Net Pay Cutoffs from Capillary Pressure

Cutoffs are used to define “net pay” and “reservoir rock.” Reservoir rock is sometimes referred to as “net sand.” Reservoir rock, as used herein, is rock that is permeable enough to allow hydrocarbons to move to the well bore. It meets the permeability cutoff and/or the porosity cutoff, volume of shale cutoff, and other appropriate reservoir quality criteria.

Net pay, as used herein, is reservoir rock that meets the water saturation cutoff. The water saturation cutoff is generally thought of as the highest Sw (or Swe in an effective porosity methodology) that will still produce hydrocarbons, the Sw where the oil cut is roughly 1% or higher.

Hydrocarbons exist in most reservoirs in rocks that are not considered either net pay or reservoir rock. Using conventional petrophysical and reservoir engineering methods, these hydrocarbons will not be included in the computation of hydrocarbons-in-place. So, by using cutoffs to compute net pay, a portion of the hydrocarbon-in-place is excluded. Great care must be used in selecting cutoffs so that the hydrocarbons excluded are truly not producible using current and foreseeable technology.

Capillary pressure analysis, backed up with proper core measurements, can provide justification for cutoffs. This sort of analysis can also be used to help select the proper cutoffs. The talk describes one method of building a capillary pressure model, how the parameters are selected, and how the model can be used to pick appropriate cutoffs in a heavy oil reservoir.

Cutoffs from a Capillary Pressure Model

  • Justifying cutoffs is difficult in the absence of extensive test data
  • Proper core measurements and a capillary pressure model can define reservoir rock and net pay
  • Cutoffs are important because all hydrocarbons-in-place are not equal
  • Provides a profile of the hydrocarbon/water transition zone for accurate reservoir simulation – Sw is not a constant or a simple function of porosity!
Capillary Pressure

Capillary Pressure

Capillary Pressure

Traditional OWC

Traditional OWC

Neutral Wet Example

Neutral Wet Example

Corey’s Function

Corey Function

Sw (critical)

SW critical