Geochemistry, specifically gas chromatography and isotope analysis, offers a solution to the problem of properly allocating gas production to specific reservoirs when more than one formation is commingled in a well. Current allocation methods involving zone isolation are limited because the commingled flow rates obtained on the surface are not necessarily representative of actual individual formation contributions.

The proposed solution to this problem was the derivation of three models that accounts for variable isotope and composition characteristics, while also providing a trend in which all such data could be correlated. Isotope analysis in this application involves obtaining the geochemical "fingerprint" of a natural gas sample using detailed chemical and isotopic characteristics of the gas sample.1  From these models, it is possible to determine a commingled production allocation from calculated isotope data, given only the gas chromatography results, specifically the composition and specific gravity of a given sample.

This approach will permit adjusting the commingled production allocations over the life of the well under changing reservoir conditions. Once each13C endmember is established, the fingerprint remains constant for the life of the well. Early data suggests that the use of isotope analysis to determine production allocations should produce accurate results once a calibration technique is established

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