Flowing wellhead equipment is generally composed of three main parts: casing head, tubing head and the oil or gas production wellhead body. In the past, casing heads were rarely used on oil wells in China, so the production wellhead equipment commonly referred to in field practice usually included only the tubing head and the wellhead body. In special areas such as offshore fields and frontier oilfields, a downhole safety valve is also required to ensure oilfield safety.
1. Casing Head and Tubing Head
A casing head usually consists of a casing-head body and a casing hanger. It is installed at the lowest end of the wellhead equipment. Its function is to connect and suspend each casing string and seal the annular space between casing strings. The casing may be suspended with a threaded hanger or a slip-type hanger. The structure of a casing head for two casing strings is shown in Figure 1. The surface casing is connected to the casing head through a flange screwed onto the casing thread. The production casing passes through the casing head and is connected to the internal thread of the casing head by thread. The structure of a casing head connecting three casing strings is shown in Figure 2. The surface casing is connected to the lower casing head by thread. The intermediate casing is seated on the lower casing head through a slip-type hanger, also called a casing hanger. The production casing is connected by thread to a hanger seated on the upper casing head. The upper and lower casing heads are sealed with a ring gasket and connected with bolts. The annular spaces between casing strings are sealed with sealing gaskets. To meet the requirements of different types of oil wells, casing heads may also be made in four-stage or five-stage structures.
A tubing head usually consists of a tubing-head housing, also called a casing cross or large cross, and a tubing hanger. It is connected above the casing head. Its function is to suspend the tubing string in the well and seal the annular space between tubing and casing. Tubing hangers have two structural forms: conical and cylindrical. The outer cone of a conical tubing hanger seats on the inner cone of the tubing head. In deep wells, because the load is often large, the tubing hanger and tubing head may be difficult to separate. In addition to single-tubing tubing heads, there are also dual-tubing and triple-tubing tubing heads.
Figure 1. Two-stage casing head
1. Lower flange of casing cross; 2. steel ring; 3. production casing; 4. casing head; 5. surface casing
Figure 2. Three-stage casing head
1. Tubing hanger; 2. tubing head; 3. tubing; 4. threaded hanger; 5. upper casing head; 6. production casing; 7. intermediate casing; 8. sealing gasket; 9. casing hanger; 10. lower casing head; 11. surface casing
2. Downhole Safety Valve
On offshore oil production installations, a downhole safety valve is required to prevent rupture of pipelines above the seabed. On land production installations, it is required to prevent accidental damage to the wellhead, including war damage. It is installed below the seabed or more than ten meters below the wellhead. The safety valve consists of a safety-valve assembly and a control system. The safety-valve assembly is built by installing a ball valve inside a tubing sub, and the through bore of the ball valve is the same as the tubing bore. Under downhole crude-oil pressure, this ball valve remains closed. A control cylinder is installed above the ball valve, and a steel pipe or copper pipe outside the tubing connects with the control cylinder. This pipe is led out from the cross at the wellhead and connected to the control system. The control system consists of a hydraulic pump, hydraulic valves, motor and related parts. When pressure oil from the control system enters the control cylinder of the safety valve, the downhole safety valve opens for production. If there is no pressure in the control line, the downhole safety valve closes by itself and provides the safety function. The model specification of the downhole safety-valve assembly currently produced is AF62/250. Its bore is 62 mm, the maximum working-medium pressure is 250 kg/cm2, and the opening pressure required at maximum working pressure is 360 kg/cm2. The overall dimensions are 114 x 1770 mm, the connection thread is 2-1/2 in. external-upset tubing thread, and the weight is 71 kg.
3. Oil Production Wellhead Equipment
1. Structure and types of oil production wellhead equipment
Oil production wellhead equipment is mainly used to control crude-oil output from flowing oil wells. It can also be used as wellhead equipment on water-injection wells. During downhole operations, except fracturing, other wellhead equipment does not need to be replaced.
Oil production wellhead equipment usually consists of a bottom flange, tubing head, casing gate valve, master gate valve, tubing cross or tee, production gate valve, choke, pressure-gauge shutoff valve and related parts. Its outline is shown in Figure 3. The figure contains
Slab Gate Valve
six valves of the same model and specification. They are given different names only according to their functions. The figure shows two production gate valves and two casing gate valves. In actual production, one of each may also be used. The quantity and position of Slab Gate Valve units in this type of wellhead equipment can be combined freely according to service conditions. The arrangement in the figure is called a double-wing type. If all Slab Gate Valve units are placed on one side and the pressure gauge is placed on the other side, a single-wing type is formed. Two Slab Gate Valve units may also be installed on each wing. Most Slab Gate Valve units used for oil production wellhead equipment are rising-stem wedge Slab Gate Valve units. The valve structure is shown in Figure 4. The valve body is generally made of cast steel. For high-pressure Slab Gate Valve units or Slab Gate Valve units requiring high sulfide-resistance performance, the body is forged from alloy steel. The wedge gate must be wear resistant and corrosion resistant, so wear-resistant alloy is overlay welded on the gate surface. To make opening and closing flexible and convenient, two sets of thrust bearings are installed on the valve-stem drive nut. The Slab Gate Valve may use flange connection or clamp connection. The connection shown in the figure is clamp type. Some low-pressure oil production wellhead equipment also uses ball valves. A ball valve is equivalent to a Plug Valve and can be opened or closed by rotating 90 degrees. This valve is not suitable for frequent operation and is not suitable for sealing high-pressure oil wells, otherwise machining requirements are very high. Therefore, ball valves are currently mostly used on simplified 150-atmosphere oil production wellhead equipment.
Figure 3. Oil production wellhead equipment outline
1. Paraffin-cleaning gate valve; 2. small cross; 3. production gate valve; 4. choke; 5. master gate valve; 6. casing gate valve; 7. cone-seat tubing head; 8. pressure gauge; 9. pressure-gauge shutoff valve
Figure 4. Slab Gate Valve
1. Handwheel; 2. thrust bearing; 3. valve stem; 4. gate; 5. valve seat
Another main component in oil production wellhead equipment is the tubing head. It has four outlets, so it is also called a large cross. It is mainly used to suspend tubing, and each outlet is connected to a corresponding component. Its structure is shown in Figure 5. Because the outer surface of the tubing hanger is conical, it is called a cone-seat tubing head. The lower end connects to the bottom flange on the casing, and the left and right outlets communicate with the casing annulus. The upper part connects to the master valve and is the outlet for crude oil in the well. To prevent the tubing from being pushed upward by high internal well pressure, set screws are installed on the upper part of the tubing head. The tubing-head housing is generally cast or forged from alloy steel. The tubing-hanger area requires precision machining so that it can both seal the space between tubing and casing and fully contact the conical surface of the tubing hanger, preventing the tubing hanger from being jammed by tubing weight. Simple or low-pressure tubing heads may also be made as tee structures by reducing one casing-annulus outlet.
Figure 5. Cone-seat tubing head
1. Tubing pup; 2. round sealing ring; 3. thread protector; 4. tubing hanger; 5. red-copper gasket ring; 6. set screw; 7. large cross
In oil production wellhead equipment, the component that directly controls production rate is the choke, also called the Choke Bean holder. It consists of a Choke Bean, choke body and heating jacket, as shown in Figure 6. The Choke Bean is the outlet for crude oil from the well. It is made from high-carbon alloy steel after heat treatment, with a hole diameter of 2 to 20 mm. It must be wear resistant and convenient to replace. The heating jacket is used to heat the Choke Bean so that crude oil does not solidify and block the Choke Bean in cold areas. Hot water or steam is passed through the heating jacket for heating.
Figure 6. Choke Bean
1. Choke Bean; 2. choke body; 3. heating jacket
2. Common specifications of oil production wellhead equipment
At present, there is only one standardized type of oil production wellhead equipment in China, model KY250/65. Its performance specifications are as follows:
(1) Nominal bore: 65 mm
(2) Working medium: crude oil, clean water and non-corrosive liquid
(3) Working pressure: 250 kg/cm2
(4) Strength test pressure: 500 kg/cm2
(5) Seal test pressure: 250 kg/cm2
(6) Maximum bore of large cross: 157 mm
(7) Connection type: clamp connection
(8) Overall dimensions (L x W x H): 1405 x 580 x 1440 mm
(9) Weight: 550 kg
All seals between components in the complete oil production wellhead use low-carbon-steel gasket rings. The KY250/65 oil production wellhead uses two types of gasket rings, and their dimensions are expressed as D x d x H. Type B14 gasket ring is 100.5+0.15 x 77.1+0.10 x 15-0.10 mm; type B27 gasket ring is 222.1+0.30 x 200.1+0.13 x 15-0.10 mm. The four sides of the gasket ring are chamfered 23 degrees +/- 30 minutes.
In addition to KY250/65 oil production wellhead equipment, some oilfields also produce certain types of their own. The Qing-150 type is used more often. Its working pressure is 150 kg/cm2 and its gate valves are ball valves. A paraffin-cleaning gate valve is installed on the top. This type of oil production wellhead equipment is simple in structure and light in weight at 152 kg, and is sometimes called a miniature oil production wellhead.
4. Gas Production Wellhead Equipment
1. Structure of gas production wellhead equipment
The structure of gas production wellhead equipment is basically similar to that of oil production wellhead equipment. However, because the physical properties of gas and oil are different, there are certain structural differences. Gas wells generally have the following characteristics: (1) under the same formation pressure, the wellhead pressure of a gas well is higher than that of an oil well, because the specific gravity of oil is higher than that of gas in liquefied state. The required condition for oil to flow out of the wellhead is that formation pressure is higher than the hydrostatic pressure of the oil column, while the gas "liquid" column pressure is lower than that of oil. Therefore, under the same formation pressure, oil and gas show different wellhead pressures. (2) Natural-gas wells often contain hydrogen sulfide, which is a highly corrosive and toxic gas. (3) Gas sealing is much more difficult than liquid sealing. (4) Natural gas, a hydrocarbon gas, is flammable and explosive, and accidents can occur if it is handled improperly. (5) Wear on wellhead equipment is high when gas passes through it, because natural-gas production often carries sand, water and other impurities, and the pressure differential is relatively large. Oil may also carry sand and water, but it has a certain lubricating property and can avoid some mechanical abrasive wear. Because of these characteristics, the structural differences between gas production wellhead equipment and oil production wellhead equipment are: (1) all components use flange connections. (2) The casing gate valves and master gate valves are arranged in pairs; one is for service and the other is standby. (3) The choke part uses a
needle valve
instead of a fixed-orifice Choke Bean, making it convenient to adjust production rate at any time. (4) All components must have hydrogen-sulfide resistance.
The internal structures of the components in gas production wellhead equipment are similar to those in oil production equipment. The external structure is shown in Figure 7. The Slab Gate Valve of a gas production wellhead differs slightly from that of an oil production wellhead. The valve-stem sealing structure is changed from a round seal to a gland-packing U-shaped sealing-ring structure, and the connection is changed to flange connection. The needle valve marked 6 in Figure 7 is not used in oil production wellhead equipment. Turning the regulating handwheel changes the flow area, so the production rate can be controlled conveniently. The detailed structure is shown in Figure 8.
Figure 7. External structure of gas production wellhead equipment
1. Pressure gauge; 2. pressure-gauge shutoff valve; 3. test valve; 4. small cross; 5. production valve; 6. needle valve; 7. tubing head; 8. casing valve
Figure 8. Needle valve
The material selection and manufacturing process of gas production wellhead equipment have special features. During oilfield use, special attention must be paid when replacing parts and carrying out repairs, and the original requirements for the gas production wellhead equipment must be followed. These features and requirements are: (1) all valve stems of valve components are made from high-chromium stainless steel, grade 318 steel, and the gate surfaces are overlay welded with wear-resistant alloy. (2) All soft seals use fluororubber, and self-sealing packing uses composite packing made from fluororubber and PTFE. (3) Welding repair is not allowed on any cast-steel body, and the heat-treatment hardness of all structural steel must be strictly controlled at about HRC 22.
2. New gas production wellhead equipment
According to the characteristics of natural-gas production areas, where downhole pressure is relatively high and production rate is relatively large, several new types of gas production wellhead equipment have been successfully trial manufactured in China. Their models are KQ-400, KQ-700 and KQ-1000. The main differences between them and ordinary gas production wellhead equipment are:
(1) A Slab Gate Valve is used instead of a wedge-type Slab Gate Valve. The main advantages of the Slab Gate Valve are good sealing performance and self-sealing action in the closed state. The higher the working-medium pressure, the higher the sealing pressure. The principle of self-sealing is that both the valve seat and the gate are floating. The structural schematic is shown in Figure 9. Because this self-sealing principle is completely different from that of the wedge gate, the service life is longer. The opening and closing torque is also much smaller than that of the wedge gate.
Figure 9. Slab Gate Valve structure schematic
1. Stem nut; 2. valve stem; 3. gate; 4. valve seat; 5. wave spring; 6. balancing stem; 7. packing; 8. lower bonnet; 9. valve body; 10. upper bonnet; 11. handwheel
(2) A hydraulic automatic-control device is added so that, when a fault occurs in the outlet pipeline of the oil or gas well site, the wellhead equipment can close automatically. It mainly consists of a wellhead safety valve and a hydraulic control console. The installation schematic is shown in Figure 10.
Figure 10. Automatic-control device installation schematic
The difference between the wellhead safety valve and other Slab Gate Valve units is that it adds a hydraulic mechanism for pushing the parallel gate. It can be operated manually or hydraulically. Pressure gas is led from the pipeline into a pneumatic slide valve, equivalent to a sensor. When pipeline pressure is abnormal, the safety valve can close automatically. It is installed at the position of the second master gate valve, replacing one commonly used master gate valve that can only be operated manually.
The hydraulic control console is equipped with a hydraulic pump, accumulator, various valves, pressure gauges and related parts. The working principle of the hydraulic system in the console is shown in Figure 11. In this system, four valves mainly control opening and closing of the wellhead safety valve: the U-type pilot valve, bypass valve, closing valve and control valve. These four valves are connected along the routes shown in Figure 12. After this device is installed, the wellhead safety valve can be opened and closed not only by turning the handwheel directly at the wellhead, but also by hydraulic operation. At the same time, when the pressure in the gas-transmission pipeline is too high or too low, the system automatically closes the wellhead safety valve. The positions of the four valves under different operating conditions are shown in Figure 12. In Figure 12(a), the wellhead safety valve is in the automatic closed state, and the U-type pilot valve is in the oil-drain state because the pressure in the gas-transmission pipeline is higher than the normal value. The operating handle of the bypass valve is in the automatic closing position. Although there is no pressure in the hydraulic-control port of this valve, its position is still the one shown in the figure. This position leaves the hydraulic-control port of the closing valve without pressure, and the closing valve is moved to the illustrated position by its spring. At this time, the hydraulic-control port of the control valve has pressure, moving the control-valve handle to the "close" position. The wellhead safety valve is therefore closed. When all valves are in the state shown in Figure 12(a), the control valve cannot be used to open the wellhead safety valve, because the hydraulic-control port has pressure and the handle cannot be moved. When the gas-transmission pipeline returns to normal, the U-type pilot valve moves to the normal position. At this time the wellhead safety valve remains closed, as shown in Figure 12(b). To open the wellhead safety valve, move the control-valve operating handle toward "open". Figures 12(c) and 12(d) show the valve positions when the gas-transmission pipeline is abnormal, or when workover operations require opening or closing of the wellhead safety valve. In this case, move the bypass-valve handle to the manual opening and closing position, and then operate the control valve to close or open the wellhead safety valve.
Figure 11. Hydraulic system schematic
Figure 12. Position diagram of operating valves
(3) Tools for replacing the master valve and casing valve are added; they are sometimes called service tools. On ordinary gas production wellhead equipment, replacing the master gate valve and casing gate valve requires killing the well. The design feature of the new wellhead equipment is that Slab Gate Valve units can be removed and installed without killing the well. When a valve needs to be removed or installed, a special tool is used to screw a special plug into the thread machined at each of the four outlets of the large cross, blocking the outlet so that the valve can be removed or installed. Finally, a special tool is used to remove the plug and the wellhead is put back into production.
3. Main accessories of gas production wellhead equipment
The main component of oil and gas production wellhead equipment is the Slab Gate Valve, which is a moving sealing component. The parts replaced most often are the valve seat, gate, valve stem and various rubber elements.
5. Series and Specifications of Wellhead Equipment
(1) Code:
In model KQ350, K is the code for wellhead equipment products. Q indicates gas production, from the Chinese initial for "gas". 350 indicates that the maximum working pressure is 350 atmospheres.
(2) Pressure rating
The original pressure ratings produced in China were 150, 250, 350 and 600 kg/cm2. To approach API specifications, new pressure-rating standards will be adopted: 140, 210, 350, 700 and 1050 kg/cm2.
(3) Connection dimensions and connection type
Oil production wellhead equipment generally uses clamp connection. Gas production wellhead equipment is specified with flange connection. The flange connection dimensions are consistent with the relevant dimensions proposed in API specifications.
(4) Specifications of wellhead equipment
The oil production wellhead equipment previously produced in China used old models. Since 1980, wellhead equipment has gradually moved closer to API specifications. The products listed in Table 1 are arranged according to the pressure ratings of the new standard. Original product pressure ratings are shown in parentheses. In the table, the Slab Gate Valve bore refers to the vertical bore; the side bore may be selected according to need.
Table 1. Wellhead equipment specifications
| No. | Specification | Wellhead equipment model | |||||
|---|---|---|---|---|---|---|---|
| KY-140(150) | KY-210(250) | KQ-210(250) | KQ-350 | KQ-700(600) | KQ-1050(1000) | ||
| 1 | Working pressure | 140(150) | 210(250) | 210(250) | 350 | 700(600) | 1050(1000) |
| 2 | Seal pressure | 140(150) | 210(250) | 210(250) | 350 | 700(600) | 1050(1000) |
| 3 | Strength test pressure | 280(300) | 420(500) | 420(500) | 350 | 700(600) | 1050(1000) |
| 4 | Gate-valve bore, mm | 65, 78 | 65, 78 | 52, 65, 78, 113 | 52, 65, 78, 113 | 52, 65, 78, 113 | 52, 65, 78, 113 |
| 5 | Gate-valve type | Ball, wedge type | Wedge type | Wedge parallel plate | Wedge parallel plate | Wedge parallel plate | Wedge parallel plate |
| 6 | Connection type | Clamp type | Clamp type | Flange type | Flange type | Flange type | Flange type |
| 7 | Applicable medium | Oil, water | Oil, water | Sour gas, oil, water | Sour gas, oil, water | Sour gas, oil, water | Sour gas, oil, water |
Because casing heads were rarely used in China in the past and almost no domestic production existed, they are not listed in the specifications. However, this does not affect relevant manufacturers producing casing heads of different types and specifications to meet the requirements of deeper oil and gas wells.
