NONLINEAR VISCOELASTIC BEHAVIOR OF SEDIMENTARY-ROCKS, PART I - EFFECTOF FREQUENCY AND STRAIN AMPLITUDE

Sedimentary rocks display nonlinear elastic behavior. This nonlinearit y is a strong function of frequency strain amplitude, and the properti es of the saturating fluid. Experimental observations and potential me chanisms that cause these nonlinearities are presented in this and a c ompanion paper, Young's moduli and Poisson's ratios obtained from ultr asonic laboratory measurements (50 kHz, 100 kHz, 180 kHz and 1 MHz). l ow-frequency measurements (1-2000 Hz) and static measurements (0.001-0 .05 Hz) show significant differences under identical stress conditions . A comparison of the laboratory-measured quantities with log-derived moduli measured at 20 kHz indicates that E-ultrasonic > E-log > E-lowf req > E-static. This shows clearly that a wide variety of sandstones d emonstrate frequency-dependent elastic behavior (viscoelastic behavior ) over a range of frequencies. Differences between static low-frequenc y, high-strain amplitude) velocities and ultrasonic velocities call be explained partially by differences in frequency as predicted by grain contact models. Such models. however, do not explain the strain ampli tude dependence observed II our data. A series of uniaxial stress cycl ing measurements were carried out to investigate the influence of stra in amplitude on elastic moduli. These low-frequency measurements (0.01 Hz) clearly show that the Young's modulus decreases with strain ampli tude for a nide variety of sandstones. Attenuation increases with stra in amplitude, The strain amplitude dependence does not change when the rocks are saturated with brine although the rocks soften measureably.