One of the primary jobs of drilling mud is to control formation pressure, the pressure of the fluids in the rock pores. The relationship between the mud’s hydrostatic pressure and the formation pressure decides whether a well is drilled safely. This article defines the key pressure terms and the two ways of controlling the well: overbalance and underbalanced drilling.
Basic Definitions
- Pressure is the force exerted on a unit cross-sectional area.
- Pressure gradient is the pressure exerted per unit length (for example psi/ft).
- Hydrostatic pressure (PH) is the pressure due to the weight of a static column of fluid, here the drilling mud.
- Formation pressure (PF) is the pressure of the fluids inside the pores of the rock.
The pressures used in drilling are explained in more detail in types of operational pressures.
1. Overbalance Drilling
This is currently the most popular technique. The drilling mud exerts a hydrostatic pressure on the formation that is greater than the formation pressure. The safe mud weight limit is confirmed with a leak-off test.
PH = PF + POB
- PH = hydrostatic pressure
- PF = formation pressure
- POB = overbalance pressure, or simply the overbalance
Example: if the formation pressure is 4,000 psi and the mud gives a hydrostatic pressure of 4,300 psi, the overbalance is 300 psi.
If the formation pressure becomes greater than the hydrostatic pressure, formation fluid flows into the wellbore. This is known as a kick, and it must be controlled quickly to avoid a blowout.
2. Underbalanced Drilling
Underbalanced drilling is a specialised technique in which the influx of formation fluid into the wellbore is deliberately controlled, to minimise or avoid borehole problems such as formation damage. The hydrostatic pressure is designed to be less than the formation pressure. It is a controlled kick in which the volume of fluid flowing in and mixing with the wellbore fluid is known.
PH = PF − POB
Most conventional wells are drilled overbalance, but recent developments in drilling technology have led to the adoption of underbalanced drilling to improve well productivity.
| Overbalance | Underbalanced | |
|---|---|---|
| Hydrostatic vs. formation pressure | PH greater than PF | PH less than PF |
| Fluid influx | None (an influx is a kick) | Deliberate and controlled |
| Main advantage | Simple and safe well control | Less formation damage |
| Usage | Most conventional wells | Specialised cases |
The regulation of the relationships between these pressures is crucial to a successful drilling operation.
Frequently Asked Questions
What is a kick?
An unwanted flow of formation fluid into the wellbore, which happens when formation pressure becomes greater than the hydrostatic pressure of the mud.
Why is overbalance used in most wells?
It keeps formation fluids in the rock and makes well control simpler and safer.
Key Takeaways
- Overbalance means PH > PF; underbalanced means PH < PF.
- A kick happens when PF exceeds PH unintentionally.
- Mud weight is limited by formation pressure below and fracture pressure above.
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