Page 212 - Geology of Carbonate Reservoirs

DETECTING FRACTURED RESERVOIRS 193

is used in conjunction with caliper, drilling time, and mud logs, including the lithol-
ogy log from cuttings. When fractured zones with dramatically increased interval
acoustic transit time are encountered in the borehole, the log scale changes as
many as 2 to 4 times almost instantly, depending on the difference in acoustic
transit time between fractured and unfractured rock. Rapid scale changes, increased
drilling rate, and the presence of saddle dolomite in cuttings were correctly inter-
preted as fracture indicators in Mississippian “ mud - mound ” reservoirs in Texas
(Ahr and Ross, 1982 ). Caliper logs are useful for locating washouts and enlarged
boreholes that may have been susceptible to washing out because the fractured
borehole wall was highly friable and tended to cave in. Drilling time logs are
useful to compare drilling rate with borehole diameter, lithology log, and records
of mud circulation. Fractured rock is more easily drilled than unaffected matrix
rock; therefore drilling time will decrease, sometimes dramatically. Fractured zones
commonly cause lost mud circulation or problems with maintaining mud circula-

tion, and these events coincide with drilling “ breaks ” — or high rates of penetration
by the bit. These changes in mud circulation patterns are reflected on the mud

log. Lithology logs prepared by qualified and observant persons may show traces

of large crystals or veins of fracture - filling minerals. Calcite or exotic minerals
such as saddle dolomite cements are commonly found in cuttings as mineral fi ll-
ings from subsurface fractured rock. Saddle dolomite is particularly useful in this
regard (Ahr, 1982 ). Borehole imaging logs have become the principal tool for
fracture detection in today ’ s industry. Both acoustic and electrical imaging systems
are used. Acoustic imagers such as Schlumberger ’ s UBI (ultrasonic borehole
imager) and FMI (formation micro imager) are typical examples. The acoustic tool
is used mainly in wells drilled with oil - based muds in which the resistivity tool
does not function well. The imaging logs produce an unrolled “ picture ” of the
borehole (Figure 7.6 ). With digital data and workstation interpretation software,
the dip and strike of all features such as fractures and sedimentary structures can
be calculated automatically. The dipmeter is another device commonly used to
detect natural fractures. The four - pad dipmeter can function as a caliper log by
comparing borehole enlargement in two directions at 90 ° from each other, and it
can function as a four - pad resistivity device that measures vertical displacements
in resistivity response to the four pads. The resistivity responses are assumed to

be generated by fluid - fi lled fractures.
Some well testing procedures are widely used by engineers as indirect methods
for detecting fractured reservoirs. One of the most commonly used ones is the pres-
sure buildup test because it can be performed at any time in the life of a well. The

only requirement is that the well be shut in. The shut - in flow rate (zero) is easier to

control than a constant rate flow test; therefore buildup tests are the preferred
method of pressure - transient testing (Lee, 1992 ). Fractures are indicated by the
longer - term pressure behavior of the well in the manner illustrated in Figure 7.9 .
Borehole effects are dominant during the early stages of the pressure - transient test
and are shown as a straight - line segment on the semilog plot. This segment refl ects
reservoir behavior influenced by fractures only. The response is immediate and does

not yet reflect pressure contribution from matrix permeability. The second straight -

line segment reflects the transient flow from the matrix until it stabilizes with fl ow

from fractures. At that point, the third straight - line segment represents combined
flow from fractures and matrix. This characteristic semilog plot is known as the