ARMA 15- 767
June 1, 2015

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 normal-load 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 Dilation (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.

Authors: Munir Aldin, Michael Myers, and Malik Al Salman