reservoir geomecanics

344 Reservoir geomechanics
40
35
30
m = 1.0
m = 0.6
Shear stress (MPa)
25
20
15
10
5
0
0 10 20 30 40 50 60 70 80
Effective normal stress (MPa)
Figure 11.3. Shear and normal stresses on fractures identified with borehole imaging in Cajon Pass
(triangles), Long Valley (circles), and Nevada Test Site (squares) boreholes. Filled symbols
represent hydraulically conductive fractures and faults, and open symbols represent non-conductive
fractures. From Townend and Zoback (2000) based on original data in Barton, Zoback et al.(1995).
The stereonets shown in the right column of Figure 11.2 introduce several basic
principles that will be illustrated further in the examples considered below:
Faults and fractures are often observed at many orientations, implying that they
formed at various times during the geologic history of the formation.
Simple conjugate sets of faults are not usually identifiable.
The subsets of permeable faults have orientations controlled by the current stress state
(blue dots). This is normal/strike-slip for Cajon Pass and normal for Long Valley and
the Nevada Test Site. The subset of permeable faults in any given well is not the same
as the most significant concentrations. The last point is seen most dramatically in the
Cajon Pass data set.
Figure 11.3 presents the data shown in the normalized Mohr diagrams in Figure 11.2
in a single Mohr diagram that is not normalized by the vertical stress (after Townend
and Zoback 2000). Each data point indicates the shear and effective normal stress acting
on a given fault. Colors and symbol shape distinguish the data from the three wells.
Filled symbols indicate hydraulically conductive faults and open symbols indicate
hydraulically dead faults. Note that independent of the effective normal stress acting on
agiven fault, the tendency for a fault to be hydraulically conductive depends on the ratio
of shear to effective normal stress, with the majority of conductive faults having a ratio
of shear to effective normal stress consistent with coefficients of friction between ∼0.6
and ∼0.9. The reason for this, we believe, is that in fractured and faulted siliciclastic
rocks, most geologic processes, such as precipitation, cementation and alteration of