Mechanical Characteristics of Cluster Perforation String under Running Process in Horizontal Wells with Unconventional Oil/Gas Reservoirs

Summary To improve the safety of cluster perforating tubing strings in horizontal wells with unconventional oil/gas reservoirs, a passing capability analysis model for cluster perforation string in a wellbore is established, in which the friction between downhole tools and borehole wall, fluid resistance, wellbore geometric constraint, and tool variable cross section were accounted for. The model is solved via geometric analyses and the beam-column theory. Then, the cable pump thrust test is carried out in three shale gas wells in South Sichuan, and comparing the measured data with the theoretical calculation results, the correctness of the theoretical model is verified. Based on that, the influences of the cluster numbers, the perforating gun types, the well section length, and the well types on the mechanical characteristics of the cluster perforation string are systematically analyzed. The results obtained demonstrate that, first, the number of perforating gun clusters should be as small as possible when the azimuth angle or well inclination angle changes greatly. In case of a small change of azimuth or deviation angle, the number of perforation gun clusters can be appropriately increased to improve the operation efficiency. Second, when the deviation angle and azimuth angle change greatly in field operation, it is recommended to use a small perforating gun to ensure the smooth running of the perforating string. When a large perforating gun is needed in the field, it is necessary to optimize the well trajectory (reduce the location of the big dogleg in wellbore) to ensure the safe running of the tubing string. Third, in the field well trajectory optimization design, the position of well inclination angle change and azimuth angle change should be staggered to improve the trafficability of perforating tubing strings on-site. Moreover, when increasing the well sections, the position of the knee point should be optimized to be smoother, and the change rate of the parameter value of the well trajectory should be reduced. The research results provide a theoretically sound guidance for designing and practically sound approach for effectively improving the service life of perforating tubing strings in horizontal wells with unconventional oil/gas reservoirs.