Summary

Mechanical properties play a critical role for optimizing the production from organic-rich shale reservoirs since such reservoirs have extremely low permeability. In situ stress conditions, intrinsic anisotropy due to layering, anisotropy due to fractures, mineral composition, porosity and maturity are among the key parameters affecting the mechanical properties. In this paper, we present experimental results on how Young's modulus, Poisson's ratio, and compressional velocity change with overburden and horizontal stresses for core samples from the oil window of the Eagle Ford shale, Green River immature oil shale and outcrop Mancos shale. These shales have distinct features that are discussed in detail and the intrinsic anisotropy of the shales due to layering is presented. Moreover, challenges in obtaining preserved shale core samples representative of the intact shale formation for the laboratory measurements and associated uncertainties in the laboratory measurements including the effect of natural and coring induced fractures parallel to bedding along with the sample size on the mechanical properties will be discussed.

Introduction

Mechanical and acoustic properties of shale formations have been investigated for many years as more than seventy percent of the formations drilled are seal shales. Shale formations have been challenging to drill and complete due to the instability of wellbores, mostly attributed to the physico-chemical interaction of clay minerals with drilling fluid, pore pressure build up, temperature and time (Mokhtari and Tutuncu, 2012). Stress concentration around the wellbore could result in shear or tensile failure of rocks. Thus, managing mud weight and salinity of drilling fluids are two major approaches to mitigate wellbore instability. However, the problem is more complicated by the existence of natural fractures and the physics of the osmotic phenomena.

URTeC 1619158

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