Investigation of formation water evaporation behavior and its impact on CO2 storage within aquifers

Abstract

Evaporation is one of the primary mechanisms in subsurface fluid migration, prevalent in gas-liquid multiphase flow processes within porous media. However, less emphasis has been placed on fluid evaporation during its flow from well to formation. This study investigates the behavior of formation water evaporation and its impact on CO₂ storage through high-volume CO₂ displacement experiments coupled with online nuclear magnetic resonance testing. The retrograde crossover phenomenon of water recovery under varying temperatures is observed during high-volume CO₂ displacement. The color change in silica gel provides clear evidence of formation water evaporation which leads to the crossover. Initially, formation water migration is primarily driven by CO₂ displacement; however, as gas saturation exceeds 40 %, evaporation replacing displacement becomes the dominant migration mechanism. The primary migration mechanism shifts during this process. Evaporation typically occurs at inlet of the core, or near-wellbore area in field applications. Pronounced CO₂ override flow phenomenon is observed, which significantly enhances the water evaporation and gas channeling in the upper part of porous media. A sufficient cumulative CO₂ injection volume is necessary for significant formation water evaporation. Increasing temperature within an enclosed space does not significantly enhance water evaporation. Conversely, both isothermal depressurization and vacuum evacuation with an open boundary can markedly increase water evaporation. These outcomes document that open boundary, fluid flow, and high-volume CO₂ injection are prerequisites for effective formation water evaporation. Furthermore, high formation temperature, large pressure difference, and slow injection speed promote earlier and more intense formation water evaporation. The effects of evaporation on filtration are twofold: on one hand, the reduction in irreducible water saturation enhances permeability; on the other hand, salt precipitation resulting from evaporation decreases permeability. Therefore, rationally utilizing formation water evaporation mechanism can lower flow resistance near wellbore, reduce injection pressure, improve sweep efficiency, and increase CO₂ storage capacity.