ULTRASONIC ELASTIC CHARACTERISTICS OF SIX KINDS OF METAMORPHIC COALS IN CHINA UNDER ROOM TEMPERATURE AND PRESSURE CONDITIONS

Abstract

Coal elasticity is one of the important mechanical characteristics reflecting the material composition and structure of coal. Under the constraint of drilling and well logging, applying seismic exploration to predict elastic characteristics of the coal seams would be of important engineering significance for coal mining and CBM reservoir evaluation and development. In this regard, ultrasonic experiment of coal samples is the basis to realize the seismic inversion of physical properties of coal seam. In this paper, based on 30 pieces of six kinds of metamorphic raw coals collected from six mining areas of Yima, Fukang, Huainan, Pingdingshan, Hebi and Jiaozuo in China, laboratory ultrasonic measurements of these coal samples were respectively conducted in three directions of strike, dip and perpendicular to the beddings of coal seam under room temperature and pressure. The experimental results show: the samples’ velocities of both pressure wave (P-wave) and shear wave (S-wave) decrease in turn in the three directions; there are obvious anisotropies of velocities, and the average anisotropy of P-wave velocity is stronger than S-wave's. Moreover, larger differences exist among both quality factors and modules of elasticity in the three directions, and the S-wave quality factors are greater than P-wave's; with exception of Poisson's ratio, modules of elasticity of coal are less than those common sedimentary rocks. Through the experiment and analysis, it can be demonstrated that Gardener and Castagna formulas are not suitable to represent relations of coal elastic parameters in China's coal fields, and two empirical formulas with higher precision are statistically given.