Hydrocarbon Reservoirs Chapter 10

Water Influx in Hydrocarbon Reservoirs

• Hydrocarbon reservoirs are often surrounded by water-bearing rocks, known as aquifers, which can vary significantly in size.
• A pressure differential develops between the reservoir and surrounding aquifer as reservoir fluids are produced and reservoir pressure declines.
• Fluid flow in porous media results in the aquifer encroaching into the reservoir, affecting overall reservoir performance.
• Cases exist where water encroachment is negligible, due to aquifer pore volume being similar to the reservoir's or low aquifer permeability requiring high pressure differentials.

Water Drive Mechanisms

• Many reservoirs utilize a mechanism known as water drive, which helps maintain reservoir pressure as hydrocarbons are produced.
• Natural water drive refers to aquifer response through water influx due to pressure drops from hydrocarbon production.
• Water influx is influenced by several factors:
  • Expansion of water in the aquifer
  • Compressibility of aquifer rock
  • Artesian flow from higher outcropping water-bearing formations

Classification of Aquifers

• Reservoir-aquifer systems are evaluated based on:
  • Degree of pressure maintenance
  • Flow regimes
  • Outer boundary conditions
  • Flow geometries

Degrees of Pressure Maintenance

• Three qualitative classifications exist in natural water drive:
  • Active Water Drive: Water influx rate matches total reservoir production rate, leading to a slow pressure decline.
  • Partial Water Drive: Water influx only partially offsets production rate.
  • Limited Water Drive: Insufficient water influx to significantly impact reservoir pressure.

Water Influx Rate Equation

• The balance between water influx and hydrocarbon flow is represented by the equation:
  • ( ew = Qo \cdot Bo + Qg \cdot Bg + Qw \cdot Bw )
  • Where:
    • ( ew ): Water influx rate (bbl/day)
    • ( Qo ): Oil flow rate (STB/day)
    • ( Bo ): Oil formation volume factor (bbl/STB)
    • ( Qg ): Free gas flow rate (scf/day)
    • ( Bg ): Gas formation volume factor (bbl/scf)
    • ( Qw ): Water flow rate (STB/day)
    • ( Bw ): Water formation volume factor (bbl/STB)

Cumulative Production Equation

• Water influx can also be expressed in terms of cumulative production:
  • ( ew = \frac{dWe}{dt} = Bo \frac{dNp}{dt} + (GOR - R_s) Bg \frac{dNp}{dt} + Bw \frac{dWp}{dt} )
  • Where:
    • ( We ): Cumulative water influx (bbl)
    • ( Np ): Cumulative oil production (STB)
    • ( GOR ): Gas-oil ratio (scf/STB)
    • ( R_s ): Gas solubility (scf/STB)
    • ( dNp/dt ): Daily oil flow rate (STB/day)
    • ( dWp/dt ): Daily water flow rate (STB/day)
    • ( dWe/dt ): Daily water influx rate (bbl/day)

Significance of Water Influx

• The study of water influx is crucial for understanding reservoir behavior, ensuring efficient hydrocarbon recovery, and optimizing production strategies.

Explore the role of water influx in hydrocarbon reservoirs in this Chapter 10 quiz. Understand the dynamics of aquifers, their size relative to reservoirs, and how they affect reservoir performance under pressure differentials. Enhance your knowledge of reservoir engineering concepts critical to petroleum studies.