Net pay and Net reservoir

Net Reservoir and Net Pay are required for petrophysical averaging and reservoir modelling.

1. The Difference between Net Pay and Net Reservoir, how to pick them and their Application to Reservoir Modelling Steve Cuddy

2. Outline Definitions of Net Pay and Net Reservoir How to determine Net Pay and Net Reservoir from Core and Electrical Logs How to correctly upscale Net Reservoir properties for the 3D reservoir model

3. High Sw 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

4. What is Net? 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 More difficult to pick

5. How can Net Pay be Determined? The microlog identifies mudcake which suggests movable fluids When Sxo > Sw indicates moveable hydrocarbon Mud losses – esp. for fractures Formation pressure Borehole gas chromatography Production logs NMR DST

6. Net Pay Usually defined using a Sw and/or permeability cutoff But it doesn’t include: The ratio of horizontal to vertical permeability ( Kh / Kv ) Standoff distance from the FWL Shape of the transition zone Gas and water drive Draw down Water cut Fractures Most of hydrocarbon above the FWL is potentially producible The amount of hydrocarbon produced depends on how hard we try Is Net Pay therefore a function of the oil price ? Net Pay is difficult to define

7. Net Reservoir Net Reservoir is much easier to define than Net Pay As it is defined as the portion of reservoir rock which is capable of storing hydrocarbon Knowledge of Net Reservoir is essential for: Upscaling for reservoir averages Reservoir modelling Net Reservoir is used to calculate Net/Gross

8. Net/Gross is required calculate the hydrocarbon in place

9. Net Reservoir from Core Better vertical resolution than electrical logs 2 inches compared to 2 feet Net from core analysis Useful where there are sharp boundaries Can be combined with lithology analysis Net from u ltraviolet fluorescence Only useful for oil that hasn’t been lost or evaporated Gives an upper limit

10. Net Reservoir from Core Identify intervals with fluorescence from U/V photos Only for intervals above the Free Water Level Porosity histogram picks the porosity cutoff Intervals with no fluorescence Intervals with fluorescence 0 Porosity (p.u) 30

11. Net Reservoir from Sw vs. Porosity Xplot Plot water saturation vs. porosity above the Free Water Level (FWL) for all wells in the field This xplot shows how Sw increases as the porosity decreases Defines the porosity cutoff where the reservoir rock is capable of storing hydrocarbon 0 Porosity (p.u) 20 0 Water Saturation (%) 100 Each well is shown by a different colour

12. Calculate the hydrocarbon pore height (HCPH) for each porosity cutoff for 1 p.u, 2 p.u, 3 p.u etc. Plot the HCPH as a function of the porosity cutoff This gives you the porosity cutoff where the hydrocarbon kicks in It also tells you how much hydrocarbon the reservoir model loses from by changing this cutoff HCPH = (1-Sw) * Porosity * Net Porosity cutoff 0 % change in HCPH 100 0 Porosity (p.u) 35 Net Reservoir from Hydrocarbon Pore Height HCPH = Net * (1-Sw) * porosity

13. Reservoir high above the FWL has low saturations of capillary bound water and hydrocarbon enters the smaller pores Reservoir just above the FWL, with higher porosities, 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 Net Reservoir from Water Saturation Net Porosity 25 pu Sand Shale Gas FWL Log derived Sw 100 su

14. Net Porosity 25 pu Sand Shale Gas FWL Log derived Sw 100 su The Net Reservoir cut-off varies as a function of the height above the FWL Sw varies as a function of porosity BVW = % volume of water in a unit volume of reservoir Net Reservoir from Bulk Volume of Water BVW varies only as a function of height above the FWL BVW is what is measured by electrical logs and by core analysis BVW

15. Where BVW < Porosity is Net Reservoir - Hydrocarbon bearing intervals Where BVW = Porosity is Non-Net Reservoir - Shales and tight intervals In this example: porosity > 9 p.u Net Reservoir from Bulk Volume of Water

16. The SwH Function tells us how water saturation varies as a function of the height above the Free Water Level (FWL) It tells us how the formation porosity is split between hydrocarbon and water Net from the Saturation Height Function 0 Water Saturation (%) 100 Height above FWL (Feet) FWL > Water Hydrocarbon It tells us the shape of the transition zone It is used to initialize the 3D reservoir model

17. BVW vs. Height Swh Function BVW is independent of facies type, porosity and permeability - confirmed by xplots Free Water Level   0 Bulk Volume of Water (%) 20 400 Height Above FWL (feet) z-axis colour = well 400 Height Above FWL (feet) 0 Bulk Volume of Water (%) 20 Tells us how net varies as a function of height Where: = Bulk Volume Water (Sw*Phi) = Height above FWL , = Constants  

18. Upscaling From ½ foot to the cell size of the reservoir model Upscaling porosity and permeability requires net reservoir to be identified Sw-Height functions are used to initialize the reservoir model. It is essential that the SWHF predicted water saturations upscale accurately This is done by integrating the Sw-Height function Unlike other parameters, water saturation must be pore volume averaged

19. Upscaling Water Saturations = average water saturation = average porosity = average bulk volume of water Averages are over net reservoir sections “A function that predicts BVW from height is especially appropriate to this application” Paul Worthington Pore volume averaged

20. Upscaling Permeability Log and core permeabilities represent typically 2 feet To be used in a reservoir model, the predicted permeabilities must upscale correctly Predicted permeabilities must have the same dynamic range (standard deviation s ) as the core data Core Permeability Predicted Permeability 0.01 (mD) 1000 0.01 (mD) 1000 s s

21. Core permeability upscaling Core distribution Linear Regression Fuzzy logic prediction 0.001 mD 1000 Permeability frequency plots - Colour represents data from 15 cored wells Least squares regresses towards the mean Fuzzy logic predicted permeabilities preserves the dynamic range and matches the core distribution Regression techniques are poor at predicting the extremes and therefore will be incorrect when upscaled

22. Conclusions Net Pay Is difficult, if not impossible, to define Depends on the oil price Net Reservoir is reservoir rock capable of storing hydrocarbon Net Reservoir can be determined Using core, logs and Sw-height functions Net reservoir depends on the height above the FWL Upscaling requires: Net reservoir cut-off for porosity, Sw and permeability Correct upscaling for the 3D reservoir model is essential