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Abstract
To solve the problems that the borehole depth is shallow and the borehole formation rate is low during the gas drainage drilling in soft coal seam with current cuttings removal method, a new technology of reverse circulation pneumatic cuttings removal is put forward. The CFD-DEM coupling method is used to establish the simulation model of cuttings-air two-phase flow in drill pipe. The effects of the air velocity for cuttings removal and the mass flow rate of cuttings on the flow characteristics, cuttings removal effect and pressure drop of cuttings-gas two-phase flow are analysed. The results show that the drag force of drilling cuttings becomes larger with the increase of air velocity and the stratified flow characteristic is obvious. The drill cuttings migration ratio is positively correlated with the air velocity for cuttings removal and negatively correlated with the mass flow rate of cuttings. When the mass flow rate of cuttings is constant, the increase of air velocity for cuttings removal leads to the increase of pressure drop in the inner hole of drill pipe. When the air velocity of cuttings removal is constant, the mass flow rate of cuttings and the pressure drop in the inner hole of drill pipe increases. Therefore, the appropriate air velocity should be selected considering the energy consumption during cuttings removal.
Highlights
Gas extraction is the main means to prevent and control gas outburst in coal seam
Drill cuttings conveying during the borehole drilling in coal seam is the key to ensure the drilling depth and the drilling efficiency. e mechanical removal and the hydraulic removal of cuttings have a great impact on the stability of borehole wall, which is easy to cause the borehole collapse and blockage, sticking of the drill pipe, and other accidents [1]. e current cuttings removal method with pressure air removes the cuttings through the annular hole between drill pipe and borehole wall by the pressure air through the inner hole of drill pipe
Because the cuttings speed is less than the air velocity, the drilling cuttings accelerate and form obvious cuttings flow under the drag force of air flow, and under the gravity action, the drilling cuttings flow gathers at the bottom of the inner hole of drill pipe and moves along the direction of the air flow until they are discharged from the inner hole of drill pipe
Summary
Gas extraction is the main means to prevent and control gas outburst in coal seam. And, drill cuttings conveying during the borehole drilling in coal seam is the key to ensure the drilling depth and the drilling efficiency. e mechanical removal and the hydraulic removal of cuttings have a great impact on the stability of borehole wall, which is easy to cause the borehole collapse and blockage, sticking of the drill pipe, and other accidents [1]. e current cuttings removal method with pressure air removes the cuttings through the annular hole between drill pipe and borehole wall by the pressure air through the inner hole of drill pipe. Drill cuttings conveying during the borehole drilling in coal seam is the key to ensure the drilling depth and the drilling efficiency. E current cuttings removal method with pressure air removes the cuttings through the annular hole between drill pipe and borehole wall by the pressure air through the inner hole of drill pipe. It is difficult to collect the drill cuttings at the outlet of boreholes, which is easy to cause the dust pollution [2,3,4]. A new reverse circulation pneumatic cuttings removal technology is proposed that the annular hole between drill pipe and borehole wall is used as the air inlet and the inner hole of drill pipe is used as the cuttings removal channel [5, 6]
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