Application of CO 2 Geological Storage Technology in Global Greenhouse Gas Reduction

. Greenhouse gas emission has serious impacts on human societies, ecosystems and economies as a result of global climate change. In response to this challenge, the global community has set greenhouse gas reduction targets aimed at reducing emissions of greenhouse gases such as carbon dioxide (CO 2 ). In terms of emission reduction technology, CO 2 geological storage technology is considered to be an important and potential solution. The wide application of CO 2 geological storage technology can help achieve greenhouse gas reduction targets and slow down the greenhouse effect of the earth. The research will explore the role of CO 2 geological storage technologies for global greenhouse gas emission reduction targets. Through extensive collection of relevant information on CO 2 geological storage technology around the world, comprehensive research and analysis of the principle, application cases, limiting challenges and environmental impact of this technology, to evaluate the prospects and prospects of CO 2 geological storage technology in achieving global emission reduction goals challenge. The study found that the geological storage of CO 2 can promote the reduction of global greenhouse gas emissions, and its long-term effect will be more obvious, and the advantages will be more obvious with the improvement of technology and the cooperation between international governments


Introduction
Since the beginning of the first Industrial Revolution in 1750, fossil fuels have been the main source of human energy.At the end of the 18th century, coal was widely used as a major energy source to drive steam engines, gas lighting and industrial production.With the passage of time, fossil fuels such as oil and natural gas have gradually become an important source of energy, and its widespread use began in the late 19th century and early 20th century.Up to now, human beings still regard fossil energy as the main source of energy.
The extensive use of fossil energy has had a profound impact on the development of human society.It has driven the industrialization process, accelerated urbanization, and the development of transportation.However, the use of fossil energy has also brought environmental and climate challenges for a long time, especially the emission of greenhouse gases and the intensification of climate change.The use of fossil fuels has led to a dramatic increase in atmospheric CO2 concentrations, according to a 2023 report by the International Energy Agency (IEA): In 2022, Global carbon dioxide (CO2) emissions from energy combustion and industrial processes to a new all-time high of 36.8Gt [1].
Carbon capture, utilization, and storage (known as CCUS) is a crucial technology for reaching net-zero emissions.CCUS is utilized in the Sustainable Development Scenario for the production of synthetic low-carbon fuels and for the purpose of extracting CO2 from the atmosphere.According to IEA [2], by 2060, the worldwide implementation of CCUS technology is projected to achieve a total of 115×10 9 tons of CO2 capture.93% of the captured CO2 will be securely stored underground.Furthermore, by 2070, CCUS is estimated to contribute approximately 15% to the overall cumulative reduction in global emissions.All these confirm that CCUS technology has a significant effect on improving the global greenhouse effect and reducing the existing greenhouse gases.
In the future, the importance of CO2 capture and storage for climate change mitigation will depend on a number of factors, including the commercial maturity of CCUS technology, the financial incentives provided by host governments and international organizations to implement CCUS technology, and whether CO2 storage risks can be successfully managed.
By discussing the principle of CO2 storage, analyzing and judging national policies, and referring to the existing international mature cases and how to deal with the risks posed by carbon dioxide storage, we can better understand the feasibility of CCUS technology, and the future possibilities of this technology.

Application of CO2 geological storage technology
CO2 geological storage is a technology to isolate CO2 from the atmosphere for a long time by injecting carbon dioxide into a suitable underground structure.At present, CO2 geological storage technology is suitable for geological structures such as deep saltwater aquifer structures, depleted oil and gas fields, basaltic aquifers and other geological structures.According to the technical mechanism, CO2 geological storage can be divided into two types: physical and geochemical capture mechanisms of reservoirs [3].According to the different storage position, it can be divided into land storage and sea storage.This technology is one of the important means to reduce the greenhouse gas content and mitigate the greenhouse effect.
Injecting CO2 into depleted oil (gas) fields is one of the ideal carbon dioxide storage methods, but this method has not been widely implemented in the world.At present, the number of projects using depleted oil (gas) fields for CO2 geological storage is limited in the world, while the deep saltwater aquifer structure is widely used for CO2 geological storage.The deep saltwater aquifer structure is composed of sedimentary rocks rich in high concentration brine, and its mineralization of groundwater is high, which is a favorable place for CO2 storage, also can make CO2 better mineralization and storage, and achieve the purpose of permanent storage.Currently, the depth of storage projects of CO2 in deep saltwater aquifer structure in the world is basically below 800 m.Depth control can ensure that CO2 is in a supercritical state, which not only makes effective use of underground storage space, but also ensures the safety of CO2 storage [3].
In addition to the storage of deep saltwater aquifer of carbon dioxide, basaltic structure is also one of the ideal storage places for carbon dioxide.Basalt is rich in Ca, Fe, Mg and other divalent metal ions, and its active nature makes basalt easy to react with CO2 and generate stable carbonate minerals.When carbon dioxide is injected into the aquifer in the basaltic structure, the PH of the groundwater decreases with the dissolution of carbon dioxide in the water, and the basalt is dissolved to neutralize the acidity of the groundwater and generate stable carbonate mineral precipitation, thus achieving the purpose of permanent storage of CO2 [4].
Besides, the use of carbon dioxide enhanced oil recovery (CO2-EOR) is also a widely used technology in the world today.The principle of the technology is to inject CO2 into the oil field at high pressure and form a mixture of CO2 and crude oil, so as to drive the oil into the production well.At the same time, the CO2 is also stored underground.Similar to this principle is the injection of carbon dioxide into unminable coal seams--CO2-ECBM (Enhanced Coal Bed Methane).Because the adsorption capacity of carbon dioxide in coal seam is stronger than that of methane, the coal seam will desorption CH4 while absorbing CO2, resulting in competitive adsorption between carbon dioxide and methane.
According to the data provided by CO2CRC [5] and related materials [6], the representative projects of CCS geological storage currently in operation or construction in the world are shown in Table 1, and the representative projects of CO2-EOR and CO2-ECBM are shown in Table 2.

Synergistic application of CO2 geological storage and renewable energy
In view of the irreplaceability of fossil energy, in order to achieve the goal of global greenhouse gas reduction, it is necessary to reduce the environmental impact of greenhouse gases generated by the use of fossil energy as much as possible under the premise of using renewable energy.Renewable energy sources such as wind and solar do not emit carbon, so promoting the use of renewable energy can reduce CO2 emissions, and CO2 geological storage technology can bury existing CO2 underground.The synergistic effect of both can accelerate the achievement of greenhouse gas reduction targets.
In addition, renewable energy can also help CO2 geological storage after meeting the production demand.Solar power depends on sunlight, while wind power depends on wind.During peak hours or at low loads, renewable energy may produce surplus or shortage situations.CO2 geological storage technology can use the electricity generated when these energy surpluses are Furthermore, the energy generated by renewable energy sources can be used to participate in the process of electrolyzing water, which produces H2 and O2, and after separation by a separator, the resulting H2 is first stored.Subsequently, CO2 captured from plants or the atmosphere can be converted from CO2 and hydrogen to methane via power-to-methane (PtM) technology, which can be used in oil field production, for example for enhanced oil recovery (EOR) or directly for energy generation [7].

Challenges and limitations of CO2 geological storage technology
At present, the amount of carbon dioxide stored by CO2 geological storage technology is quite limited compared with the amount of carbon dioxide emitted worldwide, and it is far from reaching the goal of carbon neutrality by relying on CO2 geological storage technology.The widespread use of CO2 geological storage technology around the world will have to deal with geological and siting constraints, as well as environmental and social risk management.Some of these limitations are unavoidable, while some can be addressed through technological improvements, policy support, and the designation of appropriate safety regulations.For now, there are limitations and challenges in the following aspects of carbon dioxide geological storage [8].

Requirements and limitations of geological storage on geological conditions, reserves and reliability
CO2 geological storage has strict requirements for stratum.First, a suitable underground reservoir for CO2 should have a suitable pore structure and sufficient storage space: the underground reservoir should have sufficient pore space to hold and fix CO2.Pore spaces can be natural gas or oil reservoirs, or pores and fractures in deep rocks.In theory, the more abundant the pore space, the more CO2 can be stored, and the greater the benefits [9].
The stratum should also be highly permeable and porous: the reservoir should be permeable and porous enough so that CO2 can flow and diffuse through the reservoir and fill the reservoir space adequately.When the porosity of the local layer is too low, the carbon dioxide is difficult to be stored, so the input cost will be greater than the economic benefits brought by the carbon dioxide storage, resulting in economic losses.
The water content in the stratum is also an important index.High water content in the stratum can accelerate the conversion of CO2 into carbonate.According to the reaction mechanism of carbon dioxide mineralization and storage, in the process of CO2 mineralization and storage, CO2 needs to be dissolved in water to form carbonic acid, so as to react with metal ions in the bottom layer to achieve permanent mineralization.Water will play a crucial role in this process, so CO2 also has a requirement for water content in the stratum.
In addition to a suitable underground reservoir, there needs to be a suitable cap rock or storage layer to prevent the leakage of CO2 to the surface or other stratum.In the storage performance of the storage layer, the storage layer should have lower permeability and higher storage performance to effectively store CO2.In addition, it is necessary to have good geological stability, and the storage layer should have sufficient geological stability to prevent the rupture or leakage of the storage layer caused by seismic activity or other geological changes [3].

Environmental and social risk management
In addition, safety issues are also the key to carbon dioxide storage projects.Although the current worldwide CCS projects are in good condition, there are no instances of CO2 leakage reported.However, a large amount of CO2 leakage will not only have a serious impact on the environment and cause economic losses, but also may cause harm to personal safety.For example, the volcanic eruption at the bottom of Lake Nyos in Cameroon in 1986 made a large amount of CO2 deposited at the bottom of the lake suddenly released.As a result, more than 1,700 people and a large number of animals suffocated to death within a radius of 25 km [10].
At the technical level, due to the huge CO2 storage project, long time span, uncertainty and complexity of the geological conditions of the storage site, it is urgent to carry out laboratory tests, field tests and numerical simulation studies on the changes of the physical and chemical characteristics of crown rock and reservoir as well as the changes of the temperature field, chemical field, seepage field and stress field after CO2 injection.In addition, for example, the long-term mechanical stability of carbon dioxide storage, the influence of the chemical coupling of CO2-water-rock on the mechanical and hydraulic properties of rock mass still needs to be solved [11].
Therefore, the storage area should be far away from seismic zones and densely populated areas, because frequent seismic activity will have a greater impact on the stability and safety of CO2 storage.

Energy and economic factors
Energy requirements and costs: CO2 geological storage requires a large amount of energy to carry out processes such as CO2 capture, compression, transport and storage.It can lead to additional energy requirements, especially when transporting CO2 to remote underground storage sites.Due to the consideration of the cost of storage, the general choice is to store CO2 nearby.According to the IPCC report, the reasonable distance between the CO2 source and the reservoir should be less than 300 km.Beyond this distance, CO2 transport costs may restrict the implementation of CO2 storage projects [3].This poses a challenge to the sustainability and costeffectiveness of energy supply [4,12].Financial investment and economic feasibility: there are few studies on mineralization storage, and there are great difficulties in technical implementation.The concept design stage has not yet been reached, and the cost is high, so the potential of mineralized CO2 storage is not optimistic [12].In addition, CO2 geological storage projects require huge capital investment, including the construction of CO2 capture facilities, transportation pipelines and underground storage facilities.These high costs can be a barrier to project implementation, especially in the absence of financial support or where carbon pricing mechanisms not robust [12].
Regulatory and social acceptance: CO2 geological storage is an emerging industry that needs to comply with relevant regulations and environmental guidelines.Developing and implementing appropriate legal frameworks and regulatory measures are keys.The lack of industry standards may lead to uneven levels of development during the development process, lacking a complete and well-adapted Occupational Health and Safety Code for CO2 geological storage.In addition, social acceptance is also an important factor, and the public's understanding of carbon dioxide geological storage projects and the possible risks of the technology are insufficient, thus affecting the public acceptance.

Technology development and innovation
In the technical field, it is necessary to strengthen the research on the mechanism of CO2 storage technology.Solving the problem of long-term mechanical stability of the storage stratum and the influence of the chemical coupling of CO2-water-rock on the mechanical and hydraulic properties of rock mass is the main way to eliminate the problems related to CO2 leakage and rock mechanical properties of crown rock.At present, there are many researches on the leakage path, which mainly focus on the fields of crown rock fracture, fault stability and well integrity.In the future, interdisciplinary research on porous media seepage mechanics, thermodynamics, rock mechanics, fluid mechanics, chemical reaction dynamics and other disciplines combined with laboratory tests and numerical simulation will be a key research direction in the future [11].

Policy and regulatory support
In order to promote the development of the program, appropriate regulatory frameworks and standards should be established at home and abroad, and economic incentives and policy support should be provided.What's more, topics such as international cooperation and knowledge sharing, how to share experience and practice through international conferences and exchanges, and how to solve common problems through transnational cooperation should be discussed.
With the gradual recognition of geological burial and storage technology from all walks of life, legislation can be adopted to ensure the legal status of the burial and storage technology and promote the active participation of relevant industries, so as to promote the large-scale implementation of the burial and storage project and alleviate the increasingly serious environmental problems.Moreover, due to the risk of leakage of CO2 geological storage, and some of the current technologies are not mature, the implementation of the technology may be protested by local residents.In order to ensure the standardization and order of industry competition, conduct reasonable evaluation of economic benefits and standardize industry operation process, it is necessary to formulate supporting industry standards and regulatory policies as soon as possible, clarify project ownership and risk control measures, and ensure the feasibility, safety and sustainability of project implementation [13] .The successful implementation of the CO2 geological storage demonstration project can not only bring valuable engineering experience to people, but more importantly, it is conducive to changing the public's attitude towards the CO2 geological storage project, enhancing the public's confidence in this technology, and win the support of the government and society.
Because carbon dioxide geological storage technology is a promising technology, countries should introduce corresponding subsidy incentive policies as soon as possible, so as to promote the mitigation of the high cost of CO2 storage technology.In the subsidy incentive process, the subsidy content can be classified and refined, and different incentive policies can be introduced for different storage types and the size of storage volume, such as tax relief of carbon storage.Similarly, it is also possible to introduce carbon storage trading market, introduce market capital, expand funding sources, and actively guide energy-related enterprises and low-carbon investment and financing institutions to participate in carbon storage projects to promote market vitality [13] .

Conclusion
, 03010 (2023) MATEC Web of Conferences MENEC 2023 https://doi.org/10.1051/matecconf/202338603010386 CO2 geological storage technology can capture and permanently store CO2 produced during large-scale industrial processes by injecting CO2 into underground reservoirs, such as underground brine or oil reservoirs, preventing its release into the atmosphere.It will help reduce greenhouse gas emissions and reduce the risk of global climate change, which has important potential and prospect in global greenhouse gas reduction.However, the technology still faces some challenges.For example, the cost of the current CO2 geological storage technology is high, including the cost of CO2 capture, transport and storage.This can be addressed through, for example, financial incentives to create a market for carbon storage.While geological storage risks, the storage of CO2 requires a suitable geological storage layer to ensure its long-term safety and stability.Assessing and monitoring risks to geological storage is an important challenge, and more research and monitoring is needed.Regarding public acceptance, CO2 geological storage technology involves large-scale CO2 treatment and geological storage, which may arouse public concern and controversy.Improving public scientific literacy, effective communication and public participation are key to addressing this challenge.
Through technological innovation, policy support, international cooperation and the promotion of diversified solutions, the technology can be better developed and applied, contribute to the fight against climate change, and can also realize its huge potential.

Table 1 .
Representative projects of carbon capture and storage (CCS) geological sequestration currently in operation or under construction worldwide.