Analysis of Time-Dependent Behavior of Dynamic and Static Parameters of Preheated Westerly Granite for Microseismic Fracture Monitoring Experiments

Abstract

AbstractA detailed analysis of time-dependent and strain-dependent behavior of dynamic and static properties of preheated to 650°C Westerly granite, which should be considered for microseismic fracture monitoring surveys, is presented in this paper. A thermally treated cylindrical Westerly granite specimen was compressed hydrostatically to 5 MPa and loaded axially to 415 kN until failure in a triaxial loading cell at the strain rate of 3.2 × 10-6 s-1. Longitudal wave (P-wave) velocities were measured periodically with triaxial ultrasonic system of transdusers along three orthogonal directions during loading time, until failure. Both dynamic (P-wave velocities along three orthogonal axes, and anisotropy of these velocities) and static (axial, lateral and volumetric strain) properties of the sample demonstrated significant variation with time (e.g., vertical P-wave velocity first increased by 58%, from 3100 m/s to 4900 m/s, and then dropped by 38%, down to 3048 m/s). Onset of fracture initiation occured at 0.5% of axial strain. During the initial stage, axial compaction prevailed in tested Westerly granite sample, inasmuch as closure of thermally generated cracks oriented horizontally was dominating. After axial strain reached 0.5%, the specimen lateral dilatancy was uncovered. Afterwards, opening of vertically oriented stress-induced fractures started to dominate, which had a response in both ultrasonic and mechanical data. It lead to presence of stress-induced anisotropy of the dynamic properties. As the velocity anisotropy, as well as the wave velocities, strongly vary with time and with increasing axial strain. At axial strain equal to 0.78% microscopic failure of the specimen was detected. For accurate localization of microseismic events during fracture monitoring a reliable time-dependent and anisotropic velocity model should be implemented.