Discrete Fracture Reservoir Simulation
Shale Gas Flow Simulation
Shale Gas Flow Simulation

FRACGEN/NFFLOW, fractured reservoir modeling software developed by NETL’s Geological and Environmental Sciences research group, is designed to simulate gas flow in naturally-fractured reservoirs, including gas storage within the matrix. It generates a stochastic network of discrete fractures that represent the main conductors of flow. The matrix provides most of the storage. The software components are:

  1. a fracture network generator (FracGen),
  2. a flow simulator (NFflow),
  3. graphical output software (FracOut)
  4. a PC-based User Interface (FracFlow)
  5. ancillary programs for input data analysis and/or manipulation

Discrete fractures are stochastically generated using one of three models.

Multilayer Fractured Reservoir

Model 1 generates randomly-located fractures, although the connectivity controls can be used to produce various degrees of clustering. Locations of fractures intersecting a bore hole can be honored.

Model 2 generates fracture swarms (elongated clusters), whereby the swarms are randomly located and can overlap. Locations of fractures intersecting a bore hole can be honored.

Model 3 generates patterns of fractures ranging from regular to uniform to random and includes regularly-spaced swarms of fractures.

All fractures extend vertically through the entire height of a layer. For a layer, each fracture set can be generated with a different model, and connectivity of fractures can be controlled. The models can generate multi-layered networks, in which fracture networks are generated for several stacked layers, with a user-specified percentage of fractures in each layer extending into the overlying layer.

A First In Gas Reservoir Simulation

The creation of FRACGEN/NFFLOW represented a significant advancement in 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. Reservoir modeling with this software is appropriate if:

  • The reservoir produces relatively dry gas, with little interference from water or oil.
  • The reservoir rock (matrix) has less than 1 millidarcy permeability.
  • Variations in fracture apertures, density and connectivity are the predominant factors affecting gas flow migration.
  • Flow conductors are oriented nearly vertical and are strata-bound (extend from bottom to top of beds that can be modeled individually).
  • Faults do not divide the reservoir into tightly sealed compartments or produce significant offsets in the productive strata.

Version 14.4 (April, 2014) added several new features:

  • Adsorption/desorption of a gaseous component in a mono-layer within the matrix porosity.
  • Fracture permeability can change as a function of fluid pressures within each fracture segment as predicted by a simple geomechanical model, whose parameters apply to all layers.
  • A new model and mode of well specification exists for vertical wells (previously all wells were required to connect to fractures via user-specified well laterals).
  • Elimination of defined limits on number of fractures, nodes, well, etc. that can be used within NFflow. FracGen continues to have array dimensions.
  • A well can now be specified through usage of the graphical tools within FracOut.
  • Substantial improvements in the User’s Guide.

The latest release is Version 14.9 (September, 2014). Its new capabilities include:

  • Better capabilities for modeling flow of slightly compressible fluids (e.g. , water).
  • Optional specification of constant pressure boundary conditions for the margins of the Flow Region (previously a no-flow boundary condition was the only option).
  • Optional representation of permeable, un-fractured layers. For example, these can be used to establish boundary conditions above and below a fractured layer.

Production from fractured layer between unfractured, permeable strata

Fracout-1.gif

Download "Naturally Fractured Natural Gas Reservoir Simulator: FRACGEN & NFFLOW", the NETL fractured reservoir modeling software today. Related publications can be found at the Office of Scientific and Technical Information, OSTI, Information Bridge by searching for either FRACGEN or NFFLOW.

FRACGEN/NFFLOW Software Compatibility

The most recent release is intended for PC application in a Windows environment. FRACGEN/NFFLOW was designed as a simulator that models fluid flow through complex fracture networks efficiently on standard desktop computers. The intended audience for this program includes small independent resource producers, academic and governmental researchers. NETL is interested in expanding the network of active users that can assist us in identifying potential bugs in the current version as well as guiding the most useful development into the future. For inclusion in future releases, please send bug fixes, modifications and comments to FRACGEN/NFFLOW Development. For notification of new releases please send an email to FRACGEN/NFFLOW Releases with the subject "Add to Release Distribution List".

The animation on this page displays sample output values for pressure from 22 time steps. A series of wells run diagonally across the display area. This enlarged version of the tight sandstone reservoir gas-flow animation more clearly shows the well locations.