Shaly Sand Formation Evaluation from logs of the Skrugard well, Southwestern Barents Sea, Norway
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The use of Archie interpretation model to estimate water saturation in clean formation has successfully been useful over the years. However, in shaly sand formation this model yields inaccurate S_w estimates (overestimate) due to shale or clay effects. Many shaly sand interpretation models have been developed, unfortunately; there is no unique model that appears to fit all shaly sand reservoirs.A comparison study of water saturation on well 7220/8-1 was carried out using four different saturation models (Archie, Indonesian, Simandoux, and Modified Simandoux). Formation permeability was then estimated using two NMR models (Timur-Coates and SDR). The results from the study have shown that the average water saturation values from Archie model were higher (14.3%) than that of shaly models. The Indonesian models yields water saturation values close to that given by Archie model of about 13.6% average. Again the result from the Simandoux model (12.8%) which is slightly lower than that of Archie model but close to Indonesian model. The even lowest average water saturation values were given by the Modified Simandoux model (9.8%). The average permeability values were 1837.308mD and 566.1611mD for Timur-Coates and SDR models respectively. Because of the lowest average water saturation values given by the Modified Simandoux model relative to base model (Archie), this is the best model to be applied for the study. The Timur-Coates model is the best model for the study due to good agreement with the core-derived permeability. The SDR underestimated the formation permeability for the entire hydrocarbon interval.