Understanding reservoir complexities such as compartmentalisation and compositional gradients early in the life of a field is crucial for its optimal development. That's especially true for deepwater reservoirs. Deepwater exploration and development has become a key area for most E&P companies because of the increased global demand for oil and gas and the limited accessibility of other potential unexploited resources. Modern wireline formation testers can provide a wide range of downhole rock and fluid properties at in-situ reservoir conditions and can help diagnose deepwater reservoir issues such as compartmentalisation and compositional grading early on. Advanced downhole fluid analysis (DFA) performed by wireline formation testing tools provides real time quantitative measurement of variation in asphaltene content as manifested though fluorescence and fluid coloration. Gas-oil ratio (GOR), another key determinant in asphaltene stability and gradients, is also measured downhole with the DFA technique. Building on recent advances in asphaltene molecular and colloidal physics, a new asphaltene equation of state (EoS) can be used to model asphaltene gradients in crude oils in the same way that a traditional cubic EoS is used for modeling GOR gradients.

In this paper, we analyze a deepwater case study showing increases in asphaltenes content with depth due to destabilisation resulting from secondary gas charging at the top of the column and with solid hydrocarbon/tar depositing at the base. These seemingly contradictory observations are shown to follow from a new nanoscience model of asphaltenes. A simple model is proposed for this tar mat formation and consisting of disequilibrium gas charging, large disequilibrium gradients of asphaltene and GOR, asphaltene instability in the oil, and tar in the core. Likely compartmentalisation will also be identified using DFA analysis and verified using lab data analysis, allowing integration into the reservoir modeling process.

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