TX 75083-3836 U.S.A., fax 1.972.952.9435. AbstractThe accurate measurement of produced gas during underbalanced and managed pressure drilling operations is recognized as one of the most critical requirements for determining production capability of drilled producing zones, accurate wellbore flow modeling, quantifiable reservoir characterization, and for determining methods for well control.Most underbalanced equipment service providers currently use conventional orifice meters to measure annular gas flow during underbalance drilling operations. However, there are a number of disadvantages with the use of conventional orifice metering technology for this application. Such disadvantages include:1. Inaccuracies in flow calculations if incorrect gas compositions are assumed 2. Difficulty of measuring widely varying gas-flow rates and high turn-down ratios 3. Difficulty of rapidly removing and/or changing orifice plates when flow rates change quickly 4. Possible downstream equipment damage as the result of dislodged orifice plates. An ultrasonic meter can resolve some of the above problems, since it has no physical restrictions or moving parts, and therefore, the need to physically remove and/or change a plate due to varying gas-flow rates is eliminated along with the danger of damaging downstream equipment due to dislodged plates. The local computer provides for "hands-off" measurement once the electronics are calibrated and properly programmed. The meter is capable of turn-down ratios of over 250:1 to accommodate varying gas-flow rates. The electronic capabilities also provide automatic flow correction for rapidly changing gas composition. This paper discusses several lessons learned from designing and testing a portable ultrasonic metering system in