Evaluation of hydrogen sulfide concentrations in Norwegian reservoir fluids

Abstract

Knowledge of the hydrogen sulfide content in produced hics. petroleum fluids is important for planning of [HaS] control measures and for material selection. The Norwegian Continental Shelf is known for producing sweet crude oils, and [HaS] has caused few problems due to low concentrations, usually below 10ppm. However, as the exploration was moved north into the Norwegian Sea, deeper exploration welts were drilled and an increase in the HZS concentrations were detected. Well test data clearly indicate that the concentration of HZS increases with depth of burials i.e. with increasing reservoir temperature. This thesis analyzes data from a large number of exploration welts drilled in sandstone reservoirs on the Norwegian Continental Shelf. Drill stem test of a duration of at least 10 hours are needed to get a stable and reliable level of [HaS] in the produced gas. However, the trend in the oil industry is that fewer exploration wells are tested and often only lifeline samples of reservoir fluids are available. Additionally, fast track field developments often require good estimates of expected [HaS] very early. The male objective was to establish an empirical correlation between [HaS] concentration in the reservoir fluids and the reservoir temperature. The amount of hydrogen sulfide generated by thermal cracking was believed to exibit an exponential relationship with reservoir temperature. A plot of the logarithm of the [HaS] concentration versus the inverse of the reservoir temperature was found to be linear to a good approximation. This indicates an exponential relationship between hydrogen sulfide and reservoir temperature. A certain scatter in the data can be explained by uncertainty in the [HZS] field measurements and by differences in loss and removal of [HZS] in the reservoirs, due to the presence of iron and water. Several empirical correlations were developed in this work allowing a reasonably accurate estimate of the expected [HZS] in produced gas. One is based on the assumed exponential relationship between [HZS] and reservoir temperature, the others include additional fluid parameters. This contribution is considered of particular importance for planning [HZS] control strategies and for production management.