Rock surfaces of CCS reservoirs, and their seals, either attract, repel, or are neutral to CO₂. In other words, CCS reservoir rocks and CCS reservoir-sealing rocks can be either CO₂–philic, CO₂–phobic, or CO₂–neutral. CO₂ phobicity is an underappreciated but important parameter in predicting the performance of a CCS reservoir and its seal. We present an example of well data from an inexpensive and rapid method for determining CCS reservoir phobicity, mostly from old or new drill cuttings.

The relative permeability of CO₂ is greater for potential CCS reservoirs that are more CO₂–phobic, as these reservoirs do not attract CO₂ to the rock surfaces. However, the relative permeability of CO₂ will be lesser for potential CCS reservoirs that are more CO₂–philic, as these reservoirs do attract CO₂ to the rock surfaces.

All else being equal, CCS reservoir seals will be more effective if they contain layers of CO₂–philic strata, i.e., strata with rocks having surfaces that bond strongly with CO₂. Smaller diameter pore throats are blocked to CO₂ migration in CO₂–philic rocks by CO₂ blocking.

Rock Volatiles Stratigraphy is a rapid, inexpensive method of determining CO₂ phobicity from the analyses of CO₂ gently extracted at multiple pressures, frozen and concentrated onto liquid nitrogen traps, and analysed with other volatiles through sublimation and vapor transport into a mass spectrometer. The most common rock samples analysed are drill cuttings. Cuttings can be from old or new wells. The well can have been drilled with either a rock or PDC bit. The mud can be either OBM or WBM. Archived samples from old oil and gas wells provide inexpensive or free materials for determining CO₂ phobicity.

The standard RVStrat analyses extracts CO2 and other volatiles from a single 400 microliters sample of cuttings or core at 2 pressures, 20 millibars and 2 millibars. Each extraction takes 7.5 minutes.

Figure 1 shows mole fraction of CO₂ analysed at 20 millibars versus 2 millibars, for potential CCS reservoir in Texas state waters of the Gulf of Mexico. The gray shaded areas are shales and the yellow lines are maximum flooding surfaces.

Extreme CO₂–phobicity occurs between 6250 and 7500 feet. In this depth range almost all the cuttings samples’ CO₂ is extracted at 20 millibars and very little CO₂ is extracted at 2 millibars. The lower Miocene sands in this depth range show no indication of CO₂ bonding to the surfaces of the rocks’ grains. Almost all the CO₂ from these samples is extracted at the higher pressure, 20 millibar, extraction.

Between 5450 and 6500 feet the 20 millibar and 2 millibar CO₂ curves converge, and even cross over towards the core of this zone at multiple depths. This more CO₂–philic zone composed of alternating layers of more and less CO₂–philic strata is directly above the underlying zone of CO₂–phobic sands.

The variable CO₂–philicity of this zone may be the signature of a CCS seal of superior quality. The isolated layers of CO₂–philic strata surrounded by more CO₂–phobic strata suggest a permeability trap through which CO₂ cannot pass.