Gas lift has been widely used for unconventional reservoirs, especially during recent years. Most of the previous experimental studies on gas lift were conducted at a constant mass flow rate boundary condition. The current study presents an experimental study that investigates gas lift performance at similar field conditions in which the production rates are driven by the formation pressure.
The experiments were conducted in a large-scale experimental facility to study gas lift operations in a toe-down gas lift well. Gas was injected at various rates through casing and tubing annulus to the end of the tubing. The formation gas was supplied by a gas tank regulated at two pressure conditions, which correspond to no production and the last stable production point, respectively. Water was used as the formation liquid phase and injected at the toe. Two liquid flowing conditions were investigated, which were a constant liquid flow rate and constant liquid permeability.
As expected for a production system with constant pressure boundaries, formation gas production rate increases first with increasing gas lift injection rate until it reaches a maximum, beyond which flow in the tubing becomes frictional-dominated, and gas production rate starts to decrease. It is found that gas lift affects the flow behavior at the upstream of the injection point. The total liquid inventory reaches a minimum at the point where the maximum gas production rate is observed. The impact of gas lift on system stability was also analyzed. Gas lift can help the well to produce when it is already dead at naturally flowing conditions. However, insufficient gas injection can kill the well faster than natural flow.