Dynamic CO₂ Residual Trapping in Carbonate Rock (Flexible Funding 2023)

Dr Yihuai Zhang, University of Glasgow, was awarded funding in the UKCCSRC’s Flexible Funding 2023 call to look at “Dynamic CO₂ Residual Trapping in Carbonate Rock”.

Project Overview

As the impacts of climate change become more pronounced, the need for effective strategies to reduce atmospheric CO₂ levels has never been greater. Among the promising measures under development, carbon capture and storage (CCS) stands out for its potential to securely store CO₂ deep underground, preventing its release into the atmosphere. One particular geological formation gaining attention for CCS is carbonate reservoirs, which are notable for their sizable storage capacity. However, the introduction of CO₂-saturated water into these formations can trigger chemical reactions that may alter rock pore structures over time, ultimately affecting the capacity of these reservoirs to trap CO₂. Our project aims to shed light on these complex chemical and physical processes, and how they influence the safe, long-term sequestration of CO₂ in carbonate reservoirs.

Why carbonate rock?

Carbonate reservoirs—such as limestones and dolomites—are abundant worldwide and hold immense storage capacity (e.g. the depleted oil and gas carbonate reservoirs). But they’re also reactive. When CO₂ dissolves into the formation water of the reservoir, it produces a weak carbonic acid. This acid can dissolve certain minerals in the rock matrix, effectively enlarging or connecting pores over time. Such changes can lower the resistance to fluid flow and modify how CO₂ is trapped in the reservoir’s pore spaces. Understanding these dynamics is crucial: if too much dissolution occurs, the reservoir’s ability to retain CO₂ safely could be compromised.

Our Approach: 4D High-Resolution X-Ray Imaging

To capture these rapid chemical and physical changes at the pore scale, our team employed in situ core flooding tests under 4D high-resolution X-ray synchrotron imaging at the I13 beamline in the Diamond Light Source. This cutting-edge facility allowed us to visualise the rock’s interior in real-time and observe exactly how CO₂-rich fluid travels through and alters the carbonate matrix, as well as how the CO₂ is trapped inside the sample before and after the damage. By segmenting and reconstructing these high-resolution images, we could identify zones of dissolution (e.g., channel formation) and quantify changes in porosity, permeability and capillary trapping. This allowed us to map how dissolution fronts spread and to measure how much the rock’s structure was altered.

Figure 1. The in situ core flooding set up at the I13 beamline in the Diamond Light Source.

Some Findings:

Channelled Dissolution Patterns

Our results revealed a channelled dissolution pattern, where fluid flow became increasingly focused through newly enlarged or connected pore networks. This channelling correlated with a measurable rise in porosity over time.

Reduced Capillary Entry Pressure

Through pore-scale simulations of fluid flow (drainage and imbibition), we found that as pore connectivity increased, the resistance to fluid penetration (capillary entry pressure) decreased. This could mean CO₂ can more easily invade and travel through these formations, altering the balance between retention and mobility.

Slight Reduction in Trapping Efficiency 

A key component of CCS is ensuring CO₂ gets “trapped” rather than freely migrating. Our findings show that as the pores become broader and better connected, overall trapping efficiency is likely to decrease slightly—though remaining a viable mechanism for CO₂ sequestration.

Figure 2. Examples of the raw images and 3D reconstructed image of the rock damage along the CO2 reactive brine injection.

Next Steps

Our ongoing work focuses on in situ contact angle measurements and dynamic flow effects to further refine our understanding of how local wettability and flow rates influence CO₂ trapping in real subsurface conditions. These insights will be published soon after the analysis is finished. We hope that the knowledge we’re generating can help industry and policymakers better gauge the long-term security of CCS in carbonate reservoirs. By pinpointing when, where and how dissolution-driven changes may occur, and how they can impact the residual trappings, we aim to optimise injection strategies, enhance monitoring tools and ensure that CCS lives up to its promise as a cornerstone of climate change mitigation.

Read more on Yihuai’s Flexible Funding 2023 project page.