FAULT ZONE PERMEABILITY, PORE PRESSURE AND STRESS
FIELD: LESSONS FROM RESERVOIR-INDUCED SEISMICITY.
Aderson F. do Nascimento1,2,*, Stella I. Pytharouli4, Rebecca. J. Lunn4, Zoe K. Shipton4,
James D. Kirkpatrick5, Joaquim M. Ferreira1,2, Francisco H. R. Bezerra1,3.
Programa de Pós-Graduação em Geodinâmica e Geofísica – Universidade Federal do Rio
Grande do Norte
1
2
Departamento de Geofísica – Universidade Federal do Rio Grande do Norte
3
Departamento de Geologia – Universidade Federal do Rio Grande do Norte
4
Department of Civil Engineering – University of Strathcyde
5
University of Califórnia, Santa Cruz
* aderson@geofisica.ufrn.br
Trabalho financiado pelo CNPq (INCT – Estudos Tectônicos; INCT – Geofísica do Petróleo)
e NERC (National Environment Research Council).
The study of reservoir-induced seismicity potentially provides a great opportunity to
study the interaction between flow and mechanical properties in faults. Because the interplay
between these properties is complex both in time and space, fault transport properties form
one of the greatest components of uncertainty in models of reservoir dynamics and in risk
assessment for waste disposal (see Faulkner et al., 2010 for a review).
Here we will present results from the Açu reservoir-induced seismicity. The Açu
reservoir is an earth-filled dam, 31 m high in the spillway, which has a capacity of 2.4 x 109
m3. The Açu reservoir is located in a flat area in the Archean and Neoproterozoic units
immediately south of the Potiguar Basin. Rainfall in this semi-arid region is limited to the first
few months of the year. As consequence, the reservoir area is quite reduced during the course
of the year (do Nascimento et al., 2004). The reservoir impoundment started in 1985.
Monitoring using one station started in 1987, but monitoring with local networks mostly
concentrated in the following field campaigns: 1989, 1990-1991 (analogic data) and 19941997 (digital data).
The correlation between seismicity and reservoir water level in the 1989 and 1990/1991,
as well as the spatial distribution of seismicity led Ferreira et al. (1995) to propose that the
Açu seismicity was reservoir-induced. They observed a 3.0 months delay, whereas do
Nascimento et al. (2004, 2005) observed a 4.5 months delay between maximum water level
and seismic peak. This time delay between maximum water level and subsequent seismicity
increased with depth (do Nascimento et al., 2004, 2005; El Harari et al., 2010), which is an
example of delayed response due to pressure diffusion, the drained effect (Rice and Cleary,
1976). This is consistent with the amplitude of the pressure wave decreasing with depth (Lunn
et al., 2005). It follows that as the permeability of intact crystalline rocks are too low to allow
pore-pressure variation at 2 km depth, this pressure diffusion must have occurred in faults and
fractures (do Nascimento et al., 2005).
Fault geometry, size, and kinematics were determined in the three periods of seismic
monitoring. Earthquakes occurred at well-defined NE-trending, 1.4 to 5 km deep faults over
short periods of time, which slipped as right-lateral strike-slip structures (Fig. 13). The 1989
earthquakes occurred inside the lake and migrated to two other faults in the east part of the
reservoir in the 1990/1991 period (Ferreira et al., 1995). The events in the 1994 – 1997 period
clustered in three other faults whose general attitude is 043o/88oSE (do Nascimento et al.,
2004a; Pytharouli et al., 2011). In all cases, hypocenter clusters formed 0.5 – 1.5 km long,
which may reflect that either the faults are of finite dimension, or that earthquakes occurred in
segments of the fault where the threshold for earthquakes triggering is lower than in adjacent
segments (do Nascimento et al., 2004; Lunn et al., 2005). The faults associated with the Açu
reservoir reactivate ductile shear zones .
The analysis of pore-pressure diffusion allowed the determination of faults properties
beneath the reservoir. The 1994-1997 data point that flow of fluids occurred within zones of
younger open fractures rather than within fault damage zones formed during fault growth
(Pytharouli et al., 2011). The seismicity migrated between faults and within individual faults
over different periods of time. This pattern is related to heterogeneous rock properties (do
Nascimento et al., 2004a). The flow pattern also indicates that the pressure waves become
smoother and more symmetric as they move downwards (do Nascimento et al., 2005). In
addition, these data point to values of hydraulic diffusivity of 5.5 x 103m2/day (do Nascimento
et al., 2005), seismicity migration rate of 14.8 m/day (do Nascimento et al., 2005) or between
15.5-17.5 m/day (El Harari et al., 2010), and average fracture permeability of 6 x 10-16m2 (El
Harari et al., 2010). The faults generated clusters of small events. For example, seismicity
reached a 3.1 mb maximum magnitude in the 1990-1991 period (Ferreira et al., 1995) and 2.1
mb in the 1994-1997 period (do Nascimento et al., 2004). The pressure change needed to
trigger earthquakes was predicted to be less than 0.5 kPa (i.e., a piezometric head of 0.05 m),
which implies that these faults are close to the critical value for failure (do Nascimento et al.,
2004).
REFERÊNCIAS
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