Open AccessInfluence of fluid chemistry on shear‐wave attenuation and velocity in sedimentary rocks
Author(s) -
VoThanh Dung
Publication year - 1995
Publication title -
geophysical journal international
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.302
H-Index - 168
eISSN - 1365-246X
pISSN - 0956-540X
DOI - 10.1111/j.1365-246x.1995.tb06435.x
Subject(s) - attenuation , geology , shear (geology) , mineralogy , saturation (graph theory) , shear velocity , sedimentary rock , shear waves , wave velocity , viscosity , thermodynamics , composite material , petrology , optics , materials science , geochemistry , turbulence , physics , mathematics , combinatorics
SUMMARY Measurements of shear‐wave attenuation Q s −1 and velocity V s were made in a sample of Massilon sandstone and a sample of Brauvilliers oolitic limestone in the kilohertz frequency range using a resonant bar technique. The major structural difference between these two rocks is the presence of grain contact microcracks (thin gaps) in the first and their absence in the second. Measurements were made as a function of fluid saturation S f using a drying technique; the first and second samples were saturated successively with nine and four low‐viscosity fluids, respectively, at room pressure and temperature. For water, the variations of Q s −1 and V s are larger in Massilon sandstone than in Brauvilliers limestone. In Massilon sandstone, an attenuation peak at S f ≅ 20 per cent is observed for all fluids and the quality factor Q s and velocity V s increase strongly at low saturations when S f drops from 25 to 0 per cent. These variations depend on the nature of the pore fluid. They are relatively large with water, intermediate with methanol and small with other non‐polar fluids (n‐alcanes, aromatic hydrocarbons, carbon tetrachloride), The nature of the pore fluid has little effect at saturations greater than S f = 25 per cent. The velocity slope is negative for water and methanol, and positive for non‐polar fluids. The velocity slope is always negative in fluid‐saturated Brauvilliers limestone. A model of grain contacts is used to interpret the data qualitatively. It includes two mechanisms: the gas adsorption on the solid‐solid surface and the local fluid flow in the contact gaps. The first phenomenon lowers the elastic moduli while the second raises them. Calculations are qualitatively consistent with the observations and show that the saturation dependence of velocity in Massilon sandstone is principally dominated by the adsorption of volatiles at low saturations; at high saturations the velocity is affected conjointly by gas adsorption and fluid flow. The influence of fluid flow is negligible in Brauvillers limestone.