The Dual-Water Model
Learning objectives
- Describe the bound-water and free-water split
- Solve for total water saturation and peel off the bound part
- Compute the effective (free) saturation Swe = (Swt - Swb)/(1 - Swb)
- Explain why a shaly zone can read wet yet produce clean
Two Waters, Not Cations
Dual-Water, from Clavier, Coates, and Dumanoir, keeps the Waxman-Smits physics but tells it as a story of two waters instead of clay cations, which makes it easier to apply. The clay holds a thin film of bound water at its charged surface, conductive and immovable; the rest of the pore holds ordinary free formation water. The resistivity tool sees the conductance of both together, so the model first solves for the total water saturation and then peels off the bound part to leave the producible water.
Total, Bound, Effective
Three saturations do the work. The total comes from the combined conductance of bound and free water. The bound grows with the clay content, a property of the rock. What is left, the effective or free water saturation, is what a clean-Archie answer should really have been:
The bar shows the pore split into bound water, free water, and hydrocarbon. As the shale rises the bound slice grows and eats the free slice, even while the total water barely changes.
Read Wet, Produce Clean
That is the punchline of the whole chapter. Bound water is held by the clay and cannot flow, so only the free water counts against the pay. A shaly sand can therefore read a high water saturation by clean Archie, look marginal or wet, and still produce hydrocarbon almost water-free, because most of its water is bound. Dual-Water captures this without measuring directly, which is why it became the routine choice in many modern interpretation packages.
References
- Clavier, C., Coates, G., and Dumanoir, J. (1984). Theoretical and experimental bases for the dual-water model. SPE Journal, 24(2).
- Asquith, G. and Krygowski, D. (2004). Basic Well Log Analysis, 2nd ed. AAPG Methods in Exploration 16.