(12) A Case Study: Optimized Valve Spacing in Gas Lift to Accommodate Maturing Reservoir Conditions in Permian Basin Wells

In unconventional wells unstable dynamic behavior ensuing from an exchange of energy in the casing and tubing is the biggest challenge in gas lift design. As reservoir and tubing pressures decline from early conditions, wells show erratic behavior in decreasing fluid levels and hence unload and operate at variable depths. This case study presents the full workflow of creating an effective gas-lift design from concept initiation to execution and field installation. Successful spacing in gas lift valves for an available gas injection pressure, assures unloading from the deepest point of the tubing string (i.e., packer depth). The design should provide the number of valves for the available kickoff pressure and accommodate future reservoir decline. Two equations provide this flexibility, one for the first gas lift valve (from surface) and another for all deeper unloading valves. After performing a well delivery simulation with a nodal analysis program, a production rate versus injection gas curve is generated at each of the several potential depths of injection. Production rate and corresponding injection gas rate are selected at each depth. The rate from each depth is validated with measured data for outflow and reservoir inflow. Several designs are often needed so spacing criteria, and available kickoff pressure reach the desired injection depth and match the target production rate. This iterative methodology develops the gas lift spacing design allowing for a valve at a shallower depth to ensure unloading efficiency throughout the range of conditions and a valve just a joint above the packer to make production achievable at the lowest possible reservoir pressure.