A robust and efficient method to geometrically calculate the minimum bit stick-out in a deepwater conductor jetting project

Abstract The length of the bit protruding from the running conductor assembly (i.e., bit stick-out), a key factor in deepwater drilling design, directly affects the efficiency of the injection process and the bearing safety of the conductor after installation. To improve the efficiency and safety of conductor installation, it is necessary to establish a reasonable theoretical model of the bit stick-out to provide a feasible scheme for field applications. In this study, geometric judgment criteria of the reasonable bit stick-out location are proposed that determine whether the diameter of the nozzle jet circle is smaller than that of the conductor. These judgment criteria are applied to obtain the reasonable position of the minimum bit stick-out for different diameters of conductors and tricone bits. According to these judgment criteria, we propose a mathematical model of the minimum bit stick-out during deepwater conductor jetting, which considers the influence of the heterogeneity in the nozzle space. Through theoretical analysis and experimental research, combined with on-site cases, the results indicate that if the diameter of the nozzle jet circle is larger than that of the conductor, the reasonable bit stick-out location occurs where the upper boundary of the nozzle jet beam is tangential to the inner boundary of the lower end of the conductor. Conversely, the reasonable bit stick-out location occurs where the jet beam originating from the nozzle can reach the intersection of the jet trajectory and the curved surface formed by the main and secondary cones of the cone bit at the bottom of the well. A numerical simulation study reveals that when the bit stick-out is greater than 150 mm, the hole can be enlarged during the jetting process, thus increasing the installation depth of the conductor and affecting the stability of the conductor. In addition, the models established in this work are applied in the field. In 12 wells, the average value of the calculated results is 114.76 mm, which is close to the lower limit of the interval. Overall, the research results provide a more accurate theoretical basis for bit stick-out design in deepwater conductor jetting projects. This study can maximize the jetting depth, considering that the size of the created hole optimally matches the casing size to achieve wellbore stability. In addition, the developed procedure based on the proposed mathematical model can be applied more conveniently to different combinations of conductor and bit sizes in the field.