QUANTIFYING THE INFLUENCE OF CLAY-BOUND WATER ON WAVE DISPERSION AND ATTENUATION SIGNATURES OF SHALE: AN EXPERIMENTAL STUDY

Abstract

Understanding the elastic and attenuation signatures of shales is of considerable interest for unconventional reservoir characterization and sealing capacity evaluation for CO2 sequestration and nuclear waste disposal. We conducted laboratory measurements on seven shale samples at seismic frequencies (2–100 Hz) to study the effects of clay-bound water (CBW) on their wave dispersion and attenuation signatures. With Nuclear Magnetic Resonance (NMR) and helium porosimeter, the volume of CBW in the shale samples is quantified. The forced-oscillation measurement reveals that Young’s modulus exhibits a continuous dispersion trend from 2 to 100 Hz. The extensional attenuation [Formula: see text] shows a weak frequency- and pressure-dependence on effective pressure ranging from 5 to 35 MPa. The magnitude of extensional attenuation shows a positive correlation with CBW, with an R-square value of 0.89. It is found that 4% of CBW in the rock frame causes roughly a 5% modulus increase from 2 to 100 Hz. We adopt a constant Q model for assigning frequency-dependent bulk and shear moduli to the CBW in the rock physics modeling, which can fit the experimental data of modulus dispersion and attenuation well, indicating that both the bulk and shear moduli of CBW in shales might behave viscoelastically.