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Abstract
DF1-1 gas field in the west of the South China Sea is associated with high concentration of CO₂). Many options have been assessed by the operator (CNOOC) in recent years in order to dispose the CO₂ separated from the gas stream. In this study, geological storage of CO₂ into offshore saline aquifers near the gas separation plant on the Hainan Island is considered, and a demonstration project is proposed and designed in terms of aquifer selection and assessment, CO₂ transportation and injection, and project economics. Several aquifer structures around the gas field and near the Hainan Island were investigated and
assessed with respect to geological structure, reservoir and trap features, fluid properties, storage capacity and site location. A saline aquifer (namely LT13-1), 60 km offshore the Hainan Island, was chosen as the storage site. CO₂ will be transported by a long-distance subsea pipeline at high pressure and injected into the aquifer via a subsea well-head and through a horizontal well. Reservoir simulation and injectivity analysis have been conducted to estimate the injection rate and pressure, and also to predict the movement of CO₂ after injection. A scoping economic analysis of the project was also conducted and presented in
the paper.

Introduction
Many gas fields in the west of the South China Sea, such as DF1-1, Yacheng 13-1 and Ledong, are associated with high concentration of CO₂. Currently a part of the CO₂ separated from the produced gas is used by industry, while most of them has to be emitted into the atmosphere [1]. Many options have been assessed by the operator (CNOOC) in recent years in order to dispose the associated CO₂ from DF1-1 and other gas fields, including industrial utilizations and CO₂ geological storage (CGS) [2, 3]. A survey has shown that, for industrial utilizations of CO₂, the demand in the surrounding provinces is small, and many
technological and economic difficulties exist in order to meet various standards for different users. Therefore it is not practicable to dispose the massive amounts of CO2 separated the natural gas fields via industrial utilization techniques. On the other hand, injecting CO₂ into geological structures for permanent storage has been considered as an effective way to reduce CO₂ emissions into the atmosphere and mitigate the global warming.

The available structures or traps for geological storage of CO₂ include depleted or mature oil & gas reservoirs, saline aquifers and unmineable coal beds [4-6]. The first geological storage project in the world at a large scale was carried out in the Sleipner gas field of Norway in 1996. The associated CO₂ in natural gas (9.5% CO₂) is separated and injected into the Utsira saline aquifer at a rate of 1 million tons per year [7]. This project demonstrates a new and effective technology for the disposal of CO₂. Other similar projects are then conducted for further demonstrations of the technology in recent years, namely, the Weyburn oilfield EOR and storage (Canada, 2000) [8], the In Salah gas field and aquifer storage (Algeria, 2004) [9], the K12-B gas field storage (Netherlands, 2004) [10], and the Snøhvit aquifer storage (Norway, 2009) [11]. Saline aquifer has advantages of larger storage capacity and wider availability over oil and gas reservoirs, therefore, it has been considered as the most prospective storage site for large-scale disposal of CO₂ in the future.