Characteristics of microseismic data recorded by distributed acoustic sensing systems in anisotropic media
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Fiber-optic distributed acoustic sensing (DAS) cables are now used to monitor microseismicity during hydraulic-fracture stimulations of unconventional gas reservoirs. Unlike geophone arrays, DAS systems are sensitive to uniaxial strain or strain rate along the fiber direction and thus provide a 1C recording, which makes identifying the directionality and polarization of incoming waves difficult. Using synthetic examples, we have shown some fundamental characteristics of microseismic recordings on DAS systems for purposes of hydraulic fracture monitoring in a horizontal well in anisotropic (vertical transverse isotropy [VTI]) shales. We determine that SH arrivals dominate the recorded signals because their polarization is aligned along the horizontal cable at the near offset, although SV will typically dominate for events directly above or below the array. The amplitude of coherent shear-wave (S-wave) arrivals along the cable exhibits a characteristic pattern with bimodal peaks, the width of which relates to the distance of the event from the cable. Furthermore, we find that S-wave splitting recorded on DAS systems can be used to infer the inclination of the incoming waves, overcoming a current limitation of event locations that have constrained events to lie in a horizontal plane. Low-amplitude SV arrivals suggest an event depth similar to that of the DAS cable. Conversely, steep arrivals produce higher amplitude SV-waves, with S-wave splitting increasing with offset along the cable. Finally, we determine how polarity reversals observed in the P and SH phases can be used to provide strong constraints on the source mechanisms.Keywords:
Microseism
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Microseism
Rockfall
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Summary Microseismic monitoring is an efficient method in understanding the stability of rock slopes and it has been increasingly applied in this field in recent years. An optimal network distribution could effectively improve the efficiency of a microseismic monitoring system, especially to increase the localization accuracy of seismic events. In this work, the widely accepted guidelines were used to densify a microseismic network composed of five three-component geophones, which has been working on an unstable rock face in Northern Italy since 2013. The existing 5-geophone network was progressively expanded to a 15-geophone network. The location accuracy was calculated by using synthetic data for each network. We compared the location accuracy for different networks to estimate their performance. The results showed that the additional geophones could decrease the location errors from 12–24 m for the 5-geophone network down to 4–6 m for the 15-geophone network. We also compared the channel performance of the five three-component geophones to select the channels that should be retained in the future expanded network.
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Abstract Amongst different options of hydraulic fracture geometry detection or measurement, microseismic monitoring is a commonly used method to reveal the hydraulic fracture geometry in three-dimensional space. Microseismic monitoring typically requires one or several monitoring wells within an effective range from the treatment well, in which the geophones are set to detect the microseismic events occurring during or after the treatment. In the past, most of the monitoring wells have been vertical wells. We present several recent case studies in which both the treatment and monitoring wells were horizontal wells, which produced some unique and interesting observations beyond the initial expectations. One of the prerequisites of a proper microseismic monitoring of hydraulic fracturing treatment is to place the geophone in the proper position because a long distance between the actual fracturing events and the geophone may result in signal deterioration, which influences the processing and increases the uncertainty. This problem is more severe if the treatment well is a horizontal well because the distance from the geophone to the microseismic events varies between stages. One of the methods to solve this issue is to monitor the microseismic events in a horizontal offset well. As horizontal wells are often batched drilled in clusters for tight or unconventional resource nowadays, the availability of the monitoring well is less of a problem, and the constant distance from the monitoring well to the treatment well may help to generate better data quality and more accurate interpretation result. We implemented horizontal well monitoring in two difference cases between 2018 and 2019. For case A, one horizontal monitoring well was used to monitor 54 fracturing stages in three offset wells, and for case B, we monitored 24 fracturing stages in one offset well. In both cases, the geophone arrays were shifted in multiple positions to fit the distance requirements, and both cases generate satisfying interpretation results. The microseismic results from the two cases showed less uncertainty and better precision of microseismic events after processing, as we expected. What is surprising is this type of monitoring showed a unique physical phenomenon a couple of times, which is a casing background noise indicating excessive fracturing extension over a long distance. This phenomenon was captured in both cases, even with small injection rate and fluid volumes, which can be important information for us to better understand the dynamics of fracture propagation in such geomechanical environment and help to set a new guideline and design reference in the same region.
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Summary A comparison study between borehole arrays, broadband seismometers and surface geophones is undertaken using a new passive seismic dataset from the Fox Creek, Alberta, area. Induced seismicity and microseismic events are compared based on waveform character, arrival time accuracy, and frequency content. In addition, using cross-correlation for detection, the effectiveness of each of the instruments at picking up events is compared. It is concluded that the broadband seismometers are most effective for induced seismicity monitoring, while the borehole array is most suitable for the microseismic monitoring.
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Passive seismic
Vertical seismic profile
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Abstract The exploitation of unconventional reservoirs requires hydraulic fracturing treatments, aimed at enhancing reservoir rock permeability through the re-activation of existing natural micro-fractures and the generation of new fractures in the near-wellbore area. Microseismic Monitoring (MSM), which consists of acoustic recording of the induced fracturing events, has nowadays become a common approach for real-time tracking of the fracture network evolution. The present work is based on the analysis of microseismic and production data coming from three multi-fractured wells drilled in a shale gas reservoir. Information derived from MSM is integrated with the Advanced Production Analysis (APA) results, to assess if the MSM interpretations are able to provide preliminary indications about the production behavior of the fractured wells before start-up. The spatial localization of microseismic events and their moment magnitude are derived from the interpretation of the seismic signals recorded by geophones during hydraulic fracturing. Data analysis, based on the cross-plot of the event moment magnitude versus the distance between the event and the nearest geophone, allows the determination of the spatial range bias, i.e. the effect for which the level of the recorded magnitudes depends on the distance from the geophones. A novel methodology was developed to correct the spatial range bias by combing a Gaussian function and the Gutenberg-Richter law for fitting the Frequency Magnitude Distribution. After the correction, microseismic data from different wells and frac stages can be directly compared and used for the evaluation of the Stimulation Efficiency (SE) and the Stimulated Reservoir Volume (SRV). APA is used to analyze production data from the three selected wells in order to characterize both the fractured system and the reservoir. Even if the results obtained from MSM and APA techniques are often at different scales and not quantitatively comparable, it is still possible to obtain useful information from their integration. APA confirms, indeed, the interference between wells production due to the overlap of different SRV, as initially estimated from MSM interpretation. Thus, the results of the developed methodology allow to improve well placement, stimulation operation design and field development strategies.
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A North American gas reservoir microseismic test incorporated a 22 node three component geophone array in two monitor wells spaced 1100 meters apart and synchronized with a fiber optic cable connection. In the horizontal treatment well, the geophones were above the interval of interest. A stringshot was performed to orient the geophones and to calibrate a velocity model for the formations through which the microseisms would travel.
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