Effect of Various Unloading Criteria on Rock Failure Parameters From Multi-Stage Triaxial Test - a Comprehensive Study

Jin, Guodong (Baker Hughes, a GE Company) | Ali, Syed Shujath (Baker Hughes, a GE Company) | Al Dhamen, Ali A. (Baker Hughes, a GE Company) | Saad, Bilal (Baker Hughes, a GE Company) | Hussain, Maaruf G. (Baker Hughes, a GE Company) | Chinea, Gonzalo (Baker Hughes, a GE Company) | Nair, Asok (Baker Hughes, a GE Company) | Alshanqaiti, Elham (Baker Hughes, a GE Company)

OnePetro 

ABSTRACT: This paper compared various unloading criteria used in multi-stage triaxial tests for determining the Mohr-Coulomb failure envelope. Results from single-stage triaxial tests formed the baseline for comparison. Three types of rock outcrop were used: Berea sandstone, Eagle Ford shale, and carbonate rock. Mineralogy, porosity, grain and bulk density were also measured, which are used to determine the sample heterogeneity and interpret the discrepancy of test results. Examples illustrated both zero and maximum volumetric strain criteria were not always applicable, especially for brittle rocks. The criterion of radial-strain gradient, defined as the ratio of change of radial strain and change of time, is generally suitable for any type of rocks. Irrespective of the applied confining pressure, samples were observed to break at almost the same radial-strain gradient for the same type of rocks. Failure envelopes from the radial-strain gradient method matched very well with those from the maximum volumetric strain for all samples tested. Compared to results of single-stage triaxial tests, multi-stage tests yielded a very good approximation of failure envelopes for Berea sandstone, while discrepancy was observed for Eagle Ford shale and carbonate rock because of the heterogeneity of samples.

1. INTRODUCTION

Mohr-Coulomb failure envelope (hereafter referred to as only failure envelope for simplicity) is one of the most commonly used failure criteria in many engineering application, such as borehole instability analysis (Manshad et al., 2014), sand onset prediction (Javani et al., 2017), and mud-weight window design (Gholami et al., 2014). Determination of the failure envelope usually requires to perform several single-stage triaxial (SST) tests on three or more samples or one multi-stage triaxial (MST) test on one sample at various confining pressures. Due to the scarcity and preciousness of rock samples, MST test is often the only option used for determining the failure envelope (Harouaka et al., 1995).

One major challenge associated with MST tests is to recognize the imminent failure of the sample and thereby prevent failure of the sample from occurring at each loading stage except for last stage (Crawford and Wylie, 1987). Once the imminent failure is reached, the test should be stopped and unloaded to the confining pressure of that stage. Then, the confining pressure increases to the next level and the test of next stage starts. Theoretically the imminent failure is defined as the point in the stress- strain curve where the axial stress does not increase when the axial strain increases, or more specifically the point when the axial stress reaches at the peak stress (Kim and Ko, 1979, Kovári et al., 1983, Youn and Tonon, 2010). In practice, the peak stress of a sample at a given confining pressure is never known in advance, and therefore a subjective judgment must be made regarding the imminent failure. It is not uncommon that a wrong estimation often occurs when interpreting the stress-strain curves.