Prediction and Optimization Method of Drilling Trajectory for Push-The-Bit Rotary Steering Tools
Authors: Wang Ge , Huang Wenjun , Shi Xiaolei , Gao Deli , Wang Xiaodong , Yang Guodong 
Published: September 2022

ABSTRACT:
Rotary steering drilling tool with the push-the-bit function can control directional drilling orientations with biasing unit pushing against the well wall. In this paper, taking the biasing mechanism as the research object, based on the mechanical model of the steering drilling tool, rock-bit interaction, the drilling tool structure, an inversion calculation model of the rock anisotropy index is established to reveal the drilling direction in the actual drilling process. Further considering the drift effect of biasing unit on the bit, the trajectory of the prediction section is calculated by utilizing the ‘natural parameter method.’ Based on the above theoretical method and guided by the dynamic optimization of the wellbore trajectory, the dynamic search method of biasing unit is established. The method has high operating efficiency and can provide a theoretical basis for integrating subsequent downhole uploading data and surface transmission command interface integration of push-the-bit rotary steering drilling tools.

1. INTRODUCTION
Rotary Steerable System (RSS), as the most advanced steering drilling technology, can effectively overcome the influence of complex underground conditions. It can accurately get into the target according to the designed trajectory by continuously and smoothly adjusting the bias mechanism. However, under the constraints of a series of subjective and objective factors (drilling tool structure, pushing force, bit cutting performance, drilling parameters, rock properties, etc.), the difficulty of trajectory control is still the critical problem of RSS. Therefore, it is necessary to seek a set of stable and efficient decision-making algorithms for pushing force that fulfills the integration of the confidential upload data and surface download command interface, forming a group of an automatically optimized control module.
At present, the commonly used trajectory control methods are mainly based on stress deformation analysis of bottom-hole assembly (BHA), bit-formation anisotropy, trajectory measurement and calculation, and the appropriate drilling parameters are selected to be controlled by continuously adjusting the control variables (pushing force, tool-face, weight on bit, rotational speed, etc.). For the mechanical analysis of BHA, the commonly used methods are mainly the semi-analytical method and numerical method. Bai et al. (1990) and Wang et al. (2018) established the three-bending moment equation of BHA by using the stabilizer as the node of the tubular string by the longitudinal and transverse bending method. Gao et al. (1994a) utilized the weighted residual method based on the deformation control equation of BHA. They used the Galerkin method to eliminate the residual error caused by the deflection of tubular string to solve the stress deformation problems of BHA. The above two methods are semi-analytical methods with high computational efficiency and easy programming, widely used in engineering. Numerical methods include the finite element method, finite difference method, etc. Ho (1986) used the finite element method to solve the small deflection problem of BHA. Millheim et al. (1978) used the finite difference method to solve the deformation problems of slight deflection and large deflection of BHA. The numerical method can more accurately calculate the three-dimensional deformation of tubular string and the contact condition with the wellbore. However, the numerical method has the problems such as the high dimension of nonlinear equations, slow calculation efficiency, and poor convergence, which is rarely used in practical applications.