Pervukhina, M, Dewhurst, DN, Kuila, U, Siggins, AF & Gurevich, B 2008, 'Stress Dependent Anisotropy in Shales — Measurements and Modelling', in Y Potvin, J Carter, A Dyskin & R Jeffrey (eds), Proceedings of the First Southern Hemisphere International Rock Mechanics Symposium
, Australian Centre for Geomechanics, Perth, pp. 287-300.
Shales comprise a large proportion of the sedimentary pile in many hydrocarbon basins and impact significantly on exploration, development and production costs through the effect of seismic anisotropy on imaging and depth conversion. Shales play an important role in time-lapse seismic response and pore pressure prediction. To understand stress dependent anisotropy on shales, stress dependencies of the five elastic constants are measured for four sets of shales. These new measurements are used to test the method that calculates the shale elastic constants using the silt fraction, clay packing density and set of elastic constants derived on the basis of nano-indentation technique. The results of our analysis show considerable variability of clay properties for a given clay packing density. We also relate the variation of clay elastic constants with the amount of very small and compliant pores (soft porosity). For the first time, soft porosity of shales is estimated from the experimental data. The soft porosity correlates with stress dependent variations of elastic moduli. This suggests that stress dependency of shale properties results from the closing of the soft porosity. Variations of the compressibility depend on exponential functions of the effective confining stress and the soft porosity can be estimated from the fitting coefficients. The stress dependencies of the measured elastic compliances of wet shales and of the dry compressibilities estimated using Gassmann equation are approximated by exponential functions using a nonlinear fitting based on the Levenberg–Marquardt algorithm. The soft porosities estimated from the fitting coefficients for dry compressibilities are shown to be in a reasonable agreement with the measured values.
Bayuk, I.O., Ammerman, M. and Chesnokov, E.M. (2007) Elastic moduli of anisotropic clay, Geophysics, 72(5), pp. D107–D117.
Ciz, R. and Shapiro, S.A. (2008, submitted) Stress dependent anisotropy in transversely isotropic rocks: Comparison between theory and laboratory experiment on shale, Geophysics.
Dewhurst, D.N., Jones, R.M. and Raven, M.D. (2002) Microstructural and petrophysical characterization of Muderong Shale: application to top seal risking, Petroleum Geoscience, 8, pp. 371–383.
Dewhurst, D.N. and Siggins, A.F. (2006) Impact of fabric, misrocracks and stress field on shale anisotropy, Geophysical Journal International, 165, pp. 135–148.
Dewhurst, D.N., Siggins, A.F., Kuila, U., Clennell, M.B., Raven, M.D. and Nordgård–Bolås, H.M. (2008) Rock Physics, Geomechanics and Rock Properties in Shales: Where Are The Links? SHIRMS 2008.
Hornby, B., Schwartz, L. and Hudson, J. (1994) Anisotropic effective medium modeling of the elastic properties of shales: Geophysics, 59(10), pp. 1570–1583.
Katahara, K.W. (1996) Caly mineral elastic properties: SEG Annual Meeting Expanded Technical programme Abstracts, Paper RP1.4.
Levenberg, K. (1944) A Method for the Solution of Certain Problems in Least Squares, Quart. Appl. Math, 2, pp. 164–168.
Marquardt, D. (1963) An Algorithm for Least Square Estimation of Nonlinear Parameters, SIAM J. Appl. Math, 11, pp. 431–441.
Mavko, G., Mukerji, T. and Dvorkin, J. (1998) The rock Physics Handbook – Cambrige Univeristy Press, UK.
Mavko, G. and Jizba, D. (1991) Estimating grain-scale fluid effects on velocity dispersion in rocks, Geophysics, 56(12) pp. 1940–1949.
Nishizawa, O. (1982) Seismic velocity anisotropy in a medium containing oriented cracks – transversely isotropic case, J. Phys. Of Earth, 3(4), pp. 331–348.
Ortega, J.A., Ulm, F-J. and Abousleiman, Y. (2007) The effect of the nanogranular nature of shale on their poroelastic behaviour, Acta Geotechnica,
Pervukhina, M., Dewhurst, D., Gurevich, B., Kuila, U., Siggins, T., Raven, M. and Nordgård–Bolås, H.M. (2008) Stress-dependent elastic properties of shales: measurement and modelling, The Leading edge, 27(6), pp. 772–779.
Sayers, C.M. (2005) Stress-dependent seismic anisotropy of shales, Geophysics, 64(1), pp. 93–98.
Shapiro, S.A. (2003) Elastic piezosensitivity of porous and fractured rocks, Geophysics, 68(2), pp. 482–486.
Shapiro, S.A. and Kaselow, A. (2005) Porosity and elastic anisotropy of rocks under tectonic stress and pore-pressure changes, Geophysics, 70(5), pp. 27–38.
Shermergor, T.D. (1977) Theory or elasticity of microinhomogeneous media: Nauka (in Russian), 399 p.
Thomsen, L. (1986) Weak elastic anisotropy: Geophysics, 52(10), pp. 1954–1966.
Ulm, F-U. and Abousleiman, Y. (2006) The nanogranular nature of shale: Acta Geotechnice, 1(2), pp. 77–88.
Vanorio, T., Prasad, M. and Nur, A. (2003) Elastic properties of dry clay mineral aggregates, suspensions and sandstones, Geophysical Journal International, 155, pp. 319–326.