Experimental study on distribution of negative pressure and flow rate under conditions of deformation and instability of boreholes

In order to study the effect of borehole deformation and instability on gas drainage and determine the instability and collapse area of the borehole,accord ing to the characteristics of gas flow and variable mass flow in the borehole and the borehole deformation,the gas drainage experimental system considering the deform ation and instability of boreholes was constructed. The drainage test system was tested and the negative pressure and flow distribution law of the borehole were analyz ed under the conditions of different borehole deformation and instability and different negative pressure ,and a calculation method of negative pressure loss considering borehole deformation was proposed. The results show that:①The negative pressure of complete hole drainage is approximately linearly distributed along the length of t he hole,and the negative pressure loss is relatively small,the negative pressures at different parts of borehole are similar with each other,and the drainage flow of the b orehole is also roughly linearly distributed along the length of the borehole. The larger the orifice negative pressure,the larger the borehole drainage flow,and the larger the borehole negative pressure loss. It is worth noting that borehole negative pressure is still much smaller compared with the orifice pressure accounting for only 2.9 3% ~ 3.62%.②When the bottom of the hole is collapsed,the negative pressure loss in the collapse section is larger than that in complete section. Because the flow i n the collapsed section is smal,its negative pressure loss is not large. ,accounting for only 2.75% ~ 4.03% of the negative pressure at the orifice. When the hole collaps es,the total negative pressure loss of hole collapse accounts for 4.68% ~ 6.14% of the negative pressure of the orifice,which is higher than the total negative pressure | oSs of complete hole,and the total drainage flow is slightly lower than the total drainage flow of complete hole. The hole collapse at the bottom of the hole has lttle effec t on the gas drainage effect.③When the hole is blocked at the bottom of the hole,the coal in the collapsed section of the borehole is connected with the surrounding coal to form a continuous medium,the gas flow in the collapsed section becomes seepage,and the drainage pressure becomes the gas pressure of the coal seam,which is in line with the complete hole and plugging. Compared with the drainage flow at the time of the hole,the drainage flow at the time of the hole collapse is significantly r educed until the flow rate drops to zero. Hole plugging has a great impact on the gas extraction effect.④Based on the calculation theory of borehole negative pressur e loss,the negative pressure distribution and flow distribution test results under different deformation and instability of boreholes,a calculation method of borehole negati ve pressure loss considering borehole deformation was proposed and was verified. The research results can provide theoretical basis for the determination of borehole instability and collapse area as well as the precise protection of borehole instability and collapse.