Underbalanced Drilling Definition, Features & Applications
https://www.youtube.com/watch?v=MtxRPpiFwfk&t=1s

Underbalanced drilling technique involves drilling the formation with drilling fluids having lesser hydrostatic pressure than the formation pressure. The lesser hydrostatic pressure of drilling fluids can occur naturally or it can be formulated by adding certain substances like methane, air, or nitrogen (N2) in the liquid phase. This technique results in the inflow of formation fluids that are circulated in the well and controlled on the surface.
https://www.drillingmanual.com/underbalanced-drilling/