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Recognizing naturally fractured reservoirs

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Fractures are the most abundant visible structural features in the Earth’s upper crust. They are apparent at most rock ridges, and it is likely that most reservoirs contain some natural fractures. Naturally fractured reservoirs are elusive systems to characterize and difficult to engineer and predict. It is important to establish some basic criteria for recognizing when fractures are an important element in reservoir performance and to recognize the nature and performance characteristics of a naturally fractured reservoir.

How naturally fractured reservoirs are formed

Size

Naturally fractured reservoirs are observed across a vast range of scale from microcracks to mile long features.

Types

Naturally fractured reservoirs can be open, permeable pathways, or they can be permeability baffles resulting from the presences if secondary mineralization or other fine-grained material filling the gaps.

Classification

Naturally fractured reservoirs have been classified according to the relative contribution of the matrix and fractures to the total fluid production. The following table 1 is modified form Nelson (2001).


Productivity heterogeneity

Well production heterogeneity is a characteristic of most naturally fractured reservoirs. Although various geologic processes other than fractures can lead to field wide heterogeneity, such as thin, discontinuous high-permeability strata or variable development of interconnected vugs. Heterogeneity can be used to help recognize the importance of fractures in a reservoir and production statistic for wells can provide a quick-look method to recognize heterogeneity.