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increase flow tortuosity. This result is similar to what obtained by
Matthäi et al. (1998) for seismically detectable faults in high
porosity sandstones where faults are associated to thick zones of
compactive shear bands but they cannot be extended to SSRF in
general. Fossen and Bale (2007), on the other hand, show some
field observations and mathematical calculations suggesting that
the effect of compactive shear bands during reservoir production
is small or negligible in most cases and fluid flow is only gently
affected by compactive shear bands. The contradictory results
probably stem from the scale of the problem that is tackled in
these different studies.

5. Conclusions

Sub-seismic resolution strike-slip shear structures in Pleisto-
cene porous grainstones of Northwest Sicily, have been recognized
as single compactive shear bands (CSB), zones of compactive shear
bands (ZB), and faults with discrete slip surfaces and cataclastic
material (DF). The permeability of these structures (0.01e1 Darcy)
is less than the permeability of the host rock (2e18 Darcy). In
particular, the ZB permeability measured normal and parallel to
their planes ranges from 0.1 to 0.37 Darcies, while the DF perme-
ability measured parallel to their planes ranges from 0.09 to
1.07 Darcies. The aforementioned structures, hence, have the po-
tential to pronouncedly affect fluid flow in porous carbonate rocks
as they have been demonstrated to do in siliciclastic rocks.
A new methodology aimed at incorporating and up-scaling the
permeability reduction effects of CSB, ZB, and DF structural ele-
ments on fluid flow in a standard open source numerical simulation
software has been developed in this study. This methodology is
different from the procedure commonly used in most commercial
software packages, which deal with open fractures and their
permeability enhancing effects on fluid flow and neglect the flow
buffering effects of cataclastic shear structures.
The large draw-downs (up to 38 m; four times as much as in the
un-deformed reference state) observed during the numerical ex-
periments (especially in correspondence of thick ZB) show that, at
the scale of the wellbore (a few tens of cm) and during production,
SSRF made up of CSB, ZB and DF have an influence on the efficient
drainage and sweep of the reservoir. Specifically, these structural
elements may cause water up-coning, and diminish the recovery
rate. To the best of our knowledge, for the first time in deformed
porous carbonate rocks, our results show that well placement (un-
deformed host rock, shear structures or structure-bounded com-
partments) is critical for well draw-down control.
Comparison of the draw-down numerical experiments in the
DFN and the deterministic models show that results are similar
with the exception of wells located within the ZB. The wells located
in areas of strain localization with thick ZB and DF, in fact, have a
Figure 13. Injector-producer pair steady-state draw-down numerical experiments
with MODFLOW 2005 on a model with constant head (20 m) on the boundaries. (a) draw-down that is underestimated, up to an order of magnitude, by
Draw-down shown by the equipotential lines for an Injector-producer pair in ho- the DFN with respect to the deterministic model. This needs to be
3
3
mogenous grainstone. Producer ( 20 m /day) on the left and injector (10 m /day). kept in mind where dealing with complex structural settings,
Flow lines from the injector to the producer are also indicated. (b) Same setup as in (a)
but for the model of San Vito Lo Capo containing the strike-slip faults. Note that the because in areas of intense strain localization, due to fault prox-
producer is in a zone of deformation bands and the injector is in un-deformed host imity or curvature, a flow model deriving from a DFN reservoir/
rock. (c) Same situation as in (b) but with a swap of the positions of injector and aquifer description might not well represent the “drilling risk” (just
producer. the bad lack to put a well right into a thick zone of bands). SSRF
associated with thick ZB and DF in structurally complex areas might
represent a drilling risk, because they can enhance draw-down
as we showed by up-scaling the hydraulic conductivity to the large during production and EOR activities. In other instances, howev-
cell size but not to the local flow in proximity to boreholes. er, the use of the DFN model, is an acceptable representation of the
Rotevatn and Fossen (2011) simulated the effects of low perme- heterogeneities induced by SSRF in a reservoir.
ability SSRF in a siliciclastic reservoir analogue using a The single-phase fluid flow experiments that have been per-
10 m 10 m 5 m cell size and they conclude that, during pro- formed may not capture well the effects that the type of SSRF
duction, low permeability faults such as the compactive shear studied have on multiphase flow during production of an oil/gas
bands may cause pressure compartmentalization and may reservoir, because of the capillary effects that arise in the cataclastic

Please cite this article in press as: Antonellini, M., et al., Fluid flow numerical experiments of faulted porous carbonates, Northwest Sicily (Italy),
Marine and Petroleum Geology (2013), http://dx.doi.org/10.1016/j.marpetgeo.2013.12.003