Reservoir Engineering

Mobility Ratio

Mobility Ratio: The Key to Understanding Fluid Movement in Oil & Gas Reservoirs

Mobility Ratio (M) is a fundamental concept in oil and gas reservoir engineering that quantifies the relative ability of two fluids to move through a porous medium. It plays a crucial role in understanding how fluids behave in reservoirs, particularly during oil and gas production.

What is Mobility Ratio?

Mobility ratio is simply the ratio of the mobility of the displacing fluid (e.g., water, gas) to the mobility of the displaced fluid (e.g., oil).

  • Mobility refers to a fluid's ability to move through a porous rock. It depends on the fluid's viscosity and the permeability of the rock.
  • Higher mobility means the fluid can move faster and easier through the reservoir.

Calculating Mobility Ratio:

Mobility ratio (M) is calculated as follows:

M = (k * µd) / (k * µf)

Where:

  • k: Permeability of the rock
  • µd: Viscosity of the displacing fluid
  • µf: Viscosity of the displaced fluid

Understanding Mobility Ratio:

  • M > 1: This indicates the displacing fluid is more mobile than the displaced fluid. This scenario leads to favorable mobility and can result in efficient displacement of the oil.
  • M < 1: This signifies the displacing fluid is less mobile than the displaced fluid. This creates unfavorable mobility and can result in inefficient displacement, potentially leaving behind significant amounts of oil.
  • M = 1: This indicates both fluids have equal mobility. Displacement may be more efficient than unfavorable mobility but can still lead to problems.

Impact of Mobility Ratio:

  • Reservoir Efficiency: Mobility ratio directly impacts the efficiency of oil and gas production. A favorable mobility ratio (M > 1) allows for efficient displacement of the oil, while unfavorable mobility (M < 1) can lead to bypassed oil and reduced recovery.
  • Water Flooding: In waterflooding, the mobility ratio between water and oil is critical. A high mobility ratio can lead to channeling and fingering, reducing sweep efficiency and oil recovery.
  • Gas Injection: During gas injection, a high mobility ratio can be beneficial, as gas can easily move through the reservoir and displace oil. However, if the gas injection is not well controlled, it can lead to premature breakthrough and reduced sweep efficiency.

Managing Mobility Ratio:

  • Chemical Injection: Polymers and surfactants can be injected into the reservoir to improve the mobility of the displacing fluid and make it more efficient.
  • Well Placement: Optimizing well placement can minimize the impact of unfavorable mobility by ensuring even displacement and reducing channeling.
  • Injection Rates: Controlling the injection rates of the displacing fluid can help manage the mobility ratio and ensure efficient displacement.

Conclusion:

Mobility ratio is a critical parameter for understanding fluid behavior and optimizing oil and gas production. By carefully analyzing the mobility ratio and taking appropriate measures, operators can maximize reservoir efficiency and achieve higher oil and gas recovery rates. Understanding the impact of mobility ratio allows for the development of more effective production strategies and the maximization of resource extraction.

Test Your Knowledge

Mobility Ratio Quiz

Instructions: Choose the best answer for each question.

1. What does mobility ratio measure?

a) The rate at which a fluid flows through a porous rock. b) The relative ability of two fluids to move through a porous medium. c) The pressure gradient needed to move a fluid through a porous rock. d) The volume of fluid that can be stored within a porous rock.

Answer

b) The relative ability of two fluids to move through a porous medium.

2. Which scenario indicates favorable mobility?

a) Mobility ratio (M) > 1 b) Mobility ratio (M) < 1 c) Mobility ratio (M) = 1 d) None of the above

Answer

a) Mobility ratio (M) > 1

3. What is the impact of unfavorable mobility on oil production?

a) Increased oil recovery b) Bypassed oil and reduced recovery c) Faster production rates d) No impact on oil production

Answer

b) Bypassed oil and reduced recovery

4. Which of the following methods can be used to manage mobility ratio?

a) Chemical injection b) Well placement optimization c) Injection rate control d) All of the above

Answer

d) All of the above

5. In waterflooding, a high mobility ratio can lead to:

a) Efficient displacement of oil b) Channeling and fingering c) Increased oil recovery d) Faster production rates

Answer

b) Channeling and fingering

Mobility Ratio Exercise

Scenario:

A reservoir has the following characteristics:

  • Permeability (k) = 100 mD
  • Oil viscosity (µf) = 2 cP
  • Water viscosity (µd) = 1 cP

Task:

  1. Calculate the mobility ratio (M) for water displacing oil in this reservoir.
  2. Based on the calculated mobility ratio, describe the impact of waterflooding on oil recovery in this reservoir.
  3. Suggest a possible solution to improve the oil recovery in this scenario.

Exercice Correction

1. **Mobility Ratio Calculation:** M = (k * µd) / (k * µf) M = (100 mD * 1 cP) / (100 mD * 2 cP) M = 0.5 2. **Impact of Waterflooding:** The calculated mobility ratio is 0.5 (M < 1), indicating unfavorable mobility. In this scenario, water is less mobile than oil, which will lead to inefficient displacement and bypassed oil. Water will tend to flow through preferential pathways, leaving behind oil in less permeable zones, resulting in reduced oil recovery. 3. **Possible Solution:** To improve oil recovery, consider injecting a polymer solution along with water. Polymers increase the viscosity of the water, making it more mobile. This will increase the mobility ratio, making the water a more effective displacing fluid and improving sweep efficiency. Additionally, optimizing well placement can help to minimize channeling and ensure more uniform water distribution throughout the reservoir.

Books

  • Reservoir Engineering Handbook by Tarek Ahmed (This comprehensive handbook covers mobility ratio in detail within the context of reservoir engineering)
  • Fundamentals of Reservoir Engineering by John Lee (This book provides a solid foundation in reservoir engineering concepts, including mobility ratio and its impact on production)
  • Modern Reservoir Engineering and Production by J.P. Brill (This book explores various aspects of reservoir engineering, including mobility control techniques and the role of mobility ratio)
  • Petroleum Engineering Handbook by William J. D. van Rensburg (This handbook covers various aspects of petroleum engineering, including mobility ratio and its implications for fluid flow)

Articles

  • Mobility Ratio and Its Impact on Waterflooding Efficiency by P. A. Witherspoon and J. J. Novy (This article discusses the impact of mobility ratio on waterflood performance)
  • Effect of Mobility Ratio on Gas Injection Performance by D. A. Wood and A. L. Khaleel (This article analyzes the effect of mobility ratio on gas injection processes)
  • Mobility Control in Oil Recovery by D. W. Green and G. Willhite (This article provides a detailed overview of mobility control techniques, including the role of mobility ratio)
  • Improved Oil Recovery by Mobility Control by G. A. Pope (This article explores different approaches to mobility control in oil recovery and the significance of mobility ratio)

Online Resources

  • SPE (Society of Petroleum Engineers) website: The SPE website offers a vast collection of technical papers, presentations, and publications related to reservoir engineering, including those discussing mobility ratio. You can search their database for relevant articles.
  • OnePetro (formerly IHS Markit): OnePetro provides access to a vast library of technical information on oil and gas production, including articles, reports, and data related to mobility ratio.
  • Schlumberger Oilfield Glossary: This glossary offers a clear definition of mobility ratio and related terms with examples and explanations.

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