CO2 emission was the major cause that accounted for the global warming and climate chance. How to reduce CO2 footprint to stop or slow down the global warming has been hot topic. As a developing country, China has become the largest CO2 emission nation in the world during the industrialization process to develop economy, although the CO2 emission intensity has been reduced significantly compared to previous stage. China has promised and succeeded to keep the promise reduce carbon intensity to meet the requirement of Paris Agreement. To meet the promise to attain carbon peak emission in 2030 and carbon neutrality in 2060 (CPCN), carbon capture, utilization and storage (CCUS) is an important and necessary step. Considering the high cost, high energy intensity and complex technology integrated optimization add uncertainties of CCS, utilization of captured CO2 can be of vital importance. One of the most attractive CCUS in China is CO2 enhanced oil recovery with captured CO2 (CCS EOR). CO2 EOR with captured CO2 may be one the best CCUS ways for China for the following three reasons. First, it can meet the increasing oil demand while reducing the carbon intensive coal. Second, around 66 CO2 EOR field tests have been conducted in China and experiences have ben gained. Finally, CO2 EOR in the USA was a proven technology which can increase oil production significantly and stably. Latest CCUS technology progress in China was reviewed. As of July 2021, 49 projects were carried out or under construction. Net CO2 avoided costs from 39 projects varied from 120 to 730 CNY/ ton CO2 (18.5-112.3 USD/ ton CO2). Although CCUS technology development in China was significant, the gap between global leading levels are obvious. Current challenges of CCS EOR include high CO2 capture cost, small scale, low incremental oil recovery, long-time huge capital input. The costs can be significantly reduced when the scale was enlarged to a commercial scale and transportation costs were further reduced by either pipelines or trains. CO2 transportation with well-distributed high-speed rail in China may be a feasible choice in future. If the CO2 EOR in China develops with the same speed as the USA, CO2 used for EOR in 2050 can be as high as 87.27 million tons. CO2 used for CO2 EOR in 2050 can account for 17% to 44% of the CO2 emission. CCS EOR in China will provid both domestic and international companies with good opportunities.

Polymer flooding has drawn more and more attention in the world for its high incremental oil recovery factor and relative low costs compared with water flooding and other chemically enhanced oil recovery techniques. However, for many oilfields, such as Daqing Oilfield, China, that have already been flooded with polymers, how to further improve recovery remains a big problem. Traditional intralayer, interlayer and plane heterogeneity studies cannot accurately characterize the remaining oil distribution after polymer flooding. To solve this problem, we established a method to quantitatively describe the reservoir’s architecture. Then, the architecture elements were dissected hierarchically and the interface of each architecture level in Daqing Oilfield was identified. The distribution pattern and development potential of the remaining oil after polymer flooding under the influence of reservoir architecture was analyzed. The results show that, regarding the sedimentary process from north to south in Daqing Oilfield, the channel becomes narrower, the thickness decreases, the point bar’s width increases and the thickness of the meandering river decreases. The braided bar scale becomes larger and the thickness becomes smaller in the braided river. According to the reservoir’s architecture, the remaining oil was divided into four categories of plane remaining oil (abandoned channel occlusion type, interfluvial sand body occlusion type, inter-well retention type and well pattern uncontrollable type) and three types of vertical remaining oil (in-layer interlayer occlusion type, rhythm type and gravity type). About 40% of the original oil in place (OOIP) of Daqing Oilfield has not yet been produced, which indicates that there is great potential for development. This study is important for improving oil recovery in polymer-flooded reservoirs.

Polymer flooding is drawing lots of attention because of the technical maturity in some reservoirs. The first commercial polymer flooding in China was performed in the Daqing oilfield and is one of the largest applications in the world. Some laboratory tests from Daqing researchers in China showed that the viscoelasticity of high molecular weight polymers plays a significant role in increasing displacement efficiency. Hence, encouraged by the conventional field applications and new findings on the viscoelasticity effect of polymers on residual oil saturation (ROS), some high-concentration high-molecular-weight (HCHMW) polymer-flooding field tests have been conducted. Although some field tests were well-documented, subsequent progress was seldom reported. It was recently reported that HCHMW has a limited application in Daqing, which does not agree with observations from laboratory core flooding and early field tests. However, the cause of this discrepancy is unclear. Thus, a systematic summary of polymer-flooding mechanisms and field tests in China is necessary. This paper explained why HCHMW is not widely used when considering new understandings of polymer-flooding mechanisms. Different opinions on the viscoelasticity effect of polymers on ROS reduction were critically reviewed. Other mechanisms of polymer flooding, such as wettability change and gravity stability effect, were discussed with regard to widely reported laboratory tests, which were explained in terms of the viscoelasticity effects of polymers on ROS. Recent findings from Chinese field tests were also summarized. Salt-resistance polymers (SRPs) with good economic performance using produced water to prepare polymer solutions were very economically and environmentally promising. Notable progress in SRP flooding and new amphiphilic polymer field tests in China were summarized, and lessons learned were given. Formation blockage, represented by high injection pressure and produced productivity ability, was reported in several oil fields due to misunderstanding of polymers' injectivity. Although the influence of viscoelastic polymers on reservoir conditions is unknown, the injection of very viscous polymers to displace medium-to-high viscosity oils is not recommended. This is especially important for old wells that could cause damage. This paper clarified misleading notions on polymer-flooding implementations based on theory and practices in China.

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Latest advances of ASP flooding (ASP) field tests in China are provided to focus ontwo major concerns: ASP EOR cost and technical matureness. Benefits with and without alkali for ASP were discussed. ASP flooding mechanisms in Chinese perspective are discussed. Although 54 tests were surveyed, 5 recent typical ASP field tests are discussed with special attention: the only one using horizontal wells (HASP), one combining microbial flooding (MASP), one in conglomerate reservoir (Xinjiang), one in a 81°C reservoir (Henan) and the only organic alkali one (OASP). 2D nanosheets surfactant with moderate low interfacial tension (IFT)(10-1mN/m) making highest incremental oil recovery (IOR) (Yin et al, 2019; Raj et al,2019) proves that ultra-low IFT is not essential for EOR (Figure 1). It is emulsification rather than ultra-low IFT that dominates high IOR (Guo, 2018; Cheng et al, 2019). Alkali is very important and necessary to reduce surfactant adsorption, promote emulsification and improve polymer injectivity (Figure 2). Interfacial viscosity should have been regarded more important. 29 commercial blocks in Daqing are conducted (Cheng etal,2019). ASP flooding produced 32.6 million bbl oil (Zhang, 2019) in 2018 and accounts for 14% total oil production (Figure 3). HASP test (2012-2017) in Daqing makes the highest IOR of 29.66% OOIP, 10%higher than all the other vertical well ASP (VASP). 5 horizontal wells (3 Injectors 2 Producers). ASP slug is 0.45PV and viscosity is 30 mPa.s with 10-3 mN/m IFT.14. 1millon bbl oil is produced. OASP field test with 10 injectors and 21 producers in high divalent ion heavy oil reservoir in Shengli produced 6.47 millionbbls and predicted IOR of 11.9% OOIP with 0.45PV slug (0.1%A +0.3%S+1500ppm P). Water cut dropped from 97.8% to 90.5% and daily oil production rises from 138.6 bbl to 554.4bbl. B1DD and B2XASP reported financial internal rate of return (FIRR) of 18.01% and 22.7% respectively. MASP (2008-2014) was reported IOR of 20.57% OOIP (26.7% predicted). 1.237 PV chemical slug including 0.06PV microbialslug [2% microbe fluid and 2% nutrient solution] was injected to 9 Injectors corresponding to 16 Producers. Better injectivity and economic performance are observed (FIRR=31.40%). Chemical cost is445 CNY/ton. ASPF in a conglomerate reservoir goes well. ASP flooding was commercially used in Daqing. Some commercial ASPtests failed to attain its goal (Cheng et al,2019). HASP has achieved 10% OOIP higher incremental oil recovery than all the others VASP. Ultra-low IFT should not be the most important factor aspreviously believed. Emulsification contributed by alkali is vital. However, emulsion viscosity should beoptimized. A totally different new macroscopic capillary number theory can well explain many hard-to-understand field test results. Alkali is both important and necessary.