PTA Application in Carbon Capture and Storage – Challenges, Diagnostics, Modeling

This paper documents pressure transient analysis (PTA) aspects of carbon capture and storage (CCS), from design considerations through modeling while also highlighting attendant challenges. CO2 injection in porous media has expanded in utility and increased in popularity following global consensus on decarbonization. To this end, CCS is unarguably a viable decarbonization option for the petroleum industry. However, it requires a reliable understanding of the subsurface in terms of quality, integrity, and capacity. In this study, we expatiate on the place of PTA analysis in long-term CO2 storage in porous media. This work focuses on CO2 injection in saline aquifers; nonetheless, findings here can be easily adapted to depleted oil and gas systems. We first discuss CO2 injection-falloff test design vis-à-vis expectations from PTA-driven formation description. This is followed by an overview of analysis techniques that guarantee reliable estimate of injectivity and robust characterization. We then demonstrate the use of transient modeling to obtain a first-level evaluation of storage capacity in addition to improved characterization. The challenges encountered are also discussed. Our findings show that knowledge of potential storage sites is indispensable in successful CO2 sequestration. A properly designed and carefully implemented well test remains a viable window to acquiring representative dynamic data which, upon analysis, provides valuable information about the storage media. Diagnostics and modeling must be supported with accurate description of the injection fluid as well as the in-situ formation fluid, otherwise the results may be in serious error. This paper provides guidance toward best practices on the design, diagnostics and modeling of CO2 injection-falloff tests for the purpose of robust formation description and long-term CO2 sequestration.