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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