The main Permian Reservoirs in Central Saudi Arabia have presented an evaluation challenge because of the presence of fresh-brackish water encountered directly below the hydrocarbon accumulations. If this water is used in the evaluation of the oil zone then high connate water saturation is obtained which is inconsistent with generally initial water free oil production observed in the field. Early field development evaluation pragmatism assumed a dual salinity model. Higher salinity formation water was used for the evaluation of the oil zone and fresher water used for evaluation of the underlying water zone. The oil zone formation water was assumed to be the original depositional water present at the time of oil migration. High salinity can be typical for an Arid Fluvial–Estuarine system with high evaporation. Oil migration trapped this water in place. Post-hydrocarbon migration, Paleozoic rain water from higher elevations of the Arabian Shield recharged and flushed the original water beneath the oil. Practical issues have limited confirmation of this assumption and no measurements have been taken to confirm the oil zone water salinity or the dual salinity model.
A new technique is being explored that involves extraction and analysis of small quantities of fluid from core plugs sampled at the rig site. Invasion is quantified using mud doping. Initial results show that there is indeed evidence for higher salinity water present in the hydrocarbon zone compared with fresher water in the aquifer, consistent with the proposed dual salinity model. Extended testing of the technique has been conducted which shows that it can be used to measure water salinities in even very tight rock (~micro Darcy). The potential of the technique is to provide a more reliable fluid discriminator, both in these low relief difficult to evaluate formations, and in exploration and unconventional areas.
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