This project is about so-called "chimneys" or pipe-structures in sedimentary basins, and what they imply for secure and stable CO2 storage. Currently, geophysicists observe chimneys in most sedimentary basins due to improved seismic imaging. They often see them in the caprock of reservoirs and aquifers. These chimneys appear as blow-out structures in the caprock produced by reservoir overpressure. They could be fluid escape pathways from the reservoir long time after they formed.
High reservoir fluid pressure initiates chimneys in the caprock. After initiation, it ascends towards the surface driven by the reservoir pressure. The process searches upwards because the least compressive stress is decreasing with decreasing depth. Reservoir fluids leak out through the chimneys, which leads to reduced reservoir pressure. Chimney formation continues until the reservoir pressure drops below a critical limit.
There are several large chimneys in the caprock of the Utsira Formation in the North Sea, and they are close to the storage site for CO2 from the Sleipner field. These chimneys appear to have been produced by high overpressure in the Utsira aquifer. These pipe-structures must have formed towards the end of the last glaciation because they cut through the entire caprock of glacial sediments. We need to understand how the glacial processes created high pore pressure in the Utsira aquifer, and what it means for leakage through chimneys in today's caprock. Therefore, we are currently developing models for pressure build-up by glacial processes. The question is if pressure build-up by rapid deposition of glacial sediments could produce chimneys. A related topic is the generation of additional overpressure by glacial loading.
We have for a long time studied how viscous deformations can form pipe-structures in sedimentary basins. The viscosity of sedimentary rocks is an unconstrained parameter in these models, and it is, therefore, studied.
We need more knowledge about pipes in sedimentary basins, and to what extent they are leakage pathways for fluids in reservoir units. This knowledge is important for the safe storage of CO2. It is also new and fundamental knowledge about fluid flow in sedimentary basins. The seabed above pipes should be monitored when they are observed in the caprock of CO2 store sites. Monitoring is the best way to assure that they are not leaking.
Prosjektet har studert pipestrukturer i takbergarten på CO2-deponier som Utsira fm. Disse pipene ble dannet i takbergarten av Utsira mot slutten av den siste istiden. Tilsvarende lekkarsjestrukturer har blitt observert de fleste steder i sedimentære bassenger. Vi har studert to typer modeller for dannelse av pipestrukturer: En model for viskøse deformationer av de porøse sedimentene, og en annen model for hydraulisk oppsprekking og damage av sedimentene. Begge modelltypene indikerer at disse strukturene ikke har stor nok permeabilitet til gi betydelig lekkarsje. Men vi har femdeles lite observasjoner av væskestrømning i pipestrukturer. Derfor konkluderer vi med at de bør monitorerers for lekkasje. Dette er spesielt viktig for piper som befinner seg over eller i nærheten av et CO2-deponi. Monitoreringen skjer på havbunnen.
Fluid escape pipes (chimneys) are considered leakage pathways during subsurface CO2 storage, affecting stability and safety of long term storage of CO2. The proposed project has as main objective to deliver models for chimney formation, and models for fluid flow through both forming and pre-existing chimneys. Pipe structures (chimneys) are currently observed almost everywhere in sedimentary basins, especially due to improved seismic imaging over the last years. Nevertheless, the formation of chimneys and their flow properties remain poorly understood. This project has four sub-goals to gain this knowledge: First, to model chimney formation and fluid flow through pipe structures. For this purpose, we will study processes that have led to chimney formation in the geological past and we will evaluate critical parameters for triggering chimney formation during subsurface CO2 injection. Second, we will estimate the leakage rates through existing chimneys and how fast they might discharge the stored CO2. Third, we will evaluate injection scenarios for the Smeaheia site in the North Sea with respect to chimney formation and leakage through chimneys. The case study will be based on the geological model of Smeaheia made available for this project by IFE´s collaboration with Statoil through the FME SUCCESS. Finally, the project will develop monitoring strategies for detection of CO2 leakage. The numerical codes developed and applied in the project will be made open source. Therefore, it will be straightforward for third parties to build on our results, once they are granted access to the data by Statoil.