Japan Geoscience Union Meeting 2016

Presentation information

Poster

Symbol S (Solid Earth Sciences) » S-CG Complex & General

[S-CG57] Geofluids and dynamics in subduction zones

Tue. May 24, 2016 5:15 PM - 6:30 PM Poster Hall (International Exhibition Hall HALL6)

Convener:*Ikuo Katayama(Department of Earth and Planetary Systems Science, Hiroshima University), Atsushi Okamoto(Graduate School of Environmental Studies), Tatsuhiko Kawamoto(Institute for Geothermal Sciences, Graduate School of Science, Kyoto University), Junichi Nakajima(Department of Earth and Planetary Sciences, Tokyo Institute of Technology)

5:15 PM - 6:30 PM

[SCG57-P06] Influence of pore size distribution on elastic wave velocities during evaporative drying

*Takuya Horikawa1, Tadashi Yokoyama1 (1.Department of Earth and Space Science, Graduate School of Science, Osaka University)

Keywords:Elastic wave, Water saturation, Pore size distribution, Sandstone

Elastic wave velocities (VP,VS) in porous rocks are known to be sensitive to the water saturation (S), the size and shape of pores, the distribution of fluids in pores, and the incident wave frequency. Various studies have tried to understand these dependences on the basis of Biot’s theory (Biot, 1956). The method for accurately predicting the quantitative relationship between S and VP, VS for the entire range of S (from S = 1 (saturated) to S = 0 (dry)) still remains to be fully elucidated. In this study, we measured the changes of VP and VS during drying for two Berea sandstones (permeabilities: 300 mD, 20 mD; hereafter described as Berea300 and Berea20, respectively) and Shirahama sandstone (permeability: <0.6 mD). P-wave and S-wave frequencies used in the measurements were 200 kHz and 100 kHz, respectively. The measured pore size distributions (aperture radius) showed that the predominant pore radii were ~5-100 µm for Berea300, ~1-10 µm for Berea20, and less than 0.4 µm for Shirahama. The change of VP with S for Berea300 can be classified into the following 4 stages: [(1) S=1→0.5: decrease of VP; (2) S=0.5→0.3: increase of VP; (3) S=0.3→0.1: decrease of VP; (4) S=0.1→0: increase of VP]. The change of VS with S for Berea300 can be separated to 2 stages: [(1) S=1→0.15: gradual increase of VS; (2) S=0.15→0: rapid increase of VS]. For Berea20, the VP change trend appeared to correspond to part of stage 1 plus the entire range of stages 2, 3, 4 observed in Berea300, and the VS change trend was approximately equal to that in Berea300. For Sirahama, the Vp change trend seemed to be equivalent to the stages 3 and 4 in Berea300, whereas VS decreased first and then increased, unlike in the case of Berea sandstones. When the drying proceeds, it is known that water is lost first in the larger pores, and later in the smaller ones (Nishiyama et al., 2012). Therefore, the size of pores containing water at a given S can be determined on the basis of the pore size distribution of each rock. By incorporating the information of pore size distribution and the frequency dependence of bulk modulus into previously reported models, we tried to precisely predict the change of VP and VS during drying.