Study On The Evolution Law Of Gas Seepage In Fracture Field And Optimization Of Drilling Extraction Parameter

Aiming at the coal seam geological conditions of high gas and low permeability in Longfeng coal mine,Guizhou Province,this paper has completed the numerical simulation of the distribution and evolution law of gas seepage under the fracture field,the field test and microseismic monitoring of hydraulic fracturing and permeability enhancement technology,and the optimization of drilling gas drainage parameters.The research contents and conclusions are as follows.1.In the coal sample with X-shaped fractures,the gas starts from the inlet,the seepage pressure decreases from left to right,and the gas pressure in the fracture channel is evenly distributed,which is a section of pressure buffer zone;The distribution of seepage velocity field in non fracture area is uneven.The gas seepage at the tributary of X-shaped fracture is more active than that at the confluence.The included angle between gas flow direction and fracture direction has an obvious influence on the seepage velocity.The maximum seepage velocity of gas flowing from the lower boundary is 29.5 times that from the left boundary;The seepage velocity in the fracture channel has a monotonically decreasing logarithmic function relationship with the fracture scale.The larger the fracture scale,the lower the Darcy seepage velocity.When the fracture scale is 0.68～1.23 mm,the influence on the seepage velocity is the most obvious.2.During hydraulic fracturing,the duration of pressure maintaining stage is the longest.During fracturing,the maximum water pressure is 9.6mpa,and the average water pressure is 6-8mpa.The initial velocity of gas release after fracturing is maintained between 7.8～13.0mmhg,reducing 36.9%～62.2%.After microseismic technology detection,the optimal wave velocity VP of the coal seam in the second mining area of Longfeng coal mine after fracturing is 4600m/s,the wave velocity anomaly coefficient an decreases,the stress concentration range is significantly reduced,and the outburst risk is effectively reduced.Moreover,the influence range of hydraulic fracturing is about 100m in the horizontal direction and more than 150m in the vertical direction.3.According to the law of negative exponential decay of gas flow,combined with COMSOL Mutiphysics simulation software to establish a three-dimensional extraction model to calculate the effective radius.According to the law of gas flow attenuation,the effective radius under different extraction time is deduced,and compared with the effective radius under the calculation of extraction model,the theoretical value of the effective radius of gas extraction after fracturing increases from 2.86 m to 4.83 m,a 49%increase.According to the on-site monitoring of gas extraction,the gas extraction flow in No.9-4 hole increases to the maximum 0.5m~3/min,the gas extraction concentration in No.9-4 hole increases from 60%to more than 90%,and the gas extraction flow in No.9-5 hole increases to the maximum1m~3/min,the gas extraction concentration increased from 58%to more than 90%in no.9-5 hole,the gas extraction flow increased to 0.09 m~3/min in No.9-6 hole,and the gas extraction concentration increased from 30%to more than 90%in No.9-6 hole.