8 th Asian Rock Mechanics Symposium ARMS8 14-16 October 2014, Sapporo, Japan Insights on Injection-induced Seismicity Gained from Laboratory AE Study—Fracture Behavior of Sedimentary Rocks X.-L. Lei a *, X.-Y. Li b , and Q. Li b a Geological Survey of Japan, AIST, Ibaraki, 3058567, Japan b State Key Laboratory of Geomechanics and Geotechnical Engineering, Institute of Rock and Soil Mechanics, Chinese Academy of Science, Wuhan, 43071, China * [email protected]Abstract Injection-induced seismicity associated with applications, in which fluids are intensively pressed into the deep formations of the Earth's crust such as Enhanced Geothermal System (EGS), fracking shale gas, geological sequence of CO 2 , have attracted growing attentions. Motivated by the desire to better understand the mechanism of damaging events so that they can be avoided or mitigated, we have started an integrated study on rock fracturing and fault reactivation in multiscales. In this paper, we present some preliminary results of an ongoing experimental study utilizing acoustic emission technique in laboratory scale. We systematically carried out rock fracture tests using samples of typical sedimentary rocks collected from the Sichuan Basin, China, where a number of injection-induced seismic swarms with sizable earthquakes ranging up to M4~5 have been observed in some gas/oil reservoirs. Since most injection-induced earthquakes are located in sedimentary strata of a wide range of lithology and depth, the fracturing behaviors of such rocks are thus important. Our results indicate that the Pre-Triassic rocks in the Sichuan Basin, including dolomite or dolomitic limestone and shale are strong and demonstrate brittle fracturing behaviors. Such properties are necessary conditions for maintaining high level reservoir stress and resulting seismic fracturing. Keywords: Acoustic Emission, Rock Fracturing, Sedimentary rocks, Injection-Induced Seismicity 1. Introduction In applications including Enhanced Geothermal System (EGS), fracking shale gas, and geological sequence of CO 2 , fluids of various viscosity and temperature are intensively pumped under high pressure into the deep formations of the crust. In recent years, associated with the rapid increase of such applications, injection-induced seismicity has attracted growing public and scientific attentions. Earthquakes of moderate sizes (~M5), which are damaging in areas having not experienced with large earthquakes in the history, were occasionally observed. Injection induced seismicity is not required but unavoidable in many cases (Technologies et al., 2012). The impact of induced earthquakes is a controversial issue that has caused delays and threatened cancellation of some projects, such as the recent project at Basel (Kraft et al., 2009). It is thus a scientific challenge to make clear the mechanism and geomechanical conditions of damaging events so that they can either be avoided or be mitigated. The cases, especially recent ones, of injection-induced earthquake sequences in the southwestern Sichuan Basin, which is a relatively stable region in China showing very low level of background seismicity, provide us a chance of comprehensive studies on which. During the past three 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 injections of unwanted water in deep wells in depleted gas fields. All sequences were initiated in days to a few weeks after water injection under high pressure begun. Event rate fluctuates following changes of injection rate and injection frequency and tapered quickly after shut down. Detailed studies on some recent and well monitored cases demonstrate that most events, particularly larger ones, mirror the reactivation of pre-existing faults (including joints and fractures) in the sedimentary formations or faults underlying/overlying the reservoir (Lei et al., 2013b; Wang et al., 2012). As a typical example, Fig.1 shows hypocenter distribution of injection-induced earthquakes superimposed on the simplified geological cross section and stratigraphy of a gas Insights on Injection-induced Seismicity Gained from Laboratory AE Study—Fracture Behavior of Sedimentary Rocks X. Lei, X. Li, Q. Li 947
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8th Asian Rock Mechanics Symposium ARMS8 14-16 October 2014, Sapporo, Japan
Insights on Injection-induced Seismicity Gained from Laboratory AE Study—Fracture Behavior of Sedimentary Rocks
X.-L. Leia*, X.-Y. Lib, and Q. Lib
a Geological Survey of Japan, AIST, Ibaraki, 3058567, Japan
bState Key Laboratory of Geomechanics and Geotechnical Engineering, Institute of Rock and Soil Mechanics, Chinese Academy of Science, Wuhan, 43071, China
Injection-induced seismicity associated with applications, in which fluids are intensively pressed into the deep formations of the Earth's crust such as Enhanced Geothermal System (EGS), fracking shale gas, geological sequence of CO2, have attracted growing attentions. Motivated by the desire to better understand the mechanism of damaging events so that they can be avoided or mitigated, we have started an integrated study on rock fracturing and fault reactivation in multiscales. In this paper, we present some preliminary results of an ongoing experimental study utilizing acoustic emission technique in laboratory scale. We systematically carried out rock fracture tests using samples of typical sedimentary rocks collected from the Sichuan Basin, China, where a number of injection-induced seismic swarms with sizable earthquakes ranging up to M4~5 have been observed in some gas/oil reservoirs. Since most injection-induced earthquakes are located in sedimentary strata of a wide range of lithology and depth, the fracturing behaviors of such rocks are thus important. Our results indicate that the Pre-Triassic rocks in the Sichuan Basin, including dolomite or dolomitic limestone and shale are strong and demonstrate brittle fracturing behaviors. Such properties are necessary conditions for maintaining high level reservoir stress and resulting seismic fracturing. Keywords: Acoustic Emission, Rock Fracturing, Sedimentary rocks, Injection-Induced Seismicity 1. Introduction
In applications including Enhanced Geothermal System (EGS), fracking shale gas, and geological sequence of CO2, fluids of various viscosity and temperature are intensively pumped under high pressure into the deep formations of the crust. In recent years, associated with the rapid increase of such applications, injection-induced seismicity has attracted growing public and scientific attentions. Earthquakes of moderate sizes (~M5), which are damaging in areas having not experienced with large earthquakes in the history, were occasionally observed. Injection induced seismicity is not required but unavoidable in many cases (Technologies et al., 2012). The impact of induced earthquakes is a controversial issue that has caused delays and threatened cancellation of some projects, such as the recent project at Basel (Kraft et al., 2009). It is thus a scientific challenge to make clear the mechanism and geomechanical conditions of damaging events so that they can either be avoided or be mitigated.
The cases, especially recent ones, of injection-induced earthquake sequences in the southwestern Sichuan Basin, which is a relatively stable region in China showing very low level of background seismicity, provide us a chance of comprehensive studies on which. During the past three 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 injections of unwanted water in deep wells in depleted gas fields. All sequences were initiated in days to a few weeks after water injection under high pressure begun. Event rate fluctuates following changes of injection rate and injection frequency and tapered quickly after shut down. Detailed studies on some recent and well monitored cases demonstrate that most events, particularly larger ones, mirror the reactivation of pre-existing faults (including joints and fractures) in the sedimentary formations or faults underlying/overlying the reservoir (Lei et al., 2013b; Wang et al., 2012). As a typical example, Fig.1 shows hypocenter distribution of injection-induced earthquakes superimposed on the simplified geological cross section and stratigraphy of a gas
Insights on Injection-induced Seismicity Gained from Laboratory AE Study—Fracture Behavior of Sedimentary RocksX. Lei, X. Li, Q. Li 947
Insights on Injection-induced Seismicity Gained from Laboratory AE Study—Fracture Behavior of Sedimentary RocksX. Lei, X. Li, Q. Li
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Rock Mechanic
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Insights on Injection-induced Seismicity Gained from Laboratory AE Study—Fracture Behavior of Sedimentary RocksX. Lei, X. Li, Q. Li
8th Asian R
Rock Mechanic
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Insights on Injection-induced Seismicity Gained from Laboratory AE Study—Fracture Behavior of Sedimentary RocksX. Lei, X. Li, Q. Li
8th Asian Rock Mechanics Symposium ARMS8 14-16 October 2014, Sapporo, Japan 4. Discussion and conclusions
Our results indicate that the Pre-Cambrian formations in the Sichuan Basin, including dolomite or dolomitic limestone, and shale show brittle fracturing behaviors, characterized by no significant foreshocks, rapid dynamic fracturing with large stress drop, and a large number of aftershocks. The well cemented sandstone, having a porosity of ~6% and an extremely low permeability (less than 0.1 mD), also shows brittle fracturing behaviors. Unlike dolomite and brittle shale, sandstone shows increasing foreshock activity preceding the final failure.
The finally created faults in dolomite, limestone, and sandstone samples demonstrate complicated geometry, showing bends, irregular and rough fault surface, and branches. Fault in the shale sample (notes the bedding plan is optimally oriented for fracture) shows relatively simple fault plane. It is worthy to note that sample of badly oriented bedding planes normally demonstrate very irregular shear fault patterns (Lei et al., 2013a). New fault in brittle rocks shows relatively high frictional coefficient (1.0~1.2), expected to decrease to the typical range (0.6~0.8) after certain amount of slip and worth further study.
Our experimental results are helpful for understanding the question has raised in the introduction section—why injection-induced seismicity is so significant in the Sichuan Basin. Major Pre-Triassic sedimentary rocks, including dolomite, shale, and dolomitic limestone are strong and demonstrating brittle fracturing behaviors. Such properties are necessary conditions for maintaining high level reservoir stress and leading to seismic fracturing. Insights gained from this study may shed some lights to the general earthquake seismology and provide a better understanding of why damaging injection-induced earthquakes occur so that they can either be avoided or be mitigated. In general, critically or sub-critically stressed fault of a dimension of a few kms is a necessary condition for resulting M~5 class earthquakes. On the same time, AE, or in other words, micro-seismicity monitoring is also useful in risk assessment and injection management and should be fully utilized in injection applications.
Acknowledgements
We thank Yingxiang Cui, Wenbin Fei, and Miao Jing for their help in the field work for collecting rock samples. References Kraft, T., et al. 2009, Enhanced geothermal systems: Mitigating risk in urban areas, Eos Trans. AGU,
90(32), 273, doi:10.1029/2009EO320001. Lei, X., Kusunose, K., Rao, M.V.M.S., Nishizawa, O., Satoh, T., 2000, Quasi-static fault growth and
cracking in homogeneous brittle rock under triaxial compression using acoustic emission monitoring, Journal of Geophysical Research, 105, 6127.
Lei, X., Funatsu, T., Villaescusa, E., 2013a, Fault formation in foliated rock – insights gained from a laboratory study, In Proc 8th Int Symp on Rockbursts and Seismicity in Mines (RaSim8) (ed. Malovichko A, Malovichko D), Russia Saint-Peterburg-Moscow, 41-49.
Lei, X., Ma, S., Chen, W., Pang, C., Zeng, J., Jiang, B., 2013b. A detailed view of the injection-induced seismicity in a natural gas reservoir in Zigong, southwestern Sichuan Basin, China. Journal of Geophysical Research: Solid Earth 118, 4296-4311.
Lei, X., Tamagawa, T., Tezuka, K., Takahashi, M., 2011. Role of drainage conditions in deformation and fracture of porous rocks under triaxial compression in the laboratory, Geophysical Research Letters, 38, L24310, doi:10.1029/2011GL049888.
Technologies, C.o.I.S.P.i.E., Resources, C.o.E., Engineering, C.o.G.a.G., Geodynamics, C.o.S.a., (BESR), B.o.E.S.a.R., (DELS), D.o.E.a.L.S., Council, N.R., 2012, Induced Seismicity Potential in Energy Technologies,The National Academies Press.
Wang, X., Ma, S., et al., 2012, Fine velocity structure and relocation of the 2010 Ml5.1 earthquake sequence in the Rongchang gas field, Earthquake Research in China, 20, 467-478.
