WAG Mechanisms at macroscopic field level

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WAG Mechanisms at macroscopic/ field level

• Presentation at FORCE WAG Seminar Stavanger, 18
  Mar 2009
• Anders GjesdalLeading Advisor Drainage Strategy,
  StatoilHydro

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Outline

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

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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

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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
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Effect of kv/kh, effect of horizontal restrictions

• 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

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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)

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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

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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

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Monitoring each WAG injector and associated
producers
Observe well performance

• WAG response
• Increasing WHP
• Increasing oil rate
• Increasing GOR
• Decreasing WCT

Saturation logs
Monitor gas injection efficiency
4D Seismic
Tracers
Estimate incremental reserves
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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
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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

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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