plume can be most accurately characterized, its rate of growth quantified, and CO2 flux at the reservoir top estimated. Seismic reflection amplitude maps (Figure 30) show how the topmost layer has grown from two small patches in 1999 to an accumulation of considerable lateral extent by 2002. A north-trending linear prolongation is prominent, corresponding to CO2 migrating northwards along a linear ridge at the reservoir top.
The volume of CO2 within the topmost layer was computed for the three methods of thickness determination (Table 7), assuming a mean sand porosity of 0.38 with saturations computed using a laboratory determined relationship between buoyancy forces and capillary pressure. From the topmost layer volumes, the rate at which CO2 has arrived at the top of the reservoir can be estimated. Taking, for example, the amplitude-structure thicknesses, an estimated 1.8 x
105 m3 of CO2 arrived at the reservoir top between the 1999 and 2001 surveys, an average flux of ~250 m3 per day. Between the 2001 and 2002 surveys ~1.1 x 105 m3 of CO2 arrived at the reservoir top, an average flux of ~450 m3 day-1. Between the 2002 and 2004 surveys a further
~3.1 x 105 m3 of CO2 arrived at the reservoir top, averaging ~400 m3 day-1. These volumes correspond to ~3.7%, ~6.2 % and ~6.5% of the total amount of CO2 injected during the respective periods. Measurements on the 2004 dataset are, as yet, preliminary, but the data nevertheless indicate an early increase in flux rates followed by stabilization. Comparisons of observed fluxes derived from the seismic data do not match the flow simulation in this study,
due to the possibility that chemical reactions of CO2 with mudstone mineralogies are producing new mineral phases capable of significantly reducing mudstone porosity and, by implication, permeability (Chadwich et al., 2006). The analysis indicates that, following early and quite rapid establishment of flow pathways, mudstone flow properties have remained fairly stable. This improves confidence in likely caprock stability in the presence of CO2, and more generally in the validity of longer-term simulations of plume development.
plume can be most accurately characterized, its rate of growth quantified, and CO2 flux at the reservoir top estimated. Seismic reflection amplitude maps (Figure 30) show how the topmost layer has grown from two small patches in 1999 to an accumulation of considerable lateral extent by 2002. A north-trending linear prolongation is prominent, corresponding to CO2 migrating northwards along a linear ridge at the reservoir top.
The volume of CO2 within the topmost layer was computed for the three methods of thickness determination (Table 7), assuming a mean sand porosity of 0.38 with saturations computed using a laboratory determined relationship between buoyancy forces and capillary pressure. From the topmost layer volumes, the rate at which CO2 has arrived at the top of the reservoir can be estimated. Taking, for example, the amplitude-structure thicknesses, an estimated 1.8 x
105 m3 of CO2 arrived at the reservoir top between the 1999 and 2001 surveys, an average flux of ~250 m3 per day. Between the 2001 and 2002 surveys ~1.1 x 105 m3 of CO2 arrived at the reservoir top, an average flux of ~450 m3 day-1. Between the 2002 and 2004 surveys a further
~3.1 x 105 m3 of CO2 arrived at the reservoir top, averaging ~400 m3 day-1. These volumes correspond to ~3.7%, ~6.2 % and ~6.5% of the total amount of CO2 injected during the respective periods. Measurements on the 2004 dataset are, as yet, preliminary, but the data nevertheless indicate an early increase in flux rates followed by stabilization. Comparisons of observed fluxes derived from the seismic data do not match the flow simulation in this study,
due to the possibility that chemical reactions of CO2 with mudstone mineralogies are producing new mineral phases capable of significantly reducing mudstone porosity and, by implication, permeability (Chadwich et al., 2006). The analysis indicates that, following early and quite rapid establishment of flow pathways, mudstone flow properties have remained fairly stable. This improves confidence in likely caprock stability in the presence of CO2, and more generally in the validity of longer-term simulations of plume development.
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