Compositional grading resulting from the gravity segregation in thick reservoirs is a common phenomenon observed especially in volatile oil or gas condensate reservoirs. Same phenomenon is repeatedly encountered for reservoirs with high temperature gradients. It is also not uncommon if compartmentalized reservoirs indicate areal variations in fluid properties. But drastic variation in composition and fluid properties both vertically and laterally in thin to moderate reservoirs where the reservoir seems to be connected based on initial pressure data may denote non-equilibrium conditions. In this study such conditions will be assessed for a Malay basin saturated oil reservoir and the challenges and proposed solutions are presented.

The individual available samples from appraisal and early production system (EPS) development wells covering six stacked sands each comprising of four segments are thoroughly analyzed for quality and reliability. The information from fluid samples including finger prints for hydrocarbons and CO2, is integrated with pressure and full PVT data to characterize the fluid in this field. Reconciliation of PVT and well test data was harnessed to achieve to a set of representative samples for each reservoir/segment while considering reasonable compositional grading and rate of bubble point depression. The samples were then scrutinized for reliable experimental data.

Various alternatives of non-equilibrium initialization vs. defining multiple equilibrium regions with compositional grading were considered. The pros and cons of each method is discussed in this paper and general hints for selecting appropriate approach is given based on type of reservoir being modeled, the purpose of study and available data. It is shown that a fit-for-purpose approach can be adopted to fulfill the requirements of the FDP while trying to reproduce the reservoir behavior. The role of reservoir management and updating the model regularly as per future performance is emphasized.

The non-equilibrium reservoir systems have not been extensively discussed in the industry and limited case studies and corresponding challenges and solutions have been presented. This paper discusses fundamental phenomena encountered in dealing with such systems and presents solutions based on the nature of the problem and project objectives both in theoretical and practical sense and in this regard it is novel.

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