Stress relaxations at low frequencies in fluid‐saturated rocks: Attenuation and modulus dispersion

Abstract

A new laboratory apparatus measures the complex Young's modulus of rock samples at frequencies between 4 and 400 Hz and at strain amplitudes near 10−7. There is negligible attenuation and modulus dispersion in vacuum‐dry rocks. In water‐saturated samples of sandstone, limestone, and granite there are large attenuation peaks which are caused by stress relaxations. The attenuation and modulus data can be described by Cole‐Cole distributions of relaxation times. The rock anelasticity therefore satisfies the Kramers‐Kronig integral relations. The stress relaxations occur at low frequencies, have narrow distributions of characteristic times, are thermally activated, and have activation energies (16–22 kJ/mol) which are characteristic of hydrogen bonds. The modulus defect and peak attenuation are shown to vary with different pore fluids (water, ethanol, and n‐decane). The results are explained in terms of the pore fluids lowering the surface free energy of the rock‐forming minerals. The relaxation process involves the movement of fluid molecules. The molecules, by bonding to the surface, reduce the surface energy and thus generate the frequency‐dependent softening in the rock.