Microseismicity at Rotokawa geothermal field, New Zealand, 2008–2012
S. SherburnSteven SewellSandra BourguignonWilliam CummingStephen BannisterC. BardsleyJeffrey WinickJaime QuinaoIrene C. Wallis
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Injection well
Microseism
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Summary Seismic monitoring is an important tool for evaluating hydraulic fracture treatments in many petroleum reservoirs. Microseismic data is used to determine the extent of fracturing due to treatment and evaluate how effectively the reservoir is stimulated. Induced seismicity monitoring has become important recently, as the occurrence of high magnitude (MW > 0) events in several locations has led to the introduction of government-mandated "traffic light" systems to mitigate the impact of induced seismicity on the general public. To better understand the reservoir conditions which lead to the generation of large events, these two different ways of measuring seismic activity can be combined, incorporating the highly accurate event location accuracy from downhole microseismic monitoring with accurate source characterization of high magnitude events from surface induced seismicity monitoring. Such a monitoring system allows the full range of seismicity related to hydraulic fracture treatments to be accurately characterized. Combining the recorded data is a technical challenge, but with attention to detail in applying relevant corrections it is possible to achieve a consistent dataset. Data from a large multi-well zipper frac employing the full-band monitoring configuration is discussed in detail to illustrate the benefits of an integrated processing workflow in terms of increased understanding of the fracture process and conditions which lead to high magnitude events.
Microseism
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In this paper we analyze the temporal distribution of fluid induced microseismicity and show which information about reservoir and source can be extracted from the seismicity rate. We assume that microseismic events induced through fluid injections are triggered by a pure diffusive process of pore pressure relaxation. We improve an existing formulation for the seismicity rate of fluid induced microseismicity, which is developed based on this assumption. In this way we derive to a formulation, which describes the temporal distribution of microseismic activity in dependency on the parameters of source and reservoir. In the next step we show that the well known Omori law, which describes the frequency of aftershock occurrence, can be transferred to the case of fluid induced microseismicity to describe the temporal distribution of events induced after injection stop. Even in seismology the controlling parameters of the characteristic p‐value of the Omori law are still under discussion. Here we identify the controlling parameters of the p‐value for fluid induced seismicity and show, which parameters of source and reservoir can be reconstructed by a p‐value analysis. Finally we apply the developed theory to synthetic data sets and to the Fenton Hill (1983) real data example.
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Abstract Mitigation of injection‐induced seismicity in Greeley, Colorado, is based largely on proximity of wastewater disposal wells to seismicity and consists of cementation of the bottom of wells to eliminate connection between the disposal interval and crystalline basement. Brief injection rate reductions followed felt events, but injection rates returned to high levels, >250,000 barrels/month, within 6 months. While brief rate reduction reduces seismicity in the short term, overall seismicity is not reduced. We examine contributions to pore pressure change by injection from 22 wells within 30 km of the center of seismicity. The combined injection rate of seven disposal wells within 15 km of the seismicity (Greeley Wells) is correlated with the seismicity rate. We find that injection from NGL‐C4A, the well previously suspected as the likely cause of the induced seismicity, is responsible for ~28% of pore pressure increase. The other six Greeley Wells contribute ~28% of pore pressure increase, and the 15 Far‐field Wells between 15 and 30 km from the seismicity contribute ~44% of pore pressure increase. Modeling results show that NGL‐C4A plays the largest role in increased pore pressure but shows that the six other Greeley Wells have approximately the same influence as NGL‐C4A. Furthermore, the 15 Far‐field Wells have significant influence on pore pressure near the seismicity. Since the main mitigation action of cementing the bottom of wells has not decreased seismicity, mitigation based on reduced injection rates and spacing wells farther apart would likely have a higher potential for success.
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For getting deep informations of the geothermal field and applied effect of new techniques,we carried out geo-noise measurements, geo-noise source examina-tion and short supervision survey on microseisms network in YBJ Geothermal field in 1989. From these works mentioned above geo-noise of the shallow heat-storage reservoir is characterized by stable wave frequency and large vibrational extent in south of the geothermal field. The anomaly extension Is consistant with known thermal reservior. In north of the geothermal field, wave spectrum of geo-noise is characterized by high main frequency and small vibrational extent. Microseisms activity was first recorded in the thermal field. Activity to the comprehensive Study,explorating prospect for high-temperature geothermal reservior is considered to be meaningful and three perspective regions are suggested on this basis.
Microseism
Geothermal exploration
Seismic Noise
Anomaly (physics)
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Welcome to the second half of TLE's two-part special section on passive seismic and microseismic. This month, we focus again on monitoring hydraulic fracturing with microseismic with five articles, but also expand beyond “micro” seismicity, to include unintended “induced” seismicity that may occur during injection. Five articles in this special section focus on induced-seismicity topics. In this introduction, we will highlight various issues related to undesired induced seismicity which may be caused by hydraulic fracturing and deep, underground salt water disposal.
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Special section
Section (typography)
Passive seismic
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Abstract The injection of water into geothermal systems is an important procedure required to recover subsurface water resources and enhance permeability for increasing the reservoir volume. The injected water often leads to microseismic events during migration, which can be used to directly track the location of the injected water. However, in rare cases, unexpectedly large induced seismicity occurs after the injection termination. For risk control, understanding the differences between cases that cause post-termination seismicity and those that do not is necessary. For this purpose, we used microseismic monitoring to examine the behavior of water during two injection tests, including their post-termination periods, in Okuaizu geothermal field, Japan. In this field, a new remote microseismic cluster, apart from the injection well, was created in the post-termination period of the first injection test. However, this cluster was not well activated in the second injection test. As a result, we revealed that this microseismic cluster was created on a structure that was different from the target fracture of the injection, possibly owing to pore-pressure migration in the post-termination period of the first injection. Its inactivation in the second post-termination period may be attributable to the lower magnitude of pore-pressure migration derived from the smaller amount of injected volume compared with that of the first injection test. The lower pore-pressure migration was insufficient to reactivate the seismicity. We concluded that the occurrence of seismicity after injection termination may depend on the magnitude of pressure in the injection well at the shut-in time. The Kaiser effect (i.e., a fault is not reactivated by stress that is less than the maximum stress loaded previously) could explain the observed phenomena.
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Injection site
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