Abstract

Although unconventional gas shales have become accepted as the game changer for natural gas (International Energy Agency’s 2009 World Energy Outlook), laboratory as well as simulated data of openhole completion and production accounting for the inherent anisotropy and low permeability of gas shales are still extremely limited, due to the great challenges in testing and modeling of shale under in-situ openhole conditions. First, shale frequently exhibits substantial mechanical anisotropy that affects the stress distribution around an open hole. Second, the intrinsic low permeability of shale, sometimes on the order of nano-Darcies, magnifies the effects of time-dependent pore pressure on the effective stresses and could cause complex rock failure behavior. In this study, the response of anisotropic thick wall cylinder shale samples under realistic laboratory time-dependent loading, simulating near-wellbore or near-perforation production stresses, is modeled and analyzed taking into account fully coupled fluid and shale matrix interaction to address these challenging issues. Extensive experimental results including thick wall cylinder tests on an anisotropic low-permeability shale were analyzed using the developed analytical simulator. The analysis showed that depending on shale permeability and loading rate, solids production could result from either shear collapse mechanism or a combination of shear and tensile failure of the rock matrix. Thick wall cylinder tests on Woodford Shale and Barnett Shale were also simulated using experimentally measured mechanical and petrophysical properties. The results showed that the stress required to initiate solids production significantly depends on the loading rate as well as sample size, and is considerably higher than predicted by conventional elasticity theory. The presented modeling will be essential for designing and interpreting thick wall cylinder tests on gas shales as well as the ensuing modeling of gas production from these formations. The results on Woodford Shale and Barnett Shale will serve as guidelines for field operations and future laboratory testing.

You can access this article if you purchase or spend a download.