# Feasibility Assessment of Acid Gas Injection in an Iranian Offshore Aquifer
###### Abstract
The primary objective of acid gas injection operations is the disposal of H\({}_{2}\)S. However, substantial amounts of CO\({}_{2}\) are injected simultaneously due to the economic impossibility of separating the two gases [5]. The primary method of acid gas injection involves the injection of a stream consisting mainly of H\({}_{2}\)S and CO\({}_{2}\), obtained from the sweetening plant. This stream is compressed and piped to an injection well, where it is directed downward into a subsurface formation typically intended for disposal [2]. The general injection scheme, which includes the sweetening plant and associated processes, can be represented by a block diagram (Figure 1). Regulatory agencies in Western Canada are currently granting approval for several parameters related to acid gas injection. These include the maximum allowed fraction of H\({}_{2}\)S, the maximum injection pressure and rate at the wellhead, as well as the maximum injection volume. Acid gas injection operations are currently conducted in 51 distinct formations, located at 44 different sites across the Alberta Basin in the provinces of Alberta and British Columbia. The injection of acid gas occurs in various types of formations at different sites. Specifically, it takes place in deep saline formations at 27 sites, depleted oil and/or gas reservoirs at 19 sites, and in the underlying water leg of depleted oil and gas reservoirs at 4 sites. These different types of formations act as places for the injection process. Of all the sites, 29 rely on carbonates as the primary reservoir formation for acid gas injection. Conversely, the remaining 21 sites predominantly use quartz-rich sandstones as the dominant reservoir formation. In most cases, shales act as caprocks, serving as an upper confining unit for injection zones. However, in the remaining injection zones, narrow limestones, evaporites, and anhydrites are responsible for the confinement of the injected substances and their effective containment [7]. Figure 1: Representation of AGI by a block diagram [6]. Numerous sedimentary regions worldwide possess varying degrees of suitability for CO\({}_{2}\) storage. In general, geological storage sites must have the following characteristics to be suitable for CO\({}_{2}\) storage:  
* Sufficient capacity and injectivity to accommodate the CO\({}_{2}\) being injected. * An effective sealing caprock, or confining unit, to prevent CO\({}_{2}\) leakage. * A geologically stable environment that ensures the long-term integrity of the storage site, minimizing the risk of any potential compromise [8]. Three geological formations have been extensively studied for the purpose of storing CO\({}_{2}\): oil and gas reservoirs, deep saline formations, and non-mineable coal beds. In all three cases, the process of geological storage of CO\({}_{2}\) involves its injection in dense form into a subsurface rock formation [9]. Rock formations that are porous and capable of holding, or that have previously held, fluids such as natural gas, oil, or brine, such as depleted oil and gas reservoirs, are considered suitable options for CO\({}_{2}\) storage. Suitable storage formations for CO\({}_{2}\) can be found in both onshore and offshore sedimentary basins. These basins are large-scale natural depressions in the earth's crust that are filled with sediments [10]. A thorough site characterization of a deep saline aquifer is crucial to assess its safety and long-term viability for effective geological storage of CO\({}_{2}\). Various techniques, including core analysis, well-logging analysis, and geological modeling, are employed to gather essential information and to form the basis for evaluation. Detailed analysis of sediment cores and interpretation of well-log data are valuable for understanding the vertical and lateral heterogeneity caused by changes in depositional environments within the framework of sequence stratigraphy [11]. However, these data primarily provide information about geology and petrology near the wells.