STRUCTURAL GEOMETRIES – IMPLICATIONS FOR CO2 STORAGE

Structural geology tends to attract people with good spatial skills, and those skills are further developed and honed through the solution of structural geology and tectonics problems. Those skills are also valuable for CO₂ injection problems, where CO₂ is injected into the Earth for long-term storage. One of the most reliable CO₂ storage sites includes structural traps – porous volumes of rock enclosed in rocks that impede the migration of buoyant fluids, like super-critical CO₂. Trap analysis – the set of processes used by structural geologists in the petroleum industry to evaluate the adequacy of a trap for containing oil and gas fluids – is fundamentally a geometric problem in which different potential leak geometries are considered and appraised. Geometric leak points include:

• Synclinal spill points developed in structural saddles
• Fault juxtaposition leak points where permeable beds on either side of a fault are connected
• Stratigraphic leak points where permeable beds are connected across seals either by processes of erosion or non-deposition

In many cases the imaging data used to define these leak connections are imprecise or inadequate, so these geometric relations are diagnosed by indirect means like the distribution of different fluid types or pressure data. The inversion of these indirect data types for leak geometries can sometimes be complex, and good 3-D spatial skills are required.

Trap analysis processes are adopted for CO₂ storage site selection, but differences in work processes arise to account for fundamental differences in the goal of the analysis. In oil and gas exploration, some level of trap failure is tolerated on an economic basis, but trap leakage to surface for CO₂ storage is probably intolerable. Because subsurface data are imperfect for defining leak geometries, this uncertainty must be accounted for in any analysis of CO₂ storage site. We propose a method for maximizing the amount of CO₂ injected into the Earth while minimizing the risk of CO₂ leakage to surface by considering a larger geometric problem that provide redundancy in trap volumes.