What It Solves
The screener gives a first-pass risk ranking for crude oils that may precipitate asphaltenes as pressure falls. It is intended for early depletion reviews where SARA and basic PVT data exist, but high-pressure microscopy, filtration, or measured AOP data are not yet available.
Colloidal Instability Index
The Colloidal Instability Index compares destabilizing fractions to peptizing fractions in a SARA assay:
CII = (Saturates + Asphaltenes) / (Aromatics + Resins)
| CII | Screening Class |
|---|---|
| < 0.7 | Stable |
| 0.7-0.9 | Metastable |
| > 0.9 | High risk |
CII is a SARA stability metric. It is distinct from de Boer pressure-density screening, ASI-style instability indices, and Yen-Mullins molecular architecture concepts.
de Boer Plot
The de Boer plot screens asphaltene risk using live-oil density against reservoir pressure minus bubble-point pressure. Petropt does not reproduce the original chart from de Boer SPE 24987 (1995); it classifies the same input space into low, medium, and high risk zones for field screening.
Hirschberg-Style Onset Flag
The Hirschberg-style flag estimates whether the pressure depletion path plausibly crosses an asphaltene onset envelope above bubble point. It follows the physical screening concept in Hirschberg et al., SPE Journal 1984, while keeping the tool lightweight for early-phase decisions.
Worked Example
Given: SARA = 55% saturates, 30% aromatics, 10% resins, 5% asphaltenes.
CII = (55 + 5) / (30 + 10)
= 60 / 40
= 1.5
Petropt verdict: high SARA instability risk.A CII of 1.5 is above the 0.9 high-risk threshold. If the PVT inputs also fall in the medium or high de Boer-style zone, the combined verdict should be treated as a strong prompt for laboratory AOP testing before final depletion or lift decisions.
Limitations
SARA-based CII is a screener, not a substitute for high-pressure microscopy, filtration, or solids deposition tests. The PVT proxy is a rough estimate when SARA is unavailable, and it should not be used as a final deposition design basis.
References
- de Boer, R.B., Leerlooyer, K., Eigner, M.R.P., and van Bergen, A.R.D. (1995). "Screening of Crude Oils for Asphalt Precipitation: Theory, Practice, and the Selection of Inhibitors." SPE 24987.
- Hirschberg, A., de Jong, L.N.J., Schipper, B.A., and Meijer, J.G. (1984). "Influence of Temperature and Pressure on Asphaltene Flocculation." SPE Journal.
- Wang, J.X. and Buckley, J.S. (2001). "Asphaltene Stability in Crude Oil and Aromatic Solvents - The Influence of Oil Composition." SPE 64994.