Water Saturation

Water saturation (Sw) is the second key parameter that must be estimated to determine net pay. Three water saturation values need to be specified to determine net pay adequately.

The first is irreducible water saturation for the formation, or Swirr, which is the water bound by capillary forces to the matrix that will not move.

The second is water saturation in the reservoir at the time the well logs were obtained, or Sw. If Sw is equal to the irreducible Sw, then the zone should produce no water. As Sw increases above Swirr, higher and higher water cuts can be expected.

At some point the third Sw value (Swcritical) is reached, where the water cut is unacceptable. This impacts net pay determination is the Sw where water cuts become uneconomic.

Because the SPE definition of reserves includes an economic caveat, factors such as water-disposal costs and lifting costs enter into the equation. The current Sw is generally calculated from log data once the Sw model has been calibrated to irreducible Sw data. The Swirr can be obtained from core capillary-pressure data and from core-calibrated Nuclear Magnetic Resonance (NMR) log data. The NMR log should have a calibrated T2 value to discriminate mobile and nonmobile fluids because the default value of 33 ms does not apply to all reservoirs. Swcritical can be obtained from core relative-permeability data and fractional flow calculations if available. A more commonly used technique is to compare Sw values from the logs with actual water cuts in the field.

Once the Sw model is calibrated to Swirr from core capillary-pressure data or NMR log data, the Sw in the reservoir that results in uneconomic produced water cuts is probably a reasonable first estimate of the critical Sw value in the absence of relative permeability and fractional flow data from cores. This Sw value is typically in the 50- to 70-percent range in most light oil and gas reservoirs where high water cuts are unacceptable, the lower Sw critical values usually being associated with higher permeability rocks. The value is also a function of viscosity, for heavy-oil values as low as 25 to 30 percent Sw have resulted in uneconomic water cuts.