Abstract

Sequestration of carbon dioxide in a saline aquifer is currently being evaluated as a possible way to handle carbon dioxide emitted from a coal-fuelled power plant in Svalbard. The chosen reservoir is a 300 m thick, laterally extensive, shallow marine formation of late Triassic-mid Jurassic age, located below Longyearbyen in Svalbard. The reservoir consists of 300 m of alternating sandstone and shale and is capped by 400 meter shale.

Experimental and numerical studies have been performed to evaluate CO2 storage capacity and long term behaviour of the injected CO2 in rock pore space. Laboratory core flooding experiments were conducted during which air was injected into brine saturated cores at standard conditions. Analysis of the results shows that the permeability is generally less than 2 millidarcies and the capillary entry pressure is high. For most samples, no gas flow was detected in the presence of brine, when employing a reasonable pressure gradient. This poses a serious challenge with respect to achieving viable levels of injectivity and injection pressure.

A conceptual numerical simulation of CO2 injection into a segment of the planned reservoir was performed using commercial reservoir simulation software and available petrophysical data. The results show that injection using vertical wells yields the same injectivity but more increases in field pressure compare to injection through horizontal wells. In order to keep induced pressure below top-seal fracturation pressure and preventing the fast propagation and migration of CO2 plume, slow injection through several horizontal wells into the lower part of the "high" permeability beds appears to offer the best solution.

The high capillary pressure causes slow migration of the CO2 plume, and regional groundwater flow provides fresh brine for CO2 dissolution. In our simulations, half of the CO2 was dissolved in brine and the other half dispersed within a radius of 1000 meter from the wells after 4000 years. Dissolution of CO2 in brine and lateral convective mixing from CO2 saturated brine to surrounding fresh brine are the dominant mechanisms for CO2 storage in this specific site and this guarantees that the CO2 plume will be stationary for thousands of years.

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