Drilling Cuttings Transportation And Erosion Risk Assessment For CBM Multi-branch Wells Based On Particle Model

Owing to the fragility of the well-wall rock of the coal bed methane(CBM)well and its characteristic demand on the open-hole well completion,as well as the complicated structure of the multi-branch horizontal wells,drilling cuttings in large size are much more likely to appear.The large size cuttings not only tend to deposit and heap up locally,hindering the drilling circulation,but also aggravate the collapse risk of the well wall due to the erosion,which probably gives rise to the drilling accidents like pipe sticking or pump blocking.In this thesis,works are focused in interests as :(1)the dynamics of irregular particles in big size in viscous flow;(2)migration patterns in well-hole;(3)operating mechanism of double circulation drilling system and tool optimizing;(4)probability model for well-wall collapse risk with the particle erosion involved;(5)risk classification by colors and 3D visualization.Firstly,the GSM is adopted to solve the fluid problems which behaves well with unstructured grids or even distorted grids.The pressures and viscous forces on the irregular particles surfaces are numerically computed and the total torques are obtained.By which,the rotation mechanism,the orientation,as well as the generation and shedding of the vortexes at the top end are revealed.In addition,the differences on the forces and the reactions among different types of particles as well as effects from the aspect ratio are discussed and the reaction time is defined and compared.Moreover,the free falling of single and multiple irregular particles is simulated and the effects from the aspect ratio and shapes on the falling are got.Furtherly,the discrete element model(DEM)for the non-sphere big-sized drilling cuttings particles is built with the equivalent diameter.The Di Felice drag model and the Hertz-Mindlin elastic contact model are adopted with the rolling friction between particles considered in order to implement the coupling CFD-DEM parallel computing,which is verified by the experiment data as adequately practicable and accurate.Using the coupling method,the transportation of the big-sized irregular drilling cuttings particles in the well-hole is numerically simulated and the influences on the moving velocity,transport ratio from the drilling fluid velocity,volume fraction and the shapes of the cuttings are analyzed.Additionally,another CFD-DEM model for the enlarged junction of the main well and production vertical well is established to numerically represent the migration and sedimentation of drilling cuttings at the junction,and influences of the volume fraction,size of cuttings,as well as the drilling fluid displacement are studied.Meanwhile,study is conducted on the mechanism of a novel drilling technique to discharge the drilling cuttings: the double circulation drilling technique.A whole well simulation model based on the trajectory data of the real well and its operating parameters is developed for the double circulation system using the low-viscosity fluid.The calculation reveals how it works and confirms the positive function on the transportation of the drilling cuttings.Also,optimization analysis is carried out on the key bottom component: the jet injector and the optimized injection angle as well as the placed position are obtained.Furthermore,on one hand,a computing model for the stresses nearly around the well is developed and through the compiled codes,the stress distribution can be accurately calculated.On the other hand,with the FLUENT erosion model,the erosion weight on the well-wall is got and reflected on the change of the well size.Combined with size effect equations,the effect of the erosion on the well-wall collapse pressure can be quantified.Moreover,an index set of wellwall collapse risk in drilling is formed including the risk of erosion.Using the Latin Hypercube method and test data of well ZS-3H,the probability distribution functions for parameters of coal rock and ground stress are got.In addition,based on what is got from our work,the probability distributions of erosion risk factors are obtained.By introducing the Hoek-brown criteria to the probability model,the risk prediction model of the well-wall collapse risk in drilling is established.At the end and as a sum,the collapse risk of well-wall is classified for five grades and attached with different colors.The well of ZS-3H is exampled.Integrated with the 3D well trajectory technique,a technique of 3D visualization of the drilling risk for the whole well group of the CBM multi-branch wells is developed and included by the expanded digital-well concept.