Experimental studies suggest that the fracture toughness of rocks increases with the confining pressure. Among many methods to quantify this dependency, a so-called burst experiment (Abou-Sayed, 1978) may be the most widely applied in practice. Its thick wall cylinder geometry leads to a stress state resembling the subsurface condition of a pressurized wellbore with bi-wing fractures. The fracture toughness of a sample, under a given confinement pressure, can be recovered from the critical pressure upon which the bi-wing cracks propagate. Traditionally, this critical pressure is thought to correspond to a sudden drop in injection pressure. However, as the standard configuration was deliberately designed to obtain stable fracture growth at the onset, propagation can take place well before this drop in pressure, and one may overestimate the fracture toughness from measured pressures. Here, we study crack stability in the burst experiment and propose modifications to the experimental design which promotes unstable fracture growth and makes the critical pressure less ambiguous to interpret. We found that experiments with the original, stable design can lead to inconsistent measurement of fracture toughness under confining pressure, while results from unstable configurations are more consistent. Our claim on the stability was also supported by the recorded acoustic emissions from both stable and unstable experiments.
|Seiten (von - bis)||427-436|
|Fachzeitschrift||Rock mechanics and rock engineering|
|Publikationsstatus||Elektronische Veröffentlichung vor Drucklegung. - 10 Okt. 2022|
Bibliographische NotizFunding Information:
The authors are grateful for financial support from Chevron to conduct the experiments through a grant to the University of Pittsburgh. The initial stability analyses of the burst experiments were performed by Dr. Erwan Tanné and his contributions are greatly acknowledged. The experimental data used in this study can be requested to the corresponding author. The files generated for computational data are available at https://doi.org/10.5281/zenodo.6323745 . Part of this work was conducted, while BB was the A.K. & Shirley Barton Professor of Mathematics at Louisiana State University (USA).
© 2022, The Author(s), under exclusive licence to Springer-Verlag GmbH Austria, part of Springer Nature.