Propagation of waves with a wide range of frequencies in digital core samples and dynamic strain anomaly detection: carbonate rock as a case study

Abstract

SUMMARYRecent advancements in various types of seismic measurement methods, such as sonic logging, vertical seismic profiling (VSP) and surface seismic surveys, have allowed the high-quality measurement of seismic wave propagation over a broad frequency range. To elucidate the relationship between seismic wave propagation captured by various seismic methods at widely different frequencies and in highly heterogeneous zones (e.g. fractures, vuggy zones) developed in carbonate reservoirs, laboratory measurements have been conducted over a broad frequency range. However, existing laboratory methods measure the effective properties over an entire core sample. Furthermore, there are few laboratory measurements of individual fracture stiffnesses. We propose a method to indirectly estimate the local properties caused by local anomalies (e.g. fractures) in a core sample over a broad frequency range from the dynamic elastic properties of a dry core sample using synthetic seismic waveforms generated from a digital volume of the core sample. 3-D numerical simulations were conducted over a broad frequency range using a digital core model produced by X-ray computed tomography. The proposed method was applied to numerical models and two types (fractured and vuggy) of carbonate core plugs acquired in an Abu Dhabi oil field, with the frequency ranging from 200 Hz to 100 kHz in the tests. Local strain anomalies and their frequency dependencies were successfully detected in the fractured core plug. Such frequency-dependent local responses could be associated with the micromechanics of incomplete solid–solid contacts at fractures or the heterogeneity of core samples, and thus with the frequency-dependent fracture stiffness. It was also demonstrated that the heterogeneity-induced local strain in a vuggy core plug may affect the accuracy of existing low-frequency laboratory measurements.