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.

Underground gas storage reservoirs (UGSRs) are used to keep the natural gas supply smooth. Native natural gas is commonly used as cushion gas to maintain the reservoir pressure and cannot be extracted in the depleted gas reservoir transformed UGSR, which leads to wasting huge amounts of this natural energy resource. CO₂ is an alternative gas to avoid this particular issue. However, the mixing of CO₂ and CH₄ in the UGSR challenges the application of CO₂ as cushion gas. In this work, the Donghae gas reservoir is used to investigate the suitability of using CO₂ as cushion gas in depleted gas reservoir transformed UGSR. The impact of the geological and engineering parameters, including the CO₂ fraction for cushion gas, reservoir temperature, reservoir permeability, residual water and production rate, on the reservoir pressure, gas mixing behavior, and CO₂ production are analyzed detailly based on the 15 years cyclic gas injection and production. The results showed that the maximum accepted CO₂ concentration for cushion gas is 9% under the condition of production and injection for 120 d and 180 d in a production cycle at a rate of 4.05 kg/s and 2.7 kg/s, respectively. The typical curve of the mixing zone thickness can be divided into four stages, which include the increasing stage, the smooth stage, the suddenly increasing stage, and the periodic change stage. In the periodic change stage, the mixed zone increases with the increasing of CO₂ fraction, temperature, production rate, and the decreasing of permeability and water saturation. The CO₂ fraction in cushion gas, reservoir permeability, and production rate have a significant effect on the breakthrough of CO₂ in the production well, while the effect of water saturation and temperature is limited.

The meaningful utilization of artificially created multiple fractures in tight formations is associated with the performance behavior of such flow channels, especially in the case of thermal energy extraction from sedimentary geothermal system. In this study, an innovative idea is presented to develop a numerical model for geothermal energy production based on concrete physical performance of an artificially created tensile multi-fracture system in a simplified manner. The state-of-the-art software FLAC3D^plus-TOUGH2MP-TMVOC are integrated to develop a coupled thermo-hydro-mechanical (THM) fictive model for constructing a multi-fracture scheme and estimating heat extraction performance. By incorporating the actual fracture width of newly created subsequent fracture under the effect of stress shadow, cubic law is implemented for fluid flow and geothermal energy production. The results depict that fracture spacing plays a vital role in the energy contribution through multiple fractures. Afterwards, a field case study to design huge multiple hydraulic fractures was performed in the geothermal well GB X1 in North Germany. The attenuation of fracture propagation becomes more significant when massive multiple fracturing operation is performed especially in the case of lower fracture spacing. The fictive model results will be extended to study the geothermal utilization of the North German basin through massive multiple fractures in our future work.