Effect Mechanism Of Gas On Ultrasonic P-wave Velocity Of Coal And It’s Verification

Abnormal gas pressure is one of the important factors causing coal and gas outburst.It is of great significance to master the change of gas pressure in coal seam during mining to prevent gas dynamic disaster.At present,the gas pressure in coal seam is mainly obtained by drilling,it is difficult to get the gas pressure distribution of the whole working face.As a non-contact detection method,seismic exploration can detect abnormal areas inside the stratum without destroying the stratum.In the oil and gas exploration field,time-shifting seismic exploration technology has been used to monitor reservoir fluid changes,but there are few researches on monitoring gas in coal seam by using this technology.One of the reasons is that the pore fluid in oil and gas reservoir mainly exists in a free state,while the gas in coal seam mainly exists in the adsorption state.The variation law of seismic parameter under the influence of free and adsorbed gas is not clear.Therefore,this paper carried out ultrasonic testing of coal containing gas under triaxial stress loading,and studied the influence mechanism of gas on ultrasonic P-wave velocity of coal.The main achievements are as follows:(1)The ultrasonic test system of coal is set up.Based on the principle of multi-process coupling test of coal-rock stress,seepage and temperature,an ultrasonic experimental system was designed,the system mainly include: the flow control module and the constant pressure adsorption module in the pressure control system can realize the control of radial constant displacement and the constant pressure adsorption of gas respectively;The axial strain measurement module in the core clamping system can measure the overall axial strain of the coal sample;The pressure probe module in the ultrasonic testing system can complete ultrasonic velocity testing under the combined action of confining pressure and gas pressure.(2)The influence law of free gas on P-wave velocity of coal is revealed.The bulk modulus of coal under different helium pressures and the P-wave velocity of coal under constant displacement and confining pressure boundary conditions were measured by using non-absorbent helium gas as a substitute gas for free gas.The measurement results show that the bulk modulus of free gas and its compression on coal matrix have little effect on P-wave velocity.The P-wave velocity of coal decreases with the increase of gas pressure.The free gas affects the porosity of coal by changing the effective stress of coal,and finally affects the P-wave velocity of coal.(3)The influence law of adsorbed methane on P-wave velocity of coal is expounded.Methane was used as experimental gas and helium was used to correct the influence of free methane.The volumetric modulus before and after methane adsorption and the P-wave velocity after methane adsorption under constant displacement and confining pressure boundary conditions were measured.The p-wave velocity of coal only affected by adsorption effect was obtained.The results show that the weakening of methane on the mechanical properties of coal has little influence on the P-wave velocity of coal;The adsorption effect has two opposite effects on the P-wave velocity of coal:(1)the coal porosity is reduced and the P-wave velocity is increased by the inward adsorption expansion;(2)The coal porosity is increased and the P-wave velocity is reduced by outward adsorption expansion.However,the effect of inward adsorption expansion is stronger than outward adsorption expansion,and the P-wave velocity of coal still shows an upward trend under the combined action of the two.(4)The influence law of free and adsorbed gas on the P-wave velocity of coal is obtained.Under the condition of low confining pressure,the P-wave velocity of coal decreases monotonically with methane pressure increasing.When the confining pressure is high,the P-wave velocity of coal increases first and then decreases.The influence of free gas pressure and adsorption expansion on P-wave velocity is related to confining pressure: as confining pressure increases,the influence of free gas pressure decreases.The P-wave velocity under uniaxial strain boundary condition also increases first and then decreases,but the upward trend is more obvious.The influence of free gas pressure and adsorption expansion on P-wave velocity is related to the type of constraints.(5)The P-wave velocity model of coal containing gas is established.Considering the different influence modes of gas pressure and coal matrix adsorption expansion on coal porosity,the P-wave velocity calculation model of gas-bearing coal was obtained.The phenomenon of "first rising and then falling" of P-wave velocity with gas pressure is well simulated,the effect of gas pressure and confining pressure on coal P-wave velocity can be accurately described.(6)Based on coalbed methane seismic exploration data,the P-wave velocity model of gas-bearing coal is verified.Firstly,the P-wave velocity of coal seam is retrieved from 3d seismic data;Secondly,the quantitative relationship between coal P-wave velocity and gas content is established by using the P-wave velocity model of gas-bearing coal,and the gas content distribution of coal seam in exploration area is obtained by inversion.Comparing the predicted results with the measured data of coal bed methane well,the two trends are basically consistent.There are 70 figures,21 tables and 109 references in this dissertation.