[SSS10-P05] Influence of water saturation and pore structure in rocks on elastic wave velocities
Keywords:Elastic wave velocities, Water saturation, Pore size distribution, Sandstone
It is widely known that velocities (VP,VS) and attenuation of elastic waves through a rock depend on the permeability and pore fluid. The most simplified method to examine the dependence is to change the water/air ratio of pore fluids (water saturation Sw). Various studies have reported viscoelastic effects of unsaturated rocks on VP. However, the relationship between Sw and VP has not been understood quantitatively. In this study, we have measured VP (100 kHz - 700 kHz), VS (100 kHz) and strain changes during evaporative drying of three Berea sandstones with different pore size distributions (porosity: 22.6, 19.1, 11.6 %; permeability: 170, 230, 5 mD; hereafter described as Berea A, Berea B, Berea C, respectively).
The VP values decrease between ca.0.2<Sw<1.0 with frequency dependences. The meniscus size at a specific water saturation can be estimated from pores size distribution data by applying the capillary flow concept. The permeability as a function of Sw is calculated by the modified Katz and Thompson model (Nishiyama and Yokoyama, 2014) under the assumption that the wave induced water flow in an unsaturated rock occurs through the water-filled pores. The obtained frequency dependent Sw-VP relationships are discussed by taking into account of the permeability for two theoretical models describing the viscoelasticity of unsaturated rocks: Global flow model (Biot, 1956) and Patchy saturation model (White, 1975). The theoretical predictions from these extended models showed that the Sw-VP relations could be understood quantitatively by combining the viscoelastic effects of these models. On the other hand, VS increases drastically for Sw<c.a. 0.2 associated with the drying shrinkage observed by the strain change measurements.
These results suggest that the frequency dependence of Sw-VP relationships could be interpreted by using the extended Global flow and Patchy saturation model considering the dependence of pore water distribution on water saturation Sw.
The VP values decrease between ca.0.2<Sw<1.0 with frequency dependences. The meniscus size at a specific water saturation can be estimated from pores size distribution data by applying the capillary flow concept. The permeability as a function of Sw is calculated by the modified Katz and Thompson model (Nishiyama and Yokoyama, 2014) under the assumption that the wave induced water flow in an unsaturated rock occurs through the water-filled pores. The obtained frequency dependent Sw-VP relationships are discussed by taking into account of the permeability for two theoretical models describing the viscoelasticity of unsaturated rocks: Global flow model (Biot, 1956) and Patchy saturation model (White, 1975). The theoretical predictions from these extended models showed that the Sw-VP relations could be understood quantitatively by combining the viscoelastic effects of these models. On the other hand, VS increases drastically for Sw<c.a. 0.2 associated with the drying shrinkage observed by the strain change measurements.
These results suggest that the frequency dependence of Sw-VP relationships could be interpreted by using the extended Global flow and Patchy saturation model considering the dependence of pore water distribution on water saturation Sw.