In ultra-deep well drilling, the drill bit undergoes severe axial-torsional-lateral coupled vibrations, such as stick-slip and whirling, significantly increasing drilling risks. The evolution laws of these complex vibrations have not been fully revealed, making it challenging to design effective vibration mitigation strategies. Therefore, this paper aims to build a new drill bit dynamic model considering axial and lateral cutting processes to reveal the evolution laws of stick-slip and lateral vibrations. Firstly, by considering the three-dimensional cutting force while breaking rock, cutting limits, and nonlinear friction, the axial-lateral-torsional coupled vibration model of the drill bit was established. Next, the contact determination conditions between the drill bit and rock were extended, refining the three-dimensional bit-rock interaction model. Additionally, considering the complex time delay caused by axial-lateral-torsional vibrations, the realistic cutting profile for the drill bit was constructed. Consequently, the evaluation indexes for the stick-slip vibration and whirling of drill bit were developed. Finally, the validity of the model was confirmed through field data validation and comparison with the RGD model. The results indicate that the lateral motion of the drill bit undergoes a “collision-cutting-separation” process. The lateral alternating cutting motion intensifies stick-slip vibrations, reduces torsional amplitude stability, and raises the rotational speed threshold for escaping stick-slip vibration. As rotational speed increases, stick-slip vibration transitions to intermittent stick-slip vibration and eventually escapes from stick-slip vibration. The lateral cutting limit and torsional sticking state can lead to the lateral sticking state. With increasing rotational speed, the lateral motion changes from “collision-cutting-sticking-separation” to “collision-cutting-separation”, with the lateral motion evolving from random backward motion to quasi-periodic backward motion and ultimately entering periodic backward whirling. This study provides a theoretical foundation for drilling parameter optimization and drill bit selection.

It is generally believed that cuttings have a significant impact on the forces of tubular string in extended-reach drilling. However, there are few studies attempted to investigate and quantify it. In this paper, a three-layer transient model for cuttings transport is established to simulate the characteristics of dynamic cuttings transport over time under various conditions. The simulation results indicate that the change in drilling parameters like ROP (rate of penetration) and flow rate of drilling fluid will lead to the non-uniform distribution of cuttings bed. And the alternation of drilling and circulation will lead to a clear wavy distribution of cuttings bed in the wellbore. Then, the effect of cuttings on tubular string is obtained through a large number of numerical simulations and the nonlinear regression method, and this influence is introduced into the conventional stiff rod model of tubular string. Finally, the transient model for cuttings transport is coupled with the modified tubular mechanic model and applies to a case study of extended-reach drilling. The results show that there is a delay effect for the effect of the changes in drilling parameters on the ground torques because the changes in drilling parameters occur instantaneously, while the changes in cuttings bed distribution are slow due to its low transport velocity. Based on the coupling analysis of transient cuttings transport and tubular mechanical behaviors, the drilling parameters are optimized, including the recommended adjustment period and adjustment range for the ROP, the proper drilling time for the increased flow rate. Furthermore, the circulation and back reaming are optimized. For circulation, the keys are choosing appropriate time interval between the two adjacent circulations and the time for each circulation. To avoid pipe stuck, at least 20 min of circulation is required to remove the cuttings bed near the large-sized BHA ((Bottom Hole Assembly)) before back reaming, and the maximum back reaming velocity should be smaller than the minimum transport velocity of the uniform bed.

In extended-reach or long-horizontal drilling, cuttings usually deposit at the bottom of the annulus. Once cuttings accumulate to a certain thickness, complex problems such as excessive torque and drag, tubing buckling, and pipe stuck probably occur, which results in a lot of non-productive time and remedial operations. Cuttings bed remover can efficiently destroy deposited cuttings in time through hydraulic and mechanical stirring effects. This paper aims to build a method for hole cleaning evaluation and installation spacing optimization of cuttings bed remover to improve the wellbore cleaning effect. Firstly, a Computational Fluid Dynamics approach with Eulerian–Eulerian multiphase model was utilized to investigate the mechanism of cuttings transportation, and a new type of cuttings bed remover was designed. Next, an evaluation method of hole cleaning effect of remover was established. After that, the effects of several drilling parameters on hole cleaning including flow rate of drilling fluid, rotational speed of drillpipe, rate of penetration, wellbore size, rheological property of drilling fluid, and remover eccentricity on the performance of cuttings bed remover were investigated. The results demonstrate that the new type of remover with streamline blade performs better than conventional removers. The efficiency of hole cleaning is greatly improved by increasing the rotational speed of drillpipe, flow rate of drilling fluid, remover eccentricity, and 6 rpm Fann dial reading for drilling fluid. While higher rate of penetration and large wellbore size result in worse hole cleaning. These findings can serve as an important guide for the structure optimization design of cuttings bed remover and installation spacing of removers.

Casing wear and casing corrosion are serious problems affecting casing integrity failure in deep and ultra-deep wells. This paper aims to predict the casing burst strength with considerations of both wear and corrosion. Firstly, the crescent wear shape is simplified into three categories according to common mathematical models. Then, based on the mechano-electrochemical (M-E) interaction, the prediction model of corrosion depth is built with worn depth as the initial condition, and the prediction models of burst strength of the worn casing and corroded casing are obtained. Secondly, the accuracy of different prediction models is validated by numerical simulation, and the main influence factors on casing strength are obtained. At last, the theoretical models are applied to an ultra-deep well in Northwest China, and the dangerous well sections caused by wear and corrosion are predicted, and the corrosion rate threshold to ensure the safety of casing is obtained. The results show that the existence of wear defects results in a stress concentration and enhanced M-E interaction on corrosion depth growth. The accuracy of different mathematical models is different: the slot ring model is most accurate for predicting corrosion depth, and the eccentric model is most accurate for predicting the burst strength of corroded casing. The burst strength of the casing will be overestimated by more than one-third if the M-E interaction is neglected, so the coupling effect of wear and corrosion should be sufficiently considered in casing integrity evaluation.