1. Net Pay Cutoffs from Capillary Pressure Andy May February 24, 2014

2. 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!

3. Origin of Capillary Pressure Sealed Open Gas Water In a water-wet rock, the pressure in the water on both sides of the tube is the same. This is the hydrostatic pressure. The gas in the left tube has a higher pressure which holds the heavier water below the gas/water interface. The difference between the water pressure and the gas pressure is the capillary pressure.

4. Capillary Pressure Free Water Level

5. Traditional OWC Alt OWC Pe

6. Neutral Wet Example Oil below FWL? FWL OWC

7. Corey’s Function J 100 is the J value for the bubbling pressure(Pb) or the Pc at the critical oil-phase Saturation Substitute (J 100 /J) for (P b /P c ) Brooks and Corey, 1966 (eq. 24) Derived from eq. 11, Amaefule, 1988

8. Critical Oil Phase Saturation The Sw where Kro comes off zero in drainage mode is the critical oil saturation, the point at which the oil crosses the pore system. Imbibition Steady State Rel K Drainage, followed by Imbibition, also steady state

9. Sr - Irreducible Sw Sw from OBM core Hg Rel Perm & Porous Plate Sr as a function of Rmh Needed to compute Swe*

10. λ and J 100 Lambda (λ) is the inverse of the slope, usually it is between.4 and 4. Corey calls this the “pore distribution index.” A value of 1.07 indicates highly variable pore sizes. J 100 is the intercept at Swe* where Kro comes off 0 (drainage). Sw(Kro=0, imbibition)=73, Sr=21, Swe*=66. J 100 =.46

11. Interfacial Tension (σ) Interfacial tension (IFT or σ) helps determine the amount of curvature between the liquids in pore space(θ). It can be measured in the laboratory. The units are dynes/cm. In heavy oil, the value is usually 10 to 20 In light oil it is 20 to 45. Ref: Firoozabadi & Ramey, 1988 After Leverett, 1940

12. Effect of Roughness on θ Surface roughness diminishes the apparent contact angle. Anderson, Oct. 1987, JPT Use zero in water-wet reservoirs if no other data Higher values are appropriate in neutral to oil-wet reservoirs, purely oil-wet reservoirs or neutral wet reservoirs are likely to have effective contact angles of about 90 degrees. Range: 0 to 140 or more.

13. Capillary Pressure Model vs. Data

14. 20 md Traditional OWC

16. Sw Cutoff Cross 1 st water Sw (Total) cutoff = 59% 91% to 99% water cut 99% water cut Sw cutoff = 59%

17. 15 md Sw = 59%

18. Core K vs. Core Sw 23 md Settled on a perm cutoff of 20 md.

19. Capillary Pressure Summary A capillary pressure model rigorously describes the transition zone The model provides a check on Sw The model provides a clear backup for cutoffs The model provides the best way to tie log and core data and the most comprehensive way to feed petrophysics to a reservoir simulator