Abstract - The statistical streamline technique for
estimating effective vertical permeabilities in reservoirs with stochastic
shale populations has been adapted to estimate the effects on horizontal
effective permeabilities (Ke) of a population of impermeable sub-seismic
faults, which act as baffles to flow in a shale-free sandstone matrix of
permeability K. Permeability measurements on small faults sampled in outcrop
show that they can be regarded as barriers to flow on the production time-scale.
The effects of power-law size populations of faults are taken into account
by calculating tortuosity on explicit fault sizes rather than on the mean
size. The results of the simple statistical method compare well with those
of numerical simulations for both 2D and 3D cases and the statistical technique
is used to calculate Ke/K ratios for a realistic range of fault densities
and size distributions. The effects of typical North Sea sub-seismic fault
populations are significant and Ke may be more than an order of magnitude
less than K. A contour map of Ke/K is presented for a North Sea field based
on fault population analysis and statistical flow simulations for 21 sub-areas.
However, realistic spatial input of sub-seismic faults in flow simulations
is not yet possible. Uncertainty concerning the hydraulic characteristics
of real faults is thought to be a more important limitation than the shortcomings
of the modelling procedures.
In: North Sea Oil and Gas Reservoirs III, (edited by
Aasen, J. O. et al.) Kluwer Academic Publishers, 173-183, 1994.