The main objective of this study is to understand the vertical sweep efficiency with miscible CO2-water-coinjection as a secondary recovery method, from multiple perspectives: phase behavior, total relative mobility, fluid densities/viscosities, the driving forces and consequent phase distributions etc. We also seek to provide insights into modeling approaches for representing the injection process by comparing compositional simulation results to those of the fractional-flow method and the model of Stone and Jenkins ( Stone, 1982 ; Jenkins, 1984 ).

We combine compositional simulation and analytical models to interpret the dynamics that affect vertical sweep efficiency in miscible CO2-water-coinjection. Stone’s model for gravity segregation at steady state predicts three phase-distribution zones: mixed zone, override zone and underride zone. In addition to these three zones, we identify from simulations an extended mixed zone and extended override zone in miscible CO2-water-coinjection, contributing to additional oil recovery and CO2 trapping. The extended zones are a result of dispersion that reflects physical and numerical dispersion in the gas-oil displacement front. To the extent that it reflects numerical dispersion, the extended zones can be considered as a numerical artifact.@en

China has achieved significant progress on wastewater treatment and aquatic environmental protection. However, leakage (in- and exfiltration) of sewer systems is still an issue. By using the statistical data of water and wastewater in 2016 in China, and the person loads (PLs) of water and wastewater in Singapore, the leakage fractions of hydraulic flow, organic carbon (COD), nitrogen (N) and phosphorus (P) mass loading, and in-sewer COD biological removal in the sewer systems of China (except Hong Kong, Macau and Taiwan), Shanghai, Guangzhou and Beijing were reported for the first time. The fractions of hydraulic flow infiltration (13%, Shanghai and Guangzhou) and exfiltration (39%, China) were calculated. Except Beijing, whose sewer networks are under appropriate management with small leakage fractions, the exfiltration fractions of COD (including in-sewer biological COD removal) ranged from 41% (Shanghai) to 66% (China) and averaged 55%; N ranged from 18% (Shanghai) to 48% (China) and averaged 33%; and P ranged from 23% (Shanghai and Guangzhou) to 44% (China) and averaged 30%. The exfiltrated sewage, COD, N and P not only wastes resources, but also contaminates the aquatic environment (especially groundwater) and contributes to 'black and odorous water bodies'. In- and exfiltration in the sewer network leads to low influent COD concentration, C/N ratio and high inorganic solids and inert particulate COD concentrations of many municipal wastewater treatment plants (WWTPs) causing high cost for nutrient removal, poor resource recovery, additional reactor/settler volume requirement and other operational problems. Therefore, tackling sewer leakage is of primary importance to today's environment in China. Recommendations for the inspection of sewer systems and the rehabilitation of damaged sewers as well as the development of design and operation guidelines of municipal WWTPs tailored to the specific local sewage characteristics and other conditions are proposed.

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The influence of highly nitrogen-doped mesoporous carbon spheres (NMCSs) (internal pore size of 5.4–16 nm) on the electrochemical response of low carbon steel (St37) in model alkaline solutions of pH 13.9 and 12.8 was studied, using Open Circuit Potential (OCP) monitoring, Electrochemical Impedance Spectroscopy (EIS) and Cyclic Voltammetry (CV). Prior to adding the NMCSs in the relevant solutions, they were characterized in the same model medium by measuring their Zeta-potential, hydrodynamic radius and particle size distribution, using dynamic light scattering (DLS) and transmission electron microscopy (TEM). In alkaline environment of pH 13.9 and 12.8, which simulates the concrete pore water of fresh and mature concrete, the DLS measurements indicated that the hydrodynamic radius of NMCSs particle varied from 296 nm to 183, respectively. According to the Zeta-potential measurements in the same solutions, the NMCSs were slightly positively charged.

The addition of 0.016 wt.% of NMCSs to the model medium induced certain variation in the electrochemical response of the tested steel. In alkaline solutions of pH 12.8, the presence of NMCSs in the passive film/solution interface induced a delay in the formation of a stable passive film. On the other hand, in solutions of pH 13.9, the higher corrosion activity on the steel surface, enhanced by high pH, was limited by adsorption of NMCSs on the film/substrate interface. In addition competing mechanisms of active state, i.e., enhanced oxidation on the one hand, and particles adsorption on anodic sites and oxidation limitation, on the other hand, was relevant in solution of pH 13.9 inducing larger fluctuations in impedance response and stabilization only towards the end of the testing period.

Except steel electrochemical response, the properties of the cement-based bulk matrix were investigated in the presence of the aforementioned additives. The mortar bulk matrix properties were highly affected by NMCSs. The lowest electrical resistivity values were recorded in mortar specimens with mixed-in 0.025 wt.% NMCSs (with respect to dry cement weight). Furthermore, the addition of 0.025 wt.% NMCSs increased the compressive strength when compared to control specimens. The presence of F127 as a dispersing agent for NMCSs was found to be not suitable for reinforced concrete applications, which is in view of the reduced mechanical strength and electrical resistivity of the cement-based bulk matrix. This is in addition to the adverse effect on the formation of electrochemically stable passive layer on steel surface in alkaline medium in its presence.
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