P. Gouze et al., Computing permeability change in sedimentary reservoirs including clays: application to the Bray fault zone (Paris Basin), B SOC GEOL, 172(4), 2001, pp. 427-436
We propose a model for simulating the changes in porosity and permeability
caused by hydrothermal diagenesis in sedimentary aquifer where salinity, te
mperature and fluid flow vary in space and time. Such modifications of the
hydrodynamic properties of the medium are bounded to geochemical reactions
and groundwater flow. Fluid velocity is particularly low in deep reservoirs
(typically less than 1m/year). Then, the local equilibrium simplification,
which is justified by a set of world-wide data of the chemical composition
of groundwater, can be implemented toward straightforward transient calcul
ations. In the model presented here, the coupled processes of fluid flow, t
emperature and chemical species transport are solved using well established
methods. The originality of the model is the development carried on to pre
dict the permeability evolution controlled by the mineral dissolution and p
recipitation. Usually to simulate permeability changes modelers use the cla
ssical porosity - permeability model based on statistical analyses of in si
tu or laboratory measurements. However, hydraulic conductivity changes are
not controlled solely by porosity changes, but also depend on pore-scale st
ructure transformations. Depending on the mineral type, the precipitation o
r dissolution of the same quantity of volumetric quantity will induce very
different changes in the hydraulic conductivity. Principally clay minerals
depict a wide range of atypical organisations of different microstructural
characteristics of the porous media. The spatial distribution of these char
acteristics cannot be modelled at basin scale. Away from both too complicat
ed and too unrealistically simplified approach, the model presented here is
based on the calculation of the permeability evolution from the change in
the mineral fraction due to mineral precipitation and dissolution. To simpl
ify, the minerals are divided into two groups : clay minerals and non - cla
y minerals. The specific contribution of clay minerals is controlled by a s
ingle weighting coefficient. This coefficient is associated to the proporti
on of poorly connected porosity that characterize clay structure, albeit it
is presently impossible to propose any quantitative relationship between t
he value of this parameter and the microstructural characteristics of the d
iagenetic clays.
The model is tested here to simulate the evolution of the porosity and the
permeability in a peculiar zone of the Paris Basin. The study area of sever
al hundred meters large is inside the Dogger aquifer, close to the Bray fau
lt zone where invasion of saline water from Triassic formation takes place.
This zone is characterised by high thermal and salinity gradient as well as
by the superposition of sub-horizontal regional flow and ascendant fault-c
on trolled flow : it is an ideal case study for examining the importance of
taking into account the specific contribution by clay minerals when comput
ing permeability evolution. This study is proposed as a parameter sensibili
ty analysis :
- to compare the relative influence of the clay weighting coefficient, the
temperature, the salinity, and the cementation exponent on the computed evo
lution of the permeability,
- to discuss the consequences of the introduction of the clay weighting coe
fficient in comparison to the classical porosity
- permeability evolution model,
- to simulate various evolution scenarios of past and future thermal and ge
ochemical constraints and their consequences on the evolution of the permab
ility changes in the Bray fault zone taking into account uncertainties on t
he value of the clay weighting coefficient and on the cementation exponent.
Forty-one simulations of one million years were necessary to cover a large
spectrum of the expected variations of each parameter. The results show tha
t :
- the local variation of the permeability depends on the time evolution of
temperature and of salinity, and on the values of the cementation exponent
of the porosity
- permeability law and of the clay weighting coefficient. Within reasonable
ranges of these four parameters, their influence on the permeability chang
es is of the same order of magnitude,
- the influence of the clay weighting coefficient on the porosity evolution
is negligible. Feedback effects of permeability evolution on the porosity
evolution, through the change in the flow regime, is minor,
- by the use of a classical model without a clay wheighting coefficient, pe
rmeability and porosity present the same pattern of evolution : they both i
ncrease or decrease. By the use of the clay weighting coefficient, in some
places the permeability and porosity can show opposite evolution. One incre
ases when the other decreases even for low values of the coefficient,
- in the vicinity of the fault, the model predict an increase of permeabili
ty independently of potential temperature and salinity modifications and wh
atever the clay mineral weighting coefficient is : Bray fault scaling is un
likely as long as head gradient is maintained in the fracture zone.