Conventional drilling techniques do not suffice in many wells when ROP optimization, well productivity, and formation damage are concerns. This can be especially true in mature fields and horizontal drilling applications. During underbalanced drilling (UBD), the pressure within the wellbore is kept lower than the pressure of fluid in the formation. The resulting inflow of fluid from the reservoir can be controlled during the entire drilling process to eliminate near-wellbore damage. By preventing formation damage, UBD increases well productivity and recovery. UBD also reduces drilling problems, such as differential sticking, drilling fluid losses, and low ROP.