Coupled Reactive Transport Models of Acid Gas Injection (AGI) in Siliciclastic and Carbonate Reservoirs: Understanding Fundamental Controls on Injection Performance and Storage Security
Authors: Xiao Yitian , Maze William , Teletzke Gary F. , Wilkinson John Roland , Xu Tianfu , Pruess Karsten 
Published: May 2016

Abstract
One of the commonly proposed methods for reducing CO2 and other acid gas emissions is to inject them in petroleum reservoirs for Enhanced Oil Recovery (EOR) or to store them in deep saline aquifers. Any successful acid gas injection (AGI) project needs to meet three key objectives:Safety, Health and Environment (SHE);efficient sweep of pore space; andreliability of operations.
A fundamental approach that couples process-based models with laboratory data and field observations will allow us to better understand the key controls on AGI performance and storage security and therefore make better design decisions from appraisal to development to monitoring.
Large-scale AGI into subsurface reservoirs will induce a complex interplay of multiphase flow, capillary trapping, diffusion, convection, and chemical reactions that may have significant impacts on both injection performance and storage security. In addition, raw CO2 from either produced gas in a reservoir or flue gas from a power plant may contain other non-hydrocarbon gases such as H2S and/or SO2 that increase the complexity and cost of the operation. Co-injection of CO2 with H2S and SO2, if feasible, requires less energy to separate from flue gas or coal gasification process and therefore might be the preferred disposal option. Residual, solubility, and mineral trapping of CO2 and other acid gases could immobilize acid gases at different spatial and time scales. Depending on the spatial distribution and temporal evolution of injected acid gases and associated mineral dissolution and precipitation (artificial diagenesis), the gas-fluid-rock interactions could have a significant impact on AGI. The objective of this study is to apply Reactive Transport Modeling (RTM) to investigate the fundamental controls, such as injection design, and key natural variables on AGI performance and long-term storage security. The results from RTM provide valuable insights for analyzing and assessing the dynamic behaviors of AGI, identifying and characterizing potential storage sites, and managing injection performance and reducing costs.
This presentation will summarize recent results from RTM of AGI in both siliciclastic and carbonate reservoirs. In particular, we investigated AGI in a feldspar rich and a quartz dominated sandstone reservoirs, and a limestone and a dolomite dominated carbonate reservoirs. We studied key technical issues onthe short- and long-term behavior of gas-fluid-rock interactions in geological formations;the role of reservoir mineralogy on injection performance and storage security;the effect of gas mixtures (e.g., H2S and SO2) on CO2 storage; andthe physical and chemical processes associated with CO2 and AGI in the primary storage reservoir.
The key conclusions from the simulations include:· The behavior of CO2 and other acid gases during injection and storage is controlled by the gas mixture composition, reservoir mineralogy, timing, and injection design. Co-injection of H2S yields similar behavior compared to CO2 injection in both siliciclastic and carbonate reservoirs.· Co-injection of SO2 can potentially result in the formation of sulfuric acid and extremely low pH close to the well. Corrosion of pipes and well abandonment are potential risks.