An approach for assessment of CO2 leakage using mechanistic modelling: CO2 injection in deep saline aquifer of Lithuanian basin in presence of fault and fractures

1. Introduction

Carbon dioxide (CO₂) leakage is a pressing environmental issue that arises from various industrial processes, particularly those related to the extraction and storage of fossil fuels. The inadvertent release of CO₂ during these operations can have detrimental effects on both the environment and human health [1]. CO₂ leakage can occur due to several factors, including inadequate well integrity, faults or fractures in underground storage reservoirs, and failures in transportation pipelines [2-4]. In the context of carbon capture and storage (CCS), which involves capturing CO₂ emissions from power plants and industrial facilities and storing them underground, leakages can result from improper storage site selection, poor monitoring, or human error during injection or storage operations [5].

The injection of CO₂ into deep saline aquifers offers significant potential for large-scale and long-term storage of carbon dioxide. These aquifers, characterized by their high storage capacity and widespread distribution, are considered one of the most promising geological formations for CO₂ storage [6]. Potential places for CO₂ sequestration around the world are shown in Fig. 1. Different aspects of CO₂ storage in Baltic basin have been investigated from pore scale modelling to simulation based assessment of storage [7, 8], showing significant CO₂ storage potential. The presence of faults and fractures in these reservoirs introduces challenges in maintaining the integrity of the storage system and preventing CO₂ leakage, see Fig. 2, which shows a conceptual picture of possible leakage during storage of CO₂. Previous studies have also shown that fault and fracture networks can significantly impact the migration and containment of CO₂ within deep saline aquifers [2-4].

The consequences of CO₂ leakage are far-reaching and encompass environmental, economic, and public health impacts. Environmental consequences include the acidification of water bodies, deterioration of soil quality, and negative effects on vegetation and biodiversity [5]. The release of large quantities of CO₂ into the atmosphere can exacerbate climate change, contributing to global warming and further disrupting delicate ecosystems [6]. Economically, CO₂ leakage can lead to substantial financial losses. Industries investing in CCS projects may face increased costs due to the need for additional monitoring and remediation efforts. Moreover, the potential damage to ecosystems and agricultural lands can have indirect economic consequences, affecting sectors dependent on these resources [5]. From a public health perspective, CO₂ leakage can pose significant risks. High concentrations of CO₂ in the air can displace oxygen, leading to asphyxiation. Furthermore, the release of CO₂ may be accompanied by the release of other toxic gases or substances, intensifying health hazards for nearby communities. Therefore, addressing CO₂ leakage is crucial for achieving global climate change mitigation targets and ensuring the long-term viability of CCS as a viable solution.

Fig. 1Figure showing suitable storage regions of the world for CO2 storage, modified from [9]

Possible leaks Prevention is essential to prevent the release of significant amounts of greenhouse gases into the atmosphere and to maintain public safety. Implementing robust monitoring systems and adopting stringent regulatory frameworks can help identify and rectify leaks promptly, minimizing their impact on the environment and human health [5]. By addressing CO₂ leakage effectively, we can contribute to mitigating climate change, protecting ecosystems, and safeguarding public well-being. In this paper, we investigate possible scenarios for CO₂ leakage with the help of models when it’s stored in deep saline aquifer formations. Models are used to investigate the impact of the presence of fault and fractures on CO₂ leakage. Particular case of deep saline aquifers in Lithuanian geology are investigated and findings are presented.

The paper is organized as follows: Firstly, aspects of CO₂ storage in deep saline aquifer and depleted hydrocarbon fields is discussed. Then a discussion on high level risk associated with the CO₂ storage are presented and way to mitigate these risks are also discussed. Next method used in this study for modelling is briefly presented followed by key results and conclusions.

2. Aspects of CO₂ storage in saline aquifer and depleted hydrocarbon reservoirs

2.1. CO₂ storage in unconfined aquifers

In an unconfined aquifer, there is no confining layer restricting the vertical movement of fluids. This lack of a confining layer means that there is little resistance to the upward migration of injected substances, such as CO₂. When CO₂ is introduced into the unconfined aquifer, its buoyancy causes it to tend to move upward within the geological formation. The buoyancy of CO₂, combined with the absence of a confining layer, makes it prone to vertical migration, posing a risk of escape towards the surface (Fig. 3(a)).

To prevent the undesired upward movement of CO₂ and secure its containment within the targeted aquifer, it is crucial to have a caprock or another form of confining layer. A caprock serves as an impermeable barrier above the aquifer, inhibiting the vertical migration of fluids, including CO₂. By providing a sealing layer, the caprock helps trap the injected CO₂ within the aquifer, preventing its escape and ensuring the success of carbon capture and storage (CCS) or other subsurface injection processes.

Fig. 2Potential leakage routes, modified from [4]

2.2. CO₂ storage in confined aquifer

In a confined aquifer, there is a confining layer above the aquifer that prevents the CO₂ from migrating upward. CO₂ is injected into the aquifer, and it is trapped below the confining layer. (Fig. 3(b)). The pressure of the injected CO₂ may increase over time, and this may cause the CO₂ to migrate into adjacent rock formations. Therefore, monitoring and management of the CO₂ storage is important to ensure its safety and effectiveness.

Fig. 3CO2 Sequestrations in an unconfined aquifer and confined aquifer

a) Unconfined aquifer

b) Confined aquifer

3. Risk associated with CO₂ sequestration in saline aquifers

Saline aquifers are underground water-bearing formations that contain high concentrations of salt and are unsuitable for human consumption or irrigation. They are considered a promising option for CO₂ storage because they have large storage capacity and are widely distributed. However, there are several specific risks associated with this type of storage, including:

4. Method

In Lithuania’s Baltic Basin, research is still in its early stages regarding the long-term fate of geological CO₂ storage [7, 8]. This study focuses on the deep saline aquifers, Syderiai of the Baltic Basin (shown in Fig. 4) and aims to demonstrate the effect of Fault and fracture on CO₂ injection. Lithuanian Basin has a number of deep saline aquifers and depleted hydrocarbon reservoirs in onshore and offshore settings. A detailed ranking of these subsurface storage sites has been analyzed and presented in [7, 8]. The focus of this paper is on the deep saline aquifer. The largest in terms of storage are two onshore deep saline aquifers Syderiai and Vaskai. In this study we focus on Syderiai aquifer only. High level geological models are built using mechanistic modelling approach whereby average properties are used to make a layer cake model. Geology of the region is represented using simple gaussian distribution. The models are then infused with high permeability faults and fractures to test various leakage scenarios based on the magnitude of fault and fracture permeability.

Fig. 4Location of saline aquifers in Lithuanian geological basin

The methodology employed in this study aimed to develop a mechanistic model of the Syderiai deep saline aquifer within the Lithuanian basin, which has been shows to have a significant representative storage volume [7]. The model was constructed based on key geological parameters, including average permeability, porosity, net-to-gross ratio (NTG), and aquifer thickness. The primary focus of this research was to address the issue of leakage in the deep saline aquifer resulting from the injection of carbon dioxide (CO₂).

To investigate the relationship between CO₂ leakage volume and fracture permeability, a box model was created. The box model comprised of a fracture network and a top seal layer, see Fig. 5. We assumed there was no CO₂ present in the deep saline aquifer when we started simulation. Additionally, Python programming language and relevant libraries were utilized for the implementation of the model and the calculation of leakage volumes from fractures. Simulations are conducted using T-navigator simulation software [13].

Fig. 5a) Permeability distribution grid block for 1000 md fracture, b) soluble CO2 in water for 1000 md fracture after 100 year

a)

b)

5. Result

The results of our study indicate a clear dependence of CO₂ leakage volume on fracture permeability. An initial observation reveals a positive correlation, as the leakage volume of CO₂ tends to increase with higher levels of fracture permeability. This finding aligns with the expectation that greater permeability provides enhanced pathways for fluid movement, resulting in a higher potential for CO₂ migration. As fracture permeability continues to increase, the sealing effects become more pronounced, leading to a subsequent decrease in CO₂ leakage volume. Fig. 5, shows simulation results on a synthetic case where faults and fractures are present and CO₂ is injected and it can be seen that CO₂ migration is not fast enough to leak through the fracture network due to the distance between injection wells and the fractures present in the system.

6. Conclusions

This paper tests the leakage volumes using different fracture configurations and fracture permeability values. Optimal conditions for minimizing leakage were identified. Investigations were made for the temporal dynamics of CO₂ leakage volumes. Analysis was carried out to look at factors such as cap rock degradation through geochemical reactions and pressure differentials, to understand how the leakage volume evolved over time.