Probabilistic methodologies have yielded new insights into oil volume and recovery for Tengiz Field, Republic of Kazakhstan. Tengiz is one of the deepest super-giant oil fields in the world, and is located on the shore of the Caspian Sea. Knowledge of the uncertainties inherent in oil recovery is a key to proper reservoir management of Tengiz Field.

Both structured Monte-Carlo and Experimental Design techniques have been used to investigate the interaction of key uncertainties for this deep, super-giant oil field. Probabilistic approaches have provided information about the most likely oil volume and production and give insights into the probable ranges of oil volume and production. This paper describes the methodology and technical studies conducted in support of the 2005 reservoir model for Tengiz Field.

A structured Monte-Carlo Technique has been used to construct several thousand static models of Tengiz Field. Construction of Monte-Carlo models is very fast and allows the impact of key uncertainties to be explored. Key reservoir uncertainties include structural elevation, geostatistical parameters (such as variogram length), the Bulk Volume Water (BVW) constants for each stratigrahic interval, the location of the boundary between facies intervals, and the extent of porous facies and altered (dolomitized) zones, etc.

Investigating the uncertainty in the dynamic performance of the model is much more computationally intensive. Experimental Design techniques minimize the number of computationally expensive simulation runs. Dynamic variables investigated include: oil in place, permeability enhancements, vertical to horizontal permeability ratio, well productivity, the water oil contact depth, and production capacity.

At Tengiz, uncertainties in variables which are "random" or vary spatially across the field have much less impact than uncertainties which can have systematic uncertainty. Examples of systematic uncertainty include wire-line to core calibration of porosity and saturation. This study also showed that for a specific development scenario the oil in place, gasoil relative permeability, and ratio of vertical to horizontal permeability, had the most impact on the estimated oil recoveries.

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