Waterflooding: Secondary and Tertiary Oil Recovery

Introduction to Waterflooding

• Waterflooding is a secondary and tertiary oil recovery technique.

Type of Recovery

• Primary recovery involves the initial extraction of oil, relying on natural reservoir pressures.
• Secondary recovery methods, like waterflooding, are implemented when natural pressure is insufficient.
• Tertiary recovery, or Enhanced Oil Recovery (EOR), aims to recover oil left behind after primary and secondary methods have been exhausted.

Primary Recovery

• This is the first phase of oil production, extracting "easy" oil.
• Natural pressure drives oil through rock or sand formations towards a low-pressure wellbore.

Secondary Recovery

• This method becomes necessary when underground pressure cannot move oil.
• Waterflooding is a standard technique where water sweeps oil towards the producing wellbore.

Enhanced Oil Recovery (EOR)

• EOR aims to recover oil that remains after primary and secondary processes are no longer economical.

EOR vs IOR

• Improved Oil Recovery (IOR) uses methods supplementing reservoir forces and energy.
• EOR focuses on recovering oil left behind after primary and secondary recovery.
• Some IOR techniques are: -Optimizing artificial lift -Increasing well stimulation -Reactivating idle wells -Changing completion strategy -Debottlenecking facilities -Upgrading facilities -Infill/Step-out drilling
• Some EOR techniques are: -Gas/Water flooding (immiscible or miscible) -Water alternating gas (WAG) -Microbial (MEOR) -Dilute surfactant/chemical -Polymer -Thermal

Water Injection vs Waterflooding

• Water Injection involves injecting water into the aquifer for: -Maintaining reservoir pressure using external water injection -Wastewater disposal
• Waterflooding is injecting water into the reservoir for: -Maintaining reservoir pressure using internal water injection -Increasing volumetric sweep efficiency

Primary Reservoir Driving Mechanisms

• Rock and liquid expansion yields 3-7% oil recovery.
• Solution gas drive yields 5-30% oil recovery.
• Gas cap expansion yields 20-40% oil recovery.
• Water drive yields 35-75% oil recovery.
• Gravity drainage yields <80% oil recovery.
• Combination drive yields 30-60% oil recovery.

Reasons for Waterflooding

• Water is an inexpensive resource.
• Water is generally available in large quantities from various sources. -Water injection effectively boosts production well flow rates due to increased reservoir pressure.

Objectives of Waterflooding

• Maintain pressure support above the bubble point. -Keeping gas in solution, which minimizes oil viscosity.
• Achieve physical displacement of oil using the viscous force of the water. -Balancing Voidage Replacement Ratio (VRR) -VRR = (Injection/Production) at reservoir conditions
• Optimizing overall oil recovery.

Voidage Replacement Ratio (VRR)

• VRR > 1: Increases pressure.
• VRR = 1: Maintains pressure.
• VRR < 1: Pressure declines.
• Injecting below VRR=1 is: -Due to Surface issues -Due to Well issues -Due to more producers with the same injectors -Due to the Unavailability of water
• Objective of VRR=1 or >1: -At early stages, you increase reservoir pressure injecting with VRR>1. -Once a target pressure is reached, VRR should not exceed 1 to avoid fracturing. -After reaching the bubble point, VRR is typically set to 1.

Time Decision for Waterflooding

• C to C1 on a phase envelope are the temperatures to consider for waterflooding.

Factors to Consider

• Reservoir geometry
• Fluid properties
• Reservoir depth
• Lithology and rock properties
• Fluid saturations
• Reservoir uniformity and pay continuity
• Primary reservoir driving mechanisms

Factors Affecting Waterflood Performance

• Recovery Factor (RF) is determined by: -Displacement efficiency (ED) -Areal sweep efficiency (EA) -Vertical sweep efficiency (Ev)
• RF = ED * EA * Ev
• Cumulative oil production (Np) = Initial oil in place (Ns) * ED * EA * Ev

Displacement Efficiency (ED)

• This is the microscopic displacement efficiency.
• ED = (Pore volume mobilized by WF/EOR agent) / (Pore volume of oil contacted by WF/EOR agent)
• ED is also the fraction of movable oil displaced from the swept zone at any time.
• ED = (Soi – Sor)/ Soi
• ED < 1.0 because immiscible gas injection/WF always leaves residual oil behind.

Areal Sweep Efficiency (EA)

• EA is the fractional area swept by the displacing fluid.
• EA = (Area contacted by the displacing fluid) / (Total area)
• Major factors determining areal sweep include: -Fluid mobility/mobility ratio -Pattern type -Areal heterogeneity -Total volume of fluid injected

Vertical Sweep Efficiency (EV)

• Ev is the fraction of the vertical section of the pay zone contacted by injected fluids.
• E v = (Length contacted by the displacing fluid) / (Total length)
• Major factors determining vertical sweep: -Vertical heterogeneity -Degree of gravity segregation -Fluid mobilities/mobility ratio -Total volume injection

Volumetric Sweep Efficiency

• the Product of EA Ev
• Macroscopic displacement efficiency
• Reservoir volume of oil contacted by displacing fluid / Reservoir volume of oil originally in place

Factors Influencing Displacement Efficiency

• Viscosities: Lower oil viscosity or higher water viscosity reduces fingering.
• Densities: Differences in density influence gravity tonguing.
• Interfacial tension: Traps oil after a waterflood.
• Wettability: Water-wet rocks are advantageous for water injection.
• Rate of displacement: Higher water velocity improves displacement efficiency.
• Capillary number: Lower repulsion between oil and water improves residual oil saturation.

Factors Influencing Areal Sweep

• Mobility Ratio: High mobility ratio causes fingering and leaves unswept areas.
• Pattern: Has a limited impact on areal sweep efficiency.
• Areal Heterogeneity: Impacts sweep efficiency.
• When M=1, water and oil move equally well.
• When M<1, oil moves easier than water.
• When M>1, water moves easier than oil.

Factors Influencing Vertical Sweep Efficiency

• Mobility ratio: A more favorable mobility ratio (M < 1) reduces the amount of water needed.
• Gravity forces: Water's density leads to preferential flow at the bottom.
• Vertical heterogeneity: This impacts vertical sweep efficiency.

Flood Patterns

• External Water Injection: -Long distance from producer wells -Smaller number of injection wells -Boost aquifer to increase encroachment strength -Recommended for high aquifer fields
• Peripheral Injection Patterns: -Injection wells at external boundary -Maximum oil recovery with minimal water production -If permeability is large then allow movent of injected water at the desired rate over the distance

Regular Injection Patterns

• These happen when oil leases divided into square miles
• Direct line drive: -Injection and production lines are directly opposed. two parameters: a = distance between wells; d = distance between lines
• Staggered line drive: -Wells are in lines, but injectors/producers are laterally displaced by a/2.
• Five spot: -Is a staggered line drive case with a = 2d
• Seven spot: -Injection wells form a hexagon with a production well at its center.
• Nine spot: -Similar to 5-spot, eight injectors surround one producer.
• "Inverted" patterns have only one injection well per pattern.

Crestal and Basal Injection Patterns

• Crestal injection places injection wells at the top of structures. Typically used for gas injection projects
• Basal injection places injection wells at the bottom with more water use. This gets additional benefits from gravity segregation

Waterflooding Performance Monitoring

• Waterflooding takes decades to complete. -Continuous monitoring and analysis are essential. -Data analysis allows modifications to the original waterflood design.
• Modification options: -Changing water allocation among injection wells -Waterflooded intervals -Drilling infill locations -Modifying pattern style
• Waterflooding has been used successfully worldwide in offshore and onshore oil fields.

Optimum Time To Waterflood

• Optimal time is determined by calculating variables: -Anticipated oil recovery -Fluid production rates -Monetary investment -Water supply -Water treatment/pumping costs -Maintenance costs -New injection well costs

Factors To Consider For Optimal Timing To Waterflood

• To initiate waterflooding determine reservoir pressure, or time, for a secondary recovery project -Must consider reservoir oil viscosity, -Free gas saturation -Cost of injection equipment. -Productivity of producing wells. -Effect of delaying investment on the time value of money. -Overall life of the reservoir.
• Reservoir oil viscosity: -Initiate injection when reservoir pressure reaches bubble-point (oil viscosity is at a minimum. Free gas saturation: -In water injection projects must have some initial small gas saturation -In gas injection projects must have zero gas saturation in oil zone