Salt Precipitation‐Driven Rock Failure Mode Transition During Geological CO2 Sequestration

Abstract

Salt precipitation has emerged as a critical factor affecting injectivity, reservoir stability, and the potential to trigger near-wellbore microseismic activity during geological CO2 sequestration. While previous studies have primarily focused on the brine acidification induced by CO2 injection, triggering geochemical reactions in carbonate rocks and leading to mechanical degradation, the mechanical behavior associated with salt precipitation in drying zones, particularly the failure mechanisms, remains poorly understood. In this work, we designed a reservoir-condition displacement system to mimic near-wellbore drying process and further investigated the rock failure modes due to salt precipitation in red sandstone samples. Our study demonstrates that, despite the densification of the pore structure due to salt precipitation, the overall mechanical performance of the rock undergoes significant deterioration. More importantly, for the first time, we observe a distinct transition of failure mode from shear-to tensile-dominated under uniaxial compression. Microstructural analysis further shows that the growth of polycrystalline and bulk crystals induces microcrack initiation and propagation, with the failure mechanism of rocks subjected to salt precipitation primarily characterized by intercrystalline damage at weak bonding interfaces under external loading.

Plain Language Summary
Geological sequestration of CO2 has emerged as a promising and viable strategy to mitigate climate change by injecting supercritical CO2 (scCO2) into deep subsurface formations for long-term containment. This process can induce salt precipitation, a phenomenon where dissolved salts crystallize out of pore brine. Such precipitation poses significant challenges, including pore blockage, reduced rock strength, and a potential contribution to microseismicity that may compromise reservoir stability. In this study, the effects of salt precipitation on the microstructure and failure characteristics of reservoir rocks were experimentally investigated under reservoir-representative conditions. Results indicate that while salt crystallization densifies the rock's pore structure, it paradoxically undermines the overall mechanical integrity. Specifically, the load-bearing capacity is significantly reduced, making the rock increasingly prone to tensile failure as opposed to shear failure under compressive stress. Given that fluid injection most commonly induces shear failure, particularly in the presence of pre-existing faults, a shift toward tensile-dominated failure makes reservoir damage more complex. Moreover, tensile failure promotes fracture opening and propagation, thereby increasing uncertainty in CO2 migration prediction and monitoring. This transition in failure mode is attributed to weak interfacial bonding between the salt crystals and the rock matrix, along with an increased development of microcracks. These findings provide critical insights into the stability of geological reservoirs during CO2 sequestration and establish a scientific basis for investigating the mechanisms of injection-induced microseismicity.