In this paper, production-modeling studies are presented for applicability of a velocity string with ESP and gas lift in production wells completed in unconventional reservoirs to prevent gas slugging for production enhancement with optimized velocity string size and length. The effect of velocity string size and length on the performances of ESP and gas lift installed wells were studied. Gas slugging behavior was reduced with an optimized velocity string length based on ESP pump setting depth or depth of gas lift orifice. The flow regimes occurring at the lateral section of the well and inlet and outlet of the velocity string were identified and pressure drops estimated.
Changing the inside diameter of the velocity string affected the operating point on the flow regime map. The flow regime can be changed at the outlet of the velocity string before the inlet of the ESP pump or gas lift orifice when the velocity string size is changed from bigger diameter to smaller diameter. For smaller velocity string sizes, the flow regime in the horizontal section of the velocity string changed from stratified/slug flow to dispersed bubble flow. However, the change in flow regime at this point of the completion did not affect the operation of ESP and gas lift systems. A shorter velocity string increased the production rates and the changes in the diameter of the velocity string became more significant and decreased the production rates. This did not take into account the potential improvement in inflow performance of the well with shorter velocity strings, which allowed production from the originally isolated frac stages in unconventional wells.
The importance of these factors is explained in this study, which will describe the applicability of velocity strings, and the challenges encountered during implementation. It will focus on an engineered approach to select and deploy a velocity string with artificial lift systems for production enhancement.