The maintenance of Western Siberia oil production levels has required the industry to evaluate and develop ever more complex reservoirs. In this case study we look at the evaluation of the heavy oil bearing field located in the north part of Western Siberia and specifically the unconsolidated Pokurskaya formation of the Cenomanian age. The specific evaluation requirements included an understanding of the oil properties, especially viscosity and its variations, and an understanding of recovery factor by an analysis of residual oil saturation. In order to accomplish these objectives an extensive suite of log data as acquired including standard triple combo data, cross-dipole acoustic, NMR, micro-images, spectroscopy and formation tester samples and pressures.

In this paper we show how an innovative use of NMR data allowed us to improve the fluid model and irreducible water saturation calculations. Combining NMR with standard data allowed a prediction of the residual oil saturation. Additionally, from the NMR data we were also able to extract oil viscosity information, however we needed a calibration point and for that a formation oil sample was required.

Unconsolidated formations saturated with viscous oils are notoriously difficult to sample with formation testers. Sand production can lead to contaminated samples, tool plugging and even borehole collapse. To avoid these issues we included dipole sonic data to estimate formation strength in order to select the optimal sampling locations. Additionally a new ‘dual intake’ straddle packer configuration was used to optimize the fluid sampling. As a result of these precautions samples were successfully acquired and the resulting PVT data was used in field development planning as well as calibration points for a continuous NMR-derived viscosity curve.

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