Seal or Leak
Learning objectives
- Frame caprock integrity as two competing failure modes: fault reactivation and hydraulic fracturing
- Compute the safe injection ceiling as the smaller of the fault-reactivation and fracture limits
- Show the storage verdict: a fault-free caprock has a large budget, a critically-oriented fault leaves almost none
- Connect to the CO2 storage sections of the Synthetic Seismic and Reservoir courses
The Two Ways a Caprock Fails
Part 9 built the machinery; this section spends it on a decision operators actually face: inject CO2, wastewater, or gas beneath a caprock, and ask whether the caprock will seal or leak. Raising the reservoir pressure threatens the seal two ways. First, it can reactivate a fault that cuts the caprock, opening a leak path along the slipped plane, the reactivation of Sections 9.1 to 9.3. Second, with no fault, enough pressure will hydraulically fracture the caprock, when the reservoir pressure reaches the minimum stress , the fracture gradient of Part 7. The safe injection ceiling is the smaller of the two limits.
On the Ogbon-1 caprock the two limits could hardly differ more. If a critically-oriented fault penetrates the seal, its reactivation pressure is barely half a MPa above the initial reservoir pressure: almost no injection budget at all. If the caprock is intact and unfaulted, the binding limit is the fracture pressure, at 46 MPa, a full 10.7 MPa of headroom above the initial 35.3. Toggle the fault and watch the safe ceiling collapse from 10.7 MPa to under one. The single most important thing about a storage site is whether a well-oriented fault cuts its seal.
Site Selection Is Stress Selection
This is why CO2 storage site selection is, at its core, a geomechanics problem. A structurally simple, unfaulted anticline with a thick, strong caprock in a relaxed stress state can absorb decades of injection with the fracture pressure comfortably out of reach. A faulted trap in a critically-stressed field, even with a perfect seal on paper, can leak the moment injection nudges a well-oriented fault. The stress model of Part 8 and the fault map of Section 9.2 are the two things a storage assessment cannot skip, and this course has now built both. The verdict reads off the reactivation and fracture limits, and the honest answer is often not a flat yes or no but a condition: seal, provided you inject slowly and stay well below the reactivation pressure of the worst fault. The same logic governs any operation that raises pressure beneath a seal, gas storage, disposal, pressure maintenance; and its mirror, depletion, lowers reservoir pressure and can breach a seal differently, by compaction and by shifting the stresses on bounding faults, which is where Part 10 begins.
The End of the Fault Story
Part 9 closes here. It took the assembled Ogbon-1 stress model and asked what it means for the faults cutting the field: which are dangerous (slip tendency), how much pressure they can take (reactivation), how large an event they could drive (the McGarr bound), and whether the caprock seals (this section). The through-line has been orientation and margin, and a critically-stressed field carries little of either to spare. Part 10 turns from faults to the reservoir itself, and to the slow, certain stress change every producing field undergoes as its pressure falls: depletion, compaction, and the surface subsidence they can drive. The cross-references to the CO2-storage treatments in the Synthetic Seismic (smk_8_6) and Reservoir Modeling (res_13_5) courses pick up the flow and monitoring sides that geomechanics hands off.
References
- Zoback, M. D., & Gorelick, S. M. (2012). Earthquake triggering and large-scale geologic storage of carbon dioxide. Proceedings of the National Academy of Sciences, 109(26), 10164-10168.
- Rutqvist, J. (2012). The geomechanics of CO2 storage in deep sedimentary formations. Geotechnical and Geological Engineering, 30(3), 525-551.
- Streit, J. E., & Hillis, R. R. (2004). Estimating fault stability and sustainable fluid pressures for underground storage of CO2. Energy, 29(9-10), 1445-1456.