The need to increase productivity by reducing formation damage during drilling is the aim of underbalanced drilling technology. The proposed solution is to drill with mud having an equivalent circulating density (ECD) less than the formation pore pressure and to avoid contact of drilling fluid with the formation. In highly depleted reservoirs, pore pressures can be very low. Analysis showed that horizontal webs could be drilled with an underbalanced system with a low instability risk. ECD values as lon as 2.0 lbm/gal were used to drill webs with no wellbore instability problems reported. Production tests showed large improvements in web productivity indices compared with conventional overbalanced drilling.