Reservoir Engineering

preflush

Preflush: Optimizing Chemical Flooding and Acid Stimulation in Oil and Gas Wells

Preflush is a crucial pre-treatment step in various oil and gas well operations, aimed at enhancing the effectiveness of subsequent chemical injections. This article explores two key applications of preflush: in chemical flooding and acid stimulation treatments.

Preflush in Chemical Flooding

Chemical flooding, a technique used to enhance oil recovery, often relies on injecting surfactant solutions into the reservoir. Here, preflush plays a vital role in preparing the reservoir for optimal surfactant performance.

1. Adjusting Reservoir Salinity and Ion Concentrations:

Preflush typically consists of injecting water prior to the surfactant solution. This injection aims to modify the reservoir's salinity and reduce the concentration of ions that can interfere with surfactant effectiveness. Surfactants work best within a specific salinity range, and preflush helps achieve the optimal conditions.

2. Obtaining Reservoir Flow Pattern Information:

Preflush can also serve as a valuable tool for understanding the reservoir's flow patterns. By injecting a tracer with the preflush water, engineers can track its movement and gain insights into the reservoir's permeability and connectivity. This information is vital for optimizing the placement of subsequent chemical injections.

Preflush in Acid Stimulation

Acid stimulation is a common method to increase well productivity by dissolving formation damage. Preflush, often referred to as a "spearhead," is used in this context to improve the effectiveness of the acid solution.

1. Preventing Premature Acid Spending:

The preflush fluid, typically a less aggressive acid or a buffer solution, is injected before the main acid solution. This helps prevent the acid from reacting with undesirable minerals near the wellbore, ensuring the bulk of the acid reaches the target zone for maximum stimulation.

2. Protecting Wellbore Integrity:

Preflush can also help protect the wellbore from damage caused by the high acidity of the main solution. The preflush fluid can neutralize some of the acid's reactivity, reducing the risk of corrosion or formation damage.

Preflush vs. Overflush

While preflush is injected before the main chemical solution, overflush is injected after the main chemical solution. Overflush serves to push the main solution deeper into the formation and to help clean up the wellbore after the treatment.

Summary:

Preflush is a crucial step in both chemical flooding and acid stimulation, ensuring optimal performance and minimizing risks. By modifying reservoir conditions, understanding flow patterns, and protecting wellbore integrity, preflush plays a vital role in improving the efficiency and effectiveness of these important oil and gas well treatments.


Test Your Knowledge

Preflush Quiz:

Instructions: Choose the best answer for each question.

1. What is the primary purpose of preflush in chemical flooding? a) To increase the viscosity of the injected surfactant solution. b) To remove dissolved gases from the reservoir. c) To prepare the reservoir for optimal surfactant performance. d) To increase the pressure within the reservoir.

Answer

c) To prepare the reservoir for optimal surfactant performance.

2. How does preflush help prevent premature acid spending in acid stimulation? a) By injecting a stronger acid solution first. b) By injecting a less aggressive acid or buffer solution before the main acid. c) By increasing the temperature of the acid solution. d) By using a specialized acid-resistant material for the wellbore.

Answer

b) By injecting a less aggressive acid or buffer solution before the main acid.

3. Which of the following is NOT a benefit of preflush in chemical flooding? a) Adjusting reservoir salinity. b) Increasing oil viscosity. c) Obtaining reservoir flow pattern information. d) Reducing the concentration of interfering ions.

Answer

b) Increasing oil viscosity.

4. What is the main difference between preflush and overflush? a) Preflush is injected before the main chemical solution, while overflush is injected after. b) Preflush is used for chemical flooding, while overflush is used for acid stimulation. c) Preflush is more effective in sandstone formations, while overflush is better for carbonate formations. d) Preflush is a more expensive treatment than overflush.

Answer

a) Preflush is injected before the main chemical solution, while overflush is injected after.

5. Preflush can help protect wellbore integrity by: a) Increasing the pressure within the wellbore. b) Reducing the risk of corrosion or formation damage. c) Removing any existing damage from the wellbore. d) Increasing the viscosity of the injected fluid.

Answer

b) Reducing the risk of corrosion or formation damage.

Preflush Exercise:

Scenario: An oil well is being prepared for a chemical flooding treatment. The reservoir contains high concentrations of calcium and magnesium ions, which can interfere with the effectiveness of the surfactant solution.

Task: Design a preflush strategy for this well, including:

  • The type of fluid to be used for preflush.
  • The volume of fluid to be injected.
  • The expected benefits of the preflush treatment.

Exercise Correction:

Exercice Correction

**Preflush Strategy:**

  • Fluid: A brine solution with a carefully controlled salinity and low calcium and magnesium ion concentration. This could be a sodium chloride (NaCl) brine or a brine with added chemicals to sequester calcium and magnesium.
  • Volume: The volume of the preflush should be sufficient to displace the existing reservoir fluids and establish the desired salinity and ion concentration in the area where the surfactant will be injected. This volume will depend on the reservoir size, permeability, and other factors.
  • Expected Benefits:
    • Improved Surfactant Performance: By adjusting the reservoir salinity and reducing interfering ions, the preflush will create an environment where the surfactant can function optimally.
    • Enhanced Oil Recovery: With improved surfactant performance, the chemical flooding process will be more effective in mobilizing and recovering oil.
    • Reduced Chemical Costs: By optimizing the surfactant performance, the preflush can potentially reduce the overall amount of surfactant needed for the flooding operation, leading to cost savings.


Books

  • "Enhanced Oil Recovery" by D.L. El-Sayed (Covers chemical flooding techniques, including preflush)
  • "Acidizing Fundamentals" by J.P. Holditch (Discusses acid stimulation and the role of preflush)
  • "Petroleum Production Handbook" by T.P. Dake (Provides a comprehensive overview of production operations, including preflush)

Articles

  • "Preflush Design Considerations for Acid Stimulation" by A. Economides et al. (SPE Journal, 1994)
  • "Optimal Design of Preflush and Overflush for Surfactant Flooding" by M. Delshad et al. (Journal of Petroleum Science and Engineering, 2014)
  • "The Role of Preflush in Chemical Flooding for Improved Oil Recovery" by K. Sorbie et al. (SPE Reservoir Evaluation & Engineering, 2000)

Online Resources

  • SPE (Society of Petroleum Engineers): www.spe.org
    • Offers a vast collection of technical papers and research on preflush and related topics.
  • OnePetro: onepetro.org
    • A comprehensive database of oil and gas industry publications, including numerous articles on preflush applications.
  • Oil & Gas Journal: www.ogj.com
    • Provides industry news and technical articles, often covering preflush technology advancements.

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  • "Preflush + Acid Stimulation"
  • "Preflush + Surfactant"
  • "Preflush + Reservoir Characterization"
  • "Preflush + Well Stimulation"

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