Seismic signatures of two orthogonal sets of vertical microcorrugated fractures

Abstract

Conventional fracture‐characterization techniques operate with the idealized model of penny‐shaped (rotationally invariant) cracks and ignore the roughness (microcorrugation) of fracture surfaces. Here, we develop analytic solutions based on the linear‐slip theory to examine wave propagation through an effective triclinic medium that contains two microcorrugated, vertical, orthogonal fracture sets in isotropic background rock.

The corrugation of fracture surfaces makes the shear‐wave splitting coefficient at vertical incidence sensitive to fluid saturation, especially for tight, low‐porosity formations. Also, in contrast to the model with two orthogonal sets of penny‐shaped cracks, the NMO (normal‐moveout) ellipses of all three reflection modes (P, S1, S2) are rotated with respect to the fracture strike directions. Another unusual property of the fast shear wave S1 is the misalignment of the semi‐major axis of its NMO ellipse and the polarization vector at vertical incidence.

Our model may adequately describe the orthogonal fracture sets at Weyburn Field in Canada, where the axes of the P‐wave NMO ellipse deviate from the S1‐wave polarization direction. The results of this work can be used to identify the underlying physical model and, potentially, estimate the combinations of fracture parameters constrained by multiazimuth, multicomponent seismic data.