Due to the specificity of coal rock containing CBM (coal bed methane), large-sized cuttings from coal bed drilling present abnormal appearances which make the transport of cuttings more troublesome. Simulations of the transport of large-sized and non-spherical drilling cuttings in the horizontal well-bore annulus possesses both theoretical challenges and engineering worth. In this work, a coupled CFD-DEM model is developed and a multi-sphere method is adopted to construct the non-spherical cuttings particles including cubic and long flaky particles. Validation with the experimental data reveals the well applicability of the developed CFD-DEM model. The movements as well as the sedimentations of cuttings are discussed in detail. The influences of fluid velocity and particle shapes on the transport efficiency of cuttings are studied and more attention is paid to flaky cuttings. The results show that, with a consistent equivalent volume, relatively good performance happens to the 20 mm spherical particles while local piled bed appears to the flaky particles; however, cuttings in cubic particles behave the worst. In the chosen conditions, transport efficiency drops dramatically when the volume fraction of the flaky cuttings exceeds 10%. Work in this paper is expected to be able to be applied to CBM drilling engineering.

The dynamics (rotation and stabilization) of single irregular particles that are constrained to rotate only and until to a quasi-steady state in a two-dimensional channel with viscous flow is numerically investigated using the gradient smoothed method (GSM). The GSM is proved to be computationally stable for arbitrary, irregular geometries discretized with distorted grids and well agreements with others’ work are revealed in validation cases. This work focuses on the influences of irregular particles’ typical shapes, including ellipse, rectangle and triangle, on the relevant surface forces, the flow dynamics, and the response time of the rotation. The effects of aspect ratio and flow velocity are studied in detail for all three typical types of irregular particles. It is found that the imbalance of the total torque on the surface of the particle causes the rotation, and when the particle is approaching the final stable position, the total torque becomes nearly zero with a small fluctuation, which contributes a local oscillation around the stable direction. In our cases, under the constraints, it is found also that the broad side of the elliptical particle always tends to be along the stream. For the rectangular particle, however, the aspect ratio and the flow velocity collectively determine the final orientation which means the major side or the diagonal line is along the flow stream. In addition, the triangular particle is found to behave quite differently in terms of both rotation and stabilization. The ‘response time’ of three types of particles is finally obtained from our GSM simulations. These findings could be helpful for a better understanding on the fluid-particle interactions and maybe advisory for determining the shape factor which is a key parameter for multiple particles motions.

In this study, a numerical model of a partial wellbore is developed by coupling computational fluid dynamics (CFD) with the discrete element method (DEM) in order to simulate the operation of a double-circulation system (DCS) in coalbed methane (CBM) well drilling, which is a novel technique used for cuttings transport. The values of the flow field are computed by a fluid solver and the motions of the particle phase are calculated by the DEM, for which an additional multi-sphere particle model is integrated. After verification of the model, the analysis centers on the working mechanism, the DCS parameters, and optimization of the angle θ of the injection nozzles, which is the most critical parameter affecting the efficiency of the DCS. Simulations reveal the working details of the DCS and prove its applicability to cuttings transport, especially in the curved and horizontal parts of the well. The effects of viscosity and particle size are investigated, and a low-viscosity drilling fluid is recommended. The optimal range for θ is obtained as 30° to 50°. This work provides a reference for the engineering application of DCSs, and by feedback from drilling sites the reliability of the results can be verified.

Falling rocks are one of the essential causes to pipeline failures in the most frequently occurring geological disasters and the pipes in a thin-shell structure can hardly resist the huge impact energy. The traditional design are conducted by keeping the maximum stress smaller than the material strength which is too conservative to make a full use of the pipeline. In this work, codes for the explicit dynamics based on the Lagrange algorithm are adopted to numerically simulate the impact of falling spherical stones on the large diameter pipe with the consideration of nonlinearity of big deformation. Dynamic responses of the pipe as well as strains on pipe’s cross section are discussed in detail. Influences on the pipe strains from factors like impact speed, stone size and moving direction are investigated. Results show that, most significant effect is from the diameter-thickness ratio of the pipe and it shows the resistant capability of the pipe drops exponentially with the decreasing of the thickness and the ratio is recommended to be chosen in a range of 40∼60. Eccentricity also effects the strains exponentially and the impact extent is greatly reduced with the eccentricity being smaller than 50%. Safety check is carried out by an ultimate-strain criterion and so, the ultimate size and velocity of the falling stone are got. What is done in this paper is expected to be of referential values for geological disaster resistant design of pipeline.