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The global warming induced by the emission of greenhouse gases, especially the carbon dioxide, has become the global climate and environmental issues. China has been working in the CO2 emission reduction and carbon sinks with the purpose of becoming the carbon-neutral country by 2060. The CO2 capture, utilization and storage (CCUS) technologies and the reforestation technology represented by the Conversion of Cropland to Forestland Program (CCFP) have great potential for sinking CO2 emission. However, the trade-off among CCFP, CCS/CCUS and Water-Energy-Food (WEF) nexus are not well evaluated. In this paper, the remote-sensing data are collected and used to evaluate the sustainability of CCFP by analyzing the variation of land use and land cover (LULC), crop production, etc. The results show that 13.29% of the cropland in 2001 vanished and converted to grassland (8.3%), mosaic cropland (3%) and urban land (0.98%) in 2019, demonstrating that the CCFP is successful in both WEF nexus and carbon sink. The total crop production has increased around 50% between 2001 and 2019, implying that the CCFP will not lead to the food risk during the conversion of croplands into other types of land in China. A sustainable implementation of CCFP and other environmental Payments for Ecosystem Services (PES) policies in 2019–2060 could reach an estimated total growth of 7.462 billion m3 in comparison of that in 2018 and the total plantation forest stock of about 10.852 billion m3 in 2060, with a corresponding minimum CO2 sink of 2.90 billion tons in 2060. The estimated peak of net equivalent CO2 emissions before 2030 is about 11.0 billion tons and could not be reduced to zero by 2060 without the large-scale application of the CCS/CCUS technologies as geological sequestration of CO2. Besides, the application of CCS/CCUS can be beneficial for WEF, e.g., through replacing the water by CO2 during energy production, especially in the shale gas production in the regions with high water risks in China. In one word, CCS/CCUS and CCFP are two decided pathways of carbon sequestration and should be systematically applied to achieve China's carbon neutrality by 2060. [ABSTRACT FROM AUTHOR]

Theoretical and numerical studies of coupled hydro-mechanical processes during geothermal and hydrocarbon production, nuclear waste disposal, and CO 2 sequestration in geological media have become a research focus worldwide. However, their application in China, especially CO 2 sequestration, is still in infancy and may need several years to develop. In this paper coupled hydro-mechanical processes in CO 2 migration, formation uplift and the state of caprock integrity as a result of CO 2 sequestration have been investigated by simulation at field scale for an integrated CCS (CO 2 Capture and Sequestration) project launched in the Ordos Basin, China. The paper aims at attaining long-term safety and integrity of caprock to ensure success for subsequent project operations at field scale. The linked TOUGH2MP-FLAC3D simulator has been used to study the coupled hydraulic-mechanical responses to primary stresses, injection rate and reservoir permeability during CO 2 injection and post-injection periods by means of 4 case studies. In the short-term operation of a CO 2 sequestration project launched in the Ordos Basin 0.1 Mt CO 2 has been injected in four saline aquifers within a period of 1 year and 9 years of relaxation time allotted for simulation runs. Contrary to using hydraulic field conditions alone, a combination of hydraulic-mechanical effects accounts for lateral migration of CO 2 in aquifer layers that leads to partial volumetric expansion of the reservoir. Amongst the main factors affecting hydro-mechanical processes, injection rate has the most significant impact. This is followed by reservoir permeability that is responsible for gauging the uplift trend rate in the post-injection period through hindering the dissipation of pore pressure. Results indicate that the scope of pore pressure attained will be larger in the isotropic stress state than in the compressional stress condition, pointing to the significance of primary stress. Calculation of the maximum storage pressure within safety limits using the integrity and shear criteria reveals that failure of caprock integrity increases with the amount of CO 2 injected. During injection however, the risk of caprock failure, especially in thin sandstone-caprock interlayered systems, increases with time. This paper estimates the pressure buildup in the four injection aquifers in the Ordos Basin, as a basis to ensure safety in the long-term operation of the CO 2 sequestration project, hence providing important reference for future commercial operations at field scale. [ABSTRACT FROM AUTHOR]

CO sequestration in deep saline formations has been proved to be an effective method for reducing greenhouse emissions into the atmosphere. However, pure sequestration of CO will add to the costs incurred by both industries and governments. A win-win method of CO injection and hot brine (water) extraction can become attractive to the investors, as it will not only increase the storage capacity of the injected CO, but also offset the costs by selling and using the produced hot water for industrial, agricultural or household purposes. For instance, water from very hot geothermal reservoirs ( T ≥ 150 °C) can be used for electricity generation in power plants and water from low-medium temperature reservoirs, the most predominant in natural systems, are more popular for direct use, e.g., in heating systems, household hot water, baths, aquaculture, etc. In this paper, low-medium geothermal reservoirs widely distributed in China, especially those in the Ordos Basin, were selected for the numerical case studies using TOUGH2MP with the ECO2N module for the simulations. Generally, simulation parameters were taken from the Ordos Basin, where the first full-integration CO sequestration project had been operated since 2010. The simulations in the base case study lasted 35 years, based on the lifespan of a normal geothermal project. Shallow re-injection systems were also considered to investigate the influence of thermal breakthrough, pressure perturbation, etc. Results show that injection of cold CO causes sharp decrease in temperature in the reservoir region near the injection well, which is enlarged with continuous injection. The region near the production well is dominated by different fluid phases during the CO driven process, including a single water phase, a two-phase fluid (water and CO) and a phase of almost pure CO. Results also show that the CO breakthrough lags far behind the pressure response in the geothermal production system. Before breakthrough, the injected CO pressurizes the reservoir, improving the overall performance of the geothermal reservoir. Furthermore, the heat extraction efficiency of CO-based system is obviously higher than HO-based system. [ABSTRACT FROM AUTHOR]

Because of the high costs involved in CO sequestration in deep saline formations, it has been difficult for its widespread application in the short term. Recently, however, a new technology called 'combined geothermal production-CO sequestration technology' has made the technique more attractive not only by increasing the CO storage capacity, but also by decreasing the operational costs, through the utilization of geothermal energy, either directly or for the purpose of producing electricity. A CO-aided (or CO-involved) geothermal extraction system (CO-AGES) is presented, based on existing technologies and the new ideas of 'combined geothermal production-CO sequestration' introduced in this paper. This system can be used to extract geothermal energy from sedimentary aquifer formations (low-medium temperature) at different stages. The purpose of this paper is to set up an evaluation system for selecting a suitable site for this CO-AGES system. There are both similarities and differences in the procedure needed for selecting a site for a conventional pure CO sequestration and for this CO-AGES system. Both are carried out at different scales, including basin, region and target formation scales. The biggest difference is the temperature gradient, which plays an opposite role in the two systems. By using the preliminary ranking and screening method presented in this paper, it can be shown that the Bohaiwan, Songliao and Qiangtang basins have the highest potential for the application of this CO-AGES technology. However, if CO sequestration is considered alone, the Ordos, Tarim and Bohaiwan basins should be selected as the best three operational sites in China. While preliminary evaluation methods can provide some useful information on the selection of the best sites for the 'combined geothermal production-CO sequestration technology', more detailed work is still required because of the strong uncertainties that exist in the determination of the upper and lower boundaries for each indicator. [ABSTRACT FROM AUTHOR]

Subsurface immobilization and conversion of CO into solid mineral phases in deep siliciclastic saline formations containing silicate minerals, commonly known as 'mineral trapping', is gaining research attention as a significant option to reduce CO emissions in the atmosphere. Although mineral trapping of CO is a long-term process, a combination of short-term results from both laboratory experiments and numerical simulations can lead to some general understanding of the required long-term CO sequestration mechanisms. This is a 100 year preliminary batch simulation study of four sandstone samples, under CO saturated water at 75 °C from the Upper Permian formations in the Ordos Basin, using the TOUGHREACT/ECO2N module to simulate the CO-brine-rock interaction processes in deep siliciclastic multilayered saline aquifers. The samples approximately correspond to the four target saline formations selected by the Shenhua Group for a CO sequestration field demonstration project in the Ordos Basin, PR China. Preliminary simulation results show that the initial salinity of formation brine plays a significant role in determining the amount of CO that will be sequestered by solubility or mineral trapping in a deep saline aquifer. Minimal differences between experimental results and numerical calculation occur in low salinity waters, and significantly larger differences in high salinity waters, which is still under the maximum acceptable difference between experimental and computed data (10 %). The upper Liujiagou formation, with the highest level of salinity (ca. 88.7 g/L TDS) and lowest level of CO solubility, offers the highest mineral trapping capacity, with a maximum carbonate mineral storage of ca. 0.7 kg/m of bulk rock over a 100 year period. Regardless of the initial acidity or alkalinity of the aquifer brine, injection of CO will inflict a sudden drop in pH of the brine to acidity levels in a range of 3.0-4.6. The subsequent amount of dissolved and precipitated minerals, arising from the CO-brine-rock interaction, is site specific and mainly dependent on initial aquifer mineralogy and brine composition. [ABSTRACT FROM AUTHOR]

Deep saline aquifers still remain a significant option for the disposal of large amounts of CO from the atmosphere as a means of mitigating global climate change. The small scale Carbon Capture and Sequestration demonstration project in Ordos Basin, China, operated by the Shenhua Group, is the only one of its kind in Asia, to put the multilayer injection technology into practice. This paper aims at studying the influence of temperature, injection rate and horizontal boundary effects on CO plume transport in saline formation layers at different depths and thicknesses, focusing on the variations in CO gas saturation and mass fraction of dissolved CO in the formation of brine in the plume's radial three-dimensional field around the injection point, and interlayer communication between the aquifer and its confining beds of relatively lower permeability. The study uses the ECO2N module of TOUGH2 to simulate flow and pressure configurations in response to small-scale CO injection into multilayer saline aquifers. The modelling domain involves a complex multilayer reservoir-caprock system, comprising of a sequence of sandstone aquifers and sealing units of mudstone and siltstone layers extending from the Permian Shanxi to the Upper Triassic Liujiagou formation systems in the Ordos Basin. Simulation results indicate that CO injected for storage into deep saline aquifers cause a significant pressure perturbation in the geological system that may require a long duration in the post-injection period to establish new pressure equilibrium. The multilayer simultaneous injection scheme exhibits mutual interference with the intervening sealing layers, especially when the injection layers are very close to each other and the corresponding sealing layers are thin. The study further reveals that injection rate and temperature are the most significant factors for determining the lateral and vertical extent that the CO plume reaches and which phase and amount will exist at a particular time during and after the injection. In general, a large number of factors may influence the CO-water fluid flow system considering the complexity in the real geologic sequence and structural configurations. Therefore, optimization of a CO injection scheme still requires pursuance of further studies. [ABSTRACT FROM AUTHOR]

The emissions of greenhouse gases, especially CO2, have been identified as the main contributor for global warming and climate change. Carbon capture and storage (CCS) is considered to be the most promising strategy to mitigate the anthropogenic CO2 emissions. This review aims to provide the latest developments of CO2 storage from the perspective of improving safety and economics. The mechanisms and strategies of CO2 storage, focusing on their characteristics and current status, are discussed firstly. In the second section, the strategies for assessing and ensuring the security of CO2 storage operations, including the risks assessment approach and monitoring technology associated with CO2 storage, are outlined. In addition, the engineering methods to accelerate CO2 dissolution and mineral carbonation for fixing the mobile CO2 are also compared within the second section. The third part focuses on the strategies for improving economics of CO2 storage operations, namely enhanced industrial production with CO2 storage to generate additional profit, and co-injection of CO2 with impurities to reduce the cost. Moreover, the role of multiple CCS technologies and their distribution on the mitigation of CO2 emissions in the future are summarized. This review demonstrates that CO2 storage in depleted oil and gas reservoirs could play an important role in reducing CO2 emission in the near future and CO2 storage in saline aquifers may make the biggest contribution due to its huge storage capacity. Comparing the various available strategies, CO2-enhanced oil recovery (CO2-EOR) operations are supposed to play the most important role for CO2 mitigation in the next few years, followed by CO2-enhanced gas recovery (CO2-EGR). The direct mineralization of flue gas by coal fly ash and the pH swing mineralization would be the most promising technology for the mineral sequestration of CO2. Furthermore, by accelerating the deployment of CCS projects on large scale, the government can also play its role in reducing the CO2 emissions. [ABSTRACT FROM AUTHOR]