InterPore2025

Pore-scale Modelling of salt precipitation during CCS

20 May 2025, 14:50

15m

Oral Presentation (MS26) Mechanisms Across Scales in Subsurface CO2 storage: A Special Session in Honor of Sally Benson MS26

Speaker

Prof. Vahid Niasar (University of Manchester)

Description

Carbon Capture and Storage (CCS) is considered a necessary technology for mitigating climate change, helping to keep temperature increases within the limits set by the Paris Agreement. In CCS, CO2 is captured from anthropogenic sources and is injected into deep saline aquifers, depleted oil and gas reservoirs or other geological traps. Deep saline aquifers play an important role as their capacity for safe storage of CO2 is two orders of magnitude greater than depleted oil and gas reservoirs. Maintaining the injection of CO2 into the subsurface is a critical part of determining the success of any CCS project; however, this is not always straightforward. Former studies show that injecting dry super-critical CO2 in saline and hypersaline aquifers leads to dry-out and formation of salt precipitation in porous space. This can cause significant decrease in permeability, leading to potential loss of injectivity. Addressing this challenge requires developing a fundamental understanding and predictive capability for injectivity loss under various conditions, including thermodynamic (pressure and temperature), hydrodynamic (injection rate), and rock heterogeneity factors. Salt precipitation arises from coupled physical, chemical, and transport processes operating across multiple length and time scales, making it a complex multi-physics, multi-scale problem (Ott et al., 2012, 2014, 2015, 2021). In the present work, we develop a quasi-static, semi-dynamic pore-network model to elucidate salt-formation mechanisms at the pore scale under diverse conditions. Our model captures the interplay of capillary-driven flow (including capillary backflow), evaporation, and salt precipitation. Key features include:

1. Quasi-static (QS) two-phase pore-network modeling (extending Niasar
   et al., 2009).

2. Capillary-driven backflow implemented through cluster labeling.

3. Kelvin effect.

4. Irregular pore-network geometry (unstructured, randomly distributed)
   with triangular pore-throat cross-sections.
5. Advection-diffusion transport of vapor.
6. Hygroscopic effect of salt in the liquid phase via an additional
   potential term in the Kelvin equation.
7. Salt precipitation feedback on the pore-network geometry,
   influencing transport dynamics.

Existing pore-network models that consider evaporation (Maalal et al., 2021; Dashtian et al., 2018; Ahmed et al., 2020) typically incorporate features (1-3) and rely on idealized geometries (e.g., lattice networks) without considering how salt precipitation alters the transport behavior. In contrast, our model (4) adopts a more realistic pore-network structure, (5) captures the essential physics of evaporation, and (6-7) accounts for salt precipitation feedback on the system.

We demonstrate that our model reproduces the characteristic stages of evaporation in porous media observed in experiments. In particular, the initial high evaporation rate “Stage I” is followed by the “Stage II” falling-rate period in which the evaporation rate decreases significantly. We then examined the hypothesis, “Is a quasi-static PNM approach suitable for modeling salt precipitation?” Our results indicate that the purely advective time scale is approximately half that of the case with evaporation and salt precipitation, implying that advective flow proceeds faster than diffusive vapor transport. Hence, the quasi-static modeling assumption is justified for capturing the essential dynamics of evaporation and salt precipitation in porous media.