Study On The Direct Method Of Gas Pressure Measurement In Coal Seam Under Mine By Numerical Simulationand Engineering Direction

Gas pressure in coal seam is one of the most important fundamental parameters of gas in mine,so its accurate measurement is of practical significance for controlof gas accidentand safety production in mine and fundamental position in energy.By summarizing and analyzing the closed researching findings,I put forward some research goals as supplements.Based on physical and numerical model of gas flow in coal seam and numerical simulation soft called COMSOL Multiphysics,I conducted some constructive studies.Consequently,I discovered and concluded some results and provided certain directions to some extent for gas pressure measurement on the spot.(1)Based on certain mathematical model,I studied the influence of different factors on pressure measurement.The time of gas pressure recovery is not in proportion to dropping pressure range and it depends on concrete situations.The results showed that the more primitive gas pressure in coal seam or permeability is,the larger dropping pressure range is under certain conditions,but the time of gas pressure recovery will not be added;and that the longer exposing time is,the larger dropping pressure range is and the longer time of gas pressure recovery is under certain conditions,verifying the previous conclusion;and that the bigger borehole is,the larger dropping pressure range is and the longer time of gas pressure recovery is under certain conditions.(2)Taking slippage effect into consideration,thepermeability of gas around the borehole is in proportion to exposing time duringthe period of sealing.But this effect can fade away with distance increasing away borehole.Thereforethe value of gas pressure is lower when the slippage effect is considered,showing that slippage effect is bad for measuring in the period of sealing.After the period of sealing,thepermeability of gasis always bigger than initial value because slippage effect.It means that gas will flow in coal seam easier,showing indirectly that it is good for gas pressure measuring.(3)Taking the fluid-structure coupling of coal and gas into consideration,the value of gas pressure is higherbeforesealing.In other words,the coupling decreases the pressure loss.But as the exposing time going by,the value of gas pressure is almost equivalent under the condition of coupling and non-coupling,showing the advantage is fading away.Traditional non-coupling only considers pressure reduction funnelaround the borehole.But there are also porosity reduction funnel and permeability reduction funnel when taking the fluid-structure coupling of coal and gas into consideration.Before the sealing becomes effective,both porosity and permeability gradually reduce and its speed presents a decelerated trend.From the perspective of reducing porosity and permeability,the deeper coal seam is,the more difficult the gas pressure measurement is.But after the sealing becomes effective,especially in early periods,gas pressure increases quickly,leading to porosity and permeability also increases quickly to initial values.Besides,the fluid-structure coupling of coal and gas decreases the pressure loss.Consequently,the measuring time is shorter than under non-coupling condition.(4)I broken the limits of traditional numerical simulation and built application for simulation of gas pressure measurement,application for simulation of gas pressure measurement that considers slippage effect and application for simulation of gas pressure measurement that considers coupling,realizing fast numerical simulation.(5)By combing simulation numerical simulation results with engineering studies,I provided certain directions to some extent for gas pressure measurement on the spot.To decrease exposing time of borehole,workers,on a one hand,should improve skills,such as sealing materials preparation in advance,which can ensure sealing in time once the coal seam is exposed.On the other hand,we can choose quick sealing methods or technologies,such as yellow mud and polyurethane sealing methods and capsule-polyurethane sealing.The diameter of borehole should be between 65~95millimeter.We can also adopt technology of big diameter in sealing part and small diameter behind sealing part.For example,we can drillborehole(65~95millimeter diameter)first 20 meters according to industrial standard of the Direct Measuring Method of the Coal Seam Gas Pressure in Mine(AQ/T 1047—2007),and then drill via the small drill bit after 20 meter until exposing coal seam.