A Novel High-Stress Method to Measure Total Horizontal Stress in Mudrocks Subjected to Uniaxial Strain Loading and Unloading
Authors: Kim Kiseok , Awad M. Mohamed , Espinoza D. Nicolas 
Published: August 2024

ABSTRACT:
Accurate assessment and prediction of the in-situ horizontal stress is essential for ensuring the integrity of subsurface storage systems. Fluid injection and extraction affect the in-situ stresses. Changes of effective vertical stress are well constrained. However, changes of effective horizontal stress are not well characterized, particularly in the caprock where undrained loading and locked-in stresses can change the response significantly from drained linear poroelasticity predictions. In this study, we introduce a high-stress experimental setup designed to accurately measure horizontal stress during the consolidation process and subsequent unloading of clay-rich resedimented samples. Horizontal stress is determined by accurately measuring the tangential strain around the oedometer cell, while ensuring that this lateral strain is minimal enough to justify uniaxial strain condition. The results indicate that the K0 (ratio of horizontal to vertical effective stress) values converge to a constant value during loading (normally consolidated) as expected, but the unloading phase (over consolidated) presents a significant increase of K0 up to ∼2, increasing under undrained conditions. These results help calibrate advanced constitutive models to predict in-situ and change of effective vertical stresses in subsurface systems subjected to thermos-hydro-mechanical loadings.

1. INTRODUCTION
Geo-energy projects, such as geologic carbon storage, hydrogen storage, and geothermal energy, alter the in-situ stress state by fluid injection or extraction. While the vertical stress is mostly affected by the overburden and can be calculated from depth and mass density, predicting horizontal stress requires an accurate constitutive model and knowledge of the loading history. In the case of mudrocks, characterizing the horizontal stress requires knowledge of the stress history, pore pressure and mineral composition (Casey et al. 2016). Mudrocks serve as sealing layers in many subsurface storage systems. Thus, properly assessing the horizontal stress is essential to ensure seal mechanical integrity (Guiltinan et al. 2018, Kim and Makhnenko 2020, Zheng et al. 2022).
The coefficient of earth pressure at rest K0 is defined as the ratio between the horizontal and vertical effective stresses.
(equation)
Assuming zero lateral strain εlat = 0 is often referred to as the "K0-condition". K0 can be expressed as a function of the Poisson's ratio ν, where K0 = ν/(1-ν), under the assumption of linear isotropic poroelasticity. However, geomaterials including mudrocks generally exhibit responses that deviate siginificantly from this simplification (Prioul et al. 2004). Previous studies have developed empirical relationships from field experiments to estimate the K0 values, employing mini-frac or leak-off tests, and finding the relationship with overburden stress gradient (Brudy et al. 1997, Haimson and Chang 2002). However, fracture measurements are usually spare and difficult to obtain. Most importantly, field experiments do not capture the stress history or permit predicting changes, which is a critical factor for the evolution of horizontal stresses in mudrocks.