Seismic Anisotropy Estimation Using a Downhole Microseismic Data Set in a Shale Gas Reservoir

Shale anisotropy has a significant impact on the data processing and interpretation of microseismic monitoring in shale gas reservoirs. A geology- and rock-physics-constrained approach to estimating shale anisotropy using down-hole microseismic data sets is proposed in this study and is applied to the case of Horn River shale. A priori knowledge of shale anisotropy is obtained by integrating geological analyses and rock physics studies. This knowledge serves as an important constraint when building the initial model, minimizing the uncertainties and evaluating the results. The application to Horn River shale shows that the optimized anisotropic velocity model reduces the time misfit by about 65% compared to the originally provided velocity model. As the relocated perforation shot indicates, the event locations are significantly improved. The results also show that a high fraction of clay mineral results in strong fabric anisotropy in the Fort Simpson formation, whereas the quartz-rich shale gas reservoirs (Muskwa and Otter Park formations) show weaker fabric anisotropy. The percentage of velocity anisotropy in Horn River shale can be up to 40%. The fabric anisotropy of shale derived from the downhole microseismic data set is comparable with that of laboratory experiments. This demonstrates that downhole microseismic monitoring, as a quasi in situ experiment, has the potential to contribute to a better understanding of subsurface anisotropy beyond the laboratory. In addition, microseismic measurements of shale anisotropy are conducted in the seismic frequency band and are thus more applicable for further seismic applications

Frequency‐Dependent Moment Tensors of Induced Microearthquakes

Analysis of 984 induced microearthquakes from The Geysers geothermal reservoir in California reveals that the retrieved moment tensors depend on the frequency band of the inverted waveforms. The observed dependence is more significant for the percentages of the double‐couple, compensated linear vector dipole, and isotropic (ISO) components than for the focal mechanisms. The average root‐mean‐square of the moment tensors obtained in different frequency bands is correlated with spectra of ambient noise. The percentages of double‐couple and ISO components tend to decrease and increase with the upper cutoff frequency (fu), respectively. This suggests that shear rupture radiates energy preferentially in a lower frequency band and tensile rupture in a higher frequency band. Events displaying a strong increase of the ISO with fu are confined within the same depth interval as the injection points. This might be related to the strong thermoelastic effects in the vicinity of injection points that promote opening of small cracks adjacent to the main fractures

Micro Seismische Inversion für anisotrope Geschwindigkeitsmodell in unkonventionellen Reservoirs

The main objective of this thesis is to develop a practical, geology- and rock physics-oriented approach to constructing anisotropic velocity model for unconventional reservoirs using downhole microseismic datasets. The working procedure of the approach starts by addressing the geological sources of anisotropy. A priori knowledge of anisotropy is obtained by integrating geological information and rock physics studies. The prior knowledge serves as constraint on the microseismic inversion. The anisotropic velocity model obtained by the approach can reflect the heterogeneity of anisotropic parameters and cover the anisotropic symmetries of most importance in seismic exploration and reservoir characterization. The optimal anisotropic velocity model not only minimizes the data misfit, but also is reasonable from the perspectives of geology and rock physics. The results derived from downhole microseismic dataset are comparable with laboratory experiments. This demonstrates that the downhole microseismic monitoring, as a quasi in-situ experiment, has a potential to contribute to a better understanding of subsurface anisotropy beyond the laboratory. The approach developed in this thesis uses a layered velocity model. This approximation is adequate due to the limited spatial range of microseismic monitoring and the relatively flat sedimentary background of unconventional reservoirs. The transverse isotropy caused by the bedding-parallel fabric is defined by Thomsen parameters in each layer. The lateral heterogeneities within each layer are dismissed, while the vertical gradients of transverse isotropic parameters are kept. The fracture- induced anisotropy is only defined in a specific layer of high brittleness and is characterized by normal and tangential fracture compliance. The approach uses the arrival-time of seismic waves recorded by sensor arrays. An anisotropic ray-tracing algorithm is modified to calculate the synthesized travel-time. Parallel computing is employed to accelerate the ray-tracing program. The inherent singularity problems in the ray-tracing method are fixed by applying numerical strategies. Two nonlinear inversion methods are involved in this approach to determine different components of anisotropy velocity model. The multi-layer TI model is inverted by an iterative gradient-based optimization (the Gauss-Newton method). The fracture-induced anisotropy represented only by two parameters is obtained by a global search method. Besides, as a possible source of uncertainties in the velocity model inversion and event locations, the issues of computing triggering time (T0) are analyzed theoretically and illustrated with examples. The approach developed in this study is partially applied to a completed project of downhole microseismic monitoring in a coalbed methane reservoir to verify the capability of iterative gradient-based inversion for anisotropic velocity model and illustrate the T0 issue in the configuration of limited aperture. Then, the approach is fully applied to a downhole microseismic dataset from Horn River Basin in Canada to investigate the fabric anisotropy and fracture-induced anisotropy of shales. The fabric anisotropy of shale is caused by the alignment and lamination of the low aspect-ratio, compliant particles, such as clay minerals and organic matter. The existence of quartz minerals can prevent and interrupt such alignment and lamination and consequently weaken the fabric anisotropy of shale. Laboratory measurements show a strong positive correlation between the degree of fabric anisotropy and the volume contents of clay minerals and kerogen. Thomsen parameters ε and γ of shale samples are well correlated with each other, but not with δ. By integrating the geological information and experimental studies, the fabric anisotropy of Horn River shales is initially estimated. The quartz-rich shale gas reservoir is expected to show much weaker transverse isotropy than the overlying clay-rich shale. An iterative optimization using the gradient-based method is then implemented on this initial model. The results derived from the downhole microseismic dataset are consistent with the laboratory measurements. The optimized VTI model reduces the time misfit by about 65% compared to the originally provided VTI model. The event locations are also significantly improved. The preferred- oriented fracture set is another important source of shale anisotropy. Mechanical analyses show that the fractures in Horn River shales mainly occur in the quartz-rich formation showing much higher brittleness. According to the core analyses and fracture mechanism, the fracture planes are commonly perpendicular to the bedding plane and the dominant fracture set strikes to NE-SW direction which is parallel to the current maximum horizontal stress. The elastic behaviors of the fracture are effectively described by the normal and tangential fracture compliance (i.e., ZN, ZT) regardless of any physical details of fracture. Theoretical modeling and experimental measurements show, the magnitudes of ZN and ZT increase with the fracture dimension scale, and the ZN/ZT ratio is sensitive to fluid fills and has the value less than or slightly larger than 1. These facts are used as physical constraints in the grid search for the optimal fracture compliance. The magnitudes of ZN and ZT define the searching range and the ZN/ZT ratio is used as a quality control. The optimal ZN and ZT have the same order of magnitude as other measurements in the crosshole and microseismic scale. The ZN/ZT ratio corresponds to the extreme cases of dry or gas saturated fractures.Das Hauptziel dieser Arbeit ist die Entwicklung eines praktischen Verfahrens zur Erstellung anisotroper Geschwindigkeitsmodelle aus mikroseismischen Bohrlochdatensätzen in unkonventionellen Kohlenwasserstofflagerstätten unter Einbindung geologischer und gesteinsphysikalischer Aspekte. Der Arbeitsablauf des Verfahrens beginnt mit der Vorstellung geologischer Ursachen von Anisotropie. Apriorische Kenntnisse über Anisotropie stammen aus der Integration geologischer Informationen und gesteinsphysikalischer Studien, welche als Randbedingungen der mikroseismischen Inversion dienen. Mit dem präsentierten Verfahren gewonnene Geschwindigkeitsmodelle können Heterogenitäten anisotroper Parameter wiedergeben und decken die wichtigsten anisotropen Symmetrien in den Bereichen der seismischen Exploration und Reservoir-Charakterisierung ab. Das optimale anisotrope Geschwindigkeitsmodell verringert dabei nicht nur die Laufzeitresiduen, sondern ermöglicht auch eine bessere Beschreibung der geologischen und gesteinsphysikalischen Vorgaben. Der Vergleich mit Hilfe von Bohrlochdaten erzielter Ergebnisse gegenüber Laboruntersuchungen zeigt, dass direkt im Bohrloch durchgeführte mikroseismischen Beobachtungen als gewissermaßen In-situ-Experiment zu einem besseren Verständnis der Anisotropie im Untergrund genutzt werden können. Das präsentierte Verfahren nutzt ein geschichtetes Geschwindigkeitsmodell unter Berücksichtigung der Einschränkungen mikroseismischen Bohrloch-Monitorings und des sedimentären Charakters unkonventioneller Lagerstätten. Die durch ein schichtungsparalleles Gefüge verursachte transversale Isotropie (TI) einer jeden Schicht wird durch Thomsen-Parameter beschrieben. Laterale Heterogenitäten innerhalb einzelner Schichten werden vernachlässigt, wohingegen der vertikale Gradient der TI Parameter erhalten bleibt. Bruchinduzierte Anisotropie wird nur in Schichten mit einer hohen Brüchigkeit definiert und als Kombination aus normalen und tangentialen Nachgiebigkeiten charakterisiert. Das Verfahren basiert auf der Registrierung von Ankunftszeiten seismischer Wellen. Ein durch parallele Berechnungen beschleunigter anisotroper Raytracing-Algorithmus wird zur Bestimmung der synthetischen Laufzeiten verwendet, wobei inhärente Singularitätsprobleme durch die Anwendung numerischer Strategien behoben werden. Im Rahmen dieser Arbeit werden zwei nichtlineare Inversionsmethoden zur Bestimmung der unterschiedlichen Komponenten des anisotropen Geschwindigkeitsmodells genutzt. Das mehrschichtige TI-Modell wird mit Hilfe des iterativen gradientenbasierten Gauss-Newton-Verfahrens invertiert. Die von nur zwei Parametern beschriebene bruchinduzierte Anisotropie wird durch eine globale Suche bestimmt. Probleme bei der Berechnung von Herdzeiten (T0) werden theoretisch betrachtet und anhand von Beispielen erklärt, weil die herkömmliche Berechnungsmethode eine mögliche Fehlerquelle für Ungenauigkeiten im Geschwindigkeitsmodell und bei der Mikrobebenlokalisierung darstellt. Das vorgeschlagene Verfahren wird teilweise an einem abgeschlossenen Projekt für mikroseismisches BohrlochMonitoring in einer Flözgas-Lagerstätte ausprobiert, um die Möglichkeiten der iterativen gradientenbasierten Inversion zu erörtern und Probleme der Bestimmung von T0in limitierten Datenvolumina zu verdeutlichen. Anschließend wird das Verfahren im vollen Umfang an einem mikroseismischen Datensatz einer Schiefergaslagerstätte im Horn-River-Becken getestet, um im Besonderen die Gefügeanisotropie als auch eine durch Brüche induzierte Anisotropie in Schiefergesteinen zu untersuchen. Gefügeanisotropie in Schiefern wird durch die einheitliche Ausrichtung und Laminierung fester länglicher Partikel wie Tonminerale oder organischen Materials erzeugt. Ein zusätzlicher Quarz-Anteil kann jedoch die Ausrichtung und Laminierung der Partikel beeinträchtigen und somit die Gefügeanisotropie der Schiefer signifikant verringern. Laborexperimente zeigen eine starke positive Korrelation zwischen der Ausprägung der Gefügeanisotropie sowie den Ton- und Kerogen-Anteilen. Außerdem korrelieren die Thomsen-Parameter ε und γ sehr gut untereinander aber nicht mit δ. Die Ausgangswerte der Gefügeanisotropie der Horn-River-Schiefer werden durch Integration geologischer Informationen und experimenteller Studien abgeschätzt. Die quarzreichen gashaltigen Schichten sollten eine deutlich schwächere transversale Isotropie als die tonhaltigen Schiefer im Hangenden zeigen. Die mit Hilfe der iterativen Optimierung aus mikroseismischen Bohrlochdaten bestimmte Gefügeanisotropie entspricht dabei Ergebnissen von Labormessungen. Das optimierte VTI-Modell reduziert die Laufzeitresiduen um rund 65% im Vergleich zum ursprünglichen zur Verfügung gestellten anisotropen Geschwindigkeitsmodell. Zudem konnten die Mikrobeben mit dem optimierten VTI-Modell signifikant besser lokalisiert werden. Eine weitere wichtige Ursache von Anisotropie in Schiefern stellt ein System von Brüchen bevorzugter Orientierung dar. Mechanische Analysen von Brüchen in Schiefergesteinen aus dem Horn-River-Becken zeigen, dass diese vorwiegend in quartzreichen Formationen mit einer sehr hohen Brüchigkeit auftreten. Anhand der Analyse von Bohrlochkernen und Bruchmechanismen wird angenommen, dass die Bruchfläche normal zur Schichtung des Gefüges der Schiefer orientiert ist. Das dominante System von Brüchen zeigt ein Streichen in NO-SW-Richtung und verläuft somit parallel zur derzeitigen maximalen horizontalen Spannung. Das elastische Verhalten von Brüchen wird durch normale und tangentiale Nachgiebigkeiten (ZN, ZT) unabhängig von der genauen Bruchmorphologie beschrieben. Theoretische Modellierungen und Messungen verdeutlichen, dass die Magnituden von ZN und ZT mit der Größe der Bruchfläche steigen und dass das Verhältnis zwischen normaler zu tangentialer Nach-giebigkeit ZN/ZT kleiner oder minimal größer 1 ist sowie sensitiv auf Fluidfüllungen reagiert. Zur Bestimmung der optimalen Werte für ZN und ZT wird eine Rastersuche durchgeführt, wobei die getätigten Beobachtungen als physikalische Randbedingungen zur Abschätzung der Nachgiebigkeiten genutzt werden, um durch Angaben zur Magnitude den Suchraum einzuschränken und ZN/ZT zur Qualitätskontrolle zu nutzen. Die ermittelten Werte für ZN und ZT bewegen sich in derselben Größenordnung wie bei anderen mikroseismischen Bohrlochuntersuchungen. ZN/ZT entspricht den extremen Fällen trockener oder mit Gas saturierter Brüche

Detection of Weak Signals Under Low SNR Stochastic Resonance System

To solve the problem that weak signals are difficult to detect accurately in low signal-to-noise ratios, this paper presents a method to achieve effective detection of weak signals, applying the method of stochastic resonance to bistable systems. The principle of the method is that by transferring part of the noise energy to the signal energy, enabling the detection of weak signals at low signal-to-noise ratios. It makes it easier to extract the signal at the receiving end. This model designs a parametrized conditioning system based on the factors influencing the output power spectrum and output SNR of a stochastic resonant system. Based on the experimental results, the influence of parameters a and b on the model can be analysed, and the optimal noise intensity range of the system can be found. At the receiving end of the system, the constellation diagram and BER are used as a measure of system performance. Simulation experiments show that stochastic resonance can effectively enhance the energy of weak signals under low signal-to-noise conditions, and the demodulation performance of the system is significantly better than that of the system without the use of stochastic resonance

Nonlinear Flood Responses to Tide Level and Land Cover Changes in Small Watersheds

Regarding global warming, the threat of flooding is projected to increase due to the change in intensity and frequency of single drivers and amplification caused by multi-driver interactions. This interaction becomes more complicated in developing regions with rapidly changing land cover. As a result, demands on flood risk management are rising especially in small watersheds, which are more vulnerable to driver disturbances compared with large watersheds. Existing studies focused on large watersheds rather than small watersheds. However, the findings derived from large-scale analysis cannot be transferred to small watersheds directly. This research investigated the flood responses in the Yonghe River Watershed (YRW) (63.8 km2) in Guangzhou, China, considering the impact of land cover change. The YRW experienced a disastrous compound flood on 22 May 2020. A hydrodynamic model integrating the Hydrologic Engineering Center’s Hydrologic Modeling System and River Analysis System (HEC-HMS and HEC-RAS, respectively) was established and calibrated using the inundation depths observed during the flood. Model analysis using multiple scenarios showed that the watershed is river-dominated, and flood responses to the three factors are nonlinear but with different increasing rates. The response curves for tide levels and land cover changes increase faster at high values, whereas the rainfall intensity curves vary slightly. These findings highlight the importance of integrating tidal impacts into flood risk management, even in river-dominated coastal watersheds. The study further recommends that in small watersheds, 50% imperviousness is an indicator of the urgent demand for flood risk management measures

Residues and Dietary Risk Assessment of Imidacloprid in Bamboo Shoot (<i>Phyllostachys praecox</i>), Winter Jujube (<i>Ziziphus jujuba</i> Mill. cv. Dongzao), <i>Dendrobium officinale</i> Kimura et Migo, and <i>Fritillaria</i>

The widespread use of pesticides poses significant risks to food and environmental safety. Imidacloprid is one of the most effective neuroactive neonicotinoid insecticides and is effective against a broad spectrum of piercing–sucking pests. A rapid, efficient, and high-throughput analysis method for the determination of imidacloprid was developed in four minor crops with six matrices (bamboo shoot, winter jujube, fresh and dry D. officinale, and fresh and dry Fritillaria) by solid-phase extraction and HPLC-MS/MS. The procedure showed satisfying recoveries (72~111%) and RSDs (1~13%). A total of 288 samples were tested in China (Aba and Luan). Imidacloprid residues were 100% detected in fresh and dry D. officinale and winter jujube, with concentrations ranging from 0.048 to 1.550 mg·kg−1. Imidacloprid residues were also abundant in fresh and dry Fritillaria (maximal concentration of 0.021 and 0.063 mg·kg−1, respectively), followed by bamboo shoot, which had the lowest detection rate of imidacloprid (6%). Using the risk quotient (RQ) method, the long-term (RQc) and short-term (RQa) dietary risks of imidacloprid in bamboo shoot, winter jujube, fresh and dry D. officinale, and fresh and dry Fritillaria were further monitored. Based on the imidacloprid residues in this paper, the RQc and RQa were 15.03% and 0.0008~1.7604%, respectively. The RQ values were far less than 100%, showing that Chinese consumers face little health risk as a result of imidacloprid intake