What main feature distinguishes the underbalanced drilling technique from the traditional technique?

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Underbalanced drilling is a technique in which the hydrostatic pressure of the drilling fluid is less than the pressure of the formations being drilled through.

This pressure condition can be obtained naturally, i.e. where naturally without any intervention the drilling fluid pressure is lower than the formation pressure, another way to achieve the underbalanced condition is inducing such condition, this can occur if the drilling fluid is added different substances such as natural gas, nitrogen and air, as the three without thinning agents that lower the density of the drilling fluid, which results in a lower hydrostatic pressure of the drilling fluid.

Whatever the condition is, i.e. natural or induced, the underbalanced condition is intended to allow the fluids that are in the formation such as oil and gas to be produced early while the well is being drilled, which in turn is what is called an influx of early formation fluids, however precautions must be taken at the surface level so that the circulation of oil and natural gas from the formation to the surface does not cause a blowout.

Characteristics of the UBD

One of the features associated with underbalanced drilling is that not only can early oil and gas production be obtained by keeping the drilling fluid pressure lower than the formation pressure, but even by maintaining a balance of pressures in which the drilling fluid pressure is equal to the formation pressures, it is possible to keep the pressures in balance and obtain the same benefits as underbalanced drilling.

However, in order to understand the most salient feature, it is convenient to compare the underbalanced drilling technique with the traditional technique, in which the pressure exerted by the drilling fluid is always higher than the formation pressure.

A clear example of the conventional technique is shown in the following image:

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We can see how in the previous image the traditional technique is expressed, in which the pressure exerted by the drilling fluid in the annular space is 5500 psi, being this higher than the pressure exerted by the formation fluids which in this example is 5000 psi.

Already to understand the main distinguishing feature of the UBD with the traditional technique I will show you the following image:

Image source

In the previous image it can be seen that the pressure exerted by the drilling fluid in the annular space is 4500 psi, which is lower than the formation pressure of 5000 psi, this imbalance of pressures is intended to reduce the damage to the formation, as there will be no invasion of the drilling fluid filtrate, and thus stimulates early production of oil and natural gas, of course having the surface equipment to prevent a blowout occurs.

References.

Underbalanced drilling limitations

https://petrowiki.spe.org/Underbalanced_drilling_limitations