A restricted meniscus motion model for wave attenuation in partially fluid-saturated porous rock (original) (raw)

1997

Abstract

Seismic wave attenuation analysis, the study of how seismic waves lose their energy while they travel, relies on models to transform measured data into descriptions of the subsurface environment. Seismic attenuation in fluid bearing rock is dominated by interactions between the fluid and the rock. Standard attenuation theories model attenuation due to viscous shear within the flowing fluid (local fluid flow), shear between the pore fluid and pore walls (global fluid flow), or fluid-assisted thermal diffusion. All of these models rely solely on physical and geometrical properties of the system to describe the mechanism by which attenuation occurs. These models do not account for attenuative physicochemical interactions between the pore fluid and pore walls. Effects of physicochemical interactions between the pore fluid and pore walls are considered in this work. Physicochemical interactions between the solid crack surface and the fluid meniscus can restrict contact line motion (motion of the three phase boundary) across the solid surface. In partially saturated cracks subjected to seismic deformation, two consequences of restricted contact line motion are crack stiffening and energy loss. Fluid redistribution in a deforming, partially saturated crack occurs via meniscus deformation or contact line motion or both. If there is some resistance to contact line motion, during the initial response to fluid redistribution the contact lines remain stationary while the meniscus deforms. The meniscus deforms because the fluid pressure is changing, and these fluid pressure changes always act against further crack deformation. The fluid pressurization makes the crack more stiff than a dry crack or a crack in which only stiffening due to viscous flow is considered. If crack deformation pressurizes the fluid enough that the force applied by the deforming meniscus on a contact line exceeds the resistive force holding the contact line stationary, contact line motion occurs. Contact line motion against the resistive force requires energy, which is lost from the seismic wave causing the original crack deformation. This friction-like energy loss mechanism is responsible for the attenuation in the restricted meniscus motion model. The restricted meniscus motion model interprets attenuation and stiffness data which can not be described by attenuation theories relying solely on the physical and geometrical parameters (pore fluid viscosity, fluid and solid compressibilities) used in the standard models. The restricted geometry, meniscus motion model, born from attenuation and stiffening observations in an artificial glass crack sample, is itself composed of parameters which can be measured or estimated using experiments which are distinct from the attenuation and stiffness measurements. These independently measured model parameters are in general agreement with the model parameters used to fit attenuation and stiffness results measured on the artificial glass crack samples.

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