Physical and chemical effect of impurities in carbon capture, utilisation and storage

Deployment of carbon capture, utilisation, and storage (CCUS) technologies to mitigate climate change and overturn CO2 emissions growth would require transformational changes comprehensively. The primary focus of this manuscript is on the impurities standards and limitation that can ensure project feasibility in the long run. There is a need in the industry for guidance on purity analysis prior to capture, shipment, and storage of carbon dioxide. This is because the cost to capture and separate the stream is proving to be very costly that can make the project to be unfeasible to operate. Following this further, this manuscript discusses the previous research and best practices that establish standards for acceptable impurities that might present in the stream and its effects towards the CCUS system. Consequently, this manuscript also provides better understanding on the impurities effects towards CCUS technology system in general. Understanding these limitations, may provide cost effective solution for CCUS problems that revolves around the impurities in CO2 stream. Impurities can affect some components of the carbon capture and storage process. It is clear that even a little number of impurities can cause the carbon dioxide stream properties to change. There are two primary factors discussed in this manuscript that affect how a CCUS system responds to a CO2 stream that contains impurities: a physical and chemical effects.


Introduction
The greenhouse gas emissions (GHG) have continuously rise since the nineteenth century and every year, it managed to reach historic high where reports show that the annual rate for 2021 has increase beyond the 2011-2020 average (Lamb et al. 2021;Raza et al. 2019;Shreyash et al. 2021;World Meteorological Organization 2021). As shown in Fig. 1, the dangerous greenhouse gas composed of multiple gases where carbon dioxide CO 2 contributed the most emission at 76%, followed by methane (CH 4 ) at 16% emission and nitrous oxide (N 2 O) at 6%. The least emission is fluorinated gases (F-gas) such as hydrofluorocarbons (HFCs) and sulfur hexafluoride (SF 6 ) at 2% (Intergovernmental Panel on Climate Change 2015; United States Environmental Protection Agency 2022). Due to the emission percentage and long life of CO 2 , it has a direct impact towards global warming through the constant rise of temperature which indirectly cause frequent extreme weather with the rise of sea level and acidity (Li 2008;World Meteorological Organization 2021). These changes cause significant changes in natural ecosystems and society that require modifications in natural resource management and allocation. The growing vulnerability of natural and human systems underscores the need to mitigate climate change's effects in order to avoid extreme and pervasive events mentioned above.
As part of the efforts to limit global warming, various CO 2 reduction schemes and technologies have been proposed by multiple international organization and government which is initiated by the United Nations (Mohd Pangi and Md Yusof 2022). One of the recent initiatives was the Paris Climate Accords where the main objective revolves around limiting the release of GHG into the atmosphere and control the temperature rise by 1.5 to 2 °C each year (United Nations 2016). It is also listed the carbon capture, utilisation, and storage (CCUS) strategy as an essential technology to help the mitigation of CO 2 emissions and contribute to the goal of achieving net zero anthropogenic greenhouse gas emissions by 2050 (Budinis et al. 2018;Kearns et al. 2021;Wang et al. 2011).
CCUS technology is defined as technology that can capture, transport, store, and utilize carbon (Khalid 2021). Capturing of CO 2 from a variety of point source would be required for geological storage of CO 2 and this capture and separation technology is the most expensive steps in the CCUS chain which account 75% of the total overall cost (Nicot et al. 2013). When carbon is captured, it will be accompanied with impurities and the separation of the impurities can give a significant impact on the project cost (Khalid 2021; Wang et al. 2011). In transport section, the carbon stream is transported by either pipelines, ships, or trucks from the capture points to storage and utilization (enhanced oil recovery). In storage, the CO 2 is injected into reservoir of rock formations deep under the seabed or in saline aquifer. For short-to-medium period of storage, deep saline aquifer provides the best solution to CCUS compared to order method (Jiang 2011). The remaining amount of CO 2 is then used for utilization, where it can be converted into chemicals and also for enhanced oil recovery (EOR) (Khalid 2021). This complete CCUS chain process is depicted clearly in Fig. 2.
The CCUS technology incorporated waste to wealth system which is beneficial not only to the stakeholders but also to the environment sustainability (Khalid 2021). However, the development of these technologies is slower than anticipated and does not meet the Paris Agreement's carbon reduction commitments (International Energy Agency 2021b). In 2021, there are only 20 commercial CCUS that operate worldwide but this is expected to grow (International Energy Agency 2021a,  (Wang et al. 2019) 2021b, 2021c). The slow progress is due to the high cost of developing and utilizing CCUS compared to other mitigation approaches. This technology, however, can be very reasonable in the long run (Budinis et al. 2018;Nicot et al. 2013). However, this can be overcome with a solid collaboration model between government and private sector for investment plus scaling up purposes where this can be seen with at least 100 new CCUS projects that have been announced so far and the worldwide project pipeline for CO 2 capture capacity is expected to grow by four-fold (International Energy Agency 2021b; Khalid 2021).
Due to technical and economical constraints, there are different types of impurities could come together with CO 2 in the stream and lead to a major concern on CO 2 transport, injection and also storage (Wang et al. 2011;Wetenhall et al. 2014). The impurities are anticipated to have a major impact on the phase behaviour of CO 2 streams, which has consequences for pipeline and injection well design and operation. Most of the impurities found are classified as non-condensable impurities which can affect the temperature and properties of the stream. As an example, nitrogen (N 2 ), oxygen (O 2 ) and argon (Ar) would increase the saturation pressure of liquid CO 2 and decrease the critical temperature which in turn can cause overpressure and for transportation and injection. Other than that, impurities like sulphur oxides (SO x ) may have acidic reaction with the cap rock which can cause problems in the storage structure and injectivity (Wang et al. 2011).
Despite the importance of impurities presence in CO 2 , there remains a paucity of evidence on the acceptable impurities' percentage for CO 2 transport and storage. Many uncertainties still exist about the relation between impurities and CO 2 on design and operation of pipelines as well as its impact on geochemical and petrophysical changes during storage in the geological media. This paper assesses the significance of CO 2 impurities and its effects on the CCUS system. The first section of this paper gives a brief overview of standards and regulations by various authorities on acceptable impurities level for CCUS project. It will then go on to classification of impurities and analyze its physical impacts (phase behavior, storage capacity, injectivity, buoyancy) and chemical impacts (fluid-rock interactions and surface material). Therefore, this study makes a major contribution to advance the understanding of impurities in CCUS system. With better understanding on the matter, a guidelines or best practice are needed to make sure that the CCUS system can work seamlessly, effectively and at cost effective structure.

Standard for CCUS
There are a variety of impurities that could be present together with CO 2 and it is almost impossible to completely remove the impurities from CCUS system. Thus, standards of acceptable impurities are required to ensure that project is feasible while obeying to the rules and regulations setup by regulators (Anheden et al. 2005;Harkin et al. 2017). These specifications usually in form of upper and lower limit was developed from tons of data where it is classified into components and its application such as transport (pipelines), storage (carbon sequestration) or utilisation (enhanced oil recovery) (Harkin et al. 2017;Shirley and Myles 2019). This quality requirement can be use as guidance to meet the CO 2 capture technology validation and recommendations (Det Norske Veritas 2010).
Many international standard bodies such as Det Norske Veritas (DNV) and International Organization for Standardization (ISO) have come up with a guidelines or best practice that present a systematic approach in evaluating CCUS capture technology from fossil fuel production. These standards need to be updated frequently as the content is not always sufficient to adapt with the rapid development of CO 2 capture technology (Det Norske Veritas 2010). Due to this reason, many major governmental bodies set their own standards of purity and impurities composition that are deemed reasonable and safe towards the environments. For example, in the United States of America (USA), the US Department of Energy convened all stakeholders such as industry players, subject matter experts, governmental and non-governmental official for at least once a year. This meeting is for sharing and exchange of information with the possibility of collaboration with the end goal of standardizing the CCUS throughout United State of America (National Energy Technology Laboratory 2017).
Other than that, The People's Republic of China (PRC) which is the world largest energy consumer have recently published a standard to CCUS process that focuses on monitoring, measurement, performance, and risk. This standard was in collaboration between the government and a nonprofit scientific organization called The Chinese Society for Environmental Sciences (CSES). These standards were tabulated because nearly 90% of the energy consumption in this country was produced by fossil fuel, thus a significant action is taken by the authority to ensure that the country obeys to low-carbon emission towards ecological civilization goal (Asian Development Bank 2015). Table 1 listed the CCUS standards produced by various authority that can act as a guideline for a safe and reliable CCUS projects.

Impurities in CCUS
CO 2 can be captured using several methods, and the stream contains gaseous pollutants that is unfavourable to the stream (Walspurger 2012). Some of the impurities discovered consist of water (H 2 O), hydrogen (H 2 ), hydrogen sulfide (H 2 S), carbon monoxide (CO), nitrogen (N 2 ), oxygen (O 2 ), methane (CH 4 ), argon (Ar), sulphur oxides (SO x ), and many others (Nicot et al. 2013;Wang et al. 2011). The type and composition structure of the unwanted gasses are varied depending on various factors such as capture technology and source (International Energy Agency 2021b; Wang et al. 2011;Wetenhall et al. 2014). CCUS process can be divided into four major process which is production, capture and transport, storage and lastly utilization. In each process, the purity varied from one to another. The goal of this section is to identify the probable impurities that is produced together with H 2 O by a variety of CO 2 capture processes. Figure 3 can provide a basic understanding on the maximum limit of impurities that could present in CO 2 capture process.
Before carbon can be captured, it must be produced first. This carbon production process can be divided into two major methods, which is pre-combustion and post-combustion. In pre-combustion methods, the carbon is removed before the fuel can be converted into energy. This can be achieved by converting fuel to syngas by reforming or gasifying process depending on the raw materials. Additional to that, the utilization of water gas shift may help in getting additional hydrogen from the water. One of the advantages for these methods is low impurities that can expected at the end results because carbon dioxide is removed before combustion (De Visser et al. 2008). However, the downside for these methods is the presence of H 2 S as impurities. The purity for the carbon captures with these methods ranging from 95 to 99% (Intergovernmental Panel on Climate Change 2015; Kather 2009).
In contrast to pre-combustion, post-combustion methods refer to the process of carbon production after the fuel converted into energy where it can be divided into three types  . 3 Possibility on the maximum level of impurities that could be obtained from CO 2 captured processes (Murugan et al. 2020) depending on the type of air reacting with the fuel. In these methods, the fuel is combusted with a gas without the refining process (reforming or gasifying). For the fuel combusted with normal air, it is known as normal post-combustion methods where lots of impurities such as N 2 , O 2 , SO x and many other is expected (De Visser et al. 2008). If the normal air is change to pure oxygen for combustion, it is known as oxy-fuel methods. With the absence of N 2 , this can greatly reduce the consumption of fuel needed for combustion. The impurities present in this method is quite similar to postcombustion with the addition of other nitrogen-based gasses such as nitrogen monoxide, nitrogen dioxide and carbon monoxide (The Global CCS Institute 2012). Lasty, instead of fuel reacting with air or oxygen gas, the methane is reacted with water where this is known as steam methane reforming. From previous reports and studies, the range of impurities produce from each of this process is listed in Table 2. In many of the CO 2 capture method studied, it is possible to increase the purity of the CO 2 produced. CO 2 transport and storage systems are likely to have a trade-off between improving CO 2 purity and creating a system that is able to manage some impurity in the stream. Overall, this manuscript should significantly contribute to the body of knowledge that CCUS project developers can use to decide the optimal techniques to manage CO 2 impurities within CCUS systems.

Impurities requirement in CCUS
This section discussed the recommended limit and impurities effects towards CCUS projects based on standards and best practice all around the world. As discussed previously, these impurities need to be removed from the stream as it can cause problems not only to the CCUS system but also towards the environment. The quality of captured carbon is crucial in determining the CCUS process efficiently. These impurities can be classified into two components which is condensable and non-condensable components. Where in non-condensable impurities, can be broken down further to three class which is major impurities (more than 0.5%), minor impurities (usually in ppm level but less than 0.5%) and micro impurities (particulate matters) with different composition percentage (Fig. 4). Other than the H 2 O, CO, Nitrogen (N 2 ), Oxygen (O 2 ), Argon (Ar), Sulphur Oxides (SO x ) and Hydrogen Sulphide (H 2 S), there are other impurities that possibly presented in the stream, but the composition is considered insignificant.

Water (H 2 O)
Water is known to be the side product for combustion (Anheden et al. 2005;De Visser et al. 2008;Det Norske Veritas 2010;Forbes et al. 2008;SNC-Lavalin Inc 2004). This compound is harmless on its own, but it can cause problem to the system when it reacts with other impurities present in the stream. The presence of large amounts of water in CO 2 streams is the most difficult problem to manage which creates many problems in transportation (pipelines) and storage (injection). The water level should be kept as low as possible to avoid excessive and stress corrosion as well as hydration production (Neele et al. 2017). For example, when H 2 O reacts with CO 2 itself, it will produce carbonic acid (H 2 CO 3 ), chemical equation is shown in (1), which is a type of dibasic acid which easily decomposed at certain temperature and pressure (Forbes et al. 2008;Wang et al. 2016).
Other than that, H 2 O also can react with H 2 S to produce a very corrosive acid called sulphuric acid (H 2 SO 4 ), chemical equation as in (2)

Carbon monoxide (CO)
Carbon monoxide is a colourless and tasteless gas that is produced when carbon in fuel is not completely burned (Wilbur et al. 2012). However, the percentage for this compound can be very low percentage. Excessive CO composition in a stream may affect sequestration and utilization process. For sequestration, due to the nature of this gas to be classified as GHG, the concern revolves around storage leaks and exposure to the atmosphere. In addition to that, CO have the ability to increase the minimum miscibility pressure (MMP) that are used in utilisation process by enhanced oil recovery. If the compound present in the stream is more than 5%, it may negatively affect the EOR overall performance (SNC-Lavalin Inc 2004). From previous study, the acceptable CO range that is deemed safe and acceptable for feasible project is between 900 and 5000 µmol/mol (Harkin et al. 2017).

Nitrogen (N 2 ), oxygen (O 2 ), and argon (Ar)
Nitrogen, oxygen, and argon are known as the air gasses because these are the main composition of dry air. Even though these gases are known as light gas, it still can pose some problems in CCUS process especially in storage and utilization segments. For example, these light gasses have the ability to lower the stream density in which can limit the CO 2 storage capacity (International Organisation of Standardisation 2020; SNC-Lavalin Inc 2004). Other than that, the presence of O 2 may present some threat to the storage integrity as it may dissolve caprock (Pearce et al. 2016). However, in order it to pose significant threat to the storage structure, large quantity of oxygen is needed thus, to prevent such nature, 10 µmol/mol of oxygen is often used to be the maximum allowable limit in the stream (Wang 2015). In term of utilization segment, likewise with carbon monoxide, the presence of these gasses also can boost the minimum miscibility pressure (MMP) thus can affect the EOR performance (SNC-Lavalin Inc 2004). Other than that, the presence of oxygen may pose as fire hazard and also can increased bacteria growth in the field (Forbes et al. 2008;Shirley and Myles 2019;White et al. 2009).

Sulphur oxides (SO x ) and hydrogen sulphide (H 2 S)
SO x and H 2 S are colourless gases that have a bad odor (Bai and Bai 2019). These gasses have the ability to cause damage in the CCUS system especially to the production equipment and pipelines by either turning sour on its own or by reacting with water to produce corrosive sulphuric acid (H 2 SO 4 ) (De Visser et al. 2008; Intergovernmental Panel on Climate Change 2015; Schwartz 1989; SNC-Lavalin Inc 2004). Usually, when fossil fuel or coal is combusted, it will produce a high concentration of these acidic gas which if it is release to the atmosphere, it can cause acid rain (Mohajan 2019). Thus, a strong and stable storage are needed to prevent leak, as it will release toxic gases to the atmosphere and endanger everything in the surrounding (Intergovernmental Panel on Climate Change 2015; SNC-Lavalin Inc 2004).
Other than that, one study found that the presence of H 2 S in the stream can cause pore blockage at the storage site, this is unfavourable as it may limit the efficiency of the storage (Wang 2015). Due to the dangerous consequences that these gasses may cause, the standards or common practice for the maximum allowable limit for H 2 S is very low when compared to other impurities at 50 µmol/mol (Forbes et al. 2008).

Impurities effects on CCUS
An important part of the CCUS safety analysis is the determination of the impurities impact towards CCUS system. All of the impurities in the system does not give the same effects toward the system where some have a more substantial effect compared to the others. For example, non-condensable impurities could alter the stream thermodynamic and other properties by raising the saturation pressure while reducing the critical temperature of CO 2 (International Organisation of Standardisation 2020; Peletiri et al. 2017;SNC-Lavalin Inc 2004;Wetenhall et al. 2014). This, in turn, can alter the behaviour of the stream and effecting the system during transportation and storage. When the stream is transported though pipelines, if the stream properties is not favourable, it needed higher pressure is needed to ensure that the flow is in single phase flow. During storage, similar problem can be observed where higher injection pressure is needed due to pore blockage, hence limiting the storage capacity (Wang 2015).
There are numerous ways that impurities could affect the CCUS operations, from design and operation of pipelines to geological and storage possibilities. Since pure CO 2 behaves in a different way when compared to normal capture stream, this is the primary reason that need to be address and understand. Impurities can have physical and chemical effects on CCUS where both have the potential to impair CCUS system from working feasibly. The effect of CO 2 stream with impurities on CCUS system can be divided into two namely, physical and chemical effects. Briefly, physical effect is due to the variation of density and viscosity however, chemical effect is due to the compound reactivity with reservoir rocks (Nicot et al. 2013). The next section will discuss in detail on the physical and chemical effect of impurities in CO 2 stream.

Physical effects of impurities
Physical impacts of impurities refer to the variations in the phase behaviour and density of pure CO 2 . The density of CO 2 is affected by the presence of non-condensable impurities such as O 2 , N 2 and Argon, which cannot be liquefied at ambient temperature. These non-condensable impurities, which do not compress to the same degree as pure CO 2 , may also result in a loss in system capacity when pure CO 2 is replaced. Table 3 shows the typical CO 2 stream composition in CCUS system based on studies and handbook by previous researchers (Rumble et al. 2021;Wang 2015).

Effect on phase behaviour
From previous literature, it is found that even the slightest impurities have the ability to change the phase behaviour of CO 2 stream (Luna-ortiz 2021; Luna-ortiz et al. 2021). To ensure a stable and steady supply of CO 2 stream in pipeline, the operator must ensure that stream flow always be in a single phase in order to decrease energy usage and investment costs while also ensuring operational safety (Li 2008). Any change of phase behaviour on pipeline transportation during the supercritical phase may need the operator to increase the supply pressure in order to avoid two-phase flow from developing. This happens because the impurities increase the bubble point of the stream . If this is not monitored properly, two-phase flow condition can happen in the pipelines, and this will hinder the system to work efficiently. This will cause problems towards compressor, pump and during injection process (Wang 2015;Zirrahi et al. 2010). Furthermore, CO 2 stream that contains impurities can negatively impact the stream properties in term of pressure, temperature, and composition. The permeation and buoyancy of a CO 2 plume are affected by the permeation and viscosity of a mixture's physical qualities.
Due to their low critical temperatures, non-condensable impurities especially H 2 , N 2 , O 2 and Ar can increase bubble point and vapour liquid saturation pressure while lower critical temperature (Al-siyabi 2013; Peletiri et al. 2017;Wang 2015). Vapor-liquid equilibrium (VLE) phase studies are commonly used to better understand the phase behaviour of binary systems and multicomponent mixtures. However, the relation is non-linear as it moves closer to the VLE line due to phase change ).

Effect on storage capacity
Previous studies discovered that inert impurities can directly affect the structural trapping capacity by replacement of CO 2 and also reducing the density of the stream (Wang et al. 2012). This density reduction causes the stream to be less compressible compared to pure CO 2 stream, thus reduce the efficiency of the storage (Wang 2015). It is also noted that O 2 , Ar, and N 2 give higher density reduction largely related to higher volume compared to H 2 in the supercritical stream (Wang 2015). This effects, however, is highly dependent on the pressure and temperature of the well. Study by IEA Greenhouse Gas R&D Programme (IEAGHG) found that the storage capacity can reduce up to 40% in 15% of noncondensable impurities present in the stream at shallow reservoir, however, at deeper reservoir of more than 3800 m, the storage capacity is approximate to the pure CO 2 streams (Wang et al. 2012;Wang 2015). Similar findings were found by another group of researchers and the results is shown by Fig. 5. Due to these reasons, it is unfeasible to store CO 2 stream with impurities at shallow reservoir (Neele et al. 2017). In order to quantify the storage capacity of reservoir, (3) is used. This is the ratio of mass CO 2 per unit volume in the stream with impurities to the pure state of CO 2 .
(3) where M and M 0 refer to the mass of CO 2 with impurities and mass of CO 2 pure, respectively. While ρ and ρ 0 represent density of CO 2 with impurities and density of pure CO 2 . Lastly, m i /m CO2 represent the mass ratio between CO 2 with impurities and pure CO 2 stream. The amount of CO 2 held per unit volume of storage space reduces when impurities are present in the CO 2 stream (Wang 2015). Capacity is a key factor in determining the total cost of CCUS. The cost of injection wells and the amount of CO 2 that can be permanently stored are currently considered in cost estimating methods. Reduced CO 2 storage capacity due to impurities may cause a drop in storage capacity earlier than expected, increasing storage costs later. Furthermore, N 2 , O 2 , Ar, CH 4 , H 2 S, decreased CO 2 storage capacity for solubility trapping mechanism, while SO 2 would enhance it (Kim and Song 2017; Ziabakhsh-ganji and Kooi 2014).

Effect on injectivity
The ability of a geological formation to take CO 2 injection fluids can be characterised as injectivity (Md Yusof et al. 2021). As mentioned before, the non-condensable impurities have the ability to decrease stream density, this will in turn cause mass flux to drop over the same pressure drop (Yusof et al. 2022). However, as viscosity decrease, it will increase the mass flux which eventually effecting injectivity (Wang 2015). As density and viscosity are dependent on pressure and temperature, these effects are less likely to cause problem at deeper reservoir storage (Nicot et al. 2013). Studies found that a substantial amount of non-condensable impurities can reduce injectivity by 15% at shallow or low-pressure reservoir, however, at deeper reservoir with higher pressure of more than 20 MPa, the injectivity is almost similar to pure CO 2 stream (Wang 2015). The Darcy's law of permeation flux can be used to examine effects in a single-phase flow. A normalized permeation flux formula is shown in (4).
where M CO2 and M 0 are the mass flow per unit area for CO 2 with impurities and pure CO 2 stream respectively, while ρ and ρ 0 are the densities of the injected stream and pure CO 2 ; μ and μ 0 are the viscosities of the injected fluid and pure CO 2 .
The pressure and temperature of the system will have the greatest impact on the viscosity and density of the fluid. There have been a number of hypothetical simulations done to investigate the impact of injectivity on storage capacity, and the results suggest that compensation between density and viscosity has a significantly smaller impact on injectivity than previously thought (Wang 2015).

Effect on buoyancy
As impurities can alter the density and velocity of the stream, it will indirectly increase the buoyancy of the CO 2 plume in the reservoir. Previous study discovered that high level of impurities may increase the buoyancy by 50% depending on pressure and temperature. This, in turn, can also increase the velocity of the stream by three-fold (Wang 2015). Depending on the heterogeneity of reservoir, the rising velocity of the injected CO 2 plume could reduce residual trapping and increase lateral spreading of the plume at the caprock. Other than that, increase in buoyant force may make the CO 2 with impurities plume to be less broad when compared with pure CO 2 stream (Nicot et al. 2013). The buoyancy of a normalized CO 2 plume can be calculated by (5) (Wang 2015).
where F and F 0 are buoyancy forces for the CO 2 mixture and pure CO 2 , ρ H2O , ρm and ρ CO2 are the densities of the formation water, plume, and pure CO 2 respectively.

Chemical effects of impurities
Most significant component that gives higher chemical effect is the condensable impurities (SO x , NO x , and H 2 S). In contrast to the physical impacts of impurities, the chemical (4)

Fig. 5
Mixture density relative to the depth (Wang et al. 2012) reactions from impurities take some time to occur and require long-term monitoring.

Effect on injectivity, caprock and reservoir capacity
The presence of SO x with water can produce sulphuric acid which is a very acidic acid. This can lower pH and cause mineral precipitation of sulphate and dissolution of both carbonate with aluminosilicate rock in the reservoir (Wang et al. 2011;Wang 2015). This usually happen after injectivity process finished, the stream with impurities will migrate towards the caprock and reaction may occur resulting in mineral dissolution which is shown in Fig. 6. Previous study showed that with the presence of only 1.5% of SO x in the stream, it can increase the dissolution rate up by 50% (Wang 2015). This caprock dissolution can negatively affect the caprock integrity and increasing the chance of leak to happen. Rapid dissolution and precipitation of minerals can alter the initial reservoir rock characteristics and this can block some pore and effecting the porosity which eventually can affect the reservoir capacity and injectivity (Bacon et al. 2009;Labus and Suchodolska 2017;Wang 2015). The problem can be worsened when H 2 S present, this usually happen when both pre and post combustion source injected at the same reservoir. A substantial pore blockage in the formation could be caused by the deposition of sulphur compounds. Other than that, O 2 also can play role in the dissolution of rocks by reacting with pyrite forming iron sulphate which can cause acidic pockets, however, for this to happen it large amount of O 2 is needed to be present in the stream (Wang 2015). On top of that, impurities may alter the wettability of the rock, resulting in the rock requiring various sealing capabilities to hold the CO 2 with impurities (Li et al. 2005).

Effect on surface materials
The return of acidic impurities-containing water can harm well materials both during and after CO 2 injection by corrosion (Wang et al. 2019). Impurities such as SO x and H 2 S can be found in CO 2 injection streams and can exacerbate corrosion and this is a key issue in CCUS system (SNC-Lavalin Inc 2004). This is likely to happen after injection is finished because from thermochemical estimation shows that the acid impurities are more significant to the cement compared to rocks. This is because the chemical properties for SO x is almost identical to the CO 2 (Scholes et al. 2009). This can cause a huge problem if the protective layer of the cement lost, it can affect the steel casing and can cause corrosion. Thus, to avoid this, the project needs to invest higher in improving casing quality and cement (Wang 2015). If the cement sheaths fail to protect them, the steel casings are vulnerable, Fig. 7 depicts corrosion of well components after exposure to the CO 2 stream with impurities. An addition to that, the presence of O 2 also can escalate the corrosion rates of carbon steel in water-saturated CO 2 phase (Choi et al. 2010).

Conclusions
This paper presents a comprehensive review on physical and chemical effects of impurities on the CCUS system. Based on the extensive literature analysis the following conclusions were made: • Global standard-setting organizations have established their own recommendations or best practices for purity and the composition of impurities that are judged appropriate and safe for the environment and CCUS deployment. • In the CCUS development chain, the presence of impurities in captured CO 2 has been considered as being a significant concern. • CO 2 stream with impurities affects the viscosity and density variations result in physical consequences, whereas compound reactivity with reservoir rocks results in chemical impacts. Fig. 6 Example of the dissolution of minerals on reservoir rock over time, post CO 2 injection for storage • The thermophysical characteristics of impurities are shown to have a significant impact on CCUS functioning in term of phase behavior, storage capacity, injectivity and buoyancy of the CO 2 plume in the reservoir. • The chemical impurities knowingly affect the CO 2 storage properties such as corrosion, injectivity failure, and reservoir capacity of the geological site.

Recommendations
• As the knowledge of CCUS technologies becomes better understood, current standard and guidelines recommended to be updated regularly to incorporate new best practices as we learn more about the CCUS technologies as a whole. • Special attention during the early deployment phase is recommended to highlight which areas that require greater investigation, as this is where the potential for future development is most obvious. • Currently, there is limited research on how contaminants affect the geological storage of CO 2 , and several of the theoretical impacts investigated in the study have not yet been supported by actual evidence. • It is crucial that researcher stay informed about new experimental study that might confirm the consequences. The results of this investigation may have greatly contributed to this technology advancement and reap the potential advantages of CCUS technology which demonstrate that safe and permanent storage is feasible.