Abstract

The understating of rocks mechanical properties as a function of stress is required for many geomechanics applications. One is to estimate friction angle and cohesion values to describe the shear strength vs. normal stress over the normalload range. It is facilitated in the laboratory measurement using Single Stage Triaxial (SST) testing on multiple samples at different confining pressures. Occasionally, due to limited core availability a Multistage Triaxial test (MST) is conducted for the same objective. The uncertainty with obtained results using the two methodologies is not well quantified due the possible changes in elastic properties while loading and unloading the axial stress. Experimental results indicates the consistency of the obtained stress value at the Positive Point of Dilatancy (PPD) during MST increases the overall results and provides an adequate correlation between the two methods. This paper describes how the data should be interpreted to minimize the error with the results obtained from MST test results. Multiple rock types were evaluated representing different lithologies (Shale, Sandstone, and Chalk) to validate a possible single correction factor from MST to SST. Results appear to fit the five different rock types with and the model fits a wide range of well data from different formations, fields and basins.

1. INTRODUCTION

A single stage tri-axial test is commonly used to determine rock’s elastic properties such as Young’s modulus, Poisson’s ratio and maximum compressive strength. It is typical to repeat the measurement on multiple samples at different confining pressures for Mohr-Coulomb envelope analysis to estimate friction angle and cohesion parameters. Measured parameters are used as an input to reservoir geo-mechanical models such as borehole stability or hydraulic fracturing. This requires a suite of four to five Single Stage Triaxial (SST) measurements to determine failure envelope criterion. Due to limited available core material on times at a specific core depth or inability to obtain multiple identical samples with similar mechanical properties, a Multi Stage Triaxial (MST) test can be considered as an alternative to the SST. This paper proposes a technique where the "twinning" problem may be mitigated by the use of the "multi-stage" tri-axial test. The laboratory measurement and developed technique for real time calculation as a feedback to unload during the MST testing is critical to minimize any sample mechanical property alteration and provides consistent stress value for the MC analysis.

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