Page 230 - Fundamentals of Gas Shale Reservoirs

210 PASSIVE SEISMIC METHODS FOR UNCONVENTIONAL RESOURCE DEVELOPMENT

because the units cancel. Consider the example of a rod in Normal-faulting stress-regime
tension or compression. The linear strain of the rod is the
change in length of the rod divided by the initial length of
the rod. The sign of strain depends on whether strain is
computed with respect to the initial or deformed state of 1
the object. Contractional (shortening) strain is commonly
taken as positive in geomechanics, and extensional strain 2 3
as negative. Hydraulic fracturing produces both recover­
able (elastic) and permanent strain in the affected rock vertical
1
volume. Seismic activity, including MEQs, is a result of and horizontal
those strains. Although they are often treated separately, stress 2 3 Faults Joints Stylolites
and strain are inextricably linked as a single phenomenon. Wrench-faulting stress-regime 2
Stress always causes strain and strain always causes a
change in stress. 3
We will use the term stress state to refer to the orienta­ 1
tions and relative magnitudes of the three principal stresses
in the earth. Stress states in which the earth’s surface is a
principal plane, that is, when two of the principal stresses are
parallel to the earth’s surface and one is perpendicular, are
referred to as Andersonian stress states (Fig. 10.1) after the
structural geologist E.M. Anderson who first described them 2 vertical
in 1905 (Anderson, 1905, 1951). The Andersonian stress and horizontal Faults Joints Stylolites
states are as follows: 1 3
Reverse-faulting stress-regime 3
• Extensional (also termed normal) faulting—where Smin
and Sint are horizontal and Smax is vertical. In terms of 2 1
the horizontal and vertical stress nomenclature SHmax =
Sint, Shmin = Smin, and Sv = Smax.
• Wrench (also termed strike‐slip) faulting where Smax
and Smin are horizontal and Sint is vertical. In terms of
the horizontal and vertical stress nomenclature SHmax =
Smax, Shmin = Smin, and Sv = Sint.
• Thrust (also termed contractional or reverse) faulting vertical
3
where Smax and Sint are horizontal and Smin is vertical. and horizontal Faults Joints Stylolites
In terms of the horizontal and vertical stress nomencla­ 1 2
ture SHmax = Smax, Shmin = Sint, and Sv = Smin. FIGURE 10.1 The Andersonian stress regimes and the general orien­
tation of natural fractures that form in these regimes. Note that artificial
Andersonian stress states are commonly found in oil and hydraulic fractures will propagate in the same orientation as joints.
gas reservoirs and prevail on average over large regions of
the earth. However, borehole stress studies showed that
non‐Andersonian stress states are common on smaller If we assume that we are hydraulically fracturing an idealized
scales within oil and gas reservoirs. homogeneous medium with no discontinuities such as natural
Extensional fractures of any kind including artificial fractures, then in extensional and wrench faulting stress states
hydraulic fractures propagate perpendicular to Smin. More hydraulic fractures propagate vertically. If a horizontal well is
precisely, the propagating tip of such a fracture is always per­ drilled parallel to Smin in these stress states, then extensional
pendicular to the local Smin at the crack tip. Fractures will hydraulic fractures will propagate perpendicular to the wellbore
hence turn if the orientation of Smin varies. Many workers which maximizes the stimulated reservoir volume (SRV), that
think of extensional hydraulic fractures as propagating is the reservoir volume affected by the frac. If a well is drilled
parallel to Smax. Since Smax and Sint are both perpendicular perpendicular to Smin, then each frac stage produces a fracture
to Smin which is of course true, but this type of thinking can that runs parallel to the wellbore resulting in generation of a
lead to errors if the principal stresses are confused with single fracture and poor production.
SHmax and Shmin. The importance of correct stress nomen­ In thrust faulting stress states Smin is vertical, so that Sint
clature becomes clear when we consider hydraulic fracture and Smax are Shmin and SHmax, respectively. Confusing
propagation in Andersonian stress states. the horizontal stresses with the principal stresses leads to the