Viscosity from Geochemical Techniques: A Case Study for a Heavy Oil Field on the UKCS

Abstract

Heavy oil fields are characterised by complex chemistries and exhibit a wide range of physical properties (especially oil viscosity). This paper describes the development of two innovative and cost-effective data acquisition methods: the optimised use of mud gas data and the geochemical characterisation of cuttings extracts, both as an alternative data source for oil viscosity determination. Applications of these methods can lead to the cost reduction of data acquisition programs and potentially improvements in oil recovery. The mud gas provides real-time surface logging data. Due to the nature of heavy oil in the case study wells, the mud gas compositions contain mainly C1-C3. By comparing with the reservoir oil samples from the exploration and appraisal wells, we investigated the possibility of using mud gas while drilling to identify if the reservoir oil belongs to the high viscosity or medium viscosity group. For cutting analysis, we thoroughly investigated solvent extracts from cuttings from the exploration and appraisal wells and correlated them with the oil viscosity from the corresponding PVT samples. When such a correlation is established, we can then apply the correlation to predict oil viscosity from the cuttings of new production wells. We achieved excellent results for both real-time oil viscosity identification from mud gas data and post well oil viscosity prediction from the extraction of cuttings. The C1/(C2+C3) is the key parameter for oil viscosity identification from the standard mud gas data in the field. For the past wells, the mud gas interpretation has high accuracy for determining if the reservoir oil belongs to the high viscosity or medium viscosity regions. We established a strong correlation between the biomarkers and the oil viscosity from the exploration and appraisal wells. When we applied the correlation to new production wells, the oil viscosity prediction was very close to the viscosity measurement from the wellhead PVT samples. The geochemical techniques provide oil viscosity for critical decisions regarding well placement and reservoir management modelling. The new source of reservoir fluid data from mud gas and cuttings provides real-time fluid interpretation while drilling and close to PVT sample quality oil viscosity prediction after drilling. This new approach is cost-efficient and has a large potential to replace the downhole fluid sampling or expensive downhole logging tools for production wells.