Research On Key Technology Of Tight Radius Drilling With Multi-nozzle Rotary Jet For Coalbed Methane Horizontal Well

As a low-cost horizontal well drilling technology,tight radius drilling technology has been applied in petroleum drilling,but it is still in the experimental research stage in coalbed methane（CBM）drilling.There are still many difficulties and challenges in theoretical research,tool development,technology update,etc.Relying on the“13^th Five-Year”National Science and Technology Major Project“Research on tight radius drilling technology and equipment based on high pressure water jet”（2016ZX05045-002-005）,this paper focused on the two technical problems of multi-nozzle rotary jet efficient drilling and safe steering of tight radius drilling in CBM wells.Guided by the theories of rock mechanics,computational fluid dynamics,mechanical design and drilling engineering,the method of combining theoretical analysis,testing test,numerical simulation,theoretical research and technical development have been carried out on hole-forming mechanism of coal breaking by self-propelled multi-nozzle rotary jet bit,underground steering equipment,movement law of coal chips and influencing factors,etc.,the pilot test of radial horizontal branch well with tight radius drilling technology in CBM well has been carried out in typical coal mine.The following research results and understandings have been obtained:（1）The distribution law of jet flow field and the characteristics of coal-breaking and hole-forming under the conditions of different jet pressure,rotation speed,jet angle and target distance are expounded.The mechanism of coal breaking and large diameter borehole-forming by multi-nozzle rotary jet is revealed.The rotary jet impinges the coal body mainly by the inclined impact,and forms the forward impact pressure and the surface parallel force（namely the radial force and the tangential force）on the coal surface.Coal breaking forms include dense core,tensile,water wedge damage caused by forward impact pressure and tensile shear failure caused by surface parallel force.Multiple annular broken pits were formed on the coal wall by rotary jets in front of the drill bit.As the bit was pushed down by the backward nozzle jet,different annular broken pits gradually extend inward and penetrate each other,forming connected boreholes.At the same time,the formed borehole wall was broken by the rotary jet at the side of the bit,the borehole diameter was increased.The effects of jet pressure,rotation speed,jet angle,target distance and jet environment on jet flow field and coal breaking pore formation were studied.（2）Self-propelled multi-nozzle rotary jet bit was designed and developed,which was characterized by three-stage drilling,two-stage detritus,anti-stall and speed control.The bit is mainly composed of rotary nozzle,rotary body,propeller,speed limiting device for the bit,crushing large coal chip--anti-stall hood,magnetic nipple and other parts,the drilling hole diameter can reach more than 60 mm,and the theoretical maximum self-propelled force can reach 1013.57 N,it has the ability of high efficiency breaking coal,large pore forming,effective carrying chip and strong self-propelled and so on.（3）The design idea of a new type of multi-functional support arm steering device was proposed,and the support arm downhole steering device with extended arm length and measurable in-situ state was developed.The device is mainly composed of steering mechanical assembly,steering power assembly,control assembly and cable assembly.Through the reciprocating movement of the electric push rod and turning link,the support arm is propped up and recycled.It has many new features,such as the lifting angle of the support arm can be measured and controlled（0-90°）,the length of the rear arm can be secondary telescopic（the forward extension length is 120 mm）,the working state can be monitored and the bit position can be perceived.The turning curvature radius is 550mm.It can make multi-nozzle rotary jet bit turn safely under complex construction conditions and reduce the construction risk.（4）The motion law of coal chips in the process of tight radius drilling with multi-nozzle rotary jet was obtained.Under the action of the annular swirling flow field,the coal chips can complete 1-2 spiral movements around the high-pressure hose in the annulus,and then move forward close to the bottom of the shaft wall,the track and speed of the coal chips will fluctuate in a small range.With the decrease of flow rate and the increase of coal chip diameter,the movement time of coal chip gradually increases and the migration efficiency gradually decreases.（5）A set of tight radius drilling equipment suitable for 139.7 mm casing completion CBM well was developed.The main technical parameters of the equipment are as follows:drilling curvature radius is 550 mm,the maximum displacement of the pump is 227 L/min,the maximum pump rate is 100 MPa,and the maximum tension of the winch is 80 kN.The equipment has the construction ability of sidetracking radial horizontal branch wells for CBM wells buried in shallow coal seams with depth of 1000 meters.（6）The research results have been applied in Binchang mining area.The equipment has stable performance and good operation,and good application effect has been obtained.Four horizontal branch wells have been drilled in one CBM well,the maximum drilling length is 80.36 m,the accumulative footage is 265.75 m,and the drilling speed ranged from 0.51 m/min to 1.03 m/min.