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Water Influx

Water Influx
Water Influx

Water Influx in Reservoirs

Water influx, also known as water encroachment, plays a vital role in maintaining reservoir pressure and influencing production performance. It occurs as surrounding aquifers respond to pressure differentials caused by fluid production from hydrocarbon reservoirs.

Key Drivers of Water Influx

  1. Aquifer Expansion
    • As reservoir fluids are produced, water expands due to the drop in pressure, moving into the reservoir.
  2. Aquifer Compressibility
    • The compressibility of aquifer rock helps drive water encroachment by responding to pressure changes.
  3. Artesian Flow
    • Occurs when the aquifer’s outcrop is at a structurally higher position than the reservoir, enabling natural water movement into the reservoir.

Reservoir-Aquifer System Classification

Reservoir-aquifer systems are commonly classified based on:

  • Degree of Pressure Maintenance
  • Outer Boundary Conditions
  • Flow Regimes
  • Flow Geometries

Degree of Pressure Maintenance

  1. Active Water Drive
    • The aquifer provides substantial pressure support, leading to significant water influx as production occurs.
  2. Partial Water Drive
    • Only moderate pressure support is provided by the aquifer, resulting in a limited influx.
  3. Limited Water Drive
    • The aquifer has minimal influence on reservoir performance due to its small size or distance from the reservoir.

Aquifer Boundaries

  • Infinite Systems: Aquifers are large enough that pressure changes are not felt at their boundary.
  • Finite Systems: Aquifers experience pressure changes at their boundary due to the influx of water into the reservoir.

Flow Regimes in Water Influx

  1. Steady-State Flow
    • Constant water influx over time as pressure remains balanced between the reservoir and aquifer.
  2. Semisteady (Pseudosteady) Flow
    • Occurs when pressure drops are gradually equalized between the reservoir and aquifer.
  3. Unsteady (Transient) Flow
    • Water influx rapidly changes due to significant pressure drops in the reservoir.

Flow Geometries

  • Edge-Water Drive: Water moves laterally into the reservoir from its flanks.
  • Bottom-Water Drive: Water moves upward into the reservoir from below, with significant vertical flow.
  • Linear-Water Drive: Water influx occurs in a straight line from one side of the reservoir.