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Valve Mechanics
- Gas lift valves are constructed where all components must be fitted into a small cylindrical shaped tube. A typical valve must have a closing force, an opening force, and a flow regulating orifice.
- If tubing pressure causes gas flow regulation, it is referred to as a tubing or fluid operated valve (or “automatic valve”).
- If casing pressure causes gas flow regulation, it is referred to as a casing or gas operated valve (or as “human operated”).
Mandrel & Gas Lift Valve
Packer Gas Lift Mandrel Production string Perforations
Casing and Tubing Valves Casing operated valve (“human operated valve”) Tubing operated valve (“automatic valve”)
Valve Mechanics
- Gas lift valves operate in accordance with certain basic principles, and are mostly compared with pressure regulators. The component parts are: 1. Body 2. Loading Element - Spring, gas, or combination of both 3. Responsive Element - Metal bellows, piston, or rubber diaphragm 4. Transmission Element - Metal rod or rubber diaphragm 5. Metering Element - Orifice or port
Casing operated valve:
- A regulator responding to upstream pressure.
- A control valve at the surface operates the downhole valve.
- Pinching down on manual valve at surface reduces casing pressure, which pinches down on the gas lift valve, thereby reducing gas flow.
- Opening the surface valve increases casing pressure, causing the gas lift valve to open and increases the gas flow thru the valve.
Advantages & Disadvantages
Casing operated:
* pumper has surface control
* can only gas lift one tubing string
* inefficient – pressure drops to unload are lost energy
Tubing Operated:
* multiple tubing strings gas lifting from one annular
gas column
* automatic – requires minimal pumper intervention
Opening Pressure of Valve Under Operating Conditions
- The force balance for valves (figure 5) which are closed, but on the
verge to opening will be: F
c
= F
o F c is the force in pounds holding the valve closed and Fo is the force in pounds trying to open the valve.
- Setting these expressions equal, dividing by A b , and solving for P o , [pressure to open] = [bellows effect] + [spring effect] – [tubing effect]
Tubing Effect (T.E.) For example, if there is a pressure in the tubing opposite the flow valve of 300 psig and the ratio A v /A b is 0.11, the Tubing effect (T.E.) is: 𝑇. 𝐸. = 300.
- 11 1 − 0. 11 = 37 𝑝𝑠𝑖𝑔 Or the opening pressure P o is reduced by 37 psig
Opening Pressure in Valve Tester
- In a valve tester, the tubing pressure opposite the valve is zero (p t = 0). With a force balance, opening and closing forces are equated,
Closing Pressure of valve under operating conditions
- The valve is now open and ready to close, and the casing pressure is operating over the total effective bellow area (figure 6.11c). The force balance is:
Tubing operated valve:
Fully opened – pressure on stem bottom = P t Fully closed – pressure on stem bottom = P csg c bT b t b v F P A S A A o tc b v o v F P (A A ) P A
𝑡𝑐
𝑡
𝑏𝑇
𝑣
𝑏
𝑜
𝑣
𝑏
𝑣
𝑏 At Fully Closed Position:
Belows Pressure at Operating Conditions