Impact of Shale softening on Permeability

Over the past century, shale gas exploration and production have grown significantly especially with horizontal drilling coupled with hydraulic fracturing that enabled the exploration of tight gas bearing formations. However, upon exposure to the aqueous fracturing fluids under high temperature and high pressure, mechanical properties of shales such as elasticity, hardness, and strength usually deteriorate. This phenomenon is termed as "shale softening" which has negative impact on the structural behavior of the shale. In addition, shale softening also limits the amount of shale gas production. With the softening of shale, its porosity, pore sizes, and pore throat sizes are reduced. In this study, the properties of intact shale as well as for soften shale obtained through the CT images were used for a three-dimensional nanoscale pore-network model that can describe the hydraulic properties of porous media. The shale softening experiments produced an average porosity of 3.7% for Haynesville with original porosity of 4.1%. A three-dimensional nanoscale pore-network model can be used to describe the hydraulic properties of porous media. The pore network for the intact and soften Haynesville was constructed and the simulated permeability showed progressive reduction in hydraulic conductivity. The predicted hydraulic conductivities of intact shale, layer in contact with fracturing fluids, 50% of the rock mass softened and 100% of the rock mass softened were 2.00E-19, 1.27E-19, 2.74E-20 and 1.22E-20 m² for, respectively. Hence, the pore network model is capable of explaining the reduction in hydraulic conductivity of shale, which is related to the variation of shale gas production rate with time due to shale softening.

Key Words: Shale softening, Swell ratio, Pore network, CT image, Shale gas production