Research On Coiled Tubing Drilling String And Swirling Jet Rock-breaking In Underground Coal Mines

Coiled tubing drilling technology has the advantages of no single connection,continuous tripping,continuous mud flow during drilling,high drilling purification efficiency,easy to realize operation automation and intelligence,etc.It has become an important research direction of safe,efficient and automatic drilling technology in underground coal mine.However,the two key technical problems need to be solved in developing coiled tubing drilling technology for near-horizontal holes in underground coal mine are coiled tubing string extension and efficient rock breaking.The buckling behavior of coiled tubing with near horizontal hole is the basis of string extension.Due to the limitation of borehole diameter and coiled tubing drilling tool classification,the drilling capacity of available mud motor is not enough,so the swirling jet rock breaking becomes the first choice of efficient rock breaking method.Therefore,the mechanical behavior of coiled tubing string in near-horizontal hole and rock breaking by swirling jet,which have important theoretical significance and application value to explore and develop coiled tubing drilling technology in underground coal mine.According to the space of coal mine roadway,borehole characteristics,mud pump capacity（pressure and flow）and coal seam drilling properties,the calculation of coiled tubing elastic bending radius,fluid friction analysis,drum structure parameter and drilling process are analysed,the swirling jet drilling method as the core of the coal mine coiled tubing drilling overall technical scheme is determined.It lays a prerequisite for the subsequent research on the mechanical behavior of coiled tubing string in near-horizontal hole and the jet rock breaking method.The buckling model of coiled tubing with near horizontal hole was constructed,and the critical loads of sinusoidal buckling and helical buckling of coiled tubing with near horizontal hole were analyzed.The relationship between critical buckling load of coiled tubing and diameter ratio r_cb(r_cb=d_coil/D_bore)and the relationship between the radial free end length of hole head and critical buckling load of coiled tubing at different r_cb were obtained by numerical simulation,and the buckling load axial force and radial free end instability were studied by the buckling test system.The results show that,the r_cbis larger,then the axial force is larger when the critical buckling load become larger.When the length of the radial free end exceeds 800 mm,the critical load of coiled tubing buckling tends to be the same at different r_cb.The optimal diameter/aperture ratio r_cb=0.454 is obtained,which provides a theoretical basis for the optimization of coiled tubing string and the control of drilling process parameters.By constructing the fluid flow equation of coiled tubing drilling,the circulation pressure loss distribution law of coiled tubing drum section,straight pipe section and borehole annulus was analyzed,and the hydraulic extension limit of near-horizontal hole was obtained.Through the establishment of a frictional model considering the drilling trajectory bending factors,the relationship between the factors affecting the mechanical extension capacity of the coiled tubing,such as wall thickness and outer diameter and friction coefficient are analyzed.It is obtained that the hydraulic and mechanical comprehensive extension limit of the coiled tubing in the near horizontal hole can reach more than 300 m,which provides a theoretical basis for the drilling capacity design of the coiled tubing.Based on the established drag-reducing model of axial vibration of coiled tubing with near horizontal hole and the evaluation method of drag-reducing efficiency,the influences of vibration tool parameters and coiled tubing parameters on friction and drag reduction effect were studied,and according to the optimization model of vibration parameters,the recommended vibration excitation amplitude of coiled tubing with near horizontal hole in coal mine was given.Finally,the fluid-structure coupling method is used to verify the drag reduction law of the water shock oscillator.The results show that the drag reduction efficiency of the vibration tool increases first and then decreases with the increase of frequency,and linearly increases with the increase of vibration amplitude.The larger the vibration amplitude is,the lower the fatigue safety factor of coiled tubing is.It is recommended that the vibration amplitude of coiled tubing drag reduction vibration tool in coal mine should not exceed 1.8 kN.Based on the analysis of the flow field of swirling jet,the paper developed the cone dynamic pressure liquid film lubricated swirling jet bit,and the relationship between the flow rate of swirling jet drilling,the drilling rate and the diameter of the drilling hole,as well as the drilling deviation rule of directional drilling of swirling jet by coal similar material breaking experiment and drilling tests are studied.The test results show that under the same jet pressure,the strength of coal rock is doubled and the diameter of borehole is reduced by about 30%.The drilling rate is faster,the hole diameter will be smaller.Otherwise,the hole diameter will be larger.The hole diameter can be controlled by controlling the pump pressure（displacement）or feed speed.Under the same conditions,the build-up rate of swirling jet directional drilling is higher and stable in the coal sample of f=0.93,while the build-up rate of swirling jet directional drilling is low and discrete in the coal sample of f=0.42 or f=0.23.The law of deviation with swirling jet directional drilling in coal with different hardness is revealed.The research results provided a theoretical basis for the development of coiled tubing drilling technology and equipment for near horizontal borehole in underground coal mine.