RS2013-1078 Insights gained from the injection-induced ... · 3. Criticality of stress inferred from statistical analysis of injection-induced seismicity In injection-induced seismicity,

6th Int. Symp. on In-Situ Rock Stress RS2013

RS2013-1078 20-22 August 2013, Sendai, Japan

Insights gained from the injection-induced seismicity in the southwestern Sichuan Basin, China

Xinglin Lei

a* and Shengli Ma

b

a Geological Survey of Japan, AIST, 1-1 Higashi, Tsukuba 305-8567, Japan

bState Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake

Administration, Beijing, China * [email protected]

Abstract

Toward the promotion of Enhanced Geothermal System (EGS), fracking shale gas, geological sequence of CO2, and other applications, in which fluids are intensively pressed into deep formations, fluid-injection-induced earthquakes have attracted growing attentions. Here, we present several case studies on injection induced earthquake sequences in a relatively aseismic region in the southwestern Sichuan Basin. During the past decades, a number of seismic sequences have been observed with sizable earthquakes ranging up to M4~5. Their timing, location and occurrence pattern involved in statistical models, convincingly suggest that these sequences were induced by water injections in deep wells in the gas fields in this region. Event rate fluctuated following change of injection rate and tapered after shut down. Most events show shear fracturing mechanisms, which together with hypocenter location data demonstrate that pre-existing faults, known or unknown, in the formations have a governing role controlling the hypocenter distribution of the induced earthquakes. In some cases of long-term injection, there are clear phases of deeper events, which mirror the reactivation of underlying faults in the crystalline basement. Faults of a relative larger scale have a two-fold role: working as a bounding interface for horizontal fluid flow and a leakage path for vertical fluid flow. Statistical analysis shows that these sequences are more swarm-like. During the earlier stages of the injections, the total fraction of seismically triggering earthquakes takes about 40-60% while the forced earthquake rate increases with injection time, indicating that the stress in the formations is originally critical or subcritical. An inhomogeneous stress field at reservoir scale is required to interpret the reactivation of badly oriented faults. Insights gained from such kind of case studies may provide a better understanding why damaging events occur so that they can be avoided or mitigated.

Keywords: Injection-induced seismicity, Gas reservoir, Fault reactivation, Pore pressure, Slip-tendency, Limestone

1. Introduction The Sichuan Basin, with the exception of its boundaries, has a rigid crust and is relatively stable.

However, in the southwestern of the basin, there are increasing seismic activity which results in a number of dense seismic clusters (Fig. 1). Water injections though deep wells, either for disposal of unwanted water from natural gas production or for dissolving mine salt, started in the 1970s in gas/salt fields in the region where fractures of various sizes in tight sand stone and limestone/dolomite formations are important repositories of natural gas and saline water. The timing and locations of these seismic clusters clearly correlate with fluid injections, indicating that these clusters were injection-induced seismicity. These cases are important because 1) they mirror fluid injections, 2) their source fault can be compared to pre-existing structure, thereby allowing the role of fluid and structure in seismogenesis to be investigated, and 3) they demonstrate how damaging small to moderate earthquakes can be.

The present paper focuses on three typical cases of injection-induced seismicity (Fig. 1, A-C) to highlight some general insights on the occurrence of induced earthquakes and stress criticality within the formations in the region. These typical cases include 1) a long-term case at Rongchang gas field, 2) two ~M5 earthquakes occurred in a salt reservoir of well-known deep structures, and 3) a well monitored recent case in the east of Zigong city.

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20-22 August 2013, Sendai, Japan

Figure 1. Map view showing location and geographical and seismological features of the southwestern

Sichuan Basin and surrounding areas. A through K mark major seismic clusters (every cluster contains one or more moderate earthquakes of a magnitude up to M4-5), which are located in gas

reservoirs and/or salt mines and are thought to be induced/triggered by fluid injection. The lower plot shows a cross section of simplified geology. (After Lei et al., submitted).

2. Evidence of reactivation of pre-existing faults 2.1 A long-term injection-induced earthquake sequence at Rongchang gas field

The seismicity in Rongchang-Longchang gas field (Fig.1, A), termed Rongchang sequence, began in 1988 and continues today, was initiated and further enhanced by intermittent injections of unwanted water to the formation ~3 km depth through several depleted wells. The ML5.2 earthquake in 1996 is probably one of the largest yet seen induced or triggered by fluid injection. Lei et al. (2008) have presented the detailed temporal evolution and statistical features of seismicity by the end of 2006.

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Unfortunately, due to the limitation of available seismic stations, it was unable to draw detailed picture of exactly how the injected fluid induced the earthquakes. A portable network of 5 stations was installed in late 2008 under a cooperation project between Chongqing Earthquake Administration and the Geological Survey of Japan (Wang et al., 2012). By the end of 2012, more than 2000 M0.5+ events, including an ML4.6 earthquake occurred in 16 Oct., 2009 have been recorded. These new data, together with the old data recorded at a few stations, provide a long time span over 20 years and detailed earthquake hypocenters, thereby permitting a more comprehensive analysis of the injection induced seismicity in this field. We used the double difference method to relocate earthquakes recorded by the temporal seismic network. Fig.2 shows a close up view of the geographical and geological features of Rongchang gas field overlapped with mechanism solutions and earthquake hypocenters organized in different categories. It is clearly shown that most earthquakes are related with pre-existing faults which were reactivated by injection at two major wells (L#4 and L#2 in Fig.2) located close to major faults near the axis of the anticline structure.

It is worth noting that there are some deeper events, which probably indicate the reactivation of underlying faults in the crystalline basement (Lei et al., 2008). Further studies are required to answer the critical question - how does water leave the formation to enter the crystalline rock?

Figure 2. A close up view of the geographical and geological features of Rongchang gas field overlapped with CMT solutions and earthquake hypocenters organized in different categories.

Note the hypocenter of earthquakes occurred before 2008 was poorly determined.

2.3 Two ~M5 earthquakes occurred in Ziliujing anticline of well-known deep structures There are two ~M5 earthquakes occurred at shallow depths beneath the Ziliujing anticline in the

western Zigong city (Fig.1, B). The deep structures of the Ziliujing anticline are well-understood from seismic prospections and boring investigations conducted for natural gas and rock salt (Ryder et al., 1991 and references therein). Fig.3 shows a simplified geological section. The M5.0 events in 1985 and the M4.8 event in 2008 are clearly resulted from the reactivation of two blind reverse faults due to water injection for salt production (Zhang et al., 1993). These events are important because there are

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some published in-situ stress measurement data obtained by hydrafracturing method (see section 4 for details).

Figure 3. Hypocenter depth distribution superimposed on simplified geological cross sections and stratigraphy (modified from Ryder et al., 1991 and references therein)

which are well constrained with seismic profile and boring data.

2.3 Injection-induced seismicity at Niufudu-Putaisi anticline--a well monitored case The 2009-2010 Zigong earthquake sequence, located at the east of Zigong city (Fig.1, C), is

closely associated with the injection of unwanted water in the limestone formation of Permian. More than 130,000 m

3 of water has been pressed into the depleted reservoir during the period from 2009 to

July 2010 under a wellhead pressure up to 6.2 MPa. During this period, more than 7,000 earthquakes, among which 5,000 events having magnitudes of greater than or equal to 0.5 have been recorded by a nearby local network and five temporary stations. Lei et al. (submitted) presented a detailed view on hypocenter distribution and temporal evolution.

In map view, the hypocenters concentrated in a NNW extended narrow zone approximately 6 km long and 2 km wide centered approximately at the injection well. The hypocenter distribution is consistent with the deep flunk at the west side of the Niufudu-Putaisi nose-like anticline. A hypocenter density map demonstrates that the hypocenters are clearly controlled by a set of pre-existing conjugate fractures. Such fractures are consistent with the anticline structure and the regional stress field.

Fig.4 shows depth distribution of hypocenters superimposed on simplified geological cross sections and stratigraphy. More than 90% of hypocenters fall in the depth range of 2.5 to 4 km, which corresponds to the Permian limestone formation. Shale and mudstone in the overlying and underlying layers act as fluid diffusion barrier and play a role in arresting fractures in the limestone. At the front of hypocenters, seismic activity was probably bounded by dipping faults leading to upward and downward migrations. In particular, at the northwest front, there are a number of very shallow and relatively deep events, probably indicating a dip fault, which is consistent with the anticline structure.

Multiple sources of evidence, such as the shear mechanism, the pattern of hypocenter distribution, and small elevated pore pressure as compared with the least principal stress in the region indicate that the induced seismicity resulted from the reactivation of known or unknown pre-existing faults. In addition, the largest events occurred in the very early stage (2 weeks delay) of injection under high pressure. Thus, the pre-existing faults are probably the most important factor in the occurrence of induced earthquakes. Injected fluids diffused outward along pre-existing faults, which are critically or subcritically stressed, play a role in weakening the faults and lead them to reactivation.

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In a summery, all cases presented in this paper demonstrate that major earthquakes occurred in the gas/salt reservoirs in the South-western Sichuan Basin are resulted from rupture of pre-existing faults or joint, which reactivated by the water injections into the reservoirs.

Figure 4. Hypocenter depth distribution superimposed on simplified geological cross sections and stratigraphy. The stratigraphy is drawn while referring to the deep structure of the nearby Ziliujing anticline at which the stratigraphy and deep structures are well constrained with boring data. The

depth distribution of hypocenters indicates that more than 90% of earthquakes are located in a depth range between 2.5 and 4 km. Note that the injection interval is 2.45 to 2.55 km beneath the surface,

which has an elevation of 340 m. (after Lei et al., submitted).

3. Criticality of stress inferred from statistical analysis of injection-induced seismicity In injection-induced seismicity, fluid-forced activity is always accompanied by seismically

triggered activity. The fraction of the seismic triggering is thought to be an indication of the criticality of the local stress field in the formation in which earthquakes located. The epidemic-type aftershock sequence (ETAS) model (Ogata, 1992), which incorporates the Omori law by assuming that each earthquake has a magnitude-dependent ability to trigger its own Omori-law-type aftershocks, is an useful tool for quantifying forced background seismicity and triggered aftershocks (i.e., Hainzl and Ogata, 2005; Lei et al., 2008; Lei et al., 2011). In the ETAS model, the total occurrence rate is described as the sum of the rate triggered by all preceding earthquakes and a forcing rate λ0(t) that represents the background activity (injection-induced) :

}:{

)(

00 )()(),()()( min

tti

p

i

MM

i

i ctteKtttt (1)

where Mmin is the low cut-off magnitude of the catalog and α is a constant that specifies the degree of magnitude dependence. For injection-induced seismicity, the forcing rate is non-stationary since it depends on injection factors as demonstrated by ETAS model of piecewise constant forcing rate for the long-term Rongchang-Longchang sequence (Lei et al., 2008). For non-stationary injection-induced seismicity, a time-varying forcing rate is recommended to avoid under-estimation of the forcing rate and the α value,(Lombardi et al., 2010; Marsan et al., 2013).

In order to estimate the time-varying forcing rate and other parameters, we applied a sophisticated algorithm (see Lei et al. submitted for details) which is inspired from Zhuang et al. (2002) and Marsan et al. (2013).

Fig.5 shows ETAS modeling results for the Zigong 2009-2010 sequence. During the earlier stages, the total fraction of forced activity is ~50%, indicating that injection-inducing and seismic triggering are both significant. During the later stages, the forced activity takes more than 80%, thus seismic

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triggering becomes a minor factor. As seeing from Fig.6, ETAS results of the Rochang long-term sequence agree fairly well with the Zigong case.

These results demonstrate that the stress in the formation was originally critical or subcritical and thus seismic triggering plays an important role. Seismic activity releases stresses within the formation and may thus lead to a decreasing tendency of seismic triggering with increasing injection period.

Figure 5. ETAS parameters K, α, p, and AIC for the M≥1.0 earthquakes of the Zigong 2009-2010 sequence, together with the total fraction of forced seismicity.

Figure 6. ETAS parameters K, α, p, and the total fractions of forced seismicity (f.s.) for three distinct

periods of M≥1.75 earthquakes occurred in Rochang gas field.

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4. Stress regime and mechanism of induced earthquakes Mapping earthquake focal mechanisms of aftershocks on the 3D Mohr diagrams is helpful for the

estimation of fluid pressure change caused by the flow of overpressured fluid from a deep reservoir after large earthquakes (Terakawa et al., 2013). By examining the fault orientation relative to the regional tectonic stress pattern, Terakawa et al. (2012) used the so termed focal mechanism tomography technique to identify packets of high fluid pressure in the source region of earthquake sequences. In the case of injection-induced sequences, the upper limit of the pore pressure can be estimated from injection pressure and assumed hydraulic properties and thus the same technique can be applied to infer stress heterogeneities. In this study, we combine the 3D Mohr diagrams and slip-tendency analysis to address issues associated with the stress field within the formations.

The tendency of an arbitrarily oriented plane to undergo slip under a given stress pattern depends on its frictional coefficient and the ratio of shear to normal stress acting on the plane. Slip-tendency analysis is a technique that visualizes the stress tensor in terms of its associated slip-tendency distribution and the relative likelihood and direction of slip on interface of all orientations (Morris et al., 1996).

Based on the Coulomb failure law, the critical condition for rupturing on a pre-existing fault is

)( fe P (2)

where τ and σ are shear and normal stresses acting on the fault plane, respectively, Pf is pore pressure, and μ represents sliding friction of the fault plane. The slip tendency of the fault is defined as the ratio of the shear stress and normal stress (Morris et al., 1996) and thus equals to the friction coefficient.

eTs / (3)

Under a uniform regional stress field, the most optimally oriented fault has the maximum slip-tendency. A fault of greater value of slip tendency, or in other words a fault having more optimal orientation, would be easier to rupture.

Under the principal stress coordinate system (s1, s2, s3), the shear and the normal stresses on a surface of given direction cosines (l, m, n) can be calculated from the three principal stress magnitudes (σ1, σ2, σ3) as:

22

3

22

2

22

1

2

222

13

222

32

222

21

2

nml

lnnmml

(4)

In most cases, the stress tensor is not fully defined but only the direction of the of the principal stresses and the stress difference ratio (R) , or equivalently, the shape ratio (φ), are given (Etchecopar et al. 1981; Gephart & Forsyth 1984),

31

21

R (5)

31

321

R (6)

In addition, σ1-σ3 is not well constrained, and can be expressed as an unknown parameter k. By further assuming that the frictional sliding envelope is tangential to the (σ1, σ3) Mohr circle, then the principal stresses are given by:

k

kR

k

13

12

1 2/)1)sin(/1(

(7)

where tanφ=1/tan(2θ)=μ. Inserting Eqs.7 into Eqs.4 leads to following equations for shear stress and normal stress.

22

2/122222222

)1(2

1)csc(

1

nmk

lnnmmlk

(8)

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Thus, the slip tendency is independent of the choice of the unknown parameters k, and we can get a slip tendency normalized by the maximum. For such a partially defined stress field, we can draw the 3-D Mohr circles for shear and normal stresses normalized by k or the maximum shear stress.

It is convenient to define an overpressure coefficients λ for fluid pressure (Terakawa et al., 2013):

0max

0

PP

PPf

(9)

where P0 is the critical pore-pressure required to initiate rupture on the optimally oriented fault of given friction coefficient, Pmax (=σ3) is the maximum pore pressure above which hydrofracture occurs. For mapping earthquake focal mechanisms to the 3-D Mohr diagram and plots of slip-tendency or expected slip angle in strike-dip coordinates, we have to select the true fault plane from two nodal planes of the mechanism solutions. In this study, we chose the nodal plane of larger slip-tendency as the true fault.

Figure 7. a) Plot shows relationship between focal mechanism and pore fluid pressure in the 3-D Mohr circles for a estimated stress field of Ziliujing field. The position in the Mohr diagram shows the fault orientation relative to the stress pattern. The Mohr–Coulomb failure lines with a friction coefficient of 0.7 under various pore fluid pressures, P0, and Pf. b) Slip tendency stereoplots under the given stress pattern and a pressure Pf = P0+5 MPa. c) Slip tendency and expected slip angle as a function of strike

and dip of virtual fault plans superimposed with the focal mechanism of the 2010 M4.8 event.

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For the Ziliujing case presented in 2.1, some in-situ stress measurements at depths 468-818m were conducted by means of hydrafracturing which give:

HS

HS

HS

v

h

H

62.24

64.2234.3

06.5820.0

(10)

where Sh, SH, Sv, and H are the minimum horizontal stress, the maximum horizontal stress, vertical stress in MPa and depth in km, respectively (Zhang et al., 1993). In this site, the azimuth of Sh 20 degree. Fig.7 explores the relationship between focal mechanism solution and pore fluid pressure in the 3-D Mohr circles for a depth of 3 km. The position in the Mohr diagram shows the fault orientation relative to the stress pattern. The Mohr–Coulomb failure lines with a friction coefficient of 0.7 under various pore fluid pressures, P0, and Pf. Slip tendency stereoplots under the given stress regime and a pressure Pf = P0+5 MPa. Focal mechanism solution of the 2010 M4.8 Zigong earthquake is mapped on each plot. The focal mechanism solution agrees well with the pre-existing reverse fault, which is badly oriented for rupture under the stress field. Over pore pressure of 20 MPa, or a low friction coefficient ~0.35 is required for the reactivation of the fault if Eq.9 works at the focal depth.

For the case of the Niufudu-Putaisi anticline, there is no published stress data. Since it is close to the Ziliujing field, we thus use the same stress regime. Aided with waveform data recorded by the temporal stations, mechanisms of 55 events are determined (Zhang et al., 2012). Instead of mapping each individual mechanism solution, we mapped two M4+ events and the rough distributions of all mechanisms (Fig.8). Many events have a relatively high dip angle. Events of a dip angle in the range of 30-45 degree, demonstrate a dominant strike direction along NW-SE, which agrees fairly well with the spatial pattern of the hypocenter distributions.

For the Rongchang long-term case, by referring stress data obtained from analysis of CMT (Central Moment Tensor) catalogues (Wan, 2010), we assume that the azimuth of the maximum horizontal stress is 120, σ2 axis is vertical, and stress ratio R is 0.6. In Fig.9, we mapped mechanisms of some large events. Some mechanisms are mapped in points of small value of slip-tendency and the misfit of slip angle is large, possibly indicating heterogeneous local stress field within the formation.

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Figure 8. a) Plot shows relationship between focal mechanism and pore fluid pressure in the 3-D Mohr circles for a referred stress field. The position in the Mohr diagram shows the fault orientation relative to the stress pattern. The Mohr–Coulomb failure lines with a friction coefficient of 0.7 under various

pore fluid pressures, P0, and Pf. b) Slip tendency stereoplots under the given stress regime and a pressure Pf = P0+5 MPa. c) Slip tendency and expected slip angle as a function of strike and dip of virtual fault plans. Dashed lines indicate ranges of major focal mechanism solutions of the induced

earthquakes. The mechanisms of two M4+ events are also mapped. Most events show either strike-slip or reverse mechanisms with strike and dip angles distributed in wide ranges.

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Figure 9. a) Plot shows relationship between focal mechanism and pore fluid pressure in the 3-D Mohr circles for a regional stress field in Rongchang gas field. The Mohr–Coulomb failure lines with a

friction coefficient of 0.7 under various pore fluid pressures, P0, and Pf. b) Slip tendency stereoplots under the given stress regime and an over pressure constant of 0.1. c) Slip tendency and expected slip angle as a function of strike and dip of virtual fault plans superimposed with the mechanism solutions

of seven large events.

In a summery, we observed many induced earthquakes with unfavorable focal mechanisms to the suggested background stress fields. By mapping the focal mechanisms of relatively large events on the 3-D Mohr diagram and slip-tendency plots reservoir by reservoir, we confirmed that some events occurred on optimally oriented faults but others occurred on misoriented faults. Pore pressure increased by injection is a main factor, while inhomogeneity of the local stress at reservoir scales, and low friction coefficients are also important factors. Further studies are required to make clear the role of these factors.

5. Conclusions

In agreement with each other, typical cases of injection-induced seismicity in Sichuan Basin, demonstrate following general features. 1) More swarm-like characterized by smaller α (normally 1~1.5 ) in the ETAS model. 2) Most events occurred from reactivation of pre-existing weak planes including faults, fractures, joints, and bedding surfaces. 3) During the earlier stages of injection the total fraction of seismically triggering earthquakes takes about 40-60%, indicating that the stress in the formations is critical or subcritical and seismic triggering and injection forcing are both important for earthquake occurrence. 4) As a natural implication of 2), the maximum magnitude of potential earthquakes is determined primarily by the size of pre-existing faults in which the pore pressure has been raised by injection. 5) Induced earthquakes, together with possible aseismic slips, could release

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the stress in the formation significantly and thus it is commonly observed that the forced earthquake rate increases with injection time. 6) A uniform regional stress field is insufficient to interpret all mechanisms observed which demonstrates the local stress filed at reservoir scale significantly inhomogeneous. Acknowledgements

This study was supported by the State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake under the project LED 2011B06. References

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RS2013-1078 Insights gained from the injection-induced ... · 3. Criticality of stress inferred from statistical analysis of injection-induced seismicity In injection-induced seismicity,