Net Pay and Net Reservoir

Common definitions of Net:

• Net Sand removes the shaly intervals
• Net Reservoir removes the low porosity intervals
• Net Pay removes the intervals of high water saturation

The Difference between Net Pay and?Net Reservoir

Net Reservoir

• The portion of reservoir rock which is capable of storing hydrocarbon
• Required for upscaling and reservoir modelling
• Relatively easy to pick

Net Pay

• “The portion of reservoir rock which will produce commercial quantities of hydrocarbon” (SPWLA) or the portion of reservoir rock which will produce or help support production of hydrocarbon over field development timescales
• Sometimes required to select perforation intervals
• Can be difficult to pick

Net Pay

How can Net Pay be Determined?

• The microlog identifies mudcake which suggests movable fluids
• When Sxo > Sw indicates moveable hydrocarbon
• Mud losses – especially for fractures
• Formation pressure, borehole gas chromatography
• Production logs, NMR, DST

Problems with the definition of Net Pay

Usually defined using a Sw and/or permeability cutoff. But this doesn’t include:

• The ratio of horizontal to vertical permeability (Kh/Kv)
• Standoff distance from the FWL and the shape of the transition zone
• Gas and water drive, draw down, water cut and fractures

Most of the hydrocarbon above the FWL is potentially producible. The amount of hydrocarbon produced depends on how hard we try to extract it.

• Net Pay is difficult to define and a function of the oil price

Net Reservoir is much easier to define

Defined as the portion of reservoir rock which is capable of storing hydrocarbon. Net Reservoir is required for calculating Net/Gross, upscaling reservoir averages and for reservoir modelling.

Determining Net Reservoir from Sw vs. Porosity crossplot

• Plot water saturation vs. porosity above the Free Water Level (FWL) for all wells
• This crossplot shows how Sw increases as the porosity decreases
• It defines the porosity cutoff where the reservoir rock is capable of storing hydrocarbon

Determining Net Reservoir from the Water Saturation

Hydrocarbon enters the smaller reservoir porosity far above the FWL where there are low saturations of capillary bound water. Reservoir just above the FWL contains high saturations of capillary bound water and there is a no room available for hydrocarbons. Consequently, the Net Reservoir cut-off varies as a function of the height above the FWL.

For instance, the two intervals below have similar porosities but the deeper one has 100% Sw whereas in the upper one has 50%.

Determining Net Reservoir from the Bulk Volume of Water (BVW)

BVW (Porosity x Sw) is the volume of water in a unit volume of reservoir.

In hydrocarbon bearing intervals BVW < porosity and is Net Reservoir. In shales and tight intervals BVW = porosity and is Non-Net Reservoir.

BVW is determined solely by its height above the FWL.?If the porosity is 10 p.u. hydrocarbon enters the remaining space.?If the porosity is 20 p.u. the BVW is the same and extra available porosity is completely filled with hydrocarbon.?

In this example the Net Reservoir is 9 p.u.?

Determining Net Reservoir from the Saturation Height Function (SwH)

The SwH Function tells us how water saturation varies as a function of the height above the FWL and how the formation porosity is split between hydrocarbon and water. This function is used to initialize the 3D reservoir model.

The SwH Function * is BVW=aH^b *Sometimes called the FOIL or Fractal Function

Where:

BVW= Bulk Volume Water (Sw*Phi)

H??????= Height above FWL

a, b??= Constants

BVW is independent of facies type, porosity and permeability as shown by this crossplot.

This function tells us how Net Reservoir varies as a function of height above the FWL.

Conclusions

Net Pay is difficult, if not impossible, to define and depends on the oil price.

Net Reservoir is reservoir rock capable of storing hydrocarbon and depends on the height above the FWL.

#petrophysics #spwla