With conventional (overbalanced) drilling practices, problems include lost circulation, differential sticking, low drilling rates and formation damage. Underbalanced drilling, although initially more costly, can reduce these problems and in the long run reduce costs.
Underbalanced Drilling simply stated is the drilling of a well with a circulation medium which has an effective pressure gradient which is less than that of the formation pressure of the section to be drilled.
The main benefits of drilling underbalanced is to minimize formation damage and increase penetration rates and. During conventional drilling there is a positive differential pressure in that the drilling fluids pressure is greater than the formation pressure and the drilling fluids will flow into the formation. In conventional drilling a filter cake is formed (hopefully) and acts as a protective barrier, reducing damage to formation permeability from drill cuttings. When drilling horizontal wellbores, drill cuttings are ground into fine powder and if a good filter cake is not formed, this powder is carried into the formation, greatly reducing near-wellbore permeability. In underbalanced drilling there is a negative differential pressure between the formation and the wellbore stimulates the production of formation fluids and gasses that will flow into the well bore thus preventive formation damage and the negative differential pressure also helps increases penetration rates in some case increasing penetration rates by 3 times.