Abstract
Gas mixing in the subsurface could have crucial implications on CO2 storage capacity and security. This study illustrates the impact of gas mixing in the “Captain X” CO2 storage site, an open saline aquifer and subset of the greater Captain aquifer, located in the Moray Firth, North Sea. The storage site hosts several abandoned hydrocarbon fields where injected CO2 could interact and mix with any remaining hydrocarbon gas left in the depleted structures. For this study, compositional simulation of CO2 injection into the Captain X storage site reservoir model was conducted to quantify the impact of mixing. Results show mixing of CO2 with the remaining trapped hydrocarbon gas makes the plume considerably less dense and more mobile. This increases the buoyancy forces acting on the plume, causing it to migrate faster towards the shallower storage boundaries and therefore, reduces the storage capacity of the site. Mixing also compromises the storage security as it mobilises the structurally trapped hydrocarbon gas from within the abandoned fields. Informed injector placement helps to manage and reduce the impact of mixing. Correct assessment of mixing is also considerably dependent on the volume and property of the trapped hydrocarbon gas. To provide a correct long term understanding of storage capacity and security, the impact of mixing, therefore, needs to be correctly considered in all large-scale CO2 storage operations.
Highlights
Despite continued development of renewable energy sources, fossil fuels are expected to remain a key source of energy for the foreseeable future [1,2]
We develop this scenario with an initial guess of 60 MT for CO2 storage capacity in Captain X, similar to that estimated in the ETIUKSAP study in that CO2 will be injected into the model for 20 years followed by 1000 years shut-in period to allow plume migration simulation during that period
This study shows that mixing between injected CO2 and trapped hydrocarbon gas within the storage site has significant impact on the storage capacity and security, in the Captain X open saline aquifer system
Summary
Despite continued development of renewable energy sources, fossil fuels are expected to remain a key source of energy for the foreseeable future [1,2]. Generates large volumes of greenhouse CO2 which contributes significantly to global warming and climate change. Subsurface storage of CO2 has been proposed as a suitable mitigation strategy to stop the rising levels of CO2 in the atmosphere in near future. The United Kingdom is committed to reduce its net global greenhouse gas emission to zero by the year 2050 [4]. One effective and secure strategy to mitigate CO2 emissions is the geological storage of CO2 in saline aquifers located in the United Kingdom Continental Shelf (UKCS) [5,6]. Saline aquifers, located in sedimentary basins all over the world, are excellent targets for CO2 storage as they offer considerably larger storage capacities than many other settings [7]
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