Abstract

In Priobskaya Field, one of the largest operated oil fields in Central Siberia, the production data from each well are gathered on a daily basis. In addition, well tests are conducted periodically to determine the efficacy of the fracture (every well is hydraulically fractured) as well as the near well bore permeability value. The field is under water flood, and as a result, the average pressure is above bubble point.

While analyzing the long term production data, it was observed that the declining production period is much longer than what is expected of a field producing above bubble point and is supported by injected water. This discrepancy was explained by using higher value of oil compressibility, although no lab evidence indicated that such compressibility exists. With the help of high value of oil compressibility, the long decline can be reproduced.

In this study, we investigated the reasons for long term decline by considering the connectivity of sands in the reservoir. We examined the fine scale models with the upscaled models and noticed that the connectivity of sands is substantially changed during the upscaling process. When a fine scale model is generated, lack of connectivity between injectors and producers resulted in some of the sands being depleted as if producing under solution gas drive mechanism. Depending on the connectivity between the injector and producer, the production from a well was a result of combination of water support and solution gas drive. The GOR of a producer well is affected as well as the decline in production. When the reservoir model is upscaled, the production decline is no longer observed due to enhanced connectivity between injector and producer. Under those conditions, the only way to match the reservoir performance is by using high oil compressibility.

The improved understanding of the relationship between fine scale connectivity and the flow behavior is very helpful in future reservoir management strategy. By observing producing gas oil ratios and relative declines, the operator would be able to determine the relative connectivity of sands between the injectors and producers. This would help re-assign injection volumes as well as potential locations of new producers and injectors

Introduction / Background

This paper presents the study to reconcile production decline with reservoir connectivity in Priobskaya field. This field is one of the largest operated oil fields in Central Siberia, Russia. This is a clastic reservoir of turbidite environment with significant heterogeneity. Figure 1 shows a vertical cross section of the sand/shale model and an areal porosity distribution of the geological model of this reservoir. As shown from this figure, it is clear that connectivity is an important issue in modeling this reservoir. The production data from each well are gathered on a daily basis. In addition, well tests are conducted periodically to determine the efficacy of the fracture (every well is hydraulically fractured) as well as the near well bore permeability value. The field is under water flood, and as a result, the average pressure is above bubble point.

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