Difference Between Overbalanced and Underbalanced Drilling?

 Variances of Overbalanced and Underbalanced Drilling

The drilling operation is a bridge between exploration and the oil and gas field development stages. Seven drilling projects were proposed to advance the existing methods but they were developed separately. These techniques have a substantial influence on performance factors like wellbore stability, reservoir deliverability, and safety. Here, we will outline how each one operates, where they are used, their positives and negatives, and the essential difference between them.

Overbalanced Drilling Explained:

Conventional drilling (Overbalanced drilling) is where the pressure exerted by the drilling fluid is greater than the formation pressure. This allows you to avoid drilling through formation fluids like oil, gas, or water flooding the wellbore, thus ensuring the well is controlled while your drilling operations continue. Drilling fluid, commonly known as drilling mud, is essential for mud cooling and lubricating to maintain wellbore stability, and controlling pressure, and carrying cuttings to the surface.

Why Overbalanced Drilling?

Pressure Differential:

The pressure exerted by the drilling fluid is always greater than that of the formation, ensuring a pressure differential in the positive side.

This stops any formation fluids from getting into the wellbore which lowers the chances of kicks and blowouts.

Wellbore Stability:

The overbalance condition prevents shale swelling and caving and stabilizes the wellbore.

Mitigates risk of hole collapse/formation damage due to uncontrolled pressure fluctuation.

Limitations of Formation Evaluation:

Wellbore communication - In overbalanced drilling the flow of fluids from the reservoir to the wellbore is artificially blocked; thus, it is difficult to obtain an accurate measure of the productivity of the formation.

Drilling fluid invasion is damaging for some wells, reducing permeability and productivity in specific scenarios.

Safety Considerations:

The importance of overbalanced drilling for safety and well control incidents.

Well pressure control is maintained by blowout preventers (BOPs) and mud weight adjustments.

Benefits of Overbalanced Drilling

✔Events for kicks or blowouts are highly reduced and excellent well control.

✔To stabilize the wellbore, and control formation collapse.

✔Works with a variety of formation types.

✔Performs efficient removal of cuttings for smooth drilling.

Limitations of Overbalanced Drilling

✔Causes formation damage due to mud filtrate invasion.

✔Can block pores with drilling fluids and lower reservoir productivity.

✔No ability for real time formation evaluation

What is the Underbalanced Drilling?

Underbalanced drilling (UBD) is one such technique that is defined as a drilling method with wellbore pressure less than formation pressure. This pressure difference causes the reservoir fluids to invade the wellbore while drilling, minimizing formation damage and enhancing productivity. In low-permeability reservoirs or in fragile formations where minimizing formation damage is critical, underbalanced drilling is widely applied.

Unbalanced Drilling and Its Characteristics

Pressure Differential:

To allow reservoir fluids to flow into the wellbore, maintain the wellbore pressure below the formation pressure.

Enabling real-time formation evaluation for accurate reservoir characterization.

Considerations on Wellbore Stability:

Underbalanced drilling decreases the risk of differential sticking and creation damage, unlike overbalanced drilling.

– Nevertheless, ensuring wellbore stability could be difficult, particularly in weak formations.

Reservoir Productivity and Formation Evaluation:

In real-time, enabling in-depth analysis of reservoir fluids helps to improve formation evaluation with UBD.

Improves hydrocarbon recovery by blocking circulation of drilling fluid into the reservoir

Safety Considerations:

Given that formation fluids are constantly flowing into the wellbore, surface equipment management is also needed to allow safe outflow.

Complex well control: this is to control a potential production increase with advanced techniques.

Benefits of Underbalanced Drilling

✔Minimizes formation damage, maintaining reservoir permeability.

✔Facilitates hydrocarbon extraction by letting the reservoir fluids to flow into the borehole.

✔And the value such a digital twin provides real-time formation evaluation to enhance decision making.

✔Minimizes Downhole Problems such as Differential Sticking and Lost Circulation

UNDERBALANCED DRILLING LIMITATIONS

✔Necessitates specialized equipment and techniques, thus heightening operational intricacy and expenses.

✔Continuous fluid influx makes it difficult to maintain well control.

✔Not looking through formations where stability is no. 1 concern

When to Use Underbalanced Drilling?

✔Where minimization of damage to the formation is critical to retain or extend the output of the reservoir.

✔In formations with low-permeability, where, as a result, the invasion of the fluid can affect the production considerably.

✔For reservoir characterization when there is a need for real-time formation evaluation.

✔For depleted reservoirs, where maintaining pressure balance can be maximized for production.

Conclusion

It is important to know the difference between overbalanced and underbalanced drilling to provide the best performance and recover the most from the reservoir. The commonly used drilling formula is overbalanced drilling in ensuring well control while maintaining stability, whereas underbalanced drilling improves both reservoir productivity and formation evaluation. Each of these methods has its uses and the decision on which to use is highly dependent on the well's geological and operational requirements. With the choice of the appropriate drilling technique, oil and gas companies can make operations more efficient, cut costs, and optimize hydrocarbons recovery.

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