Fracture Design Considerations in Naturally Fractured Reservoirs

Cipolla, C.L. (CER Corp.) | Branagan, P.T. (CER Corp.) | Lee, S.J. (CER Corp.)



The ability to effectively enhance production through hydraulic fracturing is dependent on an accurate description of the reservoir production mechanism(s) . Fracture designs may dif f er greatly depending on the production mechanism (s) . The complex nature of hydraulically fractured reservoirs in which the predominant production mechanism is a set of interconnected, naturally occurring fractures is investigated in this paper. The paper integrates general reservoir simulation results with actual field data from a naturally fractured reservoir in the Piceance Basin, Colorado.

The study investigates a variety of natural fracture/matrix properties and compares the productivity (of these naturally fractured reservoirs to homogeneous reservoirs with the same average flow capacity. The paper also investigates the influence of natural fracture anisotropy on hydraulic fracture design. The effect of damage to the natural fracture system is illustrated and compared to analogous homogeneous reservoirs. The economic considerations associated with many of the reservoir production mechanisms are presented.

The results of the reservoir simulations indicate that optimum fracture lengths for isotropic, naturally fractured reservoirs are identical to those estimated for homogeneous reservoirs having the same average flow capacity. Therefore, accepted fracture design considerations to determine optimal fracture length and conductivity can be used in isotropic, naturally fractured reservoirs based on the average flow capacity of the reservoir. However, fracture design considerations are more complex when the effects of natural fracture damage and anisotropy are encountered.


The basic fracture design criteria for homogeneous reservoirs has been discussed in detail by several authors. This literature also illustrates the interrelationship of fracture length, fracture conductivity and well productivity, and the economic impact of many fracture design considerations. However, fracture design considerations in more complex, naturally fractured reservoirs are not widely available in the literature. This paper presents reservoir simulations and field data that illustrate many fracture design considerations in naturally fractured reservoirs.

The initial requirement for designing a hydraulic fracturing treatment is an accurate description of the reservoir, including the predominant production mechanism (s) . Reservoir production mechanisms and characteristics can be obtained from log, core, geological, well test and production data. In many cases, a limited amount of data are available, and reservoir characteristics and production mechanisms are inferred from pre- and postfracture well performance. There are many uncertainties associated with inferring reservoir properties based on a limited amount of data because reservoirs with vastly different production mechanisms can produce very similar pressure /production profiles. The reservoir simulations presented will illustrate the similarities in production and pressure buildup behavior for homogeneous and naturally fractured reservoirs that have the same average flow capacity.

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