Analysis of Microseismicity Framing ML > 2.5 Earthquakes at The Geysers Geothermal Field, California
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Abstract Preparatory mechanisms accompanying or leading to nucleation of larger earthquakes have been observed at both laboratory and field scales, but conditions favoring the occurrence of observable preparatory processes are still largely unknown. In particular, it remains a matter of debate why some earthquakes occur spontaneously without noticeable precursors as opposed to events that are preceded by an extended failure process. In this study, we have generated new high‐resolution seismicity catalogs framing the occurrence of 20 M L > 2.5 earthquakes at The Geysers geothermal field in California. To this end, a seismicity catalog of the 11 days framing each large event was created. We selected 20 sequences sampling different hypocentral depths and hydraulic conditions within the field. Seismic activity and magnitude frequency distributions displayed by the different earthquake sequences are correlated with their location within the reservoir. Sequences located in the northwestern part of the reservoir show overall increased seismic activity and low b values, while the southeastern part is dominated by decreased seismic activity and higher b values. Periods of high injection coincide with high b values and vice versa. These observations potentially reflect varying differential and mean stresses and damage of the reservoir rocks across the field. About 50% of analyzed sequences exhibit no change in seismicity rate in response to the large main event. However, we find complex waveforms at the onset of the main earthquake, suggesting that small ruptures spontaneously grow into or trigger larger events.Keywords:
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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.
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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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Three series of simultaneous wave and microseism records are examined. They give a clear indication that bands of microseismic waves from different sources can be distinguished by submitting seismograph records to frequency analysis. The agreement between the results of analysis and the theoretical expectation from the prevailing meteorological conditions appears to justify the assumption that microseismic waves of different periods travel independently. Under the simple meteorological conditions that have been studied, each band of microseismic activity can be identified with a band of sea waves of twice its period. The existence of this two to one ratio between the period of waves and microseisms affords some confirmation of the theory that microseisms are produced in a region of interference between similar wave trains travelling in opposite directions either near the coast or in deep water.
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In general, microseisms are a nuisance.It is n9w quite widely agreed that microseisms are channel waves, similar in nature to the Lg and Rg waves.It is not at all unusual to observe microseisms which have traveled over distances of continental proportions.In order to determine the direction from which the microseisms come and to investigate in more detail the characteristics of microseismic disturbances, we established a tri-partite system on Mount Palomar.The array consisted of three stations, located at the vertices of a triangle, which was about 2,500 feet on a side.Two Benloft strain seismographs and one Bentoff vertical pendulum seismograph were installed in a strain vault at one location.One vertical pendulum seismograph was installed at a second location, and two horizontal and one vertical pendulum seismographs were installed at a third location.Portions of the records which were obtained on two consecutive days are included in Figure 1.The direction of approach can be calculated from the displacement between the peaks on the three verticals.These observations indicated that the waves practically always come from the coast.The direction of approach generally lies between north-northwest and south-southwest.The horizontal pendulum traces in record (a) are parallel.This indicates'that the horizontal motion is in a northeast-southwest direction and vice versa.Everything on record (a) points to a Rayleigh type motion.Shear waves predominate on record (b).The fact that the horizontal pendulums are opposed to each other indicates that the motion is in a northwest-southeast direction.The horizontal component of displacement is, therefore, perpendicular to the direction of propagation.The sum of strain components is practically zero, as it should be for a horizontally polarized shear wave.The vertical pendulums show some stoa1!motions.This is to be expected, since some Rayleigh type motion is always present.Both of these waves are character~ ized by six-second periods, but usually the Rayleigh type motion predominates.Among the many other types of microseisms which are observed are the twosecond microseisms with wavelengths between about one-third and one-half the wavelength of the six-second waves and practically the same velocity.These waves originate near the coast--probably on the continental shelf.it is generally believed that they are generated by turbulent air masses over the continental shelf.The horizontal pendulum and the horizontal strain seismographs show practically only the six-second waves and very little of the shorter waves.We were puzzled by the [act that here we had a wave which traveled with the Rayleigh wave velocity but possessed mainly a vertical component.Fortunately, Dr. Press could give us the probable solution.Drs.
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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.
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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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