Worldwide Status of CCUS Technologies and Their Development and Challenges in China

1 INRS-ETE, Universite du Québec, Québec, QC, Canada 2Energie-Forschungszentrum Niedersachsen, Clausthal University of Technology, Goslar, Germany 3State Key Laboratory of Geomechanics and Geotechnical Engineering, Institute of Rock and Soil Mechanics, Chinese Academy of Sciences, Wuhan, China 4Sino-German Energy Research Center, Sichuan University, Chengdu, China 5Institute of Petroleum Engineering, Clausthal University of Technology, Clausthal-Zellerfeld, Germany


Introduction
Fossil fuels, especially coal that is rich in carbon, constitute the highest proportion of primary energy in China [1].In recent years, the rapid urbanization and development of industries including power plants, cement factories, steel plants, biotransformation, and fossil fuel transformation plants, which are highly dependent on large consumption of fossil fuels, have been a great challenge to the Chinese environment [2,3].Since the winter of 2012/2013, most cities in China have been faced with serious atmospheric pollution from a haze formed from a combination of SO 2 , NOx, and inhalable particles within the mist, containing fine particle concentrations of up to ca. 900 g/m 3 [4].Automobile exhausts, industrial emissions, waste incineration, and fugitive dust from construction sites are the main sources of the haze.Based on statistical data from Beijing, reported by China Central Television (CCTV) in 2014, haze particles from automobile exhausts contributed 22.2%, while the burning of coal, dust, and industrial emissions accounted for proportions of 16.7%, 16.3%, and 15.7%, respectively.Therefore, a reduction in the emissions from coal and industry has become the key to improving the quality of the environment.
The increase in the concentration of greenhouse gases has had a large impact on global climate change, since industrialization.Many countries have set targets for reducing the emissions of greenhouse gases in order to mitigate global warming.Among them, top on the list of CO 2 emissions in the world, China aims at reducing 40%-45% of its CO 2 emissions per unit GDP by 2020, based on the 2005 level [5][6][7].This requires considerable changes not only in the framework of fossil fuel consumption, but also in the development of renewable energy from wind, solar, geothermal, and so on, together with an enlargement in the area covered by forests and innovations in technologies that can enable permanent storage of the CO 2 underground.
CO 2 emissions in China come mainly from the combustion of fossil fuels (90%) and during the process of cement manufacturing (10%).For example, in 2012, 68% of the emitted CO 2 was sourced mainly from the combustion of coal, while 13% came from oil and 7% from natural gas [8].According to the statistics, annual emissions of CO 2 from large stationary point sources, that is, >0.1 Mt/year, amount to 3.89 GtCO 2 , which accounts for 67% of the total emissions.Among which, 72% is from power stations [9].This demonstrates that a reduction of the CO 2 emissions from the large stationary point sources is the key to realizing China's target [10,11].
China's main target for the transformation in its energy framework is to reduce the combustion of coal, while increasing the supply of natural gas and other clean energy, and controlling the emissions of CO 2 , SO 2 , NOx, and so on.CO 2 capture and sequestration (CCS) and utilization (CCUS) technologies can be applied to store CO 2 underground effectively, thus reducing its emission into the atmosphere.This technology is now highly developed and is likely to play a significant role in China, especially when the operation costs are reduced.This paper reviews the state of the art of CCS and CCUS technologies worldwide while paying more attention on its status and development in China.The mature technology will be examined in various engineering projects.Therefore, this paper considers the state of operation of CCS and CCUS projects in detail and concludes by presenting the likely challenges to be experienced through the industrialization of these projects in China.Due to space limitation, it has not been possible to include a review of the current research status on the conversion of CO 2 to produce some commercial products or its use in the food industry, for example, as an additive in beverages or as a preservative for fruits and vegetables.Henceforth, only its utilization for geologic and geoengineering purposes such as EOR, ECBM, ESG, and EGR has been considered in this paper.
In the short term, depending on the purpose of the CCS project, CO 2 can be stored in different geological sites, including deep saline formations, depleted oil or gas reservoirs, deep unmineable coal seams, and shale formations, to reduce the CO 2 emissions [20,21], Figure 1.In comparison with the pure CCS technology, CCUS technology pays more attention to utilization (U) of the captured CO 2 while sequestration (S) plays a secondary role.CCUS can reduce the cost of sequestration and bring benefits by enhancing the production of hydrocarbons or heat energy, thus becoming very popular in recent years.Based on the purpose of the CO 2 injection, a number of related technologies have been developed including (1) Enhanced Oil Recovery (EOR), (2) Enhanced Coalbed Methane Recovery (ECBM), (3) Enhanced Gas Recovery (EGR), (4) Enhanced Shale Gas Recovery (ESG), and (5) Enhanced Geothermal System (EGS).
The engineering projects for both CCS and CCUS technologies are systematically complicated, with their success depending on rigorous research in engineering and science disciplines including geology, geoengineering, geophysics, environmental engineering, mathematics, and computer sciences.In addition, key to success in site selection for any such a project demands strict considerations of safety, economy, environment, and public acceptance at all levels of operation, that is, countrywide, basin-wide, regional, or subbasin levels [22][23][24][25][26], Figure 2.Although CCS and CCUS technologies share similarities in site selection, each will induce a series of different physical and chemical responses in the underground porous or fractured rock formations, in terms of the existing local hydrological (H), thermal (T), mechanical (M), and chemical (C) fields [27][28][29], Figure 2. Coupling of the THMC processes during and after CO 2 injection related to CCS and CCUS technologies has become a research hotspot in recent years [26,[30][31][32][33].The two technologies, however, have minor differences, in terms of purpose, storage duration, injection depth and rate, fluid and reservoir types, scheme of drilling, completion and monitoring, and so on.

CCS.
CCS is a viable option for significantly reducing CO 2 emissions from large-scale emission sources.When its only purpose is for CO 2 sequestration, the storage sites may include deep saline formations, deep unmineable coal seam, depleted oil or gas reservoir, and rock salt caverns [35][36][37][38].This technology is mature but still very expensive for widespread commercial application.

CCUS: CO 2 -EOR.
The first CO 2 -EOR field test was held in 1964 in Mead Strawn Texas, in the USA.Since the 1970s, CO 2 has been used on a commercial scale for oil production projects [20,21].Up to the present time, there have been more than 100 CO 2 -EOR projects in operation.Among them, the CO 2 -EOR project in Weyburn, Canada, is the most successful example.It uses mixed gases separated from natural gas production, coal gasification, and coal power from the Great Plains Synfuels Plant near Beulah, North Dakota, USA [39].The injection gas is mainly composed of CO 2 (96.8%), plus H 2 S (1.1%) and a minor amount of hydrocarbons that are piped to the Weyburn Basin through a pipeline 339 km in length [7].The purpose of the project is to inject 2 million tons of CO 2 into the depleting oil reservoir over a 20-year period, in order to increase oil recovery to 130 million barrels and to extend the production of oil in this oilfield to 25 years [40].

CCUS: CO 2 -ECBM.
The conventional method to produce coalbed methane is to decrease the pressure in the coalbed reservoir, making the methane desorb from the matrix.However, the recovery of coalbed methane production using this method is less than 50%.The alternative is to desorb more CH 4 from the coalbed matrix by injecting gases including CO 2 or N 2 [41][42][43][44].Studies on enhancing coalbed methane by CO 2 injection started in the 1990s [7,45].When CO 2 is injected in the coalbed layer, both the gaseous and adsorbed-state of CH 4 and CO 2 will exist in equilibrium [46].Because the coalbed has a much stronger adsorption capacity for CO 2 than CH 4 , the injection of CO 2 will make the adsorbed CH 4 desorb, thus enhancing the CH 4 recovery.
A proportion of the injected CO 2 will be stored in the coalbed formation, making it difficult for it to leak to the surface.Therefore, this technology can bring both economic benefits and also guarantee the safe storage of CO 2 [47,48].The successful injection of CO 2 to enhance coalbed methane recovery has been proved by many experimental and numerical studies.However, the production efficiency is strongly site-dependent, in relation to the permeability of the coalbed matrix, production history, gas transportation process, maturation of coal, geological configuration, completion scheme, hydraulic pressure, and so on [42][43][44][49][50][51][52].Nevertheless, the maturity of its commercial application is still very low.Pilot-scale CO 2 -ECBM projects so far include those in Alberta, Canada, which started in 1997, the Burlington project in the San Juan Basin of the USA, the RECOPOL project that started in 2001, the Yubari project in Japan, and the Qinshui basin project in China that started in 2002 [53].

CCUS: CO 2 -EGR.
Studies on injecting CO 2 into depleted gas reservoirs to enhance gas recovery started in the 1990s [54].Unlike the CO 2 -EOR technology, CO 2 -EGR technology is still at the pilot-scale stage.Its efficiency is highly dependent on reservoir type, temperature and pressure conditions, heterogeneity, production strategy, and so on [55][56][57][58][59][60].For some CO 2 -EGR projects, the gas recovered can reach 10%, while other projects have seen less or no enhancement [61][62][63].The rapid breakthrough of CO 2 in a production well, resulting in a high concentration of CO 2 , restricts the production of pure natural gas [64].Since 1999, the USA has carried out a pilot project of CO 2 -EGR in Rio Vista.The Netherlands injected 60 kilotonnes of CO 2 into a depleted gas reservoir in the K12B project during 2004 and 2009 [7].The CLEAN project in Germany started a CO 2 -EGR project in the Altmark gas fields in 2009; however, public protests have prevented CO 2 injection on the site [65].Many other countries including Australia and Norway are also positively developing this technology [64,[66][67][68][69][70][71][72][73][74].

CCUS: CO 2 -ESG.
The USA has been carrying out shale gas desorption since 1821.However, limited development of the technology made this process procedurally cumbersome and substantively difficult to apply before the 21st century.In 2000, shale gas contributed only 1% of the whole natural gas supply, while, by the end of 2011, this proportion had increased to 30% due to a breakthrough in horizontal drilling and horizontal multistaged fracturing technology.The revolution of shale gas in the USA is changing the energy structure of the world [75].
Encouraged by the successful application of CO 2 in oil and gas recovery, its application in aiding the production of shale gas began in recent years [76][77][78][79][80][81].There has also been progress in replacing water by supercritical CO 2 as the injection fluid in the fracturing technology [82][83][84][85][86].However, this process is still in the very early exploration stages.

CCUS: CO 2 -EGS.
The first study of EGS technology started in Fenton Hill, USA, in 1970 [87].Since then, many other countries, including France, Germany, Austria, Italy, Japan, and Australia, have paid attention to the development of this technology.The conventional EGS technology uses water as the injection fluid and circulation media.Based on the research in [88], CO 2 is now regarded as a more favorable circulation fluid compared with water because of its large compressibility and expansibility.This idea has already been supported by many studies (e.g., [89][90][91][92][93]).
The application of CO 2 in a geothermal system is not restricted to the hot dry rock reservoirs but also includes the conventional hydrothermal reservoirs [38,91,94].The injection of CO 2 can enhance the efficiency of reinjecting the hot wastewater by improving the porosity and permeability through the activated water-rock geochemical reactions [95].Besides being the main circulation fluid, CO 2 can also be regarded as a pressurized hydraulic fluid in the reservoir.Injection of CO 2 in a hydrothermal or hot dry rock reservoir can maintain the reservoir pressure, promoting the flow rate of the in situ water towards the production well, thus enhancing the heat recovery and even the recovery of the CH 4 dissolved in the aquifer water [96][97][98][99].Reference [38] described this process as the CO 2 -AGES (CO 2 -aided geothermal extraction system) in which three stages are involved: (1) the production of hot water when CO 2 is used as the pressurized hydraulic fluid; (2) two-phase fluid flow in the production well after the CO 2 breakthrough; and (3) and as a circulation fluid, when CO 2 fills the production well, which is similar to CO 2 -EGS.

CCS and CCUS Engineering
Projects Worldwide There are still no concrete CO 2 -ESG and CO 2 -EGS projects anywhere in the world.Only a few countries, including the USA, Canada, China, and Argentina, can commercially produce shale gas.At the end of 2015, the daily shale gas output in the USA, Canada, China, and Argentina had reached 37, 4.1, 0.5, and 0.07 Bcf, respectively [100,101].Shale gas production in the USA abruptly increased after 2000, while Canada and China successfully produced shale gas for the first time in 2008 and 2012, respectively.There are now more than 100,000 shale gas drilling wells in the USA.In China, however, only about 600 wells have been drilled in the last few years [102].The EGS technology is still at the research and development stage.Nevertheless, there are some experimental EGS plants and pilot projects, for example, at Fenton Hill, Coso, and Desert Peak in the USA, Bad Urach, Neustadt-Glewe, Bruchsal, Landau, and Unterhaching in Germany, and Soultz-sous-Forets and Bouillante in France [87].Substantially higher research, development, and demonstration efforts are needed to ensure EGS technology becomes commercially viable in the near future.1), the distribution of funding for different aspects of CCS and CCUS is shown as follows: (1) CCS (32 projects), of which all the 7 projects funded by the MOST were related to CO 2 capture technology.Several international cooperation research projects were also developed, including NZEC between China and Europe, CAGS between China and Australia, and CCERC between China and the USA; see Table 2 for further details.Based on the well test data for coalbed methane production in China, the recovery is in the range of 8.9%-74.5%,with an average value of 35%.By using CO 2 -ECBM technology, the recovery can be increased to 59% [152].Based on the preliminary evaluation of [153], the recoverable coalbed methane can increase to 1.632 × 10 12 m 3 with CO 2 storage amount of about 120.78 × 10 8 tonnes for the coalbed at a depth ranging from 300 to 1500 m.

CCUS: CO 2 -EGR in China.
According to the results from the third oil and gas reserve investigation, if 75% of the porous volume derived from gas production is used for CO 2 sequestration, there will be a potential for a CO 2 storage capacity of 5.18 billion-tons [9,154].However, the gas industry in China started late and gas production is low, which means that there will not be many depleted gas reservoirs in the short term, limiting the possibility of a commercial scale application of the CO 2 -EGR technology.From the maturity point of view of this technology, very few research institutes in China are working on the improvement of CO 2 -EGR at the present.Furthermore, the early breakthrough of CO 2 in gas production wells makes it difficult to attain good production efficiency from the application of CO 2 -EGR technology [47].
A means of reducing the costs of separating the mixed gases, CO 2 and CH 4, is required to attain the widespread application of the CO 2 -EGR technology in China.In December 2010, China drilled its first shale gas exploration well, Wei201 in Weiyuan gas field [155].In May 2012, the first shale gas horizontal well in China was drilled and operated by Yangchang oilfield, demonstrating a great breakthrough in the hydraulic fracturing technology for shale gas reservoirs.By the end of 2012, China's total shale gas production was 2.5 × 10 7 m 3 , which increased to 2.0 × 10 8 m 3 in 2013, 1.3 × 10 9 m 3 in 2014, and 4.47 × 10 9 m 3 in 2015.The production of shale gas in China has increased greatly during the last few years, especially from the Peiling shale gas field in Chongqing with a proved reserve of more than 1.0 × 10 11 m 3 .It has produced shale gas of about 1.03 × 10 9 m 3 , becoming the largest commercial shale gas field in China.

CCUS: CO 2 -ESG in
However, high production costs, a large amount of water consumption and a breakthrough in some key technologies related to shale gas production will restrict large-scale production in the near future [102].In 2012, the National Energy Administration of China set a target for shale gas production of 6 × 10 10 -1.0 × 10 11 m 3 by 2020.But after a two years' practical experience during 2012-2013, it revised this target to 3.0 × 10 10 m 3 by 2020.Using CO 2 to enhance the recovery of shale gas is now at an early exploration stage [156].

CCUS: CO 2 -EGS in China.
The 863 plan project that aims at investigating EGS was initiated by Jilin University in 2012 [157].There are now several other projects in the country using CO 2 in geothermal production (see Table 1).This demonstrates that China is interested in developing EGS to exploit the deep geothermal resources from the hot dry rocks.Many Chinese researchers (e.g., [143,[158][159][160][161][162]) have already studied the operation mechanisms of the CO 2 -EGS system and its optimization designs.A preliminary site selection system considering the role of CO 2 in the geothermal production was set up by [26].Research in this technology is still at the very early stage and requires detailed work to attain pilot scheme status.

Status of CCUS Engineering Projects in China
The CO 2 emission sources are mainly located in the middleeastern regions of China; see details in Figure 2.15, [34].Therefore, pilot-scale CCUS (mostly CO 2 -EOR) engineering projects in China are also located in these regions (Figure 7, Table 3).Based on published government and industrial reports and personal communications, the progress of pilotscale CCUS engineering projects in China is as follows: (1) A CO The Changling gas field was the first project to integrate natural gas production, CO 2 sequestration, and EOR technology [7].As the conventional water injection method does not provide good production efficiency in low permeable oilfields, CO 2 -EOR has played a large role in increasing production, such as in the Fuyang oilfield [137].By March 2017, oil production increased to 100 kilotons by injecting 1.1 million tons of CO 2 underground.
(3) A full chain pilot-scale CO 2 -EOR project has been injecting CO 2 at a rate of 40,000 tons/year in the Shengli oilfield (still in operation).The Sinopec Shengli oilfield cooperated with the Shengli power plant to install the largest equipment for capturing exhaust gases in a coal power plant [163].Its purpose is to reduce CO 2 emission by 30 kilotons/year and enhance oil recovery by 20.5%.This project started in 2008 and about 251 kilotons of CO 2 had already been injected in the ultralow permeable oil reservoir through 11 injection wells by April 2015.
(4) A CO 2 -EOR project operated by Zhongyuan oilfield (still in operation) injected CO 2 at a rate of 30,000 tons/year and managed to increase oil production by 3600 tons after injection of 2170 kilotons of CO 2 and 827 kilotons of water [7].By February 2017, a total amount of about 553 kilotons of CO 2 was injected underground.As a result, oil recovery is proved to have enhanced by 10% in the Zhongyuan oilfield and by 60% in the Shayixia oilfield after the pilot-scale test.
(5) The CO 2 -EOR project led by the Yangchang oilfield company was carried out in 2013 using captured CO 2 during the production of methanol and acetic acid.At present, the capture equipment designed for 360 kilotons/year of CO 2 is under construction.Pilotscale CO 2 -EOR field tests have been done in some    3) superimposed on the provincial CO 2 emission map for the year 2010 (from [34]).districts of Jinbian and Wuqi, with a total of 90 kilotons CO 2 injected.(6)  (9) A CO 2 storage project in the rock salt at Yingcheng in Hubei Province, where CO 2 will be captured by the oxy-fuel combustion technology, is in preparation [166].
(10) CO 2 sequestration by microbe algae has also been identified an effective means to reduce CO 2 concentration in the atmosphere.The two representative CO 2 sequestration projects using microbe algae are the Xin'ao and Qinghua groups both from China.
In the next few years, CO 2 -EOR engineering projects will still be the most important CCUS technology in application.After the successful experience attained from the pilot-scale CCUS projects so far, China is now planning to run 13 large-scale CCUS projects.Based on the stages of the engineering projects, the project will be divided into the following study phases: opportunity → preliminary → prefeasibility → feasibility → construction drawing design → construction → operation → completed.All the stages before the construction drawing design phase, that is, preparation of the engineering projects, could be lumped together and called the "evaluation" stage.Due to the current low oil price and a lack of the motivation policy, the progress in developing most of these large-scale planning CCUS projects lags far behind the schedule.Most of these projects are still at prefeasibility or feasibility stages and some may even be cancelled.
Although capturing and industrial utilization of CO 2 in China are not the key aims of this paper, the related projects in operation include (1)  At present, China does not execute any CO 2 -EGS field tests.However, a few engineering EGS projects exist at their early scientific field test stages.These include (1) the hot dry rock scientific drilling project in Zhangzhou Fujian province, in operation since May of 2015, with a drilling depth of 4000 m and a water temperature high enough for geothermal power generation and (2) a hot dry rock scientific drilling project in Qinghai Province, with a water temperature of 200 ∘ C at a depth of 3000 m [157].Studies on power generation in traditional hydrothermal fields located in Yangyi, Xizang, and Tengchong in Yunnan Province are also undergoing.However, there are no active engineering projects related to CO 2 -EGR and CO 2 -ESG in China.

Challenges in the Widespread Application of CCUS in China
6.1.Tackling Problems in Key Technologies.The injection of CO 2 underground for the CCS and CCUS purposes involves multiple physical-chemical coupling interactions of multiple components in porous fractured media, especially the transmission and migration of fluids between porous media with a low/ultralow permeability and complex fractured network.
(a) There are mature commercial CO 2 -EOR technologies in the USA and Canada.In China, however, because of the strong heterogeneity in oil reservoirs, the CO 2 channeling effect is serious [138].Therefore, improving the sweep efficiency is the key to attaining widespread application of CO 2 -EOR in China.Other efficient methods include the alternating injection of water and CO 2 (WAG) and the addition of foaming and gelling agent [132].
(b) There are currently no commercial scale CO 2 -ECBM engineering projects being developed anywhere in the world.In China, studies on CO 2 -ECBM technology are at a very early stage of exploration.More research is required to tackle key problems like the adsorptiondesorption process between CH 4 and CO 2 in the coal seam [46,146,147], the mechanisms of the interaction between CO 2 -CH 4 -H 2 O at molecular scale [150], the impact of the coal grade, water content and composition of coal, and so on on the diffusion and migration of mixed gases in the coal seam, the dynamic changes of phase behaviour during the process of CO 2 injection, and CH 4 production and so on.
(c) In the application of the CO 2 -EGR technology, more effort is required to prevent the early breakthrough of CO 2 into the production well, thus enhancing the sweep efficiency of CO 2 .Thus more studies are needed like the understanding of migration processes of the CO 2 after its injection into the depleted gas reservoir, phase behaviour, the mixing mechanism of CO 2 and CH 4 , and so on [48,60].
(d) Multistage hydraulic fracturing in the horizontal wells has been widely used in shale gas production in China.However, this technology is still not mature enough for the production of shale gas at depths >3500 m.The large amount of water consumed in the production of shale gas is a big challenge for its large-scale production, especially in southwestern China, where the existing water resources are very poor.Using CO 2 as the fracturing fluid has become a research hot spot in China [167].Injection of CO 2 to extract brine or methane energy from the aquifers was also studied recently [168].While the feasibility of using CO 2 to enhance shale gas recovery still requires more research and field tests.
(e) The direct use of geothermal energy in China has been the priority during the last few years, while its use for power generation largely lags behind that of several countries, such as the USA, the Philippines, Japan, and Indonesia.Technologies including CO 2 -AGES, EGS, and binary cycle power plants may have a positive effect on the development of China's geothermal power system.However, before obtaining mature engineering experiences, China needs to enlarge its investment in human, physical, and financial resources in these technologies.
6.2.Negative Impacts on the Environment and Resources.The risk of leakage of the injected CO 2 in the injection/production wells may have a serious environmental impact [169][170][171][172][173].
The groundwater quality may deteriorate if the CO 2 in the injection layer leaks into the freshwater aquifer through microfractures or faults [174,175].When hydraulic fracturing is applied to shale gas or geothermal energy production, it will induce microseismic events.In addition, the toxic chemical additives in the hydraulic fluid may have a serious negative impact on freshwater aquifers when they leak into the shallow layers because of possible geological hazard.Therefore, a long-term environmental monitoring activity should be carried in parallel with the CCUS engineering project to ensure its safety [104].The dynamic migration process of CO 2 , chemical interaction among CO 2 -reservoir fluid-rock, the deformation or eruption of injection/overlying caprocks, and temperature and pressure changes in the reservoir should be monitored for a long time after the injection [29,176].The drilling cost of a geothermal production well in a hot dry rock will be much higher.The corrosion property of CO 2 requires a high quality of pipelines and ground equipment, increasing the production costs of oil, gas, and geothermal energy [182,183].

Storage Capacity
6.6.Energy Price.The slump in the international oil price has greatly affected the investment in the oil/gas production and CCUS projects.Shale gas production in Peiling shale gas field in southwestern China with good geological conditions and large reserves is just above the breakeven point.If the oil/gas price remains low in the future, many industries will be unwilling to invest in these kinds of projects.With the exception of CO 2 -EOR, it is difficult to profit from other CCUS projects.Due to completion from the increased installation capacity of wind and solar energy that have been much easier to make an economic return in recent years, the development in geothermal power generation will be continuously limited because of the difficulty in returning an economic benefit.

Social
Acceptance.This is the biggest challenge for any CCS or CCUS project.It has a substantial impact on political decision makers and the implementation of energy projects such as nuclear power and wind energy programs [184].It is the same for CCS and CCUS projects, and some CCS exploration activities in Schleswig-Holstein and Vattenfall Janschwalde in Germany, the Belchatow project in Poland, and so on were postponed or cancelled because of the lack of public acceptance over the exploration of storage sites [185,186].As the most unfamiliar technology to the general public in China, CCUS technology has been reluctantly accepted when compared with other low carbon technologies including wind power, solar power, energy efficiency, or biomass for reasons of climate change mitigation [10,187].However, there is now a positive attitude towards CCUS policies in China.In order to stimulate public acceptance, the uncertainties regarding safety and environmental risks involved in CCUS will have to be reduced at the beginning of the development stage of any CCUS technology [188].
However, this will be largely dependent on the innovation of long-term monitoring techniques in both operating and planned pilot projects [189,190].

Conclusions
( (2) The serious air pollution problems in recent years are forcing the government of China to pay more attention to the development of green and clean energy aimed at saving energy and reducing the emissions of greenhouse gases.Some local governments have increased their investment in modern coal-fired power station coupled with the CCS technology.The CCUS engineering projects, especially those related to EOR, are also developing fast.
(3) Traditional CCS projects can store a large amount of CO 2 , captured from large-scale point source emission sites, deep underground, thus effectively decreasing emissions in the atmosphere.CCUS is more attractive than the CCS technology in China because of the economic benefits accrued by using the CO 2 .China has large reserves of low permeable oil and gas reservoirs.The conventional water injection methods cannot achieve good production efficiency in such reservoirs; therefore the CO 2 -EOR and CO 2 -EGR will have a great potential in enhancing the recovery of oil and natural gas in low and ultralow permeable reservoirs, as well as storing CO 2 in the underground space.The CCUS technology will play a considerable role in controlling the reduction of CO 2 emissions related to coal-fired power stations and the coal chemical industry.For a long period of time, coal will remain the main energy source in China; thus CCUS technology is very important for cleansing the coal-based industry.CO 2 has the potential to be used in the production of geothermal energy because of its favorable physical properties including large density and small viscosity.In addition, studies on replacing water by supercritical CO 2 as the fracturing fluid in the oil/gas/shale gas reservoirs are currently being carried out by many researchers.If this method is proved to be feasible, it will greatly decrease water consumption in the production of shale gas.This is particularly meaningful in the western regions of China where there is lack of groundwater resources.

2 Figure 1 :
Figure 1: Schematic diagram of the CCUS technology in different geological reservoirs for both long and short-term sequestration of CO 2 .

Figure 2 :
Figure 2: Schematics of the two main topics, that is, the site selection system (1) and the THMC responses (2) associated with CCS and CCUS technologies.

Table 1 : 2 - 2 sequestration 2 2 - 2 - 2 NSFC 2014 confirmed 23 CO 2 2 -
The research projects of CCUS funded by China Ministry of Science and Technology (MOST) and National Natural Science Foundation of China (NSFC) and so on during 2005-2016 (amount unit: 10,000 RMB or 1,450 US$).Name of the research projects oil-water-rock multiple surface system of the porous media and their application on the transportation of supercritical CO 2 , water and oil in the low permeable porous media Dalian University of Science and Technology NSFC 2008-2011 180 Studies on microscopic flowing mechanism of supercritical CO 2 , oil and water in the low permeable oil reservoir NSFC 2013-2015 25 Studies on the CO 2 diffusion and mass transfer processes in oil and water bearing porous media NSFC 2013 confirmed 25 Factors affect the CO 2 diffusion in the porous media and its mechanism studies China University of Petroleum NSFC 2009-2011 20 QSAR studies on the thermodynamics and transportation properties of CO 2 EOR and its damage mechanism to the reservoir China University of Geosciences NSFC 2012-2015 59 Studies on the surface properties using alkylol amine capture CO 2 and processes of CO quantification and optimization of injection-production scheme of CO 2 -EOR and CO CO 2 capture from the flue gas of the large-scale coal-fired power station, EOR and storage and pilot projects injection and storage in the deep coal seam and enhanced coalbed methane recovery International 2011-2015 -Test project of deep coalbed methane production technology of China United Coalbed Methane Corp. Ltd.ECBM potential in China and related basic scientific research issues Institute of Rock and Soil Mechanics ECBM potential in China and the suitability evaluation GSC 2011 -Mechanism of using mixtures of CO 2 /N 2 displace coalbed methane in situ geological conditions and the best ratio of gas composition NSFC 2012-2014 26 Impacts of coal matrix on the coal expansion and permeability changes of CO interaction during CO 2 sequestration in the deep coal seam and simulation of the sequestration experiment mechanisms of multiple gases during CO 2 -ECBM process NSFC 2003-2005 Experimental study of coal matrix on expansion effects during CO 2 -ECBM process NSFC 2008-2010 -Two-phase gas and solid coupling effect and dual porosity effect during the CO 2 sequestration in the deep coal seam NSFC 2011-2013 20 THM coupling mechanism of CO 2 -ECBM NSFC 2012-2014 25 Dynamic model of multiphase fluid CH 4 -water flow in porous media of heterogeneous coal seam NSFC 2012 confirmed 25 Fluid-solid coupling response and mechanism of supercritical CO 2 and minerals in the coal NSFC 2013 confirmed 25 Theoretical study of CO2 sequestration in the deep coal seam and the efficiency of CH 4 recovery NSFC 2014 confirmed 300 Interaction of supercritical CO 2 and organic matter in the coal and their responses to the coal structure NSFC 2014 confirmed 25 The construction of 3D model of reservoir structure in the high grade coal and the geochemical response to the injection of CO enhances CH 4 adsorbed by the coal, the permeability characteristics and mechanisms NSFC 2015 confirmed 70 Multiphase gas-liquid-solid coupling mechanisms of CO 2 sequestration in the porous coal media NSFC 2016 confirmed 62 Interaction of supercritical CO 2 and coal during CO 2 sequestration in the deep coal seam and its impact on the CO 2 storage Shandong University of Science and Technology NSFC 2012-2015 60 Transportation mechanisms of supercritical CO 2 injection into the stress partition residual coal pillar and its displacement of CH 4 NSFC 2015 confirmed 20 Basic research on the mechanism of CO 2 -ECMB in the deep low permeable unmineable coal seam under THM coupling effect Liaoning Technical University NSFC 2009-2011 33 Microscopic mechanism of supercritical CO 2 on the recovery of CH 4 in the coal NSFC 2014 confirmed 25 Type 3: CO 2 -EGR Safety production of the CO 2 bearing gas reservoir and the utilization of CO 2 Research Institute of Petroleum Exploration & Development Major Science & Technology 2008-2010 -Pilot project of the production of the CO 2 bearing volcanic gas reservoir and utilization Jilin oilfield etc. CNPC 2008-2010 -CO 2 sequestration mechanism in the depleted gas reservoir and the transportation rules Southwest Petroleum University NSFC 2013 confirmed 80 Phase behavior of supercritical CO 2 displacing CH 4 in the porous media and the seepage characteristics Dalian University of Technology NSFC 2015 confirmed 64 Type 4: CO 2 -ESG Basic research of supercritical carbon dioxide enhanced shale gas development Wuhan University 973 Program 2014-2018 -Basic research of the supercritical CO 2 in the production of unconventional oil and gas reservoirs China University of Petroleum NSFC 2011-2014 258 CO 2 sequestration in the shale gas reservoir and mechanisms of CO 2 -CH 4 -shale interaction Chongqing University NSFC 2013-2015 25 Damage mechanism of using supercritical CO 2 as the hydraulic fluid in the shale reservoir NSFC 2014 confirmed 25 Studies on the solid-fluid coupling mechanisms of CO 2 sequestration in the shale reservoir and its effect on the recovery of shale gas NSFC 2014 confirmed 80 Brittle fracturing mechanism of supercritical CO 2 used as the hydraulic fluid in the shale reservoir and the transportation rule of the suspended sand Qingdao University of Science & Technology NSFC 2014-2016 80 The propagation evolution of the hydraulic fracture network induced by the supercritical CO 2 in the shale reservoir Institute of Geology and Geophysics (CAS) NSFC 2015-2017 85 Type 5: EGS/CO 2 -EGS Simulation and prediction of CO 2 -EGS Jilin University etc. Ph.D. program 2011-2013 -Comprehensive utilization and production of hot dry rocks 863 Program 2012-2015 -Table 1: Continued.Name of the research projects Responsible institute Funding sources Amount The flowing characteristics of the man-made fractures in the hot dry rocks and the mechanisms of multifield coupling heat and mass transfer Tianjin University NSFC 2013 confirmed 76 Large scale CO 2 utilization and storage in the innovative EGS technology Tsinghua University International 2012-2014 -Mechanisms of the production of geothermal energy in the high temperature depleted gas reservoir using CO 2 as the circulation fluid and the evaluation of potential China University of Petroleum NSFC 2016 confirmed 54 Heat transfer using supercritical CO2 enhances the geothermal recovery and the mechanisms of induced sliding of fractures Institute of Rock and Soil Mechanics (CAS) NSFC 2016 confirmed 54 Multiphase dynamic characteristics in the CO 2 plume type geothermal system and the optimization of the energy conversion Jilin Jianzhu University NSFC 2016 confirmed 20 Type 6: CO 2 capture technology R&D of the new type O 2 /CO 2 circulated combustion equipment and the optimization of system Huazhong University of Science and Technology 863 Program 2009-2011 -Key technologies of CO 2 capture by using 35 MWth oxy-fuel combustion technology, R&D in equipment and pilot projects National Sci-Tech support plan 2011-2014 -R&D in key technologies of CO 2 -oleaginous microalgae-biodiesel Xin'ao Group etc. 863 Program 2009-2011 2070 IGCC-based CO 2 capture, utilization and sequestration technologies and pilot projects Huaneng Group etc. 863 Program 2011-2013 5000 CO 2 capture and purification technology using high gravity method Shengli oilfield etc.National Sci-Tech support plan 2008-2010 -Capture of high concentration of CO 2 in 0.3 million tons coal to oil industry, geological sequestration technology and pilot scale project Shenhua Group etc. National Sci-Tech support plan 2011-2014 -Development of key technologies of reduction in CO 2 emission from the blast furnace iron making and their utilization China Association of Metal National Sci-Tech support plan 2011-2014 -Research on key technologies related with large-scale CO 2 capture from the flue gases of the coal-2 storage in the saline formation (CCS technology) Potential assessment of CO 2 geological storage in China and pilot projects China Geological Survey Ministry of Land and Resources 2010-2014 -Multiphase multicomponent reactive transportation mechanism of the sequestration of impure CO of CO 2 sequestration in the saline formations, stability of rock and the transportation rules of CO 2 the water-rock-gas interaction of the CO 2 sequestration in the saline aquifers in the pressurized of CO 2 in porous media and its quantitative relationship with the propagation rate of the CO the damage in the near-field neighboring rocks of the CO 2 storage site in the rock salt and the mechanism of the supercritical CO 2 in the low permeable porous media Qingdao University of Science & Technology NSFC 2014 confirmed 80 Mechanism studies on the physical and dissolution trapping of the supercritical CO saline water-caprocks in the CO 2 geological sequestration and the risk of CO 2 leakage NSFC 2014 confirmed 84 Mechanisms of the difference in the distribution of the CO 2 saturation based on the high reliable saline formation model the physical properties of the porous media on the two-phase (CO 2 , saline water) fluid flow and the trapping mechanism of CO 2 NSFC 2014 confirmed 80 Two-phase CO 2 -water fluid flow characteristics and trapping mechanism of CO 2 at the porous scale in the multiple porous media NSFC 2015 confirmed 64 The convective mixing of CO 2 sequestration in the saline formations and its development characteristics NSFC 2015 confirmed 21 Basic studies on the transportation of supercritical CO 2 in the geological sequestration in the saline formations NSFC 2012 confirmed 25 Basic research on the wettability of CO 2 -saline water-rock equilibrium system during the CO 2 sequestration in saline formations NSFC 2013 confirmed 25 Viscous behavior and mechanism of the supercritical CO 2 on the rock surface in the geological sequestration condition NSFC 2016 confirmed 10 Geofluids China.Encouraged by the successful exploitation of shale gas in North America, China joined the exploration of shale gas in 2005 [155].The published data from the Ministry of Land and Resources in 2002 confirms that China had a shale gas reserve of 25.1 × 10 12 m 3 .By the end of 2015, China had a technical shale gas reserve of about 1.3 × 10 11 m 3 including the increased proved technical reserve of 1.09 × 10 11 m 3 .
Projects marked with * threaten cancellation in the near future for unknown reasons.

Figure 7 :
Figure 7: Distribution of CCUS engineering projects in China excluding the South China Sea Islands (numbers defined in Table3) superimposed on the provincial CO 2 emission map for the year 2010 (from[34]).
Based on the incomplete statistics of the research projects funded by MOST and NSFC during 2005-2016 (Figures 4 and 5 and Table 4. Current Status of CCS and CCUS Technologies in ChinaSince 2005, CCS has been listed as a frontier technology in China's mid-long term technical development program in order to realize the goal of zero emissions from fossil fuel energy [103].Meanwhile, more attention has been paid to CCUS technology, especially CO 2 -EOR and CO 2 -ECBM [104-107].Between 2006 and 2015, the Ministry of Science and Technology of China (MOST) funded eight National Basic Research Programs (also known as the 973 Program) and State High-Tech Development Plans (commonly known as the 863 Program).Three of these programs were related to CO 2 -EOR and the others to the CO 2 capture technology, shale gas recovery, and the hot dry rock systems.The National Natural Science Foundation of China (NSFC) also generously funded basic research related to CCS and CCUS.

Table 2 :
China's international collaboration on CCUS projects during 2005-2016.Research projects of CCS and CCUS in China during 2005-2016 based on Table 1.
2 -EOR field test was executed for the first time in Daqing oilfield in 2003.In recent years, the industrial injection of CO 2 and the production of oil with the help of CO 2 -EOR technology operated by the Daqing oilfield are mainly located in the Yushulin and Hailaer oilfields.(2) A CO 2 -EOR project with a CO 2 injection amount of 0.8-1 million tons/year in Jilin oilfield (still in operation) since 2005 for the exploitation of the CO 2rich (21% CO 2 concentration) Changling gas field.A CO 2 -EOR experiment has been carried out by Jilin oilfield in 2006 and oil recovery enhanced by 8%-10%.

Table 3 :
Main engineering CCUS projects in China.
As the first demonstration of IGCC power station in China, the first stage of the IGCC project at Tianjin combined with the CO 2 capture and EOR technology, with an installation capacity of 265 MW, has been in operation since November 2016.(7) The CO 2 -ECBM project located in the Qinshui basin of Shanxi Province operated by China United Coalbed Methane Corporation, Ltd (completed) [7, 164].It is the only pilot-scale CO 2 -ECBM field test in China and operates at an injection rate of 40 tonnes/day of CO 2 .This is a cooperation project between the Zhonglian coalbed methane Ltd and injection pressure ranging from 6.79 to 8.63 MPa.The second production test started in 2012 with varying injection rates of 6 m 3 /h, 9 m 3 /h, 12 m 3 /h, and 15 m 3 /h and constant wellhead injection pressure of 5.7 MPa and temperature of 5 ∘ C. Another large-scale CO 2 sequestration in the deep saline formations located in Lianyungang of Jiangsu Province is in preparation.
[165]roduced by the coal liquefaction factory of the Shenhua Group, has been transported by oil tankers and injected in four saline formations and one carbonate formation[165].The first stage of injection test started in 2011, with Geofluids the wellhead CO 2 project in Jiangsu province operated by the Zhongke CO 2 Jinlong company.The CO 2 pilot-scale project in Tianjin organized by China Guodian Power is in preparation.
[180,181]2 US$/ton CO 2 /250 km, while injection and storage costs are 1-12 US$/ton CO 2 based on the prices in 2013.In China, the cost of electricity generation by coalfired power station increases by 30%-50% using CO 2 capture technology due to the extra consumption of electricity and steam.Taking the Huaneng Beijing coal-fired power station as an example, the capture price is about 24.3 US$/ton CO 2 , with the CO 2 capture efficiency of 80%-85%[179].On the other hand, simulation results of the IGCC coal-fired power station with the CCS technology in Tianjin show the capture price to range from 21.3 to 24.8 US$/ton CO 2 , accounting for 80% of the price of a full-scale CCS project[180,181].However, the uncertainty in the CO 2 capture price is high depending on different capture technologies including precombustion, postcombustion, and oxy-fuel combustion at various stationary point sources including coal-fired power stations, cement factories, and coal chemical industries.From the aforementioned point of view, the uncertainty in the investment of a specific CCS or CCUS engineering project is determined by the cost of CO 2 capture.Therefore, a reduction in the cost of CO 2 capture is the key to the widespread application of CCS or CCUS technologies.Besides, drilling costs are large for all types of CCUS engineering projects and hydrocarbon/geothermal production, taking a shale gas well as an example, it costs 5.8 million US$ for a drilling length of 2500-3000 m, and 0.72 million US$ for a general gas well.
[178]he price of electricity generated from a coal-fired power station is 82-99 US$/MWh and 83-123 US$/MWh without and with the CO 2 capture technology, respectively,[178].Depending on different situations and technologies in US, the capture cost is 42-87 US$/ton CO 2 , transportation costs range from 1) Many countries have participated in activities to tackle global climate changes during the last few years.The total CO 2 emissions for China in 2005 were 59.76 × 10 8 tonnes, accounting for 80.03% of the greenhouse gas emission of China in 2016.To perform its social responsibility, China plans to reduce its CO 2 emission per unit of GDP by 40%-45% in 2020 compared with the 2005 level.Therefore, on one hand, China needs to change its current energy framework by reducing the consumption of fossil fuels like coal energy, or applying a clean coal program, capturing the CO 2 produced by the combustion of coal.On the other hand, China needs to develop the renewable energy sector, including wind energy, solar energy, and geothermal energy.
-AGES: CO 2 aided geothermal extraction system CO 2 -ECBM: CO 2 enhanced coalbed methane recovery CO 2 -EGR: CO 2 enhanced gas recovery CO 2 -EOR: CO 2 enhanced oil recovery CO 2 -ESG: CO 2 enhanced shale gas recovery Reduction of CO 2 emission by means of CO 2 storage in coal seams in the Silesian Coal Basin of Poland SNG-EOR: Synthetic Natural Gas-Enhanced Oil Recovery USDOE: United States Department of Energy.