Saturation: Filling the Pores

What Saturation Is

Porosity tells us how much pore space there is; saturation tells us what fills it. The water saturation $S_w$ is the fraction of the pore space occupied by water; the rest holds hydrocarbon. Because the pores are always full of something, the saturations of all the fluids sum to one:

With no free gas, the hydrocarbon (oil) saturation is simply $S_o = 1 - S_w$, so finding $S_w$ is the whole game: it is the one number that says how much of the pore space is worth producing.

Drag the water saturation and watch water (blue) and hydrocarbon (green) trade places in the pores.

A Fraction of the Pores, Not the Rock

Saturation is a fraction of the pore space, not of the whole rock. To get the actual volume of water in a unit of rock, multiply by porosity: the bulk-volume water is $BVW = \phi \cdot S_w$. A clean sand at $\phi = 0.30$ and $S_w = 0.30$ holds only $0.30 \times 0.30 = 0.09$, nine percent of the bulk rock, as water, and $0.30 \times 0.70 = 0.21$, twenty-one percent, as hydrocarbon. This nesting, fluids inside pores inside rock, is why a high porosity with a high water saturation can still be a poor target.

Wettability and Irreducible Water

Which fluid touches the grains is set by wettability. Most clean sandstones are water-wet: a film of water coats every grain and the hydrocarbon sits in the centres of the pores. Some of that water, the irreducible (or connate) water $S_{wi}$, is held so tightly by capillarity that it will never flow no matter how hard we produce. A reservoir at irreducible water produces clean hydrocarbon; one above irreducible produces water too. Recognizing irreducible water is what separates a pay zone from a wet one.

References

• Asquith, G. and Krygowski, D. (2004). Basic Well Log Analysis, 2nd ed. AAPG Methods in Exploration 16.
• Tiab, D. and Donaldson, E. (2015). Petrophysics, 4th ed. Gulf Professional Publishing.