Page 214 - Geology of Carbonate Reservoirs

IDENTIFYING AND DEVELOPING FRACTURED RESERVOIRS 195

7.5 PREDICTING RESERVOIR FRACTURE SPACING AND INTENSITY

Fracture spacing and intensity result from a complex interplay between stress con-
centration, orientation, and magnitude along with material characteristics. Because
it is easier to observe and measure rock properties (material characteristics) than
to deduce stress trajectories and magnitudes, this section focuses on rock properties
and their relationship to fracture spacing and intensity.

7.5.1 Factors that Influence Fracture Spacing and Intensity

Geological factors that influence fracture spacing and intensity include carbonate
rock composition, bulk rock porosity, bed thickness, and position of the fractured
zone with respect to the geometry of faults and folds. In general, as noted by Stearns
and Friedman (1972) , rocks composed mainly of brittle constituents exhibit closer
spaced fractures than those without brittle constituents. Typical brittle constituents
common in carbonate reservoirs are dolomite, chert, and, in some instances,
calcite.

Porosity influences fracture spacing because rock strength decreases as porosity
increases. If composition and fabric do not vary, rocks with lower porosity will have
closer spaced fractures and greater fracture intensity, although this situation may

be uncommon because significant changes in carbonate porosity are generally
accompanied by variations in fabric or texture.

Bed thickness influences fracture spacing and intensity in that, all else being
equal, thin beds will have closer spaced fractures than thick ones. Nelson ’ s (2001)
work suggests that when bed thickness and structural geometry are known, fracture
spacing can be predicted. Laubach et al. (2002) have had success in using observa-
tions of microfractures to predict the character of large - scale fractures in some
sandstone reservoirs.
Finally, the influence of structural geometry on fracture spacing and intensity is

based on the premise that higher fracture intensity and closer spacing correspond
to positions on structures where the greatest rate of change in dip or bed curvature
exists per unit of horizontal distance across the structures. This premise depends on
three assumptions: (1) the rocks involved behave as brittle material and fail by
brittle fracture, (2) increases in bed curvature correspond to increases in strain, and
(3) increased strain causes higher fracture intensity.

7.6 IDENTIFYING AND DEVELOPING FRACTURED RESERVOIRS

The following checklist offers some helpful hints for use in working fractured car-
bonate reservoirs.

1. Evaluate seismic data, subsurface geology, or projections of structural geom-
etry from outcrop to subsurface to maximize the likelihood of fi nding struc-
tures with tectonic fractures. Look for the fracture patterns predicted by
Stearns and Friedman (1972) if the structures are folds. In the case of faults,
expect higher fracture intensity on the hanging walls. Regional fractures may
be present in areas that have been subjected to regional stresses, such as