# The Influencing Factors of CO2 Utilization and Storage Efficiency in Gas Reservoir
## 1 Introduction  
Since 1980, the global average temperature has risen year by year. By 2020, the global average temperature has been 1.1 \({}^{\circ}\)C higher than that at the end of the 19 th century. Since the signing of the Paris agreement, countries have declared their greenhouse gas emission reduction policies and committed to achieve carbon neutrality by 2050 or 2060 [1]. The US Congress passed a bipartisn budget bill that expanded the corporate income tax credit for CCUS [2]. Russia proposed a federal law on limiting greenhouse gas emissions in 2021 [3]. The Chinese government has also issued a series of CCUS incentive policies [4]. During The 26 th Conference of the Parties (COP26) to the United Nations Framework Convention on Climate Change in 2021, it was announced that the global average temperature will rise at least 1.8 \({}^{\circ}\)C by the end of this century, even if countries around the world achieve their emission reduction targets. At the 27 th COP, it was suggested that even a 1.8 \({}^{\circ}\)C rise in temperature would cause serious impact in vulnerable ecological regions such as Africa [5]. Therefore, countries need to further improve their tax credit policies. At the same time, the establishment of CO2 global transport chains is also required to break the CO2 source limitation and promote CO2 commercialization [6]. As there are still certain controversies and technical barriers in coal, steel and new energy industries, CCUS is currently one of the most effective measures to reduce CO\({}_{2}\) emissions. Carbon Capture, Utilization and Storage (CCUS) is a technology used to mitigate climate change and reduce carbon dioxide emission, which is developed from enhancing oil recovery by CO\({}_{2}\) injection [7; 8]. CCUS is still in the development and demonstration stage; there are many prominent problems restricting its development which need to be solved and verified, including high cost and high energy consumption, as well as uncertain safety and reliability of long-term storage [9; 10]. Depleted oil and gas reservoirs, unexploitable coal and brine layers are commonly selected for CO\({}_{2}\) storage [11; 12]. Among them, the depleted oil and gas reservoirs have the higher adaptation for storage, because of the better economic benefit and safety [13; 14]. The storage capacity of CO\({}_{2}\) in depleted gas reservoirs is estimated to reach 8 \(\times\) 10\({}^{11}\) tones, which has great potential [15]. In addition, there are more suitable porosity, permeability and better sealed caprock in depleted gas reservoirs which have advantage over oil reservoir in CO\({}_{2}\) storage [16]. Therefore, the CO\({}_{2}\) storage could be combined with enhancing natural gas recovery, which can effectively ensure profitability and promote energy conservation and emissions reduction. CO\({}_{2}\) storage with enhanced gas recovery (CSEGR) technology has been recognized internationally and has been adopted for field tests in some countries [17; 18]. However, CSEGR technology has not achieved large-scale promotion due to the key problems as follows: 90% natural gas can be produced by depletion, due to high compressibility of gas, and there is no need for energy replenishment and secondary recovery in gas reservoirs rather than in oil reservoirs. Therefore, CO\({}_{2}\) injection in gas reservoirs is still in testing phase while widely used in oil reservoirs [19]. CO\({}_{2}\) injected into serious heterogeneous reservoirs will migrate first along high-permeability channels, resulting in premature breakthrough and low recovery efficiency [20; 21]. Diffusion and convection occur during CO\({}_{2}\) injection in gas reservoirs, which are generally believed to exacerbate CO\({}_{2}\) breakthrough, pollute the natural gas, increase the cost of gas separation and reduce the recovery and storage efficiency [22; 23]. There have been lots of studies on the mechanisms and influencing factors of CCUS.