Water Alternating Gas (WAG)

Enhanced Oil Recovery

Ibrahim Muhammad

African University of Science & Technology December 2015

Content• Introduction

• Water Alternating Gas (WAG) flooding

• Classification of WAG

• Factors affecting WAG

• Types of WAG

• When to Initiate WAG

• Conclusion

Introduction• Why is Enhanced Oil Recovery (EOR) needed

• Economic quantities left after primary and secondary recovery.

• To economically recover more oil

• Infrequent discoveries of major new reservoirs

• Different types of EOR techniques;

• Gas Injection e.g. WAG

• Microbial

• Thermal

• Chemical

Water Alternating Gas (WAG)• Water Alternating Gas• Involves Gas injection followed by water Injection• Gas Injection increases displacement efficiency• Water Injection increases sweep efficiency

WAG Basic classification

Miscible WAG (MWAG);

• Reservoir pressure maintained

above MMP.

• At MMP, gas-oil miscible at all

proportions

• In reality, not completely

achieved (hard to maintain

MMP)

• About 9.7% Improved oil

recovery (Touray, 2013)

Immiscible WAG (IWAG)

• Aim is to stabilize the sweep

front and increase contact of

un-swept areas.

• High microscopic efficiency

• Poorer macroscopic efficiency

due to high M.

• About 6.4% Improved oil

recovery (Touray, 2013).

Factors affecting WAG

Success of a WAG project depends largely on;

• Reservoir characteristics.

• Reservoir Heterogeneity

• Petrophysical properties (ф, k, Sw, So, wettability)

• Fluid properties

• Viscosity of oil f(T, P, SG).

• Injection and production well arrangement

• Optimum arrangement for optimum results f(cost)

Factors affecting WAG cont’d

• WAG parameters

• WAG slug size; Optimum size for better results

• WAG ratio; Too much water will affect micro ԑ, too much gas

affects macro ԑ.

• WAG cycles; Timing to switch from gas to water.

Factors affecting WAG cont’d• Fundamental Equations • Some equations driving the WAG process include Darcy’s law,

Mobility Ratio,

Types of WAG

• Miscible WAG (MWAG)

• Injected gas miscible with reservoir

oil at MMP or higher

• Decreases oil viscosity – Increases oil

mobility

• Immiscible WAG (IWAG)

• Not miscible with reservoir oil.

Occurs below MMP

• Displaces oil while maintaining oil

phase.

• Provides a front between the 2

phases

Types of WAG cont’d

• Hybrid WAG (HWAG)

• Begins with Injection of large slug of gas

• Followed by small slugs of water and gas

Types of WAG cont’d• Simultaneous WAG (SWAG)• Water and Gas Injected at the same time• Water and gas mixed at the surface• Single Injection well

Types of WAG cont’d• Selective Simultaneous WAG (SSWAG)• Dual completions in a single Injection well

WaterGas

When to Initiate WAG• Initial WAG : Beginning of reservoir development

• Post Breakthrough WAG: Initiating WAG after Breakthrough.

Initial WAG shows slightly higher cumulative oil recovery (Morais, 2012).

Conclusion

Every Reservoir is Unique and should be treated Independently

when deciding the type of EOR technique to be used for

optimum results. Laboratory experiments are important

processes which aid in making an informed decision.