Capstone 6: CO₂-storage site characterization

The final reservoir-characterisation capstone closes the textbook with the most modern application: CO₂ geological storage. The decision-relevant question is not "what is the expected caprock thickness?" but "what is the PROBABILITY OF CONTAINMENT FAILURE?". This is a rare-event question driven by the LOWER TAIL of the thickness distribution — exactly the question geostatistical realisations are built to answer.

The integrity-rule framework

Storage-site integrity rests on the caprock seal. A typical regulatory criterion: caprock thickness must exceed a critical threshold t_crit (e.g., 30 m) EVERYWHERE within the CO₂ plume footprint. The failure event is:

This is the probability that AT LEAST ONE cell is below threshold, NOT the probability that the mean is. Note the dramatic asymmetry: a site with mean = 50 m and SD = 12 m has very low P(mean < 30) but the P(min cell < 30) across a 32×32 grid is much higher — because the min has heavier left tail than the mean.

Why realisations matter HERE more than for OOIP

OOIP integrates over many cells — central-limit-style averaging reduces tail risk. Min thickness is the OPPOSITE — it's an extreme-value statistic that is MORE sensitive to tail behavior than to the mean. Generating realisations and counting failures gives the correct rare-event probability; closed-form solutions exist only for very specific assumptions (Gaussian min) but the spatial dependence makes those unreliable.

Workflow

• Conditioning data: caprock thickness measurements at K wells (with measurement noise).
• Variogram model: spherical, range determined from sequence-stratigraphic continuity and adjacent depositional analogs.
• SGS realisations: 40+ realisations honoring well measurements and the variogram model.
• Min-thickness per realisation: for each realisation, find the minimum thickness across the storage footprint.
• P(failure) estimate: fraction of realisations with min thickness < t_crit. Repeat across alternative variogram parameters for sensitivity.

Try it

• Defaults (μ = 50, σ = 12, threshold = 30, range = 8). Mean is 50 m, threshold 30 m, safety margin 20 m — sounds safe. But ~5-15% of realisations show min thickness < 30 m somewhere. P(failure) is dramatically larger than P(mean < threshold) (which is near-zero).
• Set σ = 4 (small thickness variability). P(failure) drops dramatically. Tight variability = small left tail = safe site.
• Set σ = 25 (large variability). P(failure) explodes to 40%+. Heterogeneous caprock = thin spots somewhere = unacceptable risk.
• Set range = 14 (long-correlation caprock). Thin spots become geographically larger (correlated thin zones). P(failure) is similar to short-range case but the FAILED REGIONS are larger and more identifiable.
• Increase threshold to 40 (more conservative regulator). P(failure) shoots up. The cost of safety: more sites rejected.

A regulator demands P(failure) < 1% before approving CO₂ storage. The site characterisation shows P(failure) = 8%. The asset team proposes "we'll just monitor and shut in if leakage starts". What are the REGULATORY and TECHNICAL flaws of this proposal, and what does Bayesian decision-theory say should be the alternative?

What you now know

You can characterise CO₂-storage sites with statistical containment-failure probabilities. The same realisation-and-count machinery applies to: leak-detection-system design, monitoring-well placement, induced-seismicity risk, post-injection plume extent. CO₂ storage is one of the highest-impact applications of geostatistics — done right, it enables gigatonnes of permanent CO₂ sequestration; done wrong, it produces ineffective or unsafe projects.

This closes Part 10 — the six reservoir-characterisation capstones cover the major reservoir types and analytical decisions. Part 11 quizzes test the cumulative material; Part 12 provides the master-workflow reference card. You now have the complete OgbonLab Geostatistics curriculum.

Mark capstone complete →

References

• IEAGHG (2019). "Caprock systems for CO₂ geological storage." IEAGHG report 2019-08.
• Chadwick, R.A., Eiken, O., Lindeberg, E. (2009). "Best practice for the storage of CO₂ in saline aquifers." BGS / NORSAR / SACS.
• Bachu, S. (2008). "CO₂ storage in geological media: role, means, status." Energy Procedia 1(1), 3743-3750.
• Pyrcz, M.J., Deutsch, C.V. (2014). Geostatistical Reservoir Modeling, 2nd ed. (Closure: from petroleum to CO₂ applications.)
• IPCC (2005). Special Report on Carbon Dioxide Capture and Storage. Cambridge University Press. (Foundational regulatory framework.)