Unstable Gas Lift at the Heidrun Field
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Gas lift is used in oil wells to maintain the production by injecting gas into the tubing. The dynamics of the system often causes pressure variations and a fluctuating flow rate. Others have proposed design concepts for stable gas lift systems. However, measurements have been superficial and sparse and it has made it difficult to enable satisfactory verification of the concepts. This thesis is based on flow rate, pressure and temperature data from a well at the Heidrun field in the Norwegian Sea. The measurements have been logged at a sufficient frequency to capture the most relevant dynamics. The well considered has periods where it produces evenly with a stochastic variation around 7 % and then it might suddenly change to oscillations with over 90 % deviation in the flow rate. The wavelength of the oscillations are 7−10 min. Such shifts seams to occur without preceding disturbances, and it only affects the tubing pressure and flow rate, not the annular variables. This means, that casing heading is not the reason behind the oscillating behaviour. A gas lift model has been implemented in Matlab to predict static changes in several variables, like phase fraction and pressure. Pressure predicted by this model compared to the measurement at Heidrun coincide with a maximum deviation of 2.3 %. A dynamical model based on an inflow correlation to simulate the well pressure was developed. The predictions was not consistent with the oscillations observed from the measurements, because the inflow correlation did not compare with the measured data. The pressure response analysis did not show any sign of instabilities. However, an unstable well with oscillations was observed. The likely reason for the inconsistency is the casing heading assumption in the model. The large pressure drop across the downhole injection valve makes gas inflow insensitive to tubing variations, this decoupling prevents casing heading.