Browsing by Author "Tang, Youcai"

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    Hydraulic Injection‐Induced Velocity Changes Revealed by Surface Wave Coda and Polarization Data at a Shale Play Site in Southwest China
    (Wiley, 2020) Zhang, Yan; Niu, Fenglin; Tao, Kai; Ning, Jieyuan; Chen, Haichao; Tang, Youcai
    We investigated temporal variations of seismic wave velocity associated with hydraulic fracturing using Green's functions computed from ambient noise data. In October and November of 2014, we set up a broadband array at a shale play site inside the Sichuan basin where a pilot horizontal drilling and hydraulic injections were conducted. We first computed cross‐correlation functions using continuous data recorded by 21 three‐component broadband sensors deployed around the treatment well. We then employed a running window correlation‐based coda wave interferometry technique to measure apparent velocity changes from the daily Green's functions of all the station pairs in the frequency range of 1 to 3 Hz. We found significant velocity changes right after the hydraulic fracturing, which exhibited a clear direction‐dependent pattern. S wave velocity along raypaths parallel to the well trajectory showed a clear increase while those perpendicular exhibited a small decrease. The anisotropic changes in seismic velocity observed here were also confirmed from surface wave horizontal particle motion data. By comparing our observations with normal stress changes calculated with a half‐space elastic model, we speculate that stress changes induced by the hydraulic fracturing were likely to be responsible for the observed anisotropic changes in seismic velocity. Our results suggest that time‐lapse seismic imaging with ambient noise data provides a promising probe for monitoring geomechanical changes related to exploitation of unconventional oil and gas resources.
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    Lg attenuation in northeast China using NECESSArray data
    (Oxford University Press on behalf of The Royal Astronomical Society, 2015) Ranasinghe, Nishath R.; Gallegos, Andrea C.; Trujillo, Andrea R.; Blanchette, Alexander R.; Sandvol, Eric A.; Ni, James; Hearn, Thomas M.; Tang, Youcai; Grand, Stephen P.; Niu, Fenglin; Chen, Yongshun J.; Ning, Jieyuan; Kawakatsu, Hitoshi; Tanaka, Satoru; Obayashi, Masayuki
    The 127 station NorthEast China Extended SeiSmic Array (NECESSArray) provides large quantities of high quality seismic data in northeast China that allow us to resolve lateral variations of Lg Q or crustal attenuation at 1 Hz (Qo) to 2.0° or greater. Using the reverse two-station/event method with 11 642 Lg path-amplitudes from 78 crustal earthquakes, we obtain a 2-D tomographic image of Lg Qo with values ranging from ∼50 to 1400. A high degree of detail in the lateral variation of Lg attenuation is revealed in our tomographic image. High Qo regions are found in the Great Xing'an, Lesser Xing'an and Songen-Zhangguangcai Ranges. Low Qo regions are observed in the Songliao, Sanjiang and Erlian Basins. The lowest Qo is found near the Wudalianchi volcanic field and other Quaternary volcanic fields, the southern Songliao Basin, the western edge of the Erlian Basin and the Sanjiang Basin. Low Qo values are measured for paths that cross sedimentary basins with thick, unconsolidated sediments. Most of the high Lg attenuation in the Songliao Basin correlates reasonably well with low crustal Rayleigh wave phase velocity anomalies. The highest attenuating regions also correlate well with regions of Holocene volcanism..
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    Microseismic Monitoring of Stimulating Shale Gas Reservoir in SW China: 2. Spatial Clustering Controlled by the Preexisting Faults and Fractures
    (Wiley, 2018) Chen, Haichao; Meng, Xiaobo; Niu, Fenglin; Tang, Youcai; Yin, Chen; Wu, Furong
    Microseismic monitoring is crucial to improving stimulation efficiency of hydraulic fracturing treatment, as well as to mitigating potential induced seismic hazard. We applied an improved matching and locating technique to the downhole microseismic data set during one treatment stage along a horizontal well within the Weiyuan shale gas play inside Sichuan Basin in SW China, resulting in 3,052 well‐located microseismic events. We employed this expanded catalog to investigate the spatiotemporal evolution of the microseismicity in order to constrain migration of the injected fluids and the associated dynamic processes. The microseismicity is generally characterized by two distinctly different clusters, both of which are highly correlated with the injection activity spatially and temporarily. The distant and well‐confined cluster (cluster A) is featured by relatively large‐magnitude events, with ~40 events of M −1 or greater, whereas the cluster in the immediate vicinity of the wellbore (cluster B) includes two apparent lineations of seismicity with a NE‐SW trending, consistent with the predominant orientation of natural fractures. We calculated the b‐value and D‐value, an index of fracture complexity, and found significant differences between the two seismicity clusters. Particularly, the distant cluster showed an extremely low b‐value (~0.47) and D‐value (~1.35). We speculate that the distant cluster is triggered by reactivation of a preexisting critically stressed fault, whereas the two lineations are induced by shear failures of optimally oriented natural fractures associated with fluid diffusion. In both cases, the spatially clustered microseismicity related to hydraulic stimulation is strongly controlled by the preexisting faults and fractures.
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    Temporal Variations of Near‐Surface Anisotropy Induced by Hydraulic Fracturing at a Shale Play Site in Southwest China
    (Wiley, 2018) Zuo, Qiankun; Tang, Youcai; Niu, Fenglin; Li, Guoliang; Chen, Haichao; Tao, Kai; Chen, Benchi
    Knowledge of the geometric properties of fractures and cracks in a petroleum reservoir is important to reservoir exploitation. When aligned and partially connected, fractures and cracks can act as conduits for fluid flow and thus can significantly increase the permeability of the reservoir. The aligned fractures and cracks, on the other hand, are an effective means to generate seismic anisotropy. In this study, we utilize the seismic data recorded by a vertical array installed in a shallow borehole at a shale play site in southwest China. By applying seismic interferometry to the ambient noise data recorded by 12 three‐component geophones, we extract P and S waves propagating vertically along the borehole. The S waves show up to 20% velocity variations with respect to their polarization directions. Such large S wave anisotropy can be explained by the horizontal transverse isotropic model and is likely caused by natural fractures that are widely present in the area and align approximately in the NE‐SW direction. During the 13‐day period of hydraulic fracking treatment, we also observe large and systematic temporal variations in S wave velocity, degree of S wave polarization anisotropy, and fast polarization direction. By comparing our observations with normal strain changes calculated with a half‐space elastic model, we speculate that strain changes induced by hydraulic injection and fracturing are likely to be responsible for the observed temporal variations in seismic anisotropy. As such, seismic interferometry with shallow borehole acquisition might provide an alternative means to monitor hydraulic fracturing and wastewater injection in the future.