Rock Breaking Mechanism and Trajectory Stabilization of Horizontal Well Section with Flexible Drilling Tool
Authors: Xin Liu , Zhongwei Huang , Huaizhong Shi , Xiaoguang Wu , Han Chen , Rui Liang , Huiyong Yu 
Published: September 2023

ABSTRACT
This paper examines the mechanics of rock-breaking and trajectory issues in ultra-short radius radial horizontal wells with flexible drilling tools that are related to the vertical deviation of well trajectory. A model for rock breaking in the horizontal section of flexible drilling tools is established. Specifically, this study focuses on the movement of flexible rods and bits, rock-breaking mechanics, trajectory characteristics, rate of penetration (ROP) along the trajectory, and adaptability to formation strength. The key findings are as follows: the flexible drill tool breaks the rock in a scanning attitude, and the drill bit rotates on its axis while revolving around the well axis. The trajectory of flexible drilling tools tends to bend in the direction of gravity. Increasing torque and weight on bit (WOB) can maintain wellbore stability and ROP. In highly drillable rock formations, the deviation rate of the bit trajectory with flexible drilling tools is significant. The study also discusses previous research on flexible tool string motion and explains the reasons for vertical trajectory deviation in the oilfield.

INTRODUCTION
The ultra-short radius lateral drilling with flexible tools is a novel sidetrack drilling technology. This technology is capable of drilling one or more horizontal laterals oriented to one or more formations within a vertical borehole, which can penetrate the near-well contamination zone and greatly increase the drainage radius. This technology utilizes specially designed flexible drilling tools, milling, and build-up tools. A flexible drilling tool consists of joint-connected rods and bits. The flexible tool string trips up and down by a workover string or coil tube (CT). Each rod is 15cm in length. The max bending angle between two rods is 3-5°. The flexible tools are supplemented by a trajectory control method to complete the operating procedures of the casing milling, build-up, and horizontal drilling process in the same formation. It can drill a well with a 2-5m radius curvature build-up part and a 40-60 m horizontal part.
The application of flexible drilling tool technology started in 2000 when Gary Gourley et al (Gary, 2000) conducted flexible drilling tool experimental work on a radial well in Graham, Texas. In 2003, Abdullah Saif Al-Hady et al (Al-Hady et al., 2003) conducted two radial well tests with flexible drilling tools in the Lekhwair field. In China, Daqing oilfield (Xiao, 2022) carried out three wells flexible drilling tools ultra-short radius radial lateral drilling horizontal well test in 2022. The borehole trajectory reached the program design requirements, and the process can realize directional orientation. In 2009, Karamay oilfield (Lu et al., 2009) used ultra-short radius lateral drilling horizontal well to improve shallow-layer thick oil recovery application. The radius of curvature is 2.86m, the horizontal section borehole diameter of 120mm, and the length of more than 22m. In coalbed methane application, flexible drilling tool operation in coalbed methane exploratory well LX-29 by China Union Coal Group(Lv, 2018). In the offshore oil field, (Guo et al., 2020) CNOOC achieved the first 4-branch ultra-short radius flexible drilling tool operation with a curvature radius of 3m and a build-up rate of 16°/m offshore. The first field test of CT with flexible drilling tools was carried out in Ling 72-5CZ well in Jianghan Oilfield in 2021 (WU et al., 2022).