Multi-scale study into Fluid-Rock Interactions in reservoir and caprocks for Geological CO2 Storage

25/11/2024

Doctoral Candidate: Prescelli Annan
Supervisor: Alba Zapone, ETHZ
Output type: Literature review

Background:

The impacts of anthropogenic CO2 emissions are increasingly evident worldwide, with rising global temperatures and sea levels, and increasingly frequent extreme weather events. Deploying CO2 capture and storage technologies on a large scale is essential to avoid the worst impacts of climate change. Geological CO2 Storage (GCS) offers a way to permanently store CO2 captured from the atmosphere, which is vital for countries to meet the Net-Zero targets of the 2016 Paris Agreement. Storing CO2 in geological formations, like depleted oil and gas fields or deep sedimentary aquifers, helps reduce the impact of greenhouse gas emissions.

Sedimentary aquifers have emerged as a key focus in many recent studies on GCS, highlighting both the potential and the challenges in deploying GCS to meet climate goals [12, 21, 1, 7, 16]. An impermeable caprock, which lies above the reservoir rock, is fundamental in preventing CO2 leakage during carbon sequestration. Caprocks, such as claystones, siltstones, and shales, act as a seal that stops CO2 from migrating upwards, a process known as structural trapping.

The effectiveness of a caprock depends on its ability to maintain a seal to CO2 during high-pressure CO2 injections. Common potential leakage pathways include old wells and up faults. Supercritical CO2 (SC-CO2), being less dense than water, tends to accumulate beneath the caprock, increasing pore pressure at the reservoir-caprock interface. This pressure buildup can potentially cause micro-fractures or trigger fault reactivation, leading to CO2 leakage [13, 4]. These reactivated faults can create pathways for CO2 to escape to the surface, as shown in Fig. 1.

 Figure SEQ Figure / *ARABIC 1: Conceptual diagram showing CO2 movement within a sedimentary reservoir and caprock system. The diagram illustrates how CO2 migrates upwards, forming a "plume" and highlights the potential for fault reactivation to comprise caprock integrity, leading to CO2 leakage

Figure SEQ Figure / *ARABIC 1: Conceptual diagram showing CO2 movement within a sedimentary reservoir and caprock system. The diagram illustrates how CO2 migrates upwards, forming a “plume” and highlights the potential for fault reactivation to comprise caprock integrity, leading to CO2 leakage

Well-monitored field studies that bridge the gap between laboratory and pilot scale, such as those by [14], [20], and in-situ fault reactivation studies [8, 9, 18,10], are crucial for understanding the complexities in how caprocks respond to CO2 injection. In laboratory studies on Opalinus claystone, CO2 injection has been shown to reopen fractures and increase permeability [2, 11]. However, [17] amongst others, found that the clay exhibited sealing behaviour, suggesting that the response of clay-rich caprocks to CO2 injection remains complex and warrants further investigation.

Laboratory studies also show varied results regarding the chemical and physical property changes in claystone, especially under different conditions of CO2 exposure. This variability highlights the significant gaps in our understanding of the mechanical response of claystone, particularly how it deforms, fractures, and changes in strength when interacting with CO2-saturated brine or dry supercritical CO2 [3, 5].

An emerging alternative to traditional GCS in sedimentary reservoirs is the injection of CO2-rich fluids (AQ-CO2) into reactive rocks, such as basalts as seen in Fig. 2, where the CO2 reacts to form solid carbonate minerals—a process known as mineral trapping. Pilot projects in Iceland have successfully demonstrated CO2 injection into basalts [6, 15,19]. However, the efficiency of mineral trapping varies significantly depending on factors such as temperature, pH, and mineral composition. Further laboratory testing is needed to better define effective mineral trapping rates and understand the resulting physical property alterations of the samples.

Figure SEQ Figure/ ARABIC 2: Injection of C02-rich fluids into basaltic reservoirs. AQ-CO2 sink due higher density, and crystaline flood basalts act as barriers/caprock
Figure SEQ Figure/ ARABIC 2: Injection of C02-rich fluids into basaltic reservoirs. AQ-CO2 sink due higher density, and crystaline flood basalts act as barriers/caprock

Research Summary:

This PhD project combines laboratory and field experiments to explore how reactive fluids interact with a caprock and reservoir rock, a critical aspect of effective GCS. Using a multiscale approach that integrates novel laboratory experiments with field studies at the Mont Terri Underground Rock Laboratory, we focus on understanding the structural control of reactive fluid flow, particularly fracture-limited transport processes in both laboratory and field settings.

In the laboratory, CO2 exposure experiments of intact core samples will replicate in situ salinity, pressure and temperature levels using a batch reactor system equipped with continuous pH monitoring. Characterisation of the mechanical and physical properties of the basalt and claystone samples will be conducted before and after CO2-rich seawater exposure, using X-ray computed tomography (XRCT) of cores to provide 3D insights across various rock types and better understand controls to rock alteration. We aim to confine the impact of CO2 exposure on the hydromechanical properties of a range of rock types relevant to CO2 storage. Applications of this research will help interpret results from geophysical field campaigns and assist storage reservoir management strategies.

Scaling this to the decameter scale, we investigate the injection of CO2-saturated brine into a fault zone within Opalinus Claystone at Mont Terri — an analogue caprock. This contributes to a more comprehensive understanding of fluid injection on fault reactivation and up-fault leakage, as well as the mechanisms of claystone fracture self-sealing. Here we target leakage processes at shallow depths, important to CO2 storage security, and will investigate methods to deploy in the case of leakage.

Objective 1: To understand how CO2-rich fluid injections affect fault reactivation, caprock integrity and fault self-sealing using advanced geophysical sensing techniques.

Objective 2: To investigate how CO2 exposure affects the mechanical and physical properties of Opalinus Claystone, focusing on fracture dynamics and overall integrity.

Objective 3: To investigate how CO2 exposure affects the mechanical and physical properties of Icelandic basalt, focusing on the role of fractures in dissolution and precipitation pathways and effective mineral trapping rates.

References:

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