We have field tested a non-traditional gas sensor that shows exceptional performance for quick response and robust sensing of hydrocarbon gases in drilling fluid. Immediately realized benefits include o greatly reduced chance of missing the identification of pay zones, * better characterization of hydrocarbon shows and o increased drilling safety through the such factors as o the reduction in time between the drilling penetration of a formation and the transmission of the mud gas data to the drilling operator and the evaluation engineer/geologist, and o a robust detection and quantification of the gas regardless of over- or underbalance condition, mud weight, or mud composition (water-, oil-, synthetic- or foam-based). This sensor, based on existing membrane technology and an efficient extractor/detector configuration (Brumboiu, 2000), has been successfully field tested and its performance validated in a high pressure gas well drilled with oil-based mud. This type of sensing technology has shown tremendous potential for high-temperature/high-pressure drilling environments and underbalanced drilling, as well as traditional drilling environments. Existing technology for gas sensing while mud logging incorporates a dependence on the ubiquitous gas trap, either in traditional, inconsistently manufactured form or in modified forms attempting to quantify the true mud-gas concentration ((Wright, 1993; De Pazzis, 1989). The problems inherent with all the current applications include the need to characterize the extraction efficiency of the gas trap in a continuously changing mud return stream, the dilution of the extracted gas with large proportions of air (adversely affecting the lower detection limits), the optimal physical placement of the gas trap in an environment that is constantly changing due to both changing drilling conditions and human operator intervention, the physical size of the equipment, and the distance the extraction point remains from the detection instrumentation. The new gas probe is small, physically robust with respect to extraction efficiency and calibration, has no moving parts aside from a low-volume air pump, can be inserted into a mud stream virtually anywhere in the return mud line, and uses a simple sensor located about 2 meters from the extraction point. We will discuss the results of the field test, run in a high-density oil-based drilling fluid, by o objectively evaluating the sensor performance in terms of accuracy and consistently robust operation, * comparing the sensor performance against a traditional gas trap/total HC analyzer and comparing the results of both the gas trap and membrane sensor systems with laboratory measurements of mud gas concentration.

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