Study of Reverse Circulation Sampling Method Applied in Underground on Site Measurements for Coal Seam Gas Content

Author:Kang Jian Ning

Supervisor:hu qian ting


Degree Year:2017





Coal seam gas content is an important parameter and index for the coalbed methane development and mine gas management.The key to measure gas content accurately is quickly getting the coal sample.At present,commonly used like core pipe and compressed air ejector sampling method cannot achieve rapid sampling,which will lead to loss of gas content prediction error is bigger.In order to solve this problem,this paper draws on the technique of reverse circulation drilling in geological exploration and pneumatic conveying technology,and uses the method of theoretical analysis,numerical simulation,laboratory simulation experiment and field experiment to research the realize of reverse circulation sampling of coal seam,and the coal mine reverse circulation sampling device is successful developed.The research idea of the coal mine reverse circulation sampling technique is built,and its basic principle is analyzed theoretically.Based on the continuous equations,momentum equation,energy equation,isentropic flow equation and gas state equation of fluid mechanics,the model of anti-circulation flow field in sampling borehole was established.Based on Newton’s second law,the motor process of the solid particles in the air flow was analyzed,and the differential equation of motion of solid particles was established.The pressure loss of the gas-solid two-phase flow in the pipeline transportation process was analyzed,and the pressure loss in the center tube of the sampling system includes three aspects:the pressure loss by acceleration of the gas-solid two-phase flow,the pressure loss by friction of the gas-solid two-phase flow and the pressure loss by gravity of the suspended ascension of particle swarm.Based on the calculation of the pneumatic conveying project,the reasonable range of the inner diameter of the center pipe of the double-wall drill pipe is 21~32mm.Through theoretical calculation,the relationship between the coefficient of friction resistance λ,Reynolds number Re and annular gap h under the different gap condition of annular space in double-wall drill pipe was studied,The results show that:When the Reynolds number Re is the same,the coefficient of friction resistance X of the annular pipe decreases with the increase of the annular gap h.The product of the resistance coefficient λ and the Reynolds number is a constant value under the same annular gap condition.Under any annular gap condition,the resistance coefficient of the circular pipe λ,decreases with the Reynolds number Re increased,but the decreasing trend gradually slows down and eventually tends to level.Based on the Darcy formula,the relation equation of the on-way resistance of the annular pipe and the pipe length 1 under different annular gaps was obtained.The equation shows that when the gas flow velocity and the annular gap are the same,the resistance along the path is a variate linear function of the pipe length 1.In the same annular space,the greater the Reynolds number,the greater the resistance generated by the gas flows through the same delivery line.While the Reynolds number is the same,the greater the annular gap,the smaller the resistance generated by the gas flows through the same delivery line.Based on the Fluent numerical simulation,the annular flow field of the double-wall drill pipe was studied.The results show that the pressure drop caused by the compressed air flow through the annular space is similarly shown a linear attenuation law.The smaller the annular gap is,the larger the pressure drop is.Combined with coal mine pressure wind conditions,the center tube inner diameter of 32mm,annular gap of 5mm for 73mm external diameter double-walled drill pipe is determined.The area ratio between center tube and annulus is about 1.2.Based on the numerical simulation of Fluent,the structure and parameters of the key part to release the reverse circulation drilling flow field,the inner embedded annular injector of the drill bit and the external nozzle were studied respectively.Combined with the laboratory test,the nozzle established angle of the annular injector is determined to be 15°,the diameter is 2.1mm,the length of suction chamber is 1.5 times the size of the nozzle throat diameter;the drill external nozzle angle is 10~15°,the distance from external hole to the drill tooth center section is 2 times of the minimum distance between them.Based on the dense discrete phase model DDPM,the gas-solid two-phase flow in the borehole is simulated in the coupling effect of the annular ejector and the external nozzle,the feasibility of the reverse sampling of the drill bit was verified.Through the laboratory measurement of the samples particle size and the strength coefficient f of different tooth structure drill bit,the influence of the bit tooth structure on the sampling effect was studied.A laboratory simulation model of reverse circulation sampling system was constructed,and four kinds of flow media of air,sorghum,soybean and maize with different particle size were simulated.The mechanism of reverse circulation sampling was further understood,and the feasibility of reverse circulation sampling and the rationality of device component parameters were verified.The field application of practical coal mine reverse circulation sampling device was proceeding,the sampling depth is up to 120m,the sampling time is generally within 5min,the total sampling success rate is more than 90%,the sampling quality satisfies the amount of sample required for the direct determination of the gas content,and the accuracy of the coal seam gas content measured by the reverse circulation sampling is higher,which is verified the scientific of the paper research.