Some rock formation has a reactive tendency to water. When drilling-mud is used the water in the drill mud reacts with the formation (mostly clay) and inherently causes formation damage (reduction in permeability and porosity). Use of under-balanced drilling can help prevent this.

Why is underbalanced drilling not suitable for directional drilling?

Factors that negatively affect underbalanced drilling

Spontaneous imbibitions because of incompatibility between the base fluid used in the UBD fluid and the rock or reservoir fluid. ... Wells that require hydrostatic fluid or pressure to kill the well during certain drilling or completion operations.

What is the advantage of under balanced drilling?

5.3.

UBD is considered a drilling method to protect the reservoir by reducing formation damage during the operation. Although original impetus of UBD was reducing mud loss, which is an immediate safety issue, the most significant benefit of UBD is in keeping the reservoir integrity intact.