Angle-Dependent Ultrasonic Wave Reflection for Estimating High-Resolution Elastic Properties of Complex Rock Samples

Abstract

Due to depositional, diagenetic, and structural processes, reservoir rocks are rarely homogeneous, often exhibiting significant short-range variations in elastic properties. Such spatial variability can have measurable effects on macroscopic mechanical properties for drilling and fluid production operations. We describe a new laboratory method for the acquisition of ultrasonic angle-dependent measurements of reflected waves that delivers high-resolution, continuous descriptions of P- and S-wave velocity along the surface of the rock sample. Reflection coefficient vs. incidence angle is the main source of information about rock elastic properties. The acquired measurements are matched to numerical simulations to estimate P- and S-wave velocity and density of the porous sample and their variations within the rock specimen, hence providing continuous descriptions of sample complexity. Data collected from various locations on the rock specimen are subsequently used to construct two-dimensional (2D) models of elastic properties along the surface of the rock sample. P- and S-wave velocities estimated with this method agree well with acoustic transmission measurements for most homogeneous rocks. The spatial resolution of the method is limited by receiver size, measurement frequency, and incidence angle. At high incidence angles, the surface area sensitive to the measurements increases, and consequently, the spatial resolution of the corresponding reflection coefficient decreases across neighboring rock features.