Relaxation shift in rocks containing viscoelastic pore fluids

The interaction of pore stiffness with pore fluid moduli leads to shifts in viscoelastic relaxation times of the overall rock relative to those of the fluids alone. Crack-based and fluid substitution models indicate that stiff pores cause little shift, whereas thin, soft cracks can shift relaxation times by several orders of magnitude toward lower frequencies (longer relaxation times). Pore stiffness also causes a shift in apparent temperature dependence of rock viscoelasticity toward higher temperatures when cracks are present. As with more conventional fluid substitution problems, quantifying the effects of pore fluids on rock properties requires information about the crack and pore stiffness distributions in addition to the complex moduli and viscosity of the pure fluid.