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'Hydrocarbon Sampling in Tight Chalk'

The acquisition of representative reservoir fluid samples is a fundamental step of reservoir appraisal. Correct reservoir fluid data are of paramount importance to maximize the value of the project, by accurately quantifying the reserves and designing optimized processing and export infrastructure. This paper discusses the wireline sampling operations for a well evaluating a tight Upper Cretaceous Hod reservoir section. Acoustic impedance data from seismic surveys indicate that hydrocarbons are accumulated in a combined structural and stratigraphic trap. Large subsurface uncertainties were still present at the time of planning the appraisal well. Formation pressure was estimated in a range of 8500-11500psi. Hydrocarbon type was estimated to be Volatile Oil or Gas Condensate, with expected range of Saturation Pressure between 5000-8000psi. Petrophysical properties were also uncertain, with porosity/permeability ranges of 10-25PU and 0.01-1.0mD. The appraisal well included a pilot hole with a comprehensive data acquisition program: formation cores, electric logs, formation pressures and fluid samples. High importance was given to the acquisition of representative hydrocarbon fluid samples. As the preliminary core data became available, it was clear that formation permeability was at the very low end of the estimate range. This made the fluid sampling operation particularly challenging. The paper describes the planning and execution of the wireline sampling operations, the quality of the data obtained, results of data analysis and lessons learnt.
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