Page 201 - Geology of Carbonate Reservoirs

182 FRACTURED RESERVOIRS

as (1) fracture permeability, (2) fracture porosity, (3) fluid saturations within frac-

tures, and (4) fluid recovery factor expected from the fracture system. The kinds of
data necessary for these determinations can be obtained from well tests and, accord-
ing to Nelson, from whole core analyses. He points out that calculations from wire-
line log data do not provide accurate information for the evaluation of fracture
contributions to reservoir performance. Imaging logs such as the Schlumberger
®
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FMI (formation microresistivity imaging log) and FMS (formation microscanning
log) are, however, very useful for the identification of fractures and for determining

their orientation (Figure 7.6 ). Fracture porosity and permeability are greatly infl u-
enced by fracture morphology. Nelson (2001) has identified four categories of frac-

ture morphology: (1) open fractures, (2) deformed fractures, (3) mineral - fi lled
fractures, and (4) vuggy fractures. The following paragraphs are based on material
in Nelson (2001) .
Open fractures are those that have not been deformed and that are not plugged
with tectonic gouge or mineral precipitates. Permeability in open fractures is deter-
mined by the original fracture width, roughness, and the effective stress component
that is oriented perpendicular to the plane of the fracture. Fracture width, roughness,
and the contact area along fracture walls are usually determined by the texture of
the host rock, that is, by its constituent particle size. Open fractures greatly increase
permeability in the direction of the fracture plane but there is little change in per-
meability in the direction perpendicular to the fracture system.
Deformed fractures are produced either as ductile shear zones or as once - open
fractures later altered by shear stresses. Two different fracture morphologies may

result: gouge - filled deformation bands or slickensided fractures. Gouge is pulverized
rock produced by the grinding and sliding motion of fracture walls against each

other. Gouge partially or completely fills fractures and reduces both fracture poros-
ity and permeability. In general, reduction in permeability due to gouge fill is greater

in the direction perpendicular to the sense of sliding, or shear motion. Because
gouge is typically fine grained, it may have high S w that reduces relative permeability

to hydrocarbons. Slickensides are simply gouge material that has been melted during
cataclasis or grinding and pulverization that occurs during shear motion on fault
surfaces. Slickensides generally occupy less volume than gouge because they consist
of melted residue; consequently, the deformed rock volume around the fracture is

lower than in the case of gouge filling. Slickensides can be thought of as glassy coat-
ings that line fracture walls and that vary in thickness from less than 1 mm to a few
millimeters. Gouge, on the other hand, may be several centimeters thick depending
on the material involved, the mechanics of gouge formation, and the duration of
the process. In slickensided fractures permeability is decreased perpendicular to the
direction of the slip surface, but some increase in permeability may occur if there
is a “ mismatch ” between slickensided textures on opposing fracture walls. In effect,
the slickensides can prop - open fractures in some instances. Evaluating gouge and
slickensided fractures involves knowing the mechanical system that produced the
fractures and identifying the rock types that were more susceptible to cataclasis and
production of gouge.

Mineral - filled fractures have been partially or completely filled by diagenetic
precipitates after fracturing. As discussed in Chapter 6 , diagenesis can take place at
the surface or at great depth, it may occur once or many times, and it may decrease

or increase porosity. Mineral fillings decrease original fracture porosity but mineral