This article models the contact between the drillstring with large slenderness ratio and the extending rigid wellbore based on the multibody dynamics method. The drillstring is modeled as absolute nodal coordinate formulation beams with contact detection points. An algorithm is developed to locate the contact points and calculate the contact forces when the drillstring is sliding and rotating in the wellbore. This provides support force and friction for the moving drillstring and effectively confines it inside the wellbore. A rock penetration model is established based on the rock-breaking velocity equation. The governing equation for the full-hole drilling system including the drillstring and the contact model is established and solved. A rock penetration correction method is proposed to stabilize the computation and to model and simulate the slide drilling process. A field drilling process is modeled and simulated. The simulation result fits the experimental result well.

Slide drilling refers to the technology of creating a predetermined non-vertical wellbore with a bent housing positive displacement motor (PDM). It is widely adopted in the area of directional drilling. In practical drilling operation, the top drive on the ground introduces an angular rotation to the top of the drill string, and the PDM at the bottom of the drill string rotates accordingly. When the bend is pointed to the desired direction, the adjustment of the PDM stops and the drill string slides without rotation to make a deviation. Up till now, the relationship between the top drive displacement and the direction of the bend, namely the tool-face angle (−180° ∼ 180°), is still unclear. In this research, an indoor slide drilling experiment is carried out, and the nonlinear relationship between the top drive input and the tool-face output is recorded. The hysteretic phenomenon observed is consistent with the in-field experience, and a single-input-single-output (SISO) system is established to describe this relationship. The Volterra/ Wiener neural network (VWNN) is introduced to identify this system, and provides a one-step prediction of the tool-face output. The predicted tool-face output is verified by the experiment data.

Slide drilling is widely used in directional drilling because of its lower cost, but the tool-face adjustment during the drilling process is almost completed by the driller, and the automatic control has not yet been realized. The main difficulty is that the drill string system has a large amount of contact and friction with the wellbore. Moreover, it is also affected by the viscous force of the mud fluid, which brings more difficulty to achieve accurate dynamic model. Considering that the actual drilling is too costly, it is necessary to establish an indoor experimental system to simulate the response of the slide drilling tool-face, thus providing an experimental platform for the research of the control method. In this paper, the dynamic equation of the torsion process of the drill pipe is established. According to the model similarity theory, the similarity condition of parameters and scale correspondence between the experimental model and the drilling prototype are obtained, which provides the basis for selection of simulated drill pipe and mud. Then the expert PID method is used to realize the automatic tool-face control based on the laboratory experimental system. The experimental results show that the expert PID method can obtain better control effect rather than the traditional PID method, and it could be used in the actual slide drilling because of the dynamic similarity.