Design of a Thermally-Actuated Gas Lift Safety Valve

Gas-lifted oil wells are susceptible to failure through malfunction of gas lift valve assemblies (GLV). One failure mode occurs when the GLV check valve fails and product passes into the well annulus, potentially reaching the wellhead. This is a growing concern as offshore wells are drilled thousands of meters below the ocean floor in extreme temperature and pressure conditions, and repair and monitoring become difficult. Currently no safeguard exists in the GLV to prevent product passage in the event of check valve failure. In this paper a design and operational procedures are proposed for a thermallyactuated positive-locking safety valve to seal the GLV in the event of check valve failure. A thermal model of the well and GLV system is developed and compared to well data to verify feasibility of a thermally-actuated safety valve. A 3X scale prototype safety valve is built and tested under simulated failure scenarios and well start-up scenarios. Realistic well temperatures in the range of 20C to 70C are used. Results demonstrate valve closure in response to simulated check valve failure and valve opening during simulated well start-up. Introduction Gas lift is an artificial lifting method used to produce oil from wells that do not flow naturally. Gas is injected through the well annulus and into the well tubing at a down-well location (as shown in figure 1). The gas mixes with the oil in the tubing, aerating the oil and causing it to rise to the surface [2]. Gas lift valves are one-way valves that allow gas to pass from the annulus to the tubing but prevent oil from passing through to the annulus [2]. Most valves contain a pressurized bellows and an internal check valve (see figure 2). The bellows opens when the injection gas is pressurized above a threshold value, and the internal check valve prevents oil from passing through the gas lift valve [12] . A gas lift valve fails if it allows oil passage from the tubing to the annulus [3]. Two main criteria must be met for failure to occur: (1) the reverse-flow check valve has a leak and the tubing pressure exceeds the gas pressure , (2) a combination of high tubing pressure and low gas pressure allows the bellows valve to open. See figure 2. Failure can also occur if both bellows and check valves leak, and tubing pressure exceeds annulus pressure. The pressure relation that satisfies both of these criteria is described by the equation Popen < Pann < Ptube (1) where Popen is the annulus pressure required to open the bellows valve, Pann is the actual pressure in the annulus, and Ptube is the pressure in the tubing.