NETL has developed a state-of-the-art fractured reservoir model , FRACGEN/NFFLOW, that consists of two key components.  First, a series of fracture network generators (FRACGEN) provides detailed two- or three-dimensional representations of reservoir fracture networks. Second, a flow model (NFFLOW) estimates the interaction of the fractures with the rock matrix and simulates the flow of gas through the fracture network to one or more boreholes.

The models are available through the NETL CD ordering system under the title "FRACGEN ^TM and NFFLOW ^TM Fracture-Description and Reservoir Engineering Codes for Highly Fractured Reservoirs (Release Z)".

FRACGEN implements four stochastic models of increasing complexity that sample fitted distributions of fracture length, aperture, spacing, etc., for up to ten fracture sets. The selection of which model to use is driven by the amount of data available. The three most complex models allow for fracture termination and intersection frequencies among the different sets to be controlled by the user. Two of the models also generate fracture swarms. Clustering can be random or parallel to sub-parallel. In addition, the user can condition the network to known fracture locations as observed in a borehole. All fractures extend vertically through the entire height of the layer. Recent work allows the modeling of multi-layered networks, in which fracture networks are generated for several layers that are then stacked, with a user-specified percentage of fractures in each layer extending into the overlying layer.

NFFLOW computes flow rates or bottom-hole pressures according to user-specified pressure or rate schedules. Single, multiple, or multi-branched wells may be used, and the wells may be vertical, inclined, or horizontal. Flow is single phase and gravity effects are neglected. Fracture-bound matrix blocks drain to or recharge from the midpoint of adjacent fractures in accordance with a one-dimensional unsteady-flow model. A requirement of material balance among all intersections couples the individual recharge models together.

FRACGEN/NFFLOW represents a significant advancement in the art of gas reservoir simulation by being the first model to readily simulate gas flow and drainage in fractured reservoirs with a discrete, irregular, stochastic fracture network using a large number of fractures.

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