Recognizing Reservoir Compartments on Geologic and Production Timescales in Deep-Water Reservoirs: An Example from Genes

1. Recognizing Reservoir Compartments on Geologic and Production Timescales in Deep-Water Reservoirs: An Example from Genesis Field, Gulf of Mexico M. L. Sweet and L. T. Sumpter ExxonMobil Upstream Research Co.

2. Outline • What is Connectivity? • Genesis Field Background • Observations of Pre-Production Fluid Contacts • Production Timescale Pressure Changes • Conclusions

3. What is Connectivity? • Common fluid contacts and /or pressure gradients • Smalley and Hale (1996) • Vrolijk et al. (2005) • Sand to sand connection in geologic models • King (1990) • Ainsworth (2005) • Continuity between injectors and producers – waterflood sweep efficiency • Stiles and Magruder (1992) • Common hydrocarbon fluid properties • Smalley and Hale (1996)  The definition of connectivity depends in part on the timescale of interest

4. Geologic Time-Scale Connectivity • Describes the pre-production fluid distribution in a field established over geologic time • Timescale - tens of thousands to millions of years • Compartments can be defined by structural and/or stratigraphic elements Stratigraphically Controlled Contact Structurally Controlled Fluid Contacts

5. Production Timescale Connectivity • Related to changes in fluid contacts and pressure caused by production • Short timescale (hours to years) • Lateral and vertical permeability changes due to fault juxtaposition, low permeability fault rocks, facies changes and/or diagenesis • Need to integrate surveillance data (down-hole pressure gauges, 4D seismic, production logging) with geologic interpretation.

6. Genesis Field – Background • Field Background • Deepwater Gulf of Mexico - 2600 ft water depth • Green Canyon Block 160, 161, and 205 • Discovered 1988, operated by Chevron • Production started November, 1999 • 25 wells and sidetracks, 3D seismic • Down-hole pressure gauges in most production wells N Genesis Field • Geologic Background • Three-way structure against salt in a mini-basin • Pleistocene-age, deepwater sands • Neb reservoirs main production targets • Vertical and lateral variability of sand content • Several small throw faults cut the reservoir Compartmentalization is primarily a function of stratigraphic complexity

7. Neb 3 Reservoir Neb 3 Amplitudes on Neb 3 structure Neb 3 well log cross-section • Patchy amplitude pattern and abrupt lateral thickness and facies changes  Greater compartmentalization? • Erosion at base of Neb 3

8. Neb 3 Environment of Deposition Core Photographs from Neb 3 • Abundant rip-up clasts and amalgamated, massive sand beds (Ta) • Thick blocky sand change laterally into thinner laminated sands • Abrupt lateral thickness changes • Erosion at base of Neb 3 Tb Rip-ups Ta Neb 3 is the deposits of a laterally and vertically amalgamated, erosionally-confined channel complex

9. Channel-fill sandstones Overbank mudstones Outcrop of An Erosionally Confined Deepwater Channel-Complex Capistrano Formation near San Clemente, CA From Mike Farrell ExxonMobil Research Co.

10. Neb 3 Pre-Production Connectivity Pressure–Depth Plot From RFT Common oil-water contact, oil and aquifer gradients = one pre-production compartment Neb 3 Top Structure Map

11. Neb 3 Production Time-Scale Connectivity Down-hole pressure Each new well falls on pressure decline trend of the earlier wells Wells watered out as aquifer moved up-dip On production time-scales Neb 3 behaved as one reservoir connected to a common aquifer

12. Neb 1 Reservoir Seismic Amplitudes on Neb 1 Structure Map Neb 1 Isopach Map

13. Neb 1 Well Log Cross-Section • Continuous amplitude pattern, limited lateral thickness and facies changes  Greater connectivity? • Erosion at base of Neb 3

14. Neb 1 Environment of Deposition Core Photographs from Neb 1 • Thinly-interbedded sand and mud with abundant ripple-laminations (Tc) • Thicker wells have barrel-shaped log motif with suppressed gamma and resistivity. Thinner well have a sharp-base cleaning-up log motif • Conformable base of Neb 1 1 Foot Ta Tb Tc Neb 1 is the deposits of a channel-levee complex

15. Channel-Levee System Quaternary Amazon Fan 50 m Levees HARPs After Pirmez et. al., 1997

16. Neb 1 Pre-Production Connectivity Pressure–depth plot from RFT At least two separate oil-water contacts and oil gradients = two or more pre-production compartments Top Structure Map on Neb 1

17. Neb 1 Production Timescale Connectivity Down-hole pressure • New wells came on production at or near virgin pressure • Some wells (A14) showed rapid pressure decline • Slower pressure decline in the A5 well suggests aquifer support On production time-scales Neb 1 has multiple compartments. Some of these compartments are connected to the aquifer and others are not. Seismic and well data suggest that these compartments form due to poor connection between channel-fill and levee deposits.

18. Pressure Transient Analysis A3 A14 A2 A18 Neb 1 Isopach map showing location of channel axis (blue), gamma ray Logs and 4 well used in pressure-transient analysis (A3, A14, A2 and A18)

19. Pressure-Time Plot For Neb 1 Wells A14 was shut-in while A2, A18 and A3 were producing. Pressure increase suggests separate compartment. Pressure-TimePlot

20. Well Test Analysis of A14 Data A14 Well A14 pressure transient response is best match by the well being in a narrow channel within a weak connection to a larger volume (levee?)

21. Reservoir Architecture of Neb 1 and Neb 3 Well connected Poorly connected

22. Dynamic Pressure Compartments Drive Mechanisms Static pressure Compartments Multiple- channels separated /baffled from levees Pressure Depletion Aquifer Multiple Channel/levee Neb 1 Pressure Depletion Aquifer Multiple Multiple Semi-amalgamated channels - Neb 2 Aquifer Single Single Amalgamated channels - Neb 3 Stratigraphic Controls on Reservoir Performance

23. Results of Genesis Connectivity Analysis • Log and seismic data from the Neb 3 reservoir suggested greater potential for compartmentalization, however fluid contact and pressure data suggested a common oil-water contact. • Production data has shown a good connectivity on a production timescale within the Neb 3 oil column and between the oil column and the aquifer. • Neb 1 appeared less variable from log and seismic data, however fluid pressure and contact data suggested multiple compartments. Continuous monitoring of down-hole pressure showed an even higher degree of compartmentalization on a production timescale. • Neb 3 and Neb 1 have a similar structural style, but were deposited in different deep-water environments suggesting that stratigraphy is the main control on connectivity. • Good connectivity within erosionally confined channel complexes. • Poor connectivity between channels and levees.

24. Conclusions • Analysis of fluid contacts in the context of a field’s structural and stratigraphic framework gives an indication of pre-production compartments that can have a major impact on development planning and identification of infill drilling opportunities. • With production another level of compartmentalization may become apparent. Integration of surveillance data with an understanding of the reservoir geology is required to define these production timescale compartments and use this knowledge to guide reservoir management.

25. Acknowledgements ExxonMobil and Genesis Field partners Chevron and BHP Billiton gave permission to present this talk. Mike Maitland, Cherie Lee and Diane Schwetz (ExxonMobil Production Co.) for their insights into Genesis Field and for their help in getting data for this paper released. Sephir Arbabi and Jane Shyeh (ExxonMobil Research Co.) provided pressure transient analysis of the Neb1 well test data. ExxonMobil Research Co. Connectivity Team: Peter Vrolijk, John Snedden, Amanda Mosola, Chuck Kiven, Sofie Nollet, Ryan Ruppert, Ellen Meurer and Ilsa Kirsher.