| The water content and porosity of coal seam are important parameters for gas prediction and gas control.The research on the relationship between ultrasonic velocity and attenuation and porosity of coal seam loaded with water content is the basis for the development of acoustic geophysical methods for water content and porosity of coal seam.According to the coal samples of No.21 coal seam in Zhaogu No.2 coal mine.Based on the theory of Wood,the wave velocity parameters of water-bearing coal sample at normal temperature and pressure were measured by the method of theoretical analysis and test combined with the method of using the self-made test system of coal reservoir fracture simulation and physical property characteristics.After that,the characteristics of wave velocity,quality factor and porosity of loaded water-bearing coal sample are analyzed,the acoustic characteristics and porosity changes of dry coal samples and saturated coal samples under triaxial loading are discussed,and the feasibility of using wave velocity and quality factor of coal seam to predict porosity is studied.The main achievements are as follows:1)The P-wave velocity of coal measured at normal temperature and pressure is mainly affected by three factors,wave velocity,porosity and water saturation of dry coal.And 60%water saturation as the boundary point is suitable for different equations,and different water saturation coal samples parallel bedding and vertical bedding adapt to different equations.Before 60%,the wave velocity of coal samples in parallel bedding is basically consistent with that calculated by Wood theory,while the wave velocity of vertical bedding is basically consistent with that calculated by modified formula.After60%,the wave velocities in both directions are basically consistent with those calculated by kauster-toksoz theory.2)It is found that the quality factor of coal is more sensitive to the water content of coal by comparing the ultrasonic response characteristics of coal samples under the condition of drying and saturated water.By comparing the variation characteristics of ultrasonic wave velocity,porosity and quality factor of coal samples under dry and saturated water conditions,it is concluded that the change of wave velocity in parallel bedding direction of dry coal samples is not easy to distinguish,and the change of porosity is relatively different.The variation of wave velocity in the direction of parallel bedding is easy to be distinguished and the variation of porosity is not easy to be distinguished.3)When confining pressure is determined,the wave velocity,quality factor and porosity of coal sample change with the increase of axial pressure of 1MPa.Through the analysis of the change of wave velocity and porosity of dry and saturated coal samples and the change of quality factor and porosity,the curve of wave velocity and quality factor is consistent,while the change of porosity is opposite.4)The existence of porosity in coal sample affects the change of wave velocity and quality factor of coal sample.Through the analysis of wave velocity and quality factor of dry and saturated coal sample and porosity,it is found that there is a good relationship between them.the relationship between wave velocity and porosity of both dry and saturated coal samples conforms to the formulaφ=aIn(V_p)+b,In the formula,a value has a good positive correlation with confining pressure,b value has a good positive correlation with porosity change,and the fitting degree between saturated coal sample and formula is higher than that of dry coal sample.The relationship between porosity and quality factor of dry coal sample or saturated coal sample accords with the formulaQ=a~*x+b.In this paper,by analyzing the variation law of wave velocity of coal samples with different water saturation under normal temperature and pressure,and the characteristics of different bedding directions between triaxial loading and drying and saturated coal samples,such as wave velocity,porosity and attenuation coefficient,The difference between the dry coal sample and the saturated coal sample is defined,and the relationship among them is obtained through the analysis of wave velocity,porosity and quality factor variation,and the relationship between wave velocity,quality factor and porosity is established.The feasibility of using wave velocity and quality factor to predict porosity is obtained. |
