The equivalent Biot coefficient reveals the effects of heterogeneity on the Hydro-Mechanical behavior of fractured rocks

Understanding and predicting the hydro-mechanical (HM) behavior of subsurface porous and fractured formations is key to a number of engineering applications, including fluid injection/extraction, construction/excavation, geo-energy production and deep geological disposal. The interaction between fluid pressure, deformations and stresses is particularly affected by the subsurface heterogeneity, which may lead to non-intuitive responses, such as effective stress reduction and pressure increase during fluid extraction. While the impact of large-scale heterogeneities is acknowledged in most studies and modeling efforts, the presence of heterogeneities at smaller scales cannot be included in reservoir-scale models and it must be encompassed into equivalent properties assigned to uniform materials.

In this work, we focus on the Biot effective stress coefficient, a central property determining the HM behavior of fluid-saturated geological media. When not simply assumed as equal to 1, this coefficient is estimated experimentally at the laboratory sample-scale or analytically through expressions valid for isotropic homogeneous materials. However, these approaches are not able to estimate a representative equivalent coefficient for fractured rocks, which are strongly anisotropic and prone to sample-size effects, with fracture lengths spanning several orders of magnitudes from millimeters up to hundreds of meters. By employing a theoretical framework to quantify an equivalent Biot coefficient for a fractured rock mass from the properties of both the porous intact rock and the discrete fracture network (DFN), it is possible to analyze the variability of this coefficient with the DFN properties and highlight the implications for the rock upscaled HM behavior, in the context of natural processes and engineering applications.