Fract_002 Discrete Fracture Modelling

 

Introduction

 

The aim of this example is to demonstrate procedures for importing the modified fracture data file from Fract_001 and running a simple steady-state flow (solved in a single time step) across the fractured medium. The simulation domain is the same as the example in Fract_001. In this example, we define a set of boundary conditions for the fractured sample as follows. Higher pore pressure is prescribed on the east and south boundaries, whereas lower pore pressure is prescribed on the west and north boundaries. The history of fluid volume and flow rate on the latter two boundaries are recorded. In addition, all surfaces are prescribed with symmetry boundary conditions, i.e., displacement is restricted in the direction normal to the individual surface. The mesh size is set to be 10m.

 

fract_002_fig1

Fig. 1 Definition of boundary conditions (Top view)

 

Material Properties

The properties used in this simulation test are listed below.

 

Property

Value

Young's Modulus

5E4 MPa

Poisson's Ratio

0.25

Porosity

0.3

Density

2000 kg/m3

Permeability

1E-23 m2

 

The additional material parameters related to the porous flow field are:

 

Porous Flow Material Properties

Value

Fluid density

1000 kg/m3

Fluid viscosity

2.80E-23 MPa*Ma

Biot constant (α)

1.0

Fluid saturation (Sf)

1.0

 

The additional material parameters related to the contact surfaces are:

 

Contact Surface Properties

Value

Normal stiffness

100 MPa

Tangential stiffness

10 MPa

Tangential friction coefficient

0.1

Contact normal fluid conductivity

3153600 m/Ma

Filter cake normal fluid conductivity

3153600 m/Ma

Filter cake tangential fluid conductivity

31536000 m/Ma

 

 
Click to expand/collapseData File Description

The data file for the example is in: fract_002\Data\fract_002.dat.   The basic data includes:

1Include imports FracMan-formatted modified fracture data from Fract_001.

2Contact_global defines the Contact_flow_flag=1 so that fluid may flow on the contact surfaces.

3Contact_property defines the mechanical and flow properties on the contact surfaces that are associated with the imported fracture data.

4Fracture_set assigns the fracture data to the contact property and volume geometry.

5Global_loads prescribes pore pressure on the surfaces as described above.

6History_surface outputs the flow history data in terms of fluid volume and volume flow rate on the assigned geometry sets.

7Couple_control_data assigns respective fields that are coupled. In this case, they are geomechanical and porous flow fields.

8Porous_flow_control_data sets the solution algorithm for porous flow field.

 

Note that only key data structures are described below.
Click to expand/collapseInclude

Include imports FracMan-formatted fracture data.

 

Data File

 

 

* Include

! ---------------------------------

 Filename      "frac_sample_2.fab"

 

1FracMan-formatted fracture data is imported.

2Note that this is the modified fracture data file exported from the previous step in Fract_001.

 

 
Click to expand/collapseContact_global

Contact_global defines the Contact_flow_flag=1 so that fluid may flow on the contact surfaces.

 

Data File

 

 

* Contact_global

! ---------------------------------

   Contact_flow_flag               1

 

 

1Contact_flow_flag is set to 1 to allow fluid flowing on the contact surfaces.

 
Click to expand/collapseContact_property

Contact_property defines the mechanical and flow properties on the contact surfaces that are associated with the imported fracture data.

 

Data File

 

 

* Contact_property

! ---------------------------------

   Name                  "Frac_set1"

   Compression_model               1

   Compression_properties      IDM=1

    /"Normal Penalty"/           100

   Tangential_model                2

   Tangential_properties       IDM=2

    /"Tangential Penalty"/        10

    /"Friction coefficient"/     0.1  

   Flow_model_normal               1

   Flow_properties_normal      IDM=2

    /"Fluid Cond.(Contact)"/   3153600

    /"Fluid Cond.(Filter Cake)"/3153600

   Flow_model_tangential           1

   Flow_properties_tangential  IDM=1

    /"Fluid Cond. (Penalty)"/  31536000

   Contact_width                   5

 

 

1Contact property name "Frac_set1" is defined. The name is to be linked with fracture set data structure.

2Mechanical and flow properties on the contact surfaces in normal and tangential directions are defined accordingly.

 
Click to expand/collapseFracture_set

Fracture_set assigns the fracture data to the contact property and volume geometry as well as defining flag associated with discrete fracture surfaces.

 

Data File

 

 

* Fracture_set                NUM=1

! ---------------------------------

   Property_assignment   "Frac_set1"

   Volume                          1

   Active_flag                     1

   Split_flag                      1

   Element_size                    5

1The fracture set is assigned to the contact property named "Frac_set1".

2The fracture set is assigned to the volume geometry number = 1.

3Active_flag = 1 defines this fracture set as active.

4Split_flag = 1 splits all contact surfaces under this fracture set.

5Element_size on the fracture surfaces is defined as 5m.

 

 
Click to expand/collapseGlobal_loads

Global_loads prescribes pore pressure on the surfaces as described above.

 

Data File

 

 

* Time_curve_data            NUM=1

! --------------------------------

 Curve_type                              2

 Time_curve                 IDM=2

    0.0  1.0

 Load_factor                IDM=2

    1.0  1.0

 

* Global_loads               NUM=1

! --------------------------------

 Prescribed_pore_pressure  IDM=1 JDM=2

  /Set 1/   0

  /Set 2/   10.0

 Pres_pore_pressure_geom_set  IDM=4

   "east"

   "west"

   "north"

   "south"                

 Pres_pore_pressure_geom_ass  IDM=4

    2

    1

    1

    2

 

 

1Pore pressure is prescribed on the respective geometry sets at constant value throughout the simulation time.

2In particular, the “east” and “south” surfaces are prescribed with 10MPa, whereas the “west” and “north” surfaces are prescribed with 0MPa.

 
Click to expand/collapseHistory_surface

History_surface outputs the flow history data in terms of fluid volume and volume flow rate on the assigned geometry sets.

 

Data File

 

 

* History_surface            NUM=1

! --------------------------------

 Geometry_sets              IDM=1

   "west"

 Integration_values         IDM=1

   "Fluvol"

 

 

* History_surface            NUM=2

! --------------------------------

 Geometry_sets               IDM=1

   "north"

 Integration_values          IDM=1

   "Fluvol"  

 

1The history of “Fluvol” variables (consisting of fluid volume and volume flow rate) that are integrated respectively on “west” and “north” geometry sets are output.

 
Click to expand/collapseCouple_control_data

Couple_control_data assigns respective fields that are coupled. In this case, they are geomechanical and porous flow fields.

 

Data File

 

 

* Couple_control_data

! --------------------------------

 Field_names                IDM=2

   "Geomechanical"

   "Porous_flow"

Solution_algorithm  "Incremental"

Volume_strain_coupling     "None"

 

1Geomechanical and porous flow fields are coupled in this case using incremental solution algorithm.

2The coupling between element volumetric strain and pore pressure is not considered in this case.

 
Click to expand/collapsePorous_flow_control

Porous_flow_control_data defines the solution algorithm for porous flow field.

 

Data File

 

 

* Porous_flow_control_data

! --------------------------------

 Solution_algorithm             2  

 

1Steady state flow condition is imposed by setting Solution_algorithm = 2.

 

 

Click to expand/collapseResults

The result files for this example is available in directory: fract_002\Results. The fluid volume and flow rate on the outlet surfaces are computed as follows. These values are used as benchmark for the homogenisation simulations in Fract_004 .

 

Geometry Set

Fluid Volume (m3)

Fluid Volume Flow Rate (m3/Ma)

west  (Fract_002_001.hdh)

244.439

488.878

north (Fract_002_002.hdh)

295.113

590.226

 

 

fract_002_fig2

Fig. 2 Generation of fractured sample (top view)

 

 

fract_002_fig3

Fig. 3 Pore pressure contour in the middle cut-plane of simulation domain

 

 

fract_002_fig4

Fig. 4 Fluid velocity vector. Length of vector represents magnitude of velocity.