Fatigue properties and constitutive model of Jintan salt rock subjected to complex cyclic loading

Abstract

Salt cavern energy storage technology contributes to energy reserves and renewable energy scale-up. This study focuses on salt cavern gas storage in Jintan to assess the long-term stability of its surrounding rock under frequent operation. The fatigue test results indicate that stress holding significantly reduces fatigue life, with the magnitude of stress level outweighing the duration of holding time in determining peak strain. Employing a machine learning approach, the impact of various factors on fatigue life and peak strain was quantified, revealing that higher stress limits and stress holding adversely impact the fatigue index, whereas lower stress limits and rate exhibit a positive effect. A novel fatigue-creep composite damage constitutive model is constructed, which is able to consider stress magnitude, rate, and stress holding. The model, validated through multi-path tests, accurately captures the elasto-viscous behavior of salt rock during loading, unloading, and stress holding. Sensitivity analysis further reveals the time- and stress-dependent behavior of model parameters, clarifying that strain changes stem not only from stress variations but are also influenced by alterations in elasto-viscous parameters. This study provides a new method for the mechanical assessment of salt cavern gas storage surrounding rocks.

1. Introduction

Large-scale underground storage offers a promising solution to balance energy supply and demand in the future. It can be categorized into porous media type and hollow reservoir type based on geological conditions, as shown in Fig. 1. Salt cavern gas storage offers advantages over depleted gas reservoirs, aquifers, and chambers: 30% buffer volume [1], low-cost extraction [2], high containment due to low permeability and self-healing [3], and shorter cycles for operational flexibility [4]. Salt caverns exhibit versatility in storing a diverse range of gases, including natural gas, H2, He, CO2, and compressed air (CAES), due to their adaptability to various applications. The operation strategy and injection/production frequency may be precisely regulated depending on the specific geographic location, climatic conditions, market demand and technical conditions [1]. In such variable operating conditions in salt cavern gas storage, the surrounding rock is subject to the mechanical effects of creep and fatigue. Factors influencing the cyclic mechanical response of salt rock include force magnitude, time dependency, waveform, and stress holding. With higher stress magnitudes leading to shorter fatigue life and reduced attenuation deformation and steady-state strain rates [5]. At lower frequencies, the extent of damage diminishes during the decay and steady deformation phases, but intensifies during the acceleration phase [6].

In the study of fatigue deformation properties of materials, it is difficult to capture the universally applicable evolution laws even for a specific material [7,8]. Current research on factors influencing fatigue deformation in Jintan salt rock remains fragmented, with a lack of systematic comparison and generalization [9]. In addition, the closed-well stage of the salt cavern gas storage is flexibly adjusted closely to the fluctuation of power demand. As illustrated in Fig. 2, the holding time of the closed-well phase for well J1 of the Jintan gas storage reservoir, based on 6-month wellhead pressure data, comprises approximately 68% of the total operational duration. However, limited studies have focused on the constant pressure phase. Suo et al. [10] and Zhao et al. [11] focused on the study of fatigue-creep interaction damage evolution during discontinuous fatigue process in salt rock.