Measuring and Interpreting P and S Wave Velocity Data - An Application of a New Petrophysical Model

It is well known that acoustic wave propagation under pressure is very nonlinear and the elastic properties of rocks are hysteretic, which behavior is important for mechanical understanding of reservoirs during depletion. Pressure strongly influences the elastic parameters of rocks, thus wave velocities too. Therefore a quantitative model - which provides the physical explanation - of the mechanism of pressure dependence is required. In this paper a petrophysical model is presented which describes the connection between the propagation velocity of acoustic waves (both P and S) and rock pressure both in case of loading and unloading phases as well as explains the mechanism of acoustic hysteresis. The developed model is based on the idea that the pores in rocks close under loading and reopen during unloading. The advantage of the model is that it is not based on simple curve fitting, but gives physical explanation for the process with three-parameter exponential equations. P/S wave velocities were measured by a self-developed LabView software under pressure in laboratory on sandstone samples. The model was applied with success to measured data.