Structural design and mathematical cutting model of hydraulic section milling for high-strength casings in small wellbores

Abstract

The section milling of casings is widely applied in the side-tracking and abandonment operations of old wells. In particular, the cutting of casings is a crucial task during section milling; however, this process is associated with problems such as low cutting speeds, especially for the high-strength casings used in small wellbores. Furthermore, theoretical support and experimental investigations related to this operation remain scarce. Accordingly, based on the working principle and structural characteristics of a hydraulic cutter in this work, a geometric model of the interactions between the piston rod and blade was established. A high-performance hydraulic section milling tool suitable for 177.8 mm casings was also developed. According to the motion characteristics of the blade, a dynamic contact force model was established for the cutting process of casings, considering changes in the cutting depth and a mathematical model of the cutting depth, considering the effect of time, was also derived according to the grinding mechanism. The critical parameters of this cutting depth model for the P110 casing were determined through ground field tests and finite-element simulations, and the changes in the trajectory were inverted. The corresponding inversion results for the cutting depth exhibited a rapid increase in the significantly short primary stage, followed by a gradual increase. The theoretical torque and cutting time were also highly consistent with the field-measured values; this further verifies the rationality and correctness of the models used in this study. The results of this study are expected to serve as a theoretical basis for optimising the structural and hydraulic parameters of hydraulic cutters and making further improvements in the cutting efficiency for the side-tracking and abandonment operations of old wells.