Cyclic processes related to underground energy storage systems

RWTH Publications (RWTH Aachen)

Abstract

The growing reliance on intermittent renewable energy sources, such as wind and solar, has increased the demand for effective energy storage solutions. Underground energy storage systems (UESS), including underground pumped storage power systems (UPSP), porous formations for gas storage, and salt caverns for hydrogen storage, offer promising options. However, owing to the fluctuating nature of renewable energy supply, underground reservoirs in these systems are subjected to long-term cyclical loading and unloading, inducing complex hydraulic, mechanical, and thermal responses within geological formations. Over time, these cyclic processes can lead to stress redistribution, inelastic deformation, and potential structural failure, affecting the stability and capacity of UESS. This research adopts a comprehensive methodology encompassing a literature review, experimental studies, and numerical analyses at both laboratory and field scales. These methods are used to investigate deformation evolution, failure mechanisms, and the time-dependent responses of rock masses to cyclic mechanical, thermal, and hydraulic processes.~This thesis consists of three studies, focusing on three types of UESS, to deepen the understanding of cyclic processes affecting these systems: 1.The first study reviews the potential of repurposing abandoned coal mines as lower reservoirs for UPSP systems. It investigates the complex interaction of hydraulic, thermal, and mechanical processes that occur during cyclical operations, which affect the safety, efficiency, and stability of these underground reservoirs. The primary challenge lies in the limited knowledge of post-mining conditions, including weathering, dissolution, leaching, swelling, subsidence, fault creep, gas migration, and corrosion. This study analyzes the processes involved in cyclical pumping and discharge in UPSP, such as hydraulic discharge, cyclic loading, dry and wet cycles, fatigue, and thermal stress, which impact reservoir safety and stability. To address these challenges, the review presents different numerical solutions to comprehend and mitigate cyclical processes in abandoned mines. This research contributes to the understanding of utilizing abandoned mines for UPSP, highlighting the challenges associated with using coal mines as lower reservoirs and presenting several main processes to prevent safety and productivity issues. 2.The second study investigates the geomechanical behavior of porous sandstone formations for geological energy storage, focusing on deformation mechanisms under cyclic loading from injection/production cycles. The results from multi-level, multi-frequency cyclic loading experiments on sandstone specimens are presented, identifying three deformation regimes: 1) instantaneous elastic deformation, 2) transient and steady-state creep-related strain, and 3) accelerated deformation and fatigue failure due to micro-crack growth. Notably, fatigue failure correlates with rapid stiffness degradation. A viscoelastic-damage model combining Burger's viscoelasticity and an energy-driven damage approach is proposed and validated with experimental data. Parametric analysis highlights that higher cyclic load frequency and magnitude accelerate fatigue failure.~These findings inform design optimizations to ensure the stability of sandstone in energy storage applications. 3.The third study presents a numerical analysis of long-term thermo-mechanical (TM) behavior in underground salt caverns for hydrogen storage under cyclic TM loading In-situ TM conditions and key stability indices are outlined, followed by the use of a TM-coupled constitutive model incorporating elasticity, temperature-dependent creep, and plasticity to simulate rock salt behavior. Large 3D models of both vertical and horizontal caverns were analyzed over 60 years, revealing distinct deformation patterns at different cavern regions. Results show that exceeding temperature and pressure limits can lead to localized tensile and shear failures, threatening structural integrity. A parametric study highlights that managing stress within safe limits reduces displacement and enhances cavern capacity, with temperature variations also impacting storage performance.~These findings underscore the importance of cavern orientation and TM-coupled effects in ensuring safe, long-term hydrogen storage. This thesis enhances the understanding of cyclic processes related to UESS by examining deformation evolution, failure mechanisms, and time- and temperature-dependent behaviors of rock masses and underground systems. It investigates specific challenges in abandoned coal mines, the creep-fatigue behavior of sandstone, and TM-coupled effects in salt caverns, providing critical insights to improve the design and operation of stable, safe, and efficient energy storage systems.

Authors 1

1. Deyan Tian corresponding Aachen

   RWTH Aachen University

   Affiliation as printed

   RWTH Aachen