Underbalance in Drilling & Well Completion: A Simple Explanation

Underbalance in drilling and well completion refers to a situation where the pressure at the bottom of the well is lower than the pressure of the formation surrounding it.

This is achieved by using a drilling fluid (mud) that is lighter than the formation fluid. This creates a pressure difference that pushes the formation fluids into the wellbore, potentially leading to:

• Increased wellbore stability: The inward flow of formation fluid helps to prevent wellbore collapse, especially in challenging formations like shales.
• Improved productivity: It can help to stimulate the formation and increase the flow of hydrocarbons into the well.
• Reduced drilling costs: By minimizing the need for heavy mud weights, it can lead to lower equipment wear and tear and reduced drilling time.

However, there are also potential risks associated with underbalance:

• Formation damage: The influx of formation fluids can lead to the deposition of solids and fines, which can impair well productivity.
• Blowouts: If the pressure difference is too large, it can lead to an uncontrolled influx of formation fluids, potentially causing a blowout.
• Safety hazards: The handling of underbalanced drilling fluids requires specialized equipment and procedures to ensure safety.

Applications of Underbalance:

Underbalance techniques are widely used in various aspects of drilling and well completion:

• Drilling: It can be used to drill through challenging formations, like those with high pore pressure or those prone to instability.
• Well completion: It can be used to stimulate production, especially in tight or fractured formations.
• Stimulation: It can be combined with other stimulation techniques, like hydraulic fracturing, to enhance production.

Key Considerations:

• The use of underbalance techniques is highly dependent on the specific geological conditions and the well's objectives.
• Careful planning and execution are essential to ensure safety and prevent potential risks.

In summary, underbalance is a valuable technique for improving well performance and reducing drilling costs, but it must be carefully planned and managed to minimize risks.