Fract_002 Discrete Fracture Modelling

Contents

Introduction

The aim of this example is to demonstrate procedures for importing the modified fracture data file from Fract_001 Case02 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 and loading conditions for the fractured sample as follows:

•Pore pressure of 10 MPa is prescribed on the east and south boundaries and pore pressure of 0 MPa is prescribed on the west and north boundaries.

•The history of fluid volume and flow rate on the latter two boundaries are recorded.

•All boundary surfaces are fully fixed.

•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	10E4 MPa
Poisson's Ratio	0.25
Porosity	0.3
Density	2000 kg/m3
Permeability	1.0E-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

Data File Description

The data files for this example (to be run consecutively) is in the directory: Fract_002\Data. It is defined in two parts, namely, data generation and simulation, in order that the simulation part can be run in parallel. The data generation part can only be run sequentially.

•Part 1: Generates a data file (.dat) and corresponding geo file (.geo) for use to run a simulation. Part 1 can only be run sequentially.

fract_002_gen.dat (include file frac_sample_2.fab) -> fract_002_gen_exp.dat, fract_002_gen_exp.geo

•Part 2: Perform the simulation with the generated data (fract_002_gen_exp.dat) and geometry file (fract_002_gen_exp.geo) from Part 1. Part 2 can be run sequentially or in parallel.

Part 1 data includes the import of modified fracture data frac_sample_2.fab from Fract_001 Case02 and other data required for running a simulation (e.g. material, mesh, boundary condition, loading, history):

1Include imports FracMan-formatted modified fracture data frac_sample_2.fab from Fract_001 Case02.

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 and the keyword Export_datafile_flag set to 1 will export a data file (.dat) and corresponding geo file (.geo) for use to run the simulation in Part 2.

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.

Include

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 Fract_001 Case02.

Contact_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 flow on the contact surfaces.

Contact_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.

Fracture_set

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

Data File

* Fracture_set NUM=1

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

Name "Set_1"

Property_assignment "Frac_set1"

Volume 1

Active_flag 1

Split_flag 1

Element_size 5

Export_datafile_flag 1

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.

6Export_datafile_flag = 1 will export a data (.dat) file post-fracture intersection generation and stop. The corresponding geometry (.geo) file will also be generated automatically.

Global_loads

Global_loads prescribes pore pressure on the surfaces as described above.

Data File

* Time_curve_data NUM=1

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

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.

History_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"

Output_frequency_increment 1

* History_surface NUM=2

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

Geometry_sets IDM=1

"north"

Integration_values IDM=1

"Fluvol"

Output_frequency_increment 1

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.

Couple_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 (default).

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

Porous_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.

Results

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_gen_exp_001.hdh)	244.454	488.908
north (Fract_002_gen_exp_002.hdh)	302.890	605.780

$fract_002_fig2$

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.