An additional uncertainty arises from saturation profiling, which discerns between pores filled with brine and pores filled with CO2 above a poorly constrained level of saturation (Arts et al., 2004). It follows from the commonly used Gassmann equation (Gassmann, 1951) that: (a) very low saturations of less than a few percent are undetectable; (b) a strong correlation emerges as the gas saturation increases from a few percent to around thirty percent; and (c) for saturations much above thirty percent, it is difficult to distinguish between moderate and high saturations. It follows that the 80% gas saturation that is commonly assumed for the Sleipner plume, while reasonable (Chadwick et al., 2005; Bickle et al., 2007), remains uncertain (Lumley, 2008). If the 80% assumption represents a reasonable upper limit to the mean saturation for the plume, the lower limit could be as low as 40%, halving mass balance estimates premised on the widely assumed high-saturation value.
gas saturation
a few percent to around thirty percent
percent
saturations
above thirty percent
percent
Sleipner plume
gas saturation
80%
%
mean saturation for the plume
upper limit
80%
%
mean saturation for the plume
lower limit
40%