Determination of dynamic elastic properties of dry, wet and partially CO2 saturated coals from laboratory scale ultrasonic response

Abstract

We present the longitudinal (VP) and shear (VS) wave velocities and their anisotropy of coal-samples under dry, water, and partially CO2 saturated conditions. The elastic-constants have also been subsequently computed and presented. The results show that the VS generally increases when the pores are filled with a less compressible fluid which effectively raises the bulk modulus. VP depends on the combination of both bulk and shear modulus; it is highest for partially CO2 saturated samples when the combined values of bulk and shear moduli are highest. Velocity anisotropy of VS is of comparable order for dry and partially CO2 saturated samples (13.42 % and 15.08 %, respectively), while wet samples show the lowest VS anisotropy (5.44 %). For VP, the wet samples exhibit 2.75 % lower anisotropy than dry samples; while partially CO2 saturated samples have the highest VP anisotropy (15.62%). The relationships between VS and VP, Young’s Modulus (E) and VP under different fluid saturation in pores are quantified and presented using a linear regression method. Correlation between the ratio of the square of VP and VS and Poisson’s ratio was performed by polynomial fitting. Hypothesis testing (T-test) has been conducted on regression relationships using T statistics. We show that the regression relationships among all elastic parameters and ultrasonic wave velocities are not occurring by chance; rather true relationships prevail between the parameters under consideration. We finally conclude that under atmospheric or low confinements, the elastic properties of coal reservoirs are hardly affected by the type of pore fluids present in the reservoir.