Kill Manifold, degasser, diversion and flowback equipment, surface pressurizing equipment and other auxiliary equipment.
1. Function and Model Meaning of Wellhead Equipment
1. Concept of Wellhead Equipment
Wellhead equipment is the main equipment installed at the top of an oil or gas well to control and regulate oil and gas production.
2. Function of Wellhead Equipment
(1) Connect each casing string in the well and seal the annular space between casing strings.
(2) Seal the tubing-casing annulus and suspend the pipe string in the well.
(3) Control and regulate production from oil and gas wells, so that fluid in the well enters the separator and flowline through the specified oil outlet pipeline.
(4) Regulate and control the flow rate and direction of kill fluid during well killing.
(5) Carry out paraffin cleaning and data recording during normal oil-well production.
3. Model Designation of Wellhead Equipment
The model designation method for wellhead equipment is as follows:
1. Number of revisions or modified designs, expressed in Arabic numerals.
2. Factory code, expressed in Chinese pinyin letters.
3. Nominal bore, mm.
4. Maximum working pressure, MPa.
5. Product code, expressed in Chinese pinyin.
KY - oil production wellhead equipment; KQ - gas production wellhead equipment; KZ - water-injection wellhead equipment; KR - thermal-recovery wellhead equipment; KS - well-test wellhead equipment; KL - fracturing and acidizing wellhead equipment.
Example: KYS35/65DQ. KYS indicates Christmas tree, 35 indicates the maximum working pressure is 35 MPa, 65 indicates the nominal bore is 65 mm, and DQ indicates that the design unit is Daqing Oilfield.
2. Composition of Wellhead Equipment
Wellhead equipment consists of three parts: oil or gas Christmas tree, tubing head and casing head. Its connection forms include threaded connection, flange connection and clamp connection. Figure 1 shows a clamp-connected wellhead equipment assembly with a two-stage casing head.
Figure 1. Wellhead equipment
1. Christmas tree; 2. tubing head; 3 and 4. casing heads.
1. Tubing Head
The tubing head is installed between the Christmas tree and the casing head. It generally includes two parts: the tubing hanger and the casing cross, also called the tubing-head body. The upper flange plane is used as the reference plane for calculating tubing-head elevation and well-depth data.
(1) Function. The main function of the tubing head is to support the weight of the tubing in the well, work with the tubing hanger to seal the annular space between tubing and casing, provide transition from the lower casing head to the upper Christmas tree, and complete operations such as injecting balancing fluid and well washing through the two side outlets on the tubing-head cross. The tubing hanger is used to suspend the tubing in the well.
(2) Structure. Common tubing-head structures include flange-hanger tubing head, tapered-hanger single-flange tubing head, and tapered-hanger double-flange tubing head, as shown in Figures 2, 3 and 4.
Figure 2. Flange-hanger tubing head
1. Casing cross; 2. sealing ring; 3. set-screw flange; 4. tubing hanger; 5. top ring; 6. steel ring.
Figure 3. Tapered-hanger single-flange tubing head
1. Production casing flange; 2. anti-lift set screw; 3. tubing hanger; 4. sealing ring; 5. casing tee flange; 6. tubing pup.
Figure 4. Tapered-hanger double-flange tubing head
1. Tubing hanger; 2. set screw; 3. sealing ring; 4. casing cross.
The tapered-hanger single-flange tubing head combines the tubing head with the casing head for a single casing string. Its structural feature is that the tubing is connected by thread to the tubing hanger through a tubing pup, then seated inside the casing flange, compressing the sealing ring and sealing the tubing-casing annular space. It is tightened and pressurized with four bolts.
The tapered-hanger double-flange tubing head places the tubing hanger and set-screw flange between the upper flange of the casing cross and the lower flange opening of the original tubing head. The upper and lower ends are sealed with steel rings and bolts. The set screw prevents the tubing from being pushed out when well pressure is too high.
2. Casing Head
The casing head is a component that connects casing with various wellhead equipment. It is installed at the lowest end of the whole wellhead equipment assembly, and generally consists of body, casing hanger and sealing components.
(1) Function. The main functions of the casing head are to support the weight of the intermediate casing and production casing, seal the annular space between casing strings, provide transition connection for installing blowout preventers, tubing heads, Christmas trees and other upper wellhead equipment, and allow squeeze cementing, balancing-fluid injection and other operations through the two side outlets on the casing-head body.
(2) Structure. According to the number of casing strings suspended, casing heads are divided into single-stage casing head, dual-stage casing head and three-stage casing head. Their specific structures are shown in Figures 5, 6 and 7.
Figure 5. Single-stage casing head
1. Tubing-head body; 2. body; 3. casing hanger, slip type; 4. suspended casing; 5. connection casing.
Figure 6. Dual-stage casing head
1. Upper body; 2. lower body; 3. tubing-head body; 4. upper casing hanger, slip type; 5. upper suspended casing; 6. lower casing hanger, slip type; 7. lower suspended casing; 8. connection casing.
Figure 7. Three-stage casing head
1. Tubing-head body; 2. upper body; 3. middle body; 4. lower body; 5. upper casing hanger; 6. upper suspended casing; 7. middle casing hanger; 8. middle suspended casing; 9. lower casing hanger; 10. lower suspended casing; 11. connection casing.
3. Christmas Tree
In wellhead equipment, the part above the tubing head is called the Christmas tree.
(1) Classification of Christmas trees. According to different connection types, Christmas trees can be divided into flange-connected Christmas trees, threaded Christmas trees and clamp-connected Christmas trees. Threaded Christmas trees are now rarely seen. Specific types are shown in Figures 8 and 9.
Figure 8. Flange-connected Christmas tree
1 and 5. Choke Valve; 2. cross; 3. pressure gauge; 4. shutoff valve; 6. master gate valve.
Figure 9. Clamp-connected Christmas tree
1. Shutoff valve; 2. pressure gauge; 3. cross; 4. paraffin-cleaning gate valve; 5. Choke Valve; 6. master gate valve.
(2) Structure of Christmas tree. A Christmas tree is mainly composed of master gate valve, production gate valve, paraffin-cleaning gate valve and other gates, tees, crosses, flanges, pups and related components.
1. Master gate valve. The master gate valve is installed above the tubing head and is the main passage controlling oil and gas flow into the Christmas tree. During normal production, the master gate valve usually remains open and is closed only when long-term shut-in or another special condition is required. Because the section below the master gate valve and inside the casing gate valve is uncontrolled, repair or replacement can only be carried out after the well is killed. Therefore, in daily management, the master gate valve and casing gate valve should generally not be operated casually.
2. Production gate valve. The production gate valve is installed on the side of the tubing cross or tee. Its function is to control oil and gas flow to the outlet pipeline. During normal production, the production gate valve remains open and is closed when replacing or inspecting the Choke Bean or when the oil well is shut down.
3. Paraffin-cleaning gate valve, or well-test gate valve. The paraffin-cleaning gate valves are installed as two symmetrical valves at the top of the Christmas tree. A lubricator for paraffin cleaning or well testing can be connected above them. They are opened during paraffin cleaning and while running other testing instruments, and closed after paraffin cleaning and testing are completed.
4. Tubing cross, or tee. Also called a small cross, it connects upward and downward to the master gate valve and the paraffin-cleaning or well-test gate valve, and its two sides, or one side, connect to the production gate valve. It is both a connection component and a passage for oil and gas flow and downhole instruments.
5. Choke Valve. The Choke Valve is also called a Choke Bean. Its function is to directly control the reasonable production pressure differential of the reservoir during production and regulate oil-well output. It is the main device for adjusting and controlling reasonable operation of flowing wells and electric-pump wells. The Choke Bean is generally installed in the Choke Bean holder on one side of the Christmas tree, and may also be installed downhole or before the metering separator in the station. The oil-flow hole diameter of the Choke Bean is generally about 1.5 to 30 mm, and the proper diameter can be selected according to different operating schedules. The Choke Bean structure is shown in Figure 10.
Figure 10. Choke Bean structure schematic
(a) Threaded Choke Bean; (b) snap-in Choke Bean.
6. Sampling gate valve. The sampling gate valve is installed on the oil outlet line behind the Choke Bean and is used for sampling or for venting when inspecting or replacing the Choke Bean.
7. Back-pressure gate valve, or return-oil gate valve. The back-pressure gate valve is installed on the oil outlet line behind the Choke Bean. Its function is to prevent fluid in the gathering line from flowing backward when inspecting or replacing the Choke Bean, repairing the production gate valve, or carrying out downhole operations. Some oil wells also install a high-pressure Check Valve at this position instead of the back-pressure gate valve.
8. Pressure gauge. The pressure gauge is used to observe and record oil pressure and casing pressure.
3. Maximum Working Pressure of Wellhead Equipment
The maximum working pressure of wellhead equipment components should be determined by the maximum working pressure of the end or outlet connection. When the maximum working pressures of the end and outlet connections are different, the lower pressure value should be used.
The maximum working pressure and connection type of wellhead equipment are shown in Table 1.
Table 1. Maximum working pressure and connection type of wellhead equipment
| Connection type | Maximum working pressure | |||||
|---|---|---|---|---|---|---|
| 14 | 21 | 35 | 70 | 105 | 140 | |
| Flange | △ | △ | △ | △ | △ | △ |
| Clamp | △ | △ | △ | |||
Note: "△" indicates the maximum working pressure that should be available for wellhead equipment with different connection types.
4. Several Common Types of Wellhead Equipment
Figures 11 and 12 show several common types of wellhead equipment. Figure 13 is a schematic diagram of an eccentric wellhead structure for annulus testing of pumping wells.
Figure 11. KYS15/62DQ wellhead equipment
1. Blind plug; 2. paraffin-cleaning gate valve; 3. flange; 4. tubing tee; 5. master gate-valve core; 6. clamp; 7. tubing head; 8. set screw; 9. tubing pup; 10. casing pup; 11. pressure gauge; 12. casing gate-valve core; 13. tubing gate-valve core; 14. Hammer Union.
Figure 12. KY250/65 pumping wellhead equipment
1. Tubing pressure gauge; 2. casing pressure gauge; 3. clamp; 4. pressure-gauge control valve; 5. tubing cross; 6. production gate valve; 7. Choke Bean holder; 8. master gate valve; 9. casing gate valve; 10. tubing head; 11. upper flange of tubing head; 12. casing flange; 13. polished-rod sealing device; 14. set-screw flange; 15. joint.
Figure 13. Single-rotation eccentric wellhead structure schematic
1. Casing flange; 2. limit protective shell; 3. upper flange; 4. eccentric tubing hanger; 5. union joint; 6. oil outlet tee; 7. swivel connector; 8. test port; 9. ball bearing; 10. steel ring.
