Elastic anisotropy of the Marcellus Shale

Abstract

ABSTRACTThe Marcellus Shale is one of the largest shale gas resources in the world and is anisotropic due to fine layering and the partial alignment of anisotropic clay minerals and organic matter with the bedding. This anisotropy can be approximated as Vertically Transversely Isotropic (transversely isotropic with a vertical axis of rotational symmetry) with five independent density‐normalized elastic stiffnesses A11, A13, A33, A55 and A66 in the two‐index notation and with axis of rotational symmetry along x3. Compressional and dipole shear‐wave data acquired in a horizontal well allows estimation of A11, A55 and A66, while the same data in a vertical pilot well allows estimation of A33 and A55. The ratio of vertically propagating P‐ to S‐velocity is and has a quadratic dependence on in the Upper Marcellus with minimum occurring for the largest volume fraction of kerogen. This relation allows estimation of A33 along a lateral well using measured values of A55 in the well. Comparison of Thomsen's anisotropy parameter ε is found to be mostly greater than anisotropy parameter γ. Estimating A13 as the average of A33 – 2A55 and A11 – 2A66 as proposed recently by Yan and Vernik allows Thomsen's anisotropy parameter δ and a parameter K0 that relates the horizontal and vertical effective stresses to be estimated. The results are expected to help in estimating horizontal stress needed for design of hydraulic fractures, and in interpreting seismic amplitude variation with offset required for reservoir characterization.