### Abstract

This study looks at heterogeneity in hydraulic
conductivity at Romuvaara site. It concentrates on the
av-erage rock outside the deterministic fracture zones,
especially in the deeper parts of the bedrock. A large
number of stochastic fracture networks is generated based
on geometrical data on fracture ge-ometry from the site.
The hydraulic properties of the fractures are determined
by calibrating the networks against well test data. The
calibration is done by starting from an initial estimate
for fracture transmissiv-ity distribution based on 2 m
interval flow meter data, simulating the 10 m constant
head injection test behaviour in a number of fracture
network realisations and comparing the simulated well
tests statistics to the measured ones. A large number of
possible combinations of mean and standard deviation of
fracture transmissivities are tested and the
goodness-of-fit between the measured and simulated
results determined by means of the bootstrapping method.
As the result, a range of acceptable fracture
trans-missivity distribution parameters is obtained. In
the accepted range, the mean of log transmissivity varies
between -13.9 and -15.3 and standard deviation between
4.0 and 3.2, with increase in standard devia-tion
compensating for decrease in mean. The effect of spatial
autocorrelation was not simulated. The variogram analysis
did, however, give indications that an autocorrelation
range of the order of 10 m might be realistic for the
present data. Based on the calibrated fracture networks,
equivalent continuum conductivities of the calibrated 30
m x 30 m x 30 m conductivity blocks were determined. For
each realisation, three sets of simulations was carried
out with the main gradient in x, y and z directions,
respectively. Based on these results the com-ponents of
conductivity tensor were determined. Such data can be
used e.g. for stochastic continuum type Monte Carlo
simulations with larger scale models. The hydraulic
conductivities in the direction of the imposed gradient
(Kg) - determined based on flow in and out of the
opposite ends of the simulation cube - differed
significantly in most realisations. This means that the
Darcy flux in the direction of question differs with
location inside the cube. In such a case the behaviour of
the cube cannot be well presented with a continuum
tensor. A finer rotation angle could be used to see
whether the discontinuity in Kg-values is due to the
sparse rotation density. The fact that the non-continuity
was observed in most realisations indicates, however,
that the result reflects the be-haviour of the rock in
question rather than an 'unfortunate' selection of
simulation flow direction. Due to the spatially varying
Darcy flux components inside the cube, the least square
approximation used for determining the continuum
conductivity tensors is not the best estimate for the
present data. Alterna-tive averaging methods were,
however, not investigated as it appears the at the
continuum approxima-tion may not be a good representation
for the data and scale anyway. Regardless of the validity
of the continuum approximation, the results of the 30 m
fracture network blocks give us valuable information on
the flow distribution at this scale. Firstly, the results
can be used to evaluate possible anisotropy, which cannot
be seen from borehole well test data alone. Inspection of
the simulated results showed that there appear to be no
anisotropic effects. Secondly, the surface area
conductivities can be compared to well test
conductivities of the same scale. This comparison
indicates that the conductivities determined from the
well tests are clearly lower. The result is preliminary
in nature, and may be case- and data-specific, but it may
also be more general and related to e.g. differences in
flow and observation geometry between the two situations.

Original language | English |
---|---|

Place of Publication | Helsinki |

Publisher | Posiva |

Number of pages | 70 |

ISBN (Print) | 951-652-074-X |

Publication status | Published - 1999 |

MoE publication type | D4 Published development or research report or study |

### Publication series

Series | Posiva Report |
---|---|

Volume | 99-19 |

ISSN | 1239-3096 |

## Fingerprint Dive into the research topics of 'Estimation of block conductivities from hydrologically calibrated fracture networks - description of methodology and application to Romuvaara investigation area'. Together they form a unique fingerprint.

## Cite this

Niemi, A., Kontio, K., Kuusela-Lahtinen, A., & Vaittinen, T. (1999).

*Estimation of block conductivities from hydrologically calibrated fracture networks - description of methodology and application to Romuvaara investigation area*. Posiva. Posiva Report, Vol.. 99-19 http://www.posiva.fi/files/2701/POSIVA-99-19_web.pdf