Determination of Biot Coefficient for Low-Permeability Rocks Accounting for Undrained Loading

ABSTRACT The determination of poroelastic properties for low-permeability porous media (k < 10−18 m2) is often elusive because of long periods required to dissipate or equilibrate pore pressure in tested samples. Poroelastic properties, such as the Biot coefficient, play a key role in determining the effective stresses in-situ and changes due to thermal, hydraulic and mechanical loadings. This paper presents a method to determine the Biot coefficient of low-permeability porous media accounting for undrained loading. We conducted experiments and numerical simulations to show the effectiveness of the method and ran parametric studies by changing the loading rate, equilibrium time, and permeability. The results show that the Biot coefficient, drained bulk modulus and permeability can be determined from a single partially undrained experiment as long as undrained response is considered in the data analysis. This method is advantageous not only because it is faster than previous methods but also because it can determine permeability indirectly. Accurate determination of these properties is critical to predict the response of low-permeability materials such as seal rocks for carbon geological storage among other applications. INTRODUCTION Accurately characterizing the poromechanical response of geological formations is critical for the success of subsurface energy projects, such as oil and gas recovery, geologic carbon and hydrogen storage, geothermal energy, and nuclear waste geological storage among others. Low permeability formations (k < 10−18 m2) play a vital role in many cases as sealing layers or containment barriers. Low-permeability formations exhibit a strong hydromechanical coupling, hence, confidently predicting their poromechanical behavior is critical (Bachu 2008; Makhnenko et al. 2017; Espinoza and Santamarina 2017; Zheng et al. 2022). However, accurate measurement of poromechanical properties can be challenging for materials with ultra-low hydraulic diffusivity due to the long period needed to achieve pore pressure equilibrium when external loadings are applied (Kim and Makhnenko 2020).