ABSTRACT:

Static Young's modulus (E), Poisson's ratio (ν), bulk modulus (K), and maximum compressive strength (MCS) are among the most important parameters in rock mechanics. This work investigates a new approach to accurately estimate static E, ν, K, and MCS from multistage triaxial tests (MST) instead of single stage triaxial tests (SST) in case of having only MST data. Core samples from three zones; Zubair sandstone, Zubair shale, and Nahr Umr shale were obtained from the Rumaila field in Iraq. SST and four stages of MST for each formation were executed, and E, ν, K, as well as MCS, were acquired at each stage of MST and SST. The results showed that the average EMST from four stages of MST provided an underestimate of ESST for Nahr Umr shale and Zubair sandstone while the average EMST for Zubair shale provided an overestimate of ESST. Correction factors were proposed to calibrate the average EMST to ESST which are 1.127, 0.957, and 1.123 for Zubair sandstone, Zubair shale, and Nahr Umr shale, respectively. Average νMST resulted in an overestimate of νSST for Nahr Umr shale and Zubair sandstone while Zubair shale νMST provided an underestimate. Calibration factors were proposed for ν which are 0.645, 1.065, and 0.585 for Zubair sandstone, Zubair shale, and Nahr Umr shale, respectively. The averages of KMST and MCSMST provided an underestimate for KSST and MCSSST. Factors of 1.779, 1.35, and 1.458 were proposed for K for Zubair sandstone, Zubair shale, and Nahr Umr shale, respectively; and factors of 1.466, 1.446, and 1.358 were suggested for MCS for Zubair sandstone, Zubair shale, and Nahr Umr shale, respectively. These results can help to provide a good estimate of E, ν, K, and MCS from MST when the material is limited to perform SST.

1. Introduction

Elastic moduli such as Young's modulus (E), Poisson's ratio (ν), and bulk modulus (K), as well as rock strength parameters such as uniaxial compressive strength (UCS), internal friction angle, and cohesion, are considered as major components of the mechanical characteristics of rocks. These parameters characterize the attitude of rock during loading and unloading conditions, which can be obtained in the laboratory by utilizing various tests (Peng and Zhang, 2007; Fjær et al., 2008). Knowing and understanding rock strength parameters will contribute to accurately estimating the in-situ stresses in sub-surface zones, which in turn this knowledge and comprehension will be a key for avoiding or mitigating the costly and time-consuming issues related to the exploration, completion, and production stages (e.g. wellbore instability and sand control) (Zoback et al., 2003; Zeynali, 2012; Wang and Sharma, 2017).

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