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WAG Mechanisms at macroscopic/ field level

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  1. WAG Mechanisms at macroscopic/ field level Presentation at FORCE WAG Seminar Stavanger, 18 Mar 2009 Anders GjesdalLeading Advisor Drainage Strategy, StatoilHydro

  2. Outline • Introduction • Sweep • WAG in well pairs • WAG at field level • Concluding remarks

  3. IntroductionTo make WAG work at all levels • Levels • Microscopic core level • Macroscopic at well pair level • Macroscopic in the reservoir • Macroscopic at the field level • (Macroscopic at area level ) • WAG mechanisms has to function at all levels to be successful • Gas management – constraints • WAG development - Producers and Injectors • Injection planning and prioritisation • Surveillance

  4. Introduction – Key is Sweep • Gas displacement often gives a better microscopic sweep compared to water. Sorg is often lower than Sorw , particularly at miscible conditions • Fraction of the reservoir volume contacted by gas limits the impact of improved microscopic sweep. With good reservoir quality and long well distance often found offshore Norway this is often the case. • At field level focus is turned towards increasing the volumetric sweep • Key factors influencing volumetric sweep. • Gravity • Mobility ratio • Heterogeneity of the reservoir • Reservoir thickness and vertical communication kv/kh • Geometry, Structure, top perforations, spill points • Available gas • Available gas capacities • Injection schedule and rates • Number of producers and injectors – Well location and distance Manageable

  5. Typically we anticipate that only 10-20% of the reservoir is gas contacted between well pairs Horizontal barriers tends to increase sweep At kv/kh 0.01 we see that the gas contacted volume increases noticeably The limited gas sweep makes sweep of the attics and geometry very important for the WAG mechanism Effect of kv/kh, effect of horizontal restrictions

  6. WAG mechanisms in well pair • Attic Volumes andUpper part of stratigraphy • Gas lift • Free gas back-produced provides additional well hydraulic potential. • Voidage • Gas contributes to pressure maintenance, but voidage replacement is often reduced in gas injection periods • Lower part of stratigraphy (up-dip wag) • Pattern injection to increase sweep (not applied offshore Norway)

  7. WAG at a reservoir or field level • How can we get the most out of our WAG kit? • Maximise injection effectiveness. • Move the WAG kit around to new areas or flooding units? • Keep the kit active • Avoid excessive gas re-cycling, capacity utilization. • Ensure back-production of gas, avoid “stranded” gas. (Ensure injection and producer timing) • When to move?: Re-cycling ratios are a good indication on the gas (and water) injection efficiency

  8. Reservoir Management Team WAG Surveillance Team C-Platform A-Platform B-Platform Multidisciplinary WAG Surveillance Team Evaluate:  • Optimizing of the gas utilization • The effectiveness of the WAG process •  Interpretation of the field performance •  Tracer injection, planning and evaluation •  Data acquisition, planning and evaluation

  9. Observe well performance Monitoring each WAG injector and associated producers • WAG response: • Increasing WHP • Increasing oil rate • Increasing GOR • Decreasing WCT Saturation logs Monitor gas injection efficiency 4D Seismic Tracers Estimate incremental reserves

  10. Monitoring at field level Identify and map potential WAG oil Identify injection points and associated producers Plan and prioritize gas injection Monitor gas injection efficiency Prognosis for back-production

  11. Typical WAG behaviour • Volumetric gas sweep often low. • Gas breakthrough varies from a few months to 1-2 years. • Slug size varies from 200 MSm3 to 1-2 GSm3 • Typical gain is 1 Sm³ of oil per 500-1500 Sm³ of gas injected • Typical back-production of gas is 40-65% • Higher retained gas typically yields more incremental oil • When attics have been filled, re-cycling rate increases and gas injection efficiently drops. • Gas moves significantly faster than water

  12. Concluding remarks • WAG has successfully contributed with substantial IOR volumes • Drainage of attics is the most important and most efficient mechanism • Try to identify attics: Use 4D seismic processing and detailed geological studies to target extra oil recovery • Move the WAG into new areas • Re-cycling ratios are a good indication on the gas (and water) injection efficiency • Availability of WAG injectors important to ensure gas is injected where it gives the highest efficiency • Gas tracer velocity can give information on gas storage, reservoir communication • 4D seismic gives important information on gas movement • Gas movement and the corresponding incremental oil seem to depend on: • Structural complexity and geometry • Formation – vertical communication • Production well coverage • Distance between WAG injector and production wells • Each WAG injector has its own characteristics in terms of response time and behaviour of WAG producers • KEY to success is: Surveillance and good planning