Databank

POSIVA Report 1997-26

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Name:

Fracture Network Model of the Groundwater Flow in the Romuvaara Site

Writer:

Antti Poteri; Mikko Laitinen

Language:

English

Page count:

65

ISBN:

951-652-025-1; 1239-3096

Summary:

Working report: FRACTURE NETWORK MODEL OF THE GROUNDWATER FLOW IN THE ROMUVAARA
SITE





The new fracture data gathered during the detailed site characterisation phase demonstrated
that the characteristic properties of fractures can be estimated quite reliably from few boreholes
and outcrops. Results obtained by employing new methods, like the use of borehole-TV, changed
the fracture intensity of the potential water conducting fractures compared to the earlier model.
The fracture intensity of the potentially water conducting filled and open fractures was about 0.4
#/m in the preliminary site investigation phase and in the present data it is about 1.7 #/m. The
difference is mainly caused by the changed interpretation of the intact rock. In the preliminary site
investigation phase only the orientated fractures were used to derive the parameters of the intact
rock. In the present model all the fractures outside the known fracture zones are used.
The hydraulic conductivity tensor of the intact rock was estimated with the fracture network
model. The modelling volume of 16 x 16 x 16 m3 was used in the simulations. The estimation of
the hydraulic conductivity tensor was based on the simulated flow with 12 different hydraulic
gradient orientations and ten different fracture network realisations. The results did not show
clearly the orientations of the principal axes of the hydraulic conductivity. The hydraulic
conductivity was from the lower limit of 3·10-11 m/s to highest value of 5·10-10 m/s. The lowest
hydraulic conductivity was to the horizontal orientation.
The flow simulations were calculated for a 16 x 16 x 16 m3 rock volume and about 2000
fractures. This means an element mesh of about 30 000 elements. The flow rate distribution
through the cross sectional area of the disposal canisters was calculated for a set of ten realisations
and a large number of different canister positions. The total number of canister positions simulated
was 2200. The 99. percentile of the flow rate through the canister cross section area (5 m2) was
about 0.5 litre/a. The distribution of the flow rate was very skewed. Mean flow rate was about
0.065 litre/a and the median was 0.0045 litre/a.
The flow distribution in larger volume was studied using a method that searched the flow
routes of highest conductance. The flow routes were examined into north-south, east-west and
vertical directions. Because of the more tortuous flow routes the conductivity to vertical direction
was slightly lower than to horizontal direction. Using the average hydraulic gradient of the intact
rock at the Romuvaara site, about 0.05, resulted one flow route of flow rate 0.01 litre/a or higher
per each 100 m2 area for the horizontal flow direction and about 0.6 for the vertical flow direction.
In the simulations there were also indications that if the distance from the repository to the nearest
fracture zone is more than 10 metres the greatest flow rates are significantly reduced.
Flow distribution along flow routes along homogeneous and heterogeneous fractures were
compared. The simulations showed that also in the case of homogeneous fractures a single flow
path is developed, but the variations in the flow rate in the fracture plane are smooth. In the case
of heterogeneous fractures several separate channels are developed and the fluctuations in the flow
rate in the fracture plane are much higher.
FRACTURE NETWORK MODEL OF THE GROUNDWATER FLOW IN THE ROMUVAARA
SITE





The new fracture data gathered during the detailed site characterisation phase demonstrated
that the characteristic properties of fractures can be estimated quite reliably from few boreholes
and outcrops. Results obtained by employing new methods, like the use of borehole-TV, changed
the fracture intensity of the potential water conducting fractures compared to the earlier model.
The fracture intensity of the potentially water conducting filled and open fractures was about 0.4
#/m in the preliminary site investigation phase and in the present data it is about 1.7 #/m. The
difference is mainly caused by the changed interpretation of the intact rock. In the preliminary site
investigation phase only the orientated fractures were used to derive the parameters of the intact
rock. In the present model all the fractures outside the known fracture zones are used.
The hydraulic conductivity tensor of the intact rock was estimated with the fracture network
model. The modelling volume of 16 x 16 x 16 m3 was used in the simulations. The estimation of
the hydraulic conductivity tensor was based on the simulated flow with 12 different hydraulic
gradient orientations and ten different fracture network realisations. The results did not show
clearly the orientations of the principal axes of the hydraulic conductivity. The hydraulic
conductivity was from the lower limit of 3·10-11 m/s to highest value of 5·10-10 m/s. The lowest
hydraulic conductivity was to the horizontal orientation.
The flow simulations were calculated for a 16 x 16 x 16 m3 rock volume and about 2000
fractures. This means an element mesh of about 30 000 elements. The flow rate distribution
through the cross sectional area of the disposal canisters was calculated for a set of ten realisations
and a large number of different canister positions. The total number of canister positions simulated
was 2200. The 99. percentile of the flow rate through the canister cross section area (5 m2) was
about 0.5 litre/a. The distribution of the flow rate was very skewed. Mean flow rate was about
0.065 litre/a and the median was 0.0045 litre/a.
The flow distribution in larger volume was studied using a method that searched the flow
routes of highest conductance. The flow routes were examined into north-south, east-west and
vertical directions. Because of the more tortuous flow routes the conductivity to vertical direction
was slightly lower than to horizontal direction. Using the average hydraulic gradient of the intact
rock at the Romuvaara site, about 0.05, resulted one flow route of flow rate 0.01 litre/a or higher
per each 100 m2 area for the horizontal flow direction and about 0.6 for the vertical flow direction.
In the simulations there were also indications that if the distance from the repository to the nearest
fracture zone is more than 10 metres the greatest flow rates are significantly reduced.
Flow distribution along flow routes along homogeneous and heterogeneous fractures were
compared. The simulations showed that also in the case of homogeneous fractures a single flow
path is developed, but the variations in the flow rate in the fracture plane are smooth. In the case
of heterogeneous fractures several separate channels are developed and the fluctuations in the flow
rate in the fracture plane are much higher.

Keywords:

File(s):

Fracture Network Model of the Groundwater Flow in the Romuvaara Site (pdf) (2.3 MB)


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