Summary
Secondary fractures and faults associated with reservoir-scale faults affect
both permeability and permeability anisotropy and hence play an important role
in controlling the production behavior of a faulted reservoir. It is well known
from geologic studies that there is a concentration of secondary fractures and
faults in damage zones adjacent to large faults. Because there are usually
inadequate data to fully incorporate damage-zone fractures and faults into
reservoir-simulation models, this study uses the principles of dynamic rupture
propagation from earthquake seismology to predict the nature of
fractured/damage zones associated with reservoir-scale faults. We include
geomechanical constraints in our reservoir model and propose a generalized
workflow to incorporate damage zones into reservoir-simulation models more
routinely.
The model we propose calculates the extent of the damage zone along the
fault plane by estimating the volume of rock brought to failure by the stress
perturbation associated with dynamic-rupture propagation. We apply this method
to a real reservoir using both field- and well-scale observations. At the
rupture front, damage intensity gradually decreases as we move away from the
rupture front or fault plane. In the studied reservoir, the secondary-failure
planes in the damage zone are high-angle normal faults striking subparallel to
the parent fault, which may affect the permeability of the reservoir in both
horizontal and vertical directions. We calibrate our modeling with both outcrop
and well observations from a number of studies. We show that dynamic-rupture
propagation gives a reasonable first-order approximation of damage zones in
terms of permeability and permeability anisotropy in order to be incorporated
into reservoir simulators.
Introduction
Fractures and faults in reservoirs present both problems and opportunities
for exploration and production. The heterogeneity and complexity of fluid-flow
paths in fractured rocks make it difficult to predict how to produce a
fractured reservoir optimally. It is usually not possible to fully define the
geometry of the fractures and faults controlling flow, and it is difficult to
integrate data from markedly different scales (i.e., seismic, well log, core)
into reservoir-simulation models. A number of studies in hydrogeology and the
petroleum industry have dealt with modeling fractured reservoirs (Martel and
Peterson 1991; Lee et al. 2001; Long and Billaux 1987; Gringarten 1996; Matthäi
et al. 2007). Various methodologies, both deterministic and stochastic, have
been developed to model the effects of reservoir heterogeneity on hydrocarbon
flow and recovery. The work by Smart et al. (2001), Oda (1985, 1986), Maerten
et al. (2002), Bourne and Willemse (2001), and Brown and Bruhn (1998)
quantifies the stress sensitivity of fractured reservoirs. Several studies
(Barton et al. 1995; Townend and Zoback 2000; Wiprut and Zoback 2000) that
include fracture characterizations from wellbore images and fluid conductivity
from the temperature and the production logs indicate fluid flow from
critically stressed fractures. Additional studies emphasize the importance and
challenges of coupling geomechanics in reservoir fluid flow (Chen and Teufel
2000; Couples et al. 2003; Bourne et al. 2000). These studies found that a
variety of geomechanical factors may be very significant in some of the
fractured reservoirs.
Secondary fractures and faults associated with large-scale faults also
appear to be quite important in controlling the permeability of some
reservoirs. Densely concentrated secondary fractures and faults near large
faults are often referred to as damage zones, which are created at various
stages of fault evolution: before faulting (Aydin and Johnson 1978; Lyakhovsky
et al. 1997; Nanjo et al. 2005), during fault growth (Chinnery 1966; Cowie and
Scholz 1992; Anders and Wiltschko 1994; Vermily and Scholz 1998; Pollard and
Segall 1987; Reches and Lockner 1994), and during the earthquake slip events
(Freund 1974; Suppe 1984; Chester and Logan 1986) along the existing
faults.
Lockner et al. (1992) and Vermilye and Scholz (1998) show that the damage
zones from the prefaulting stage are very narrow and can be ignored for
reservoir-scale faults. The damage zone formed during fault growth can be
modeled using dynamic rupture propagation along a fault plane (Madariaga 1976;
Kostov 1964; Virieux and Madariaga 1982; Harris and Day 1997). Damage zones
caused by slip on existing faults are important, especially when faults are
active in present-day stress conditions because slip creates splay fractures at
the tips of the fault and extends the damage zone created during the
fault-growth stage (Collettini and Sibson 2001; Faulkner et al. 2006; Lockner
and Byerlee 1993; Davatzes and Aydin 2003; Myers and Aydin 2004).
In this paper, we first introduce a reservoir in which there appears to be
significant permeability anisotropy associated with flow parallel to large
reservoir-scale faults. Next, we build a geomechanical model of the field and
then discuss the relationship between fluid flow and geomechanics at well-scale
fracture and fault systems. To consider what happens in the reservoir at larger
scale, we use dynamic rupture modeling to theoretically predict the size and
extent of damage zones associated with the reservoir-scale faults.
© 2009. Society of Petroleum Engineers
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History
- Original manuscript received:
2 August 2007
- Meeting paper published:
14 November 2007
- Revised manuscript received:
31 July 2008
- Manuscript approved:
1 September 2008
- Published online:
7 July 2009
- Version of record:
9 September 2009
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