Page 231 - Fundamentals of Gas Shale Reservoirs

GEOMECHANICS AND NATURAL FRACTURE BASICS FOR APPLICATION TO HYDRAULIC FRACTURING 211
erroneous conclusion that hydraulic fractures will run verti­ across basins—from extensional to thrust, wrench and back
cally and parallel to SHmax. In a thrust faulting stress state, again and can rotate across a basin. Similarly, the stress state
extensional hydraulic fractures propagate horizontally and can change substantially with location in an oil or gas field or
the frac pressure is equal to the vertical stress Sv. within the volume of rock affected by a single frac. Because
Although thrust faulting stress states are the least common the earth’s surface is a free surface, stresses are reliably
worldwide, some potentially very large unconventional gas Andersonian close to the surface but deep enough to be
plays, such as some basins in China, are in active fold‐thrust removed from near‐surface topographic effects. Over large
belts. This may appear to bode ill for unconventional produc­ areas stresses tend to be Andersonian on average. However,
tion in these basins because idealized frac models predict that borehole stress studies show that non‐Andersonian stress states
only a single horizontal fracture will be driven from a horizontal are common on small scales within oil and gas reservoirs.
well. Very ductile rocks in a thrust faulting stress state may
indeed require frac pressures in excess of Sv. However, current 10.2.2 Natural Fracture Basics and Interaction with
work in the microseismic community shows that hydraulic Hydraulic Fractures
fracturing primarily affects the preexisting natural fracture This section discusses basic natural fracture types, their ori­
system rather than generating idealized bi‐wing, extensional entations relative to each other, and their relevance to inter­
hydraulic fractures. As discussed in subsequent sections, wells preting hydraulic fracture data. Lacazette (2000, 2009)
in thrust faulting stress states can be effectively stimulated at provides a complete review of fracture nomenclature (also
frac pressures below Sv if the frac exploits the natural fracture see the technical area of www.NaturalFractures.com).
system. Also, wrench‐faulting and normal‐faulting stress states Textbooks on fractured reservoir evaluation are provided by
occur locally in active fold‐thrust belts. These anomalous stress Nelson (2001) and Narr et al. (2006).
states result from geometric effects related to the complexities
of folding and faulting. 10.2.2.1 Natural Fracture Types Natural rock fractures
The upper, brittle region of the earth’s crust is a self‐ fall into three basic categories: joints, faults, and contrac­
organized critical system (e.g., Leary, 1997) in a state of fric­ tional fractures.
tional equilibrium because of pervasive fracturing (Zoback,
2010). Frictional models provide the most accurate predic­ • Joints are extensional fractures that form perpendicular to
tions of measured stress profiles in the crust (Zoback, 2010). Smin when the walls of the fracture move perpendicu­
Consequently, this equilibrium is very easy to disturb. larly outward from the plane of the joint and the propaga­
Hydraulic fracturing disturbs the equilibrium, thereby stim­ tion direction (Engelder, 1987; Pollard and Aydin, 1988).
ulating microseismic activity. • Faults result from shear movements parallel to the
Mechanical stratigraphy refers to the differing mechanical fracture plane and at angles that range from parallel to
properties, stress states, and natural fracturing of different perpendicular to the fracture propagation direction. Faults
lithogical layers (e.g., Laubach et al., 2009). Stress states can form approximately parallel to Sint and tend to initiate as
vary strongly between different lithologies. When fracing a conjugate pairs at ±30° to Smax and parallel to Sint.
ductile layer (typically a stratum with high organic and clay • Contractional fractures form perpendicular to Smax by
content), frac pressures approximately equal to Sv can result volume loss across a plane. The volume loss can result
from deformation of the layer over geologic time with from crushing (deformation bands), grain rearrangement
attendant equalization of the stresses. The stress magnitudes (compaction bands), or by chemical dissolution (stylo-
and even stress orientations in a reservoir can vary strongly lites). Although some workers object strenuously to term­
from lithology to lithology even when different lithologies ing stylolites as a type of fracture, it is well accepted in
are thinly interbedded (e.g., Evans et al., 1989). Such stress‐ both the geological and engineering literature that
state variations are very important both for interpreting chemical corrosion is an important fracture mechanism,
passive seismic results and for frac design. and Fletcher and Pollard (1981) show that simply
The World Stress Map (www.World‐Stress‐Map.org) is a reversing the sign of extensional fracture mechanics
1
worldwide map of publically available stress data. The stress equations correctly describes stylolite formation. For
states are presented in terms of the Andersonian stress states these reasons, stylolites must be classed as a type of
because over large areas stresses are generally Andersonian. fracture (a stress corrosion anticrack).
Inspecting the map shows that stress states can change rapidly
Note that extensional and shear fracturing can operate simul­
taneously as can contraction and shear. Figure 10.1 shows
1 The world stress map is important for many activities ranging from oil and the average orientations of the natural fracture types that
gas exploration to earthquake forecasting. The project benefits greatly from form in each of the Andersonian stress regimes. Different
stress data contributed by the oil and gas industry. Please encourage your
company or organization to contribute any stress data that you collect to the fracture types may transmit fluid differently depending on
World Stress Map project. their types and properties.