EFFECT OF CAPILLARY PRESSURE ON SEISMIC VELOCITIES AND ATTENUATION

Biot's theory allows incorporation of permeability and viscosity in computing seismic amplitudes for a porous medium that is fully saturated with a single-phase fluid. In its original form, Biot's theory does not explicitly account for capillary effects; for example, the surface tension between the wetting and nonwetting fluids. This paper uses a model to quantify capillary effects on velocity and attenuation. Studies that have attempted to extend Biot's poroelasticity to include capillary effects found changes in fast P-wave velocity of up to 5% between the sonic and ultrasonic frequency ranges. Simulations of wave propagation at varying capillary pressure in a rock saturated with multiphase fluid are also presented. The poroelastic equation for multiphase fluid is solved by using spectral methods with Fourier grids as collocations points in space and the Runge-Kutta scheme for numerical integration. The numerical simulations show the presence of three compressional (P)-waves, one fast and two slow compressional waves corresponding to the wetting and nonwetting phases. The results show that the slow P-wave amplitude is significantly affected by capillary pressure variations.