Laboratory observation of velocity anisotropy affected by clays and microcracks in artificial clay-rich shale samples

Abstract

Clays play important roles in velocity anisotropy. Unlike brittle minerals, e.g., quartz, ductile clays are commonly more easily affected by mechanical compaction. Hence, clays tend to have a platy structure that might introduce significant velocity anisotropy. Cracks developed in clay-rich shales also have substantial influences on the shale elastic anisotropy and hydraulic stimulation. However, crack developments are sometimes unknown, and the effects of clays and cracks are ambiguous due to the complexity of shale properties. In this study, we construct artificial clay-rich shale samples which contain 40% clays by weight (kaolinite, smectite, and illite, respectively). The artificial shale samples are cored into shale-90 samples (along the bedding-parallel direction) and shale-0 samples (along the bedding-normal direction), and the P- wave and S- wave velocities are measured as the axis pressure (Pa) increases from 15 MPa to 50 MPa, while the confining pressure (Pc) remains at 15 MPa. We evaluate the slopes of the fitting line of velocity versus pressure, and estimate the crack density. We analyze the velocity anisotropy effect from clays and cracks based on laboratory experiments. Clays with an apparent platy grain structure (e.g., illite) introduce higher anisotropy, indicating that velocity anisotropy is dependent on the clay lamination structure. The comparison suggests that the mineralogy effects are the dominant factors in the anisotropy of clay-rich artificial shale samples, and the microcrack effects further enhance the velocity anisotropy.