Abstract
Elastic wave propagation in partially saturated reservoir rocks induces fluid flow in multi-scale pore spaces, leading to wave anelasticity (velocity dispersion and attenuation). The propagation characteristics cannot be described by a single-scale flow-induced dissipation mechanism. To overcome this problem, we combine the White patchy-saturation theory and the squirt flow model to obtain a new anelasticity theory for wave propagation. We consider a tight sandstone Qingyang area, Ordos Basin, and perform ultrasonic measurements at partial saturation and different confining pressures, where the rock properties are obtained at full-gas saturation. The comparison between the experimental data and the theoretical results yields a fairly good agreement, indicating the efficacy of the new theory.
Highlights
Seismic waves induce fluid flow and anelasticity in rocks saturated with immiscible fluids [1,2,3,4,5,6,7,8]
The numerical examples of the characteristics of wave prorogation by the proposed model are shown in Figure 2, and the effects of permeability and the outer diameter of the patch on the wave velocity and attenuation are shown in Figures 3 and 4, respectively
The microcrack properties and characteristic squirt flow lengths are obtained from experimental data at different effective pressures and water saturations
Summary
Seismic waves induce fluid flow and anelasticity (the wave-velocity dispersion and dissipation factor) in rocks saturated with immiscible fluids [1,2,3,4,5,6,7,8]. Local fluid flow on meso- and micro-scales are neglected, and the Biot peak cannot explain the observed wave anelasticity at all frequencies [13]. Dissimilar pores, with different shapes (micro-fractures and intergranular pores) and/or orientations, cause mesoscopic pressure gradients and squirt flow, Johnson [22] generalized it to patches of arbitrary geometry by using a branch function. .TThheeccoommppaarirsiosonnbbeetwtweeeennththee eexxppeerrimimeenntatal lddaatataaannddththeeththeeooreretitcicaal lreresusultlstsaareremmaadde,e,sosoaasstotovveerirfiyfyththeeccaappaabbiliiltiytyoof fththee nneewwmmooddeel lininththeeddeessccrripiptitoionnoof fththoosseewwaavveepproroppeertriteies.s. White [20] proposed a patchy saturation model, by considering flow in a concentric spherical model where the inner sphere is saturated with one fluid type (gas), and the outer shell is saturated with a liquid (water), where the frame is assumed to be homogeneous. (see Appendix A in Carcione et al [44], and the Section 2.3)
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