Research On Wellbore Stability And Cleaning In Gravel Stratum Horizontal Radial Wells

A radial well is composed of a vertical well and multi-radial wells.Radial well represents the future development of water well because of its advantages of large water output,high efficiency,low management cost and strong ability of regulating groundwater level.Since the gravel stratum is generally loose,drilling radial wells potentially leads to wellbore instability and a series of engineering risks.Currently,in radial well construction,the vertical well is mostly completed by reverse circulation drilling and the multi-radial wells completed by artificially pipe-jacking underground.This process has the disadvantages of large excavation,low efficiency,poor worker safety and high cost,which severely restricts the application of the radial wells.In terms of the disadvantages mentioned,we firstly proposed using the reverse circulation dual-wall drill pipe drilling technology and the filter tube to complete the radial wells.During this process,the stability of the wellbore,the law of cuttings transportation and the new drilling equipment were studied in this paper.The rotary construction for drilling vertical well,its mechanism and implement process were elaborated out,with the drilling completion investigated.This method effectively enhances the drilling efficiency,reduces the vertical well diameter and lowers the construction cost.To solve the challenges of well drilling,trajectory control of wellbore stability,the concept of the reverse circulation dual-wall drill pipe drilling technology was proposed.The key supporting equipment such as integrated drilling rig,double rotary head,dual pipe drilling pipe,dual-channel faucet,and three different sorts of guided directional sections for this process were developed.In this paper,by using discrete element method,the wellbore stability and instability process of the water-rich gravel stratum were investigated from the microscopic point of view.Based on the results of large triaxial tests of the gravel in typical areas,the meso-mechanics model of gravel was built through calibration.On this basis,a discrete element model of water-rich gravel stratum was established.By studying the effect of different drilling fluid hydrostatic column pressure on stress and displacement of the surrounding borehole,the evolution rule of the deformation of the near-well stratum with time and space is obtained.The deformation was also simulated when drill pipe was in the wellbore.During the process of drilling and completion,the technical advantages of the outside drill pipe were demonstrated.According to the dual-wall drill pipe directional drilling process,the failure model of the borehole wall in radiation well is established.Based on the PFC^3D software,the process of borehole wall instability during drilling and completion is simulated,verifying the superiority of the double pipe drilling and providing a theoretical reference for the construction in field.Based on the computational fluid dynamics theory,numerical simulation was used to calculate the cuttings volume fraction and the distribution of annular pressure field in the reverse circulation dual-wall drill pipe drilling.By using the statistical calculation method,the regression equations of the volume fraction and the pressure field of the annulus were given.We designed and produced an indoor test platform of cuttings,carrying out laboratory tests,whose results are in good agreement with the numerical simulation.The accuracy of the regression equation is also verified.The results show that the research results are not only important for the construction of reverse circulation dual-wall drilling technology,but also significant for the construction of the rock stratum,the fractured stratum and the soft stratum,providing a good reference for efficient and safe drilling.