Storage capacity depends on quantities that are never known exactly: the area and thickness of the storage interval, its porosity, the density of CO2 at reservoir conditions, and the fraction of pore space that can realistically be used. Each has a range. Multiplying best guesses together gives one number and no sense of its reliability.
The three cases
A probabilistic estimate samples each input from its range many times and reports the distribution of results. Three points on that distribution are normally quoted:
- P90: the low case. There is a 90% probability that the true value is at least this large.
- P50: the mid case. The true value is as likely to be higher as lower.
- P10: the high case. There is only a 10% probability of exceeding it.
This is the exceedance convention. Some disciplines number the cases the other way round, so a report should always state which convention it uses.
Reading the spread
The ratio of P10 to P90 is a compact measure of how well the site is understood. A wide spread early in a project is normal and is itself a finding: it shows which measurement would narrow the range most, and so where the next unit of budget is best spent.
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