A detailed understanding of the composition of sour gas accumulations is essential for facility design and for production forecasting. Compositional data like H2S and CO2 contents, net sales gas volume and condensate gas ratio also form the basis for all project economics. The acquisition of high quality fluid compositions is therefore a priority in all sour gas well evaluations. For conventional gas accumulations, PVT analysis of down-hole or well-head samples provides accurate and representative compositions of reservoir fluids. But in many sour gas accumulations, like the Marrat in North Kuwait, the prediction of future production streams is a difficult task. H2S levels are highly variable and the ‘point measurements’ from MDT fluid samples may not be representative for the entire reservoir unit. This paper describes a new approach to derive gas compositions from the analysis of core samples so that PVT compositions can independently be verified and H2S concentrations can be extrapolated away from the down-hole sampling points. The method is illustrated with two wells from an ultra-sour gas field in the Arabian Gulf region.

Core and cuttings samples from gas reservoirs contain trace amounts of adsorbed gas and condensate, which can be thermally extracted. The desorbed fluids can then be analyzed using existing geochemical technology and the molecular compositions of the condensate can be used to derive the composition of the gas that was trapped in the reservoir. Carbon isotope ratios can be measured on the gas that is desorbed from core samples and provide a second, independent parameter to estimate the original gas composition. The obtained fluid distributions are used to reconstruct the filling history of the sour gas accumulation.

The isotopic signature of the gas, together with the composition of the organo-sulfur compounds in the condensate, allow the reconstruction of fluid compositions, which are a critical input parameter for the reservoir simulators. Combining the results from the geochemical analysis with existing PVT data allows the extrapolation of the fluid compositions and the reconstruction the field-wide compositional gradients.

With H2S and CO2 concentrations varying between 5 and 35%, the production forecasting of sour gas accumulations can be subject to large uncertainties. Reconstructing the gas compositions from core and cuttings samples with geochemical techniques substantially reduces these uncertainties.

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