Geological sequestration of carbon dioxide in deep saline aquifer is proposed as an option for mitigating CO2 emissions into the atmosphere, but multiwell injection into the same aquifer will cause pressurization. While many papers have considered long term CO2 trapping, few have addressed the role of pressure monitoring during injection as a way to ensure that the injection pressure does not exceed a regulated value below the fracturing pressure.
This work provides models for the injection falloff response that can be used for permanent pressure gauges installed at the injection interval for aquifer characterization and, in particular, for CO2 leak detection. A three region analytical composite reservoir model with sealing or constant pressure outer boundary is used to model numerically simulated falloff data for CO2 injection from a hypothetical 500 MW coal power plant. The analysis shows that regular falloff tests can monitor the advance of the dry zone and provide a reasonable estimate for the average aquifer pressure for the well injection area. The latter can be used for material balance, and simulations show that the behavior of the average pressure versus time is sensitive to the presence of a leak. Furthermore, it may be possible to determine whether the leak is located in the dry zone, two-phase zone, or the unswept brine zone.
Pressure behavior in CO2 storage aquifers has been neglected thus far in the literature. In reality, analysis of successive pressure falloff tests easily distinguishes over time whether a well is injecting into a limited volume exhibiting pseudosteady state behavior, an open aquifer with constant pressure support, or an effectively infinite aquifer. This paper spells out why pressure monitoring makes sense during CO2 injection.