Laboratory investigation of core-based stress measurement using synthetic sandstone

Abstract

This paper describes an experimental study of the anelastic strain relaxation (ASR) behaviour of a synthetic sandstone aimed at assessing the use of core-based techniques for stress measurement. The “sandstone” was compacted and “cemented” under stress, unloaded and placed in an ASR cell to monitor its strain response. The relationships between the axial and radial strains and applied stresses were studied for a number of stress fields including isotropic (7 to 28 MPa), anisotropic (7, 14 MPa) and complex (7, 14 MPa, reversed to 14, 7 MPa). The ASR response of the material was generally consistent with theory, showing greater relaxation in the direction of the larger stress, and isotropic relaxation after isotropic loading. The behaviour became more consistent at loads exceeding 10 MPa, applied for durations exceeding 12 hours. The ratio of relaxation strain to applied stress was remarkably consistent in the radial and axial directions for isotropic and anisotropic loading cases. However, the relaxation strain decreased with increasing stress, showing a non-linear “stress-strain” response. The preliminary results from the complex stress field experiments show a “memory” of both applied stress fields, which seems to be controlled by duration of loading. These findings promise to give us greater confidence in the use of the ASR stress measurement technique on sandstones.