Study on the replacement time of velocity string in production process in tight gas reservoir

Abstract Tight gas reservoir production processes have many challenges, one of which is liquid loading. Installing coiled tubing velocity string (VS) can be an effective tool to extend the production life cycle, and to increase gas recovery. This study focuses on the Daniudi tight gas reservoir in the Northern Ordos Basin, China, as an example of VS application. The basic information and production characteristics of VS wells were firstly analyzed. Then, an evaluation process of VS production effect was established, and the effect after VS installation was divided into three types. The evaluation indexes included water flooded in well, the declining of production rate, the change of cleanup measures and the increasement of the gas–liquid ratio. At last, two methods were used to determine the optimal time to replace the tubing with VS. The first method was a statistical approach, which calculated VS replacement time was no more than 700 days in Daniudi tight gas reservoir, this occurred when the tubing head pressure was more than 6 MPa and the gas–liquid ratio was 0.50–3.00 × 104 m3/m3 before VS installation. The second method was a theoretical calculation, in which the critical liquid carrying capacity was higher than the daily gas production, and the corresponding time was the time to incorporate VS. The critical liquid carrying capacity of the calculation model was divided into three parts: the vertical section, the horizontal section and the inclined section. Through calculation analysis, the critical liquid carrying flow rate of the inclined section became the critical flow rate of the well. The actual replacement times of wells were compared with calculated replacement times, to show the validity of the method used in this study.