Pressure Fall-off Method for CO2 Leakage Detection in Deep Saline Aquifers
- Christine A. Ehlig-Economides (Texas A&M U.) | Abhishek Anchliya (Chevron Energy Technology Company) | Bo Song (Texas A&M U.)
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- Society of Petroleum Engineers
- SPE EUROPEC/EAGE Annual Conference and Exhibition, 14-17 June, Barcelona, Spain
- Publication Date
- Document Type
- Conference Paper
- 2010. Society of Petroleum Engineers
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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.
Deep saline aquifers contain saline water, or brine, in rock pore space at depths of several thousand feet. The brine is too saline to be used for industrial, municipal, or agricultural purposes, and studies by NETL  and the IPCC  suggest that the underground volume in deep saline aquifers is more than sufficient for envisioned volumes of CO2 to be sequestered.
When bulk CO2 is injected into the aquifer, it accumulates around the injection well creating a zone of high CO2 saturation taking up nearly 100% of the pore space. This zone is called a dry zone because nearly all of the water is displaced away from the well. Because CO2 is less dense than the brine it also rises near the well forming a plume that spreads at the top of the aquifer. Immediately above the aquifer is a seal that prevents the brine from flowing to the shallower horizons, and the seal should also prevent upward migration of the CO2. However, if the CO2 managed to find a hole in the aquifer seal, it could rise and flow into other formations. Well above the depths envisioned for CO2 sequestration may be fresh water aquifers suitable for drinking water, irrigation or other uses. Clearly, injected CO2 must not leak from the aquifer intended for storage.
Oddly, there is little evidence in the literature that current demonstration CO2 sequestration projects have emphasized an obvious approach to monitor injection wells using pressure measurements. This paper shows how regular pressure falloff tests can monitor the expanding radius of the dry zone and would readily show the presence of leaking CO2 or brine from the aquifer. The CMG-GEM™ simulator [Nghiem, et al 2004] was used to generate simulations for pressure and saturation behavior during bulk CO2 injection. Periodic injection falloff tests were simulated to show what behavior can be observed over 30 years of continued injection.
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