Long term predictions of clay cap rock reactivity at Sleipner

CO2 is being injected into the Utsira sand, which is overlain by the 250 m thick Nordland shale acting as a cap rock. Due to buoyancy effects the injected CO2 moves upward from the injection point and accumulates under the overlying cap rock - under the current conditions CO2 is in a supercritical state. The CO2 will partially dissolve into the cap rock formation water and may thus subsequently diffuse upward into the cap rock. It is likely that this will lead to reaction with minerals present in the cap rock. Studies of CO2 migration in low-permeability caprocks indicate that diffusion is an extremely slow process (e.g. Hildenbrand et al., 2002)

Numerical modelling was used to predict the long term effects of geochemical reactions on the porosity of the cap rock (Gaus et al., 2005). A conceptual diagram for the modelled scenario is shown in Figure 1. The upward diffusion into the cap rock has been modelled via 1D reactive transport modelling combining reaction kinetics and diffusive transport. The modelling code used is PHREEQC (V2.6) (Parkhurst and Appelo, 1999).

Figure 1. Conceptual model of dissolved CO2 diffusion and reactivity into the base of the cap rock at Sleipner. "Reprinted from Chemical Geology (Vol 217, 2005, Gaus et al., p 319-337, Copyright 2005) with permission from Elsevier".

At the time of the modelling study, no cap rock core was available. The mineralogy of the Nordland shale cap rock used for the modelling was based on the composition of a selection of cutting samples taken at the base of the cap rock. As no porosity measurements were available, a porosity of 5% was assumed based on observations from the Nordland Shale in UK Quadrant 16, northern North Sea. The initial chemical compositions of the cap rock pore-waters were assessed by equilibrating the water chemistry of the Utsira formation with the caprock mineralogy by means of geochemical modelling. In the absence of more accurate data, this is a common procedure to reconstruct the pore-waters chemistry of argillaceous rocks, given the long time scale over which this equilibrium could be established.

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Figure 2. Profiles of impact of dissolved CO2 in the cap rock after 3000 years at Sleipner. A) diffusion of dissolved CO2 into the caprock. B) dissolution and precipitation of minerals. C) porosity changes "Reprinted from Chemical Geology (Vol 217, 2005, Gaus et al., p 319-337, Copyright 2005) with permission from Elsevier".

Figure 2. Profiles of impact of dissolved CO2 in the cap rock after 3000 years at Sleipner. A) diffusion of dissolved CO2 into the caprock. B) dissolution and precipitation of minerals. C) porosity changes "Reprinted from Chemical Geology (Vol 217, 2005, Gaus et al., p 319-337, Copyright 2005) with permission from Elsevier".

The diffusion of dissolved CO2, the major mineralogical changes and the porosity changes after 3000 years are shown in Figure 2. The main results can be summarized as follows:

• Diffusion of CO2 in the cap rock is predicted to be a slow process. After 3000 years, only the lower 10 meters of the 250 m thick cap rock have elevated dissolved CO2 concentrations. Depending on the reactivity of the cap rock, diffusion can be further retarded because the dissolved CO2 is consumed by feldspar alteration reactions.

• The calculated porosity change is small and limited to the bottom few metres of the cap rock. A decrease in porosity is predicted, with a maximum of 3% for the most reactive cap rock composition modelled. This will reduce the rate of diffusion and therefore improve the cap rock sealing capacity.

• At the very bottom of the cap rock some carbonate dissolution is expected to occur, leading to a minor porosity increase. However, this is predicted to not migrate further into the cap rock.

These conclusions are based on the assumption that the cap rock is a homogeneous medium and that diffusion is the only mass transport mechanism in the cap rock causing the displacement of dissolved CO2.

Further investigations should aim to reduce uncertainties and strengthen confidence in the predictions. A core sample of the base of the cap rock is needed to obtain an accurate mineralogical and chemical characterisation and to initiate a series of laboratory experiments. The presence of any fractures that might lead to preferential pathways has to be studied. Information about the behaviour of similar clay cap rocks in natural CO2 fields has to be searched for. Once information from these studies has been obtained, then numerical simulations need to be revised and a detailed sensitivity analysis should be carried out. Part of this work is currently being achieved through the CO2STORE European project (2003-2005) carried out within the 5th Framework Programme.

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