Research On Ground Reverse Circulation Sampling Technology And Drilling Tools For Determination Of Coal Seam Gas Content

Gas is one of the main threats to the safe production of coal mines.Coal seam gas content is the basic data for predicting the amount of gas emitted from coal mines,and also an important basis for formulating scientific and reasonable gas disaster prevention measures.Fixed–point and rapid sampling is the premise of accurate determination of coal seam gas content.At present,the problem of reverse circulation sampling in soft coal seams in horizontal open boreholes was solved,and fixed–point,continuous,rapid,stable sampling of shallow hole in coal mine can be realized.The commonly ground sampling method for determining the gas content in coal seams is the closed coring method,which has many problems such as complex process,long sampling period,and inability to continuously sample.In response to these problems,Chinese scholars have begun to explore the application of downhole reverse circulation technology to coal seam ground sampling.However,at this stage,there is a lack of systematic research on the influence of drilling tool structural parameters on the reverse circulation effect,which makes the existing underground reverse circulation technology unable to meet the needs of coal seam ground stable sampling.On the other hand,the traditional gas loss compensation model cannot accurately calculate the gas loss during the reverse circulation sampling on the ground of coal seam,which has plagued the accurate measurement of coal seam gas content on the ground for a long time.In view of this,the formation mechanism of ground deep hole reverse circulation of coal seam and the flow characteristics of gas–solid two–phase flow is systematically analyzed in this paper.The influence of drilling tool structural parameters on the reverse circulation effect and the flow characteristics of gas–solid two–phase flow in the central channel of double–wall drill pipe is studies.The calculation model of gas loss in ground reverse circulation sampling of coal seam is established.The drilling tools of ground reverse circulation sampling of coal seam is developed,and the engineering application is carried out in typical mining areas.The specific research work completed in this paper is as follows.(1)Based on the theory of gas–solid two–phase flow,the flow field model of hole bottom in ground reverse circulation sampling of coal seam and the motion equation of coal particles in the central channel of double–wall drill pipe are established.The basic principle of deep hole reverse circulation sampling and the mechanism of rock carrying in the flow field are expounded.The pressure loss of gas–solid two–phase flow in the central channel of a double–wall drill pipe and its influencing factors are studied theoretically,and the critical wind speed of stable transportation of particle group is analyzed.(2)The computational fluid dynamics(CFD)numerical simulation method is used to study the influence of bottom nozzle and diffuser hole on the reverse circulation effect of the drill bit with suction ratio and bottom negative pressure as the evaluation indexes of reverse circulation effect.Using orthogonal test design combined with numerical simulation method,the important order and optimal parameter combination of the influence of bottom nozzle diameter,perforation number,bottom nozzle angle,perforation diameter,bottom nozzle length and perforation angle on the reverse circulation effect of bit are determined.Through single factor numerical simulation,the influence of the above six structural parameters on the mass flow rate of the injected fluid in the annulus of the drill bit and the influence of the mass flow of the working fluid on the reverse circulation effect of the drill bit are revealed.The reverse circulation sampling bit test device is constructed to verify the reverse circulation effect of drill bit under optimal parameter combinations,and the numerical simulation results of the influence of working fluid mass flow on the reverse circulation effect of the bit.(3)On the basis of verifying the reliability of the simulation results by experiments,The computational fluid dynamics–discrete element(CFD–DEM)coupling simulation method is used to study the influence law of the diameter of the central channel of the double–wall drill pipe,the mass flow rate of the particles and the particle size on the flow state of the particle group,the velocity of the vertical direction,the average force,the contact force between the particles and the static pressure value in the central channel of the double–wall drill pipe.The static pressure value in the central channel of the double–wall drill pipe is revealed.(4)The gas desorption simulation test system of ground reverse circulation sampling of coal seam is constructed.The gas desorption characteristics of granular coal in the process of ground reverse circulation sampling are studied.The effects of adsorption equilibrium pressure,desorption environmental pressure,and particle size on the gas desorption rate and desorption of granular coal are obtained.The positive pressure environment in the center passage of the double–wall drill pipe during reverse circulation sampling is verified to inhibit the desorption of granular coal gas.The comparative test of gas desorption of granular coal under the condition of ground reverse circulation sampling and normal pressure environment is carried out,and the variation law of gas desorption amount of coal sample with time during ground reverse circulation sampling is obtained.The difference of gas desorption amount and desorption curve of granular coal under the condition of ground reverse circulation sampling and normal pressure environment is analyzed.The regression analysis is carried out by using the gas desorption data measured in the experiment,and the calculation model of gas loss in coal seam ground reverse circulation sampling is proposed and established.(5)The gas content determination process of coal seam ground reverse circulation sampling is given.The automatic determination method of gas desorption quantity based on the drainage quality method is proposed.The automatic determination device of gas desorption quantity on–site and laboratory of coal seam ground reverse circulation sampling is developed.The field industrial test is carried out in Qiyuan Coal Mine in Shanxi Province,and the research results of the paper are verified by sampling time as well as the comparison of the gas content measured by ground reverse circulation sampling in coal seam with the measured value underground.