Bulk-Volume Water

Water in the Whole Rock

Archie gives the water saturation, the fraction of the pore filled with water. Multiply it by the porosity and you get the bulk-volume water, the fraction of the whole rock that is water:

In the Ogbon-1 pay, $\phi = 23$ and $S_w = 19$, so $\text{BVW} = 0.23 \times 0.19 \approx 4.4$. Only about one part in twenty of the rock is water; the rest is grain and oil. This small number turns out to be one of the most diagnostic in the whole analysis.

Why It Goes Constant

Here is the key fact. When a rock sits at irreducible water saturation, the only water left clings to grain surfaces and the narrowest pore throats, held by capillary forces that the buoyancy of the oil column cannot overcome. The amount it can hold is set by the rock's internal surface area, which is a property of the rock texture, not of how high above the water the rock happens to be. A clean coarse sand has little surface and holds little water ($\text{BVW}\approx 0.02$); a fine silty sand has much more surface and holds more ($\text{BVW}\approx 0.08$). So across a whole zone at irreducible water, porosity and $S_w$ each vary, yet their product stays nearly fixed.

The Buckles Plot and Free Water

That constant product is a hyperbola on a plot of porosity against $S_w$, the Buckles line $\phi,S_w = \text{const}$. Its payoff is a clean test for movable water: points strung along a single Buckles line are all at irreducible water and will produce hydrocarbon without water, while a point lifted above its neighbours' line carries more water than capillarity can hold, so that extra water is free and the zone will make some water on production. BVW thus answers the question Archie alone cannot: not just how much water is there, but whether it will move.

References

• Buckles, R. S. (1965). Correlating and averaging connate water saturation data. Journal of Canadian Petroleum Technology, 4(1).
• Asquith, G. and Krygowski, D. (2004). Basic Well Log Analysis, 2nd ed. AAPG Methods in Exploration 16.