Page 255 - Fundamentals of Gas Shale Reservoirs

INTEGRATING, INTERPRETING, AND USING PASSIVE SEISMIC DATA 235
10.6 INTEGRATING, INTERPRETING, conductivity of a fracture and the resolved shear stress on that
AND USING PASSIVE SEISMIC DATA fracture (e.g., Barton et al., 1995; Ferrill et al., 1999; Heffer,
2012; Heffer et al., 1995; Hennings et al., 2012; Morris et al.,
Passive seismic data is only useful if it adds value. Interpretation 1996; Sibson, 2000; Takatoshi and Kazuo, 2003; Tamagawa
may be as simple as estimating well spacing by measuring and Pollard, 2008). Drilling campaigns targeting highly
the distance a cloud of MEQs extends from the wellbore. stressed natural fractures can achieve spectacular results (e.g.,
More sophisticated interpretation methods define the volume Hennings et al., 2012). More highly stressed fractures also gen­
of rock affected by the fracture treatment and the volume of erate more seismic activity during fracture treatments, so that
rock that is producing oil or gas (the two volumes are not seismic activity is an indicator of the hydraulic conductivity of
necessarily the same). Stress orientation, stress compart­ reactivated natural fractures whether or not they are hydrauli­
mentalization, and stress changes induced by the fracture cally connected to the wellbore.
treatment can be determined. The ultimate application of
passive seismic results is frac, well test, and reservoir simu­ 10.6.1.3 Bedding Parallel Features In most unconven­
lations to develop optimum treatment and production tional plays active today, bedding is either horizontal or
methods and to more accurately forecast reserves and pro­ close to horizontal. A thin horizontal cloud of MEQs or a
duction. This section provides an overview of interpretation horizontal fracture image is sometimes interpreted as a
methods, applications, and examples of passive seismic horizontal hydraulic fracture. This interpretation is unten­
interpretation.
able if the treating pressure is below Sv. If the treating
pressure is below Sv, then the likely cause is either slip on a
10.6.1 General Considerations horizontal fault that was induced by the fracture treatment,
or movement of fluid or at least fluid pressure through a
10.6.1.1 Dry Seismicity Not all seismicity produced by a highly permeable bed resulting in fracturing of the bed or
hydraulic fracture treatment results from the fracture fluid reactivation of pre‐existing fractures in the bed. In addition
breaking new rock or infiltrating pre‐existing natural to matrix permeability, a common cause of strong bed‐
fractures and causing slip. Tiltmeter surveys (e.g., Fisher and bounded permeability contrasts is contained jointing in
Warpinski, 2012) show that hydraulic fracture treatments which joints subperpendicular to bedding are well developed
produce measurable surface deformations even at depths (Section 10.2.2.1). As discussed earlier, vertical bed‐
exceeding 4.5 km (14,800 ft). This is due to inflation of the bounded joints are common in unconventional reservoirs.
rock volume around the wellbore caused by injection of the Contained joints develop preferentially in more brittle units,
frac fluid. This poroelastic strain wave propagates for thou­ and brittle units are preferred fracing targets. Such features
sands of feet horizontally as well as vertically. The wave can produce very flat MEQ distributions, activity clouds,
generates seismicity by several mechanisms, primarily by and fracture images if the bed that is carrying the fluid is thin
increasing the shear stress state on pre­existing fractures and relative to the resolution of the passive seismic data. This
perhaps by increasing the fluid pressure in natural fractures effect may be especially striking if the brittle bed is enclosed
by poroelastic mechanisms. (Pore‐fluid pressure and stress by rocks with high clay and organic matter content.
in the solid skeleton of the rock are coupled via poroelas­
ticity.) Also, fluid pressure can be transmitted through a
connected natural fracture network far more rapidly than the 10.6.1.4 Microseismic Response to Hydraulic Fracturing
frac fluid can physically move through the network (Lacazette in Contractional Faulting Stress States Contractional
and Geiser, 2013). In this case, there is a direct hydraulic con­ faulting stress states deserve special mention. Contractional
nection to the wellbore even though frac fluid is not present. In faulting appears unfavorable for unconventional resource
some cases where chemical tracers have been used to track frac development because purely extensional hydraulic fractures
fluid, movement of tracer over distances of 1.5 km or more run horizontally (Fig. 10.1). However, the authors of this
have confirmed that microseismic activity at long distances chapter have performed several passive seismic studies of
from the wellbore can represent frac fluid movement, not just hydraulic fracture treatments in active fold‐thrust belts. In
transmission of a pressure wave (e.g., Geiser et al., 2012). these studies:

10.6.1.2 Seismic Activity Indicates Hydraulic Conductivity • Direct fracture imaging showed that virtually all imaged
Seismic activity is an indicator (not proof) of hydraulically fractures were vertical even when MEQs showed slip on
conductive fractures even if the microseismicity is dry. A an active thrust fault near the treatment well during
key point about fluid flow in fractures is the dependence of hydraulic fracturing.
fracture hydraulic conductivity on stress. A large body of • Wrench faulting focal mechanism solutions were
published work accumulated since 1995 has shown that, in common in the stimulated reservoir volume.
general, there is a positive correlation between the hydraulic • The treatment pressures were well below Sv.