Microseismic Observations Research Articles

Microseismic Observation Enabled by High-Sensitivity Micromechanical Interferometers

High-sensitivity accelerometry has historically been employed to explore Earth sciences and advance resource-related industries, such as petroleum exploitation. The devices capable of such measurements, however, traditionally involve precise mechanical structures and consequent large dimensions. In this work, combined by micromechanical systems and grating-assisted interferometry, a closed-loop optical accelerometer with nano-g (ng) sensitivity is developed for microseismic observations. In order to lower the intrinsic thermal noise, a modified 8.84 Hz micromechanical spring-mass oscillator is designed and fabricated. Additionally, an active control system is equipped to stabilize the device temperature and thus enhance the low-frequency stability. Field tests demonstrate that the prototype has achieved a scale factor of 1195 V/g, along with a dynamic range of 1.12 mg and a bandwidth of about 7 Hz. Further, under the controlled 45 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\pm$</tex-math></inline-formula> 0.01 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^\circ$</tex-math></inline-formula> C temperature condition, complete terrestrial microsesimic peaks from 0.1 Hz to 3 Hz are tracked. After calibration by a reference seismometer, the device self-noise is determined as 2 ng/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sqrt{Hz}$</tex-math></inline-formula> from 0.2 Hz to 4 Hz with a bias stability of 7.9 ng. The transducer and control circuits are both packaged within a 13.6 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\times 13.6\times$</tex-math></inline-formula> 12.8 cm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{3}$</tex-math></inline-formula> insulated enclosure, making it advantageous for outdoor observations.

The Effects of Layer-Induced Elastic Anisotropy on Microseismic Monitoring Records in Underground Coal Mines

Real-time microseismic observations are extensively used in underground hard rock mines. However, microseismic monitoring in coal mines is not common. In this study, the features of microseismic (MS) event waveforms were discussed under conditions of thin-layered sedimentary media. The possibility of source mechanism evaluation was considered for the MS event recorded in coal mine. Moment tensor (MT) inversion of the waveforms can be challenging due to influence of thin-layered medium structure. Splitting of shear SH- and SV-waves is one of the characteristic features that affect MS records in coal mines. The effect of splitting was demonstrated using real microseismic data collected by an underground seismic array installed in the Polosukhinskaya coal mine, Russia. Velocities of shear SH- and SV-waves were estimated from travel times. The results reveal significant variations in shear wave velocities composed of a radial anisotropy of 75%, thus confirming the importance of a thin-layered model for moment tensor inversion. The thin-layered model was built for geological conditions at the Polosukhinskaya coal mine and synthetic seismograms were generated using QSEIS (Pyrocko OS). A full moment tensor of a single typical event was found employing the layered 1D velocity model.

Integrated analysis of cyclic steam stimulation in a heavy oil reservoir: Microseismicity, brittleness, and landing height

We compare microseismic observations against pumping information, landing heights, and various well logs. The data were acquired during cyclic-steam injection between September 2002 and December 2005. Ninety-five percent of the microseismicity occurred during injection and in the overburden; 70% of the events happened during the first cycle. Microseismicity in the overburden is likely caused by a greater brittleness than in the reservoir and a cluster of microseismic events in regions with a smaller landing height, thereby facilitating dry cracking due to the volumetric expansion of the reservoir. Yet, other areas with equally shallow landing heights displayed little to no microseismicity, pointing to an inhomogeneous steam front. Furthermore, recorded microseismicity is subject to the Kaiser effect in that event rates are low in subsequent cycles until the current injection pressure exceeds the previous maximum, explaining why 70% of the events occurred during the first cycle and possibly why microseismicity during production accounted for only 5%. Microseismicity in brittle formations can be caused by pore-pressure variations (wet cracking) and/or changes in the total stresses (dry cracking). Identification of pore-pressure variations in the overburden is important because it may indicate containment challenges. Analysis of the growth rate of the microseismic cloud combined with the shallow landing height indicated dry cracking to be more likely than wet cracking but analysis of additional data is required to strengthen this conclusion.

Study TOMAS Cyclone Using Seismic Array and Single Station

The continuous data from the YL array and four AU stations during the lifetime of the cyclone TOMAS in March 2010 were downloaded from IRIS. By performing frequency-wave number (F-K) analysis on the array data, it was found that the orientation of the maximum energy of the secondary microseisms (0.1~0.5 Hz) was consistent with the movement of TOMAS when the central wind speed reached the typhoon level. The high wind speed of the cyclone could generate secondary microseisms as well as the high swell. However, the large global earthquake can affect the microseismic observations using F-K. The AU stations have a better microseism observation than the YL array, which might be due to the vibrating and tilting of the hydrophone caused by the turbulence. The F-K analysis on microseisms can produce better slowness and back azimuth observations than polarization.

A Preliminary Earthquake Detection from The Seismic Temporary Network in The Opak Fault

Microseismic observation is a mandatory tool for analysing and imaging the progress of source earthquake mechanism. The observation of low-quality signal is usually found because of false detection, transient signals by natural noise, or related to seismometer condition and some human activities. Therefore, we try to figure the microseismic phenomenon after major earthquake in the Jogjakarta fault that was generated by an active fault namely Opak Fault. We used the seismic data recording from the seismic project that was installed in 2006 cover the Jogjakarta region. We used one week data to see the potential detection and highlight the possible of positive or negative false detection. With total 16 stations, we got ~500 events in only one week recording. We used a various threshold with 110 to scan all the dataset and we obtained ~450 events with 50-100 events with possible of false positives. Meanwhile, the threshold of 112 has < 50 events that could be suitable with the waveforms. The example results of 120 thresholds figure a strong event that is located by highly resolution of stack coherent from some stations with precise of P and S phase fitting. This study present the first automatic earthquake locations that can provide more detail of seismic structure information in the Jogjakarta region.

Nano-g Micro-Optics Accelerometer With Force Feedback Control and Improved Dynamic Range

Optical sensing techniques are well-known for electromagnetic immunity and thus have been widely considered in high-performance vibration sensors such as accelerometers and seismometers. In the progress of miniaturization, however, the force feedback control has rarely been developed due to associated complexity of the optical loops. Aiming to further explore the application of interferometry in geophysics, a force-rebalanced micro-optics accelerometer is developed in this work. This device operates relying on a grating-based MEMS interferometer, where a 3D sensing structure is microassembled to serve as the moveable reflector. In the meantime, a soft robber magnet is arranged on the proof mass and then the feedback mechanism is constructed in combination with a wound coil. The open-loop test indicates this feedback system can generate a drive force of 43.39 nN/mA, and possesses a resultant displacement-regulating capacity of 20.7 nm/mA. Implemented with a digital PI control, the closed-loop device demonstrates an improved dynamic range of 1.012 mg, equal to 10 times of the open-loop state, along with a scale factor of 3250 V/g and a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{R}^{{2}}$ </tex-math></inline-formula> coefficient of 0.99927. Additionally, the instrumental noise is tested as 10 ng/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sqrt {\text {Hz}}$ </tex-math></inline-formula> from 0.2 Hz to 10 Hz, and a bias stability of 36 ng is available at room temperature. Further considering the compact footprint of 4cm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times $ </tex-math></inline-formula> 6cm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times $ </tex-math></inline-formula> 3.15cm, this device exhibits great utility for microseismic observations in confined environments.

Development of a Hybrid Monitoring System for Micro-seismic Observation and Time-lapse Surveys at Geological CO2 Storage Site in Shallow Water Area

Development of a Hybrid Monitoring System for Micro-seismic Observation and Time-lapse Surveys at Geological CO2 Storage Site in Shallow Water Area

3D seismic interpretation and fault slip potential analysis from hydraulic fracturing in the Bowland Shale, UK

The Bowland Shale Formation is one of the most promising targets for unconventional exploration in the United Kingdom, with estimated resources large enough to supply the country's entire natural gas consumption for 50 years. However, development of the Bowland Shale has stalled due to concerns over hydraulic-fracturing-induced seismicity. Only three wells have been drilled and hydraulic-fractured to date in the Bowland Shale, and all three have produced levels of seismicity of sufficient magnitude to be felt at the surface. Susceptibility to induced seismicity will be determined by the presence of critically stressed faults. However, such faults can go undetected in conventional interpretation of 2D or 3D seismic surveys if they are shorter than the resolution retrievable from a seismic survey, or if they have low (and in some cases even zero) vertical displacement. In such cases, the faults that cause induced seismicity may only be visible via microseismic observations once they are reactivated. To better identify fault planes from 3D seismic images, and their reactivation potential due to hydraulic fracturing, a high-resolution fault-detection attribute was tested in a 3D seismic survey that was acquired over the Preston New Road site, where two shale-gas wells were hydraulic-fractured in the Bowland Shale in 2018 and 2019, obtaining fault planes with lengths between 400 and 1500 m. Fault slip potential was then estimated by integrating the obtained faults with the formation's stress and pore pressure conditions (with the Bowland shale also being significantly overpressured), and several critically stressed faults were identified near the previously hydraulic fractured wells. However, the faults that induced the largest seismic events in the Preston New Road site, of c. 200 m in length for seismic events of magnitudes below 3.0 (as imaged with a multicomponent, downhole microseismic monitoring array deployed during the hydraulic-fracturing stimulations), could not be identified in the 3D seismic survey, which only mapped fault planes larger than 400 m in length.

Real-time analysis and forecasting of the microseismic cloud size: Physics-based models versus machine learning

The spatiotemporal distribution of hydraulic fracturing microseismicity is complicated and depends on various mechanical and diffusional parameters. Hydraulic fracture modeling can aid in understanding fracture propagation and microseismicity. Nevertheless, the complex spatial and temporal interaction of several processes occurring within and around the fracture represents a challenge for developing real-time tools for microseismic prediction. Two approaches are developed to forecast the microseismic cloud size in real time. The first approach uses fracture propagation models to derive the cloud size directly from the microseismic observations. The second approach is based on a convolutional neural network (CNN) trained with the engineering parameters and past microseismic cloud size values. A rolling forecasting strategy is used to train consecutive CNN models in real time to make predictions at a specified time lag. A data augmentation technique known as double noise injection is used to ensure that the amount of training examples available to the machine-learning models at each time step is similar to or larger than the number of free parameters. The results indicate that the CNN outperforms the quality of predictions of the physics-based models but with a reduced prediction capability. The physics-based approach can predict growth at any time but ignores the engineering parameters. Conversely, physics-based methods lead to real-time insights into the fracturing regime, revealing whether microseismicity is most likely generated due to a leak-off-dominated or a storage-dominated regime. The CNN model can forecast the cloud size only at a single future time lag while using the engineering parameters and past cloud growth as input. However, this approach does not provide a physical interpretation of the fracture propagation regime. The prediction accuracy of both methodologies varies depending on the microseismic behavior. We postulate that CNN forecasts could be improved by including more physical constraints into the predictive model.

Near-field strain in distributed acoustic sensing-based microseismic observation

Microseismic monitoring with surface or downhole geophone arrays has commonly been used in tracking subsurface deformation and fracture networks during hydraulic fracturing operations. Recently, the use of fiber-optic distributed acoustic sensing (DAS) technology has improved microseismic acquisition to a new level with unprecedentedly high spatial resolution and low cost. Deploying fiber-optic cables in horizontal boreholes allows for very close observation of these microsize earthquakes and captures their full wavefield details. We have found that DAS-based microseismic profiles present a seldomly reported near-field strain signal between the P- and S-wave arrivals. This near-field signal indicates a monotonically increasing (or decreasing) temporal variation, which resembles the previously reported near-field observations of large earthquakes. To understand the near-field strain behavior, we use a mathematical expression of the analytic normal strain solution that reveals the near-field, intermediate-near-field, intermediate-far-field, and far-field components. Synthetic DAS strain records of hydraulic-fracture-induced microseismic events can be generated using this analytic solution with the Brune source model. The polarity sign patterns of the near-field and far-field terms in these synthetics are linked to the corresponding source mechanism’s radiation patterns. These polarity sign patterns are demonstrated to be sensitive to the source orientations by rotating the moment tensor in different directions. A field data example is compared to the synthetic result, and a qualitative match is shown. The microseismic near-field signals detected by DAS have potential value in hydraulic fracture monitoring by providing a means to better constrain microseismic source parameters that characterize the source magnitude, source orientation, and temporal source evolution and therefore better reflect the geomechanical response of the hydraulically fractured environment in the unconventional reservoirs.

Fault Triggering Mechanisms for Hydraulic Fracturing-Induced Seismicity From the Preston New Road, UK Case Study

We investigate the physical mechanisms governing the activation of faults during hydraulic fracturing. Recent studies have debated the varying importance of different fault reactivation mechanisms in different settings. Pore pressure increase caused by injection is generally considered to be the primary driver of induced seismicity. However, in very tight reservoir rocks, unless a fracture network exists to act as a hydraulic conduit, the rate of diffusion may be too low to explain the spatio-temporal evolution of some microseismic sequences. Thus, elastic and poroelastic stress transfer and aseismic slip have been invoked to explain observations of events occurring beyond the expected distance of a reasonable diffusive front. In this study we use the high quality microseismic data acquired during hydraulic fracturing at the Preston New Road (PNR) wells, Lancashire, UK, to examine fault triggering mechanisms. Injection through both wells generated felt induced seismicity—an ML 1.6 during PNR-1z injection in 2018 and an ML 2.9 during PNR-2 in 2019—and the microseismic observations show that each operation activated different faults with different orientations. Previous studies have already shown that PNR-1z seismicity was triggered by a combination of both direct hydraulic effects and elastic stress transfer generated by hydraulic fracture opening. Here we perform a similar analysis of the PNR-2 seismicity, finding that the PNR-2 fault triggering was mostly likely dominated by the diffusion of increased fluid pressure through a secondary zone of hydraulic fractures. However, elastic stress transfer caused by hydraulic fracture opening would have also acted to promote slip. It is significant that no microseismicity was observed on the previously activated fault during PNR-2 operations. This dataset therefore provides a unique opportunity to estimate the minimum perturbation required to activate the fault. As it appears that there was no hydraulic connection between them during each stimulation, any perturbation caused to the PNR-1z fault by PNR-2 stimulation must be through elastic or poroelastic stress transfer. As such, by computing the stress transfer created by PNR-2 stimulation onto the PNR-1z fault, we are able to approximate the minimum bound for the required stress perturbation: in excess of 0.1 MPa, orders of magnitude larger than stated estimates of a generalized triggering threshold.

Noise classification for the unified earthquake catalog using ensemble learning: the enhanced image of seismic activity along the Japan Trench by the S-net seafloor network

Homogeneous and accurate hypocenter distribution along the Japan Trench is important to better understand the process of plate subduction and the occurrence mode of large earthquakes. The seafloor seismic network (S-net) deployed recently along the Japan Trench has revealed new seismic activity including shallow slow earthquakes. However, conventional microseismic observations, such as those reported in the Japan Meteorological Agency (JMA) unified earthquake catalog, have been limited to land-based stations. Thus, steady seismic activity occurring at the shallow plate boundary, which is far from land is not always recorded. In the present study, we construct an automatic earthquake catalog using the nationwide observation network, including S-net. Because false detections caused by noise account for about 5% of automatically determined hypocenters, we attempted to reduce false detections using ensemble learning methods such as random forest and AdaBoost. First, we created a training dataset of earthquakes and noise by visual inspection based on the data recorded in the automatically determined catalog over a 2-month period, and we trained the dataset using the hypocenter and phase data as input. As a result of the training, AdaBoost was able to reduce the noise to about one-fifth of the total false detections, which is equivalent to 1%, while keeping the number of hypocenters above 99%. This method will contribute to significantly improving the efficiency of seismic activity monitoring and cataloging. In addition, the automatic denoised catalog data revealed that from January to August 2020, the completeness magnitude was M 1.6 along the Japan Trench. These microearthquakes were concentrated at depths of 20–50 km around the upper surface of the subducting Pacific Plate and are complementary to the slow earthquakes occurring at 10–20-km depths. Exceptionally, microearthquakes were observed off Iwate and Ibaraki prefectures, which correspond in location to areas of clustered foreshock activity. This spatial heterogeneity in microseismic activity is similar to the spatial complementary between the coseismic slip and afterslip of the 2011 Tohoku earthquake, which may be related to differences in interplate frictional properties and stress changes in the surrounding crust.

Developing Upscaling Approach for Swarming Hydraulic Fractures Observed at Hydraulic Fracturing Test Site through Multiscale Simulations

Summary This work aims to address a challenge posed by recent observations of tightly spaced hydraulic fractures in core samples from the hydraulic fracturing test site (HFTS) in the Middle Wolfcamp Formation. Many fractures in retrieved cores have subfoot spacing, which is at odds with conventional models in which usually one hydraulic fracture is initiated per cluster. Models assuming a single fracture at each cluster, although a common practice, often predict excessive fracture propagation that is inconsistent with microseismic observation. Here, we aim to develop a numerical approach to effectively account for densely spaced hydraulic fractures in field-scale simulations. Because it is impractical to explicitly model all aforementioned fractures, we develop a new upscaling law that enables existing simulation tools to predict reservoir response to fracture swarms. The upscaling law is derived based on an energy equivalence argument and validated through multiscale simulations using a high-fidelity code, GEOS. The swarming fractures are first modeled with a spacing that is much smaller than the cluster spacing; these fractures are then approximated by an upscaled, single fracture based on the proposed upscaling law. The upscaled fracture is shown to successfully match the energy input rate and produce the total fracture aperture and average propagation length of the explicitly simulated swarm. Afterward, the upscaling approach is further implemented in 3D field-scale simulations and validated against the HFTS microseismic data of a horizontal well. Our results show that hydraulic fracture swarming can significantly affect fracture propagation behaviors compared with the propagation of single fractures as assumed by conventional modeling approaches. Under the considered situations, the conventional treatment yields fast propagation speed that far exceeds that indicated by the microseismic data. We also illustrate that this discrepancy can be reduced readily through the implementation of the upscaling law. Our results demonstrate the importance of accounting for the fracture swarming effect in field-scale simulations and the efficacy of this approach to enable realistic predictions of reservoir responses to fracture swarms, without the need to model tightly spaced fractures individually.

СУЧАСНИЙ СТАН ЗАСТОСУВАННЯ МЕТОДІВ МІКРОСЕЙСМІЧНОГО КОНТРОЛЮ ЗА ПРОВЕДЕННЯМ ТА ЯКІСТЮ ГРП

The article is devoted to the review of processing and interpretation of microseismic fracturing monitoring. The need to cover the works related to the intensification of production is caused by the underdeveloped practice of such research in Ukraine. The article describes the world experience of microseismic observations and outlines their prospects on the territory of Ukraine. Considerable attention is paid to the processing and interpretation of fracturing microseismic monitoring data. The choice of technologies is associated with the correct consideration of the geological structure of the prospective area or hydrocarbon field, as well as the previous stages of development of oil and gas fields. The ways of using the world experience of microseismic monitoring of hydraulic fracturing in the conditions of oil and gas provinces of Ukraine are determined.

High-Resolution Imaging of the ML 2.9 August 2019 Earthquake in Lancashire, United Kingdom, Induced by Hydraulic Fracturing during Preston New Road PNR-2 Operations

AbstractHydraulic fracturing (HF) at Preston New Road (PNR), Lancashire, United Kingdom, in August 2019, induced a number of felt earthquakes. The largest event (ML 2.9) occurred on 26 August 2019, approximately three days after HF operations at the site had stopped. Following this, in November 2019, the United Kingdom Government announced a moratorium on HF for shale gas in England. Here we provide an analysis of the microseismic observations made during this case of HF-induced fault activation. More than 55,000 microseismic events were detected during operations using a downhole array, the vast majority measuring less than Mw 0. Event locations revealed the growth of hydraulic fractures and their interaction with several preexisting structures. The spatiotemporal distribution of events suggests that a hydraulic pathway was created between the injection points and a nearby northwest–southeast-striking fault, on which the largest events occurred. The aftershocks of the ML 2.9 event clearly delineate the rupture plane, with their spatial distribution forming a halo of activity around the mainshock rupture area. Across clusters of events, the magnitude distributions are distinctly bimodal, with a lower Gutenberg–Richter b-value for events above Mw 0, suggesting a break in scaling between events associated with hydraulic fracture propagation, and events associated with activation of the fault. This poses a challenge for mitigation strategies that rely on extrapolating microseismicity observed during injection to forecast future behavior. The activated fault was well oriented for failure in the regional stress field, significantly more so than the fault activated during previous operations at PNR in 2018. The differing orientations within the stress field likely explain why this PNR-2 fault produced larger events compared with the 2018 sequence, despite receiving a smaller volume of injected fluid. This indicates that fault orientation and in situ stress conditions play a key role in controlling the severity of seismicity induced by HF.

Locating passive sources on seismic reflection image in the presence of velocity errors

Locating passive-source positions by using microseismic events is essential for monitoring hydraulic fractures. Among all microseismic source locating approaches, time-reversal imaging (TRI) is a promising one that is based on the principle that all of the back-propagated receiver wavefields should coincide at the source position when the velocity is accurate. It can image microseismic sources by applying a focusing imaging condition to the reconstructed receiver wavefields. However, the TRI method is highly sensitive to velocity errors and it is time-consuming or even challenging to refine the velocity model when the subsurface structure is complex. Instead of updating the velocity model, we have adopted a new method to locate microseismic events on a seismic reflection image under the condition that the seismic data are colocated with surface or near-surface microseismic observations. This method does not correctly place the microseismic events in depth; rather, it makes them consistent with the seismic reflection image; thus, it is still capable of providing correct local structure information around the passive source without the need of building an accurate velocity model. We have theoretically analyzed the variation of the imaged source location when the velocity model is inaccurate. Our result shows that faster velocities cause shallower depths whereas slower velocities cause deeper depths of source locations on the image. This is similar to the behavior of the focusing depth, but it is opposite to that of the migration depth in active-source seismic reflection imaging. Then, we match the locations of microseismic events with the well-focused seismic reflection image, which is extracted by slicing the time-shift common-image gathers at the time lag where the image has the maximal energy. Finally, synthetic tests validate the effectiveness of the proposed approach.

Model for Asymmetric Hydraulic Fractures with Nonuniform-Stress Distribution

Summary Engineers commonly expect symmetric fracture wings in multiple-transverse-fracture horizontal wells. Microseismic surveys have shown that asymmetric hydraulic fractures grow away from the recent fractured wells and grow toward previously produced wells. This might be caused by the elevated stress around the recently fractured well and the reduced stress near the depleted wells. This paper presents the asymmetric fracture growth observed by the microseismic events, develops a simple model to simulate the fracture propagation, and discusses its effect on the well productivity. Motivated by the microseismic observations, we developed a simple 2D fracture model to simulate asymmetric fracture wings that can capture the behavior of fracture hits between two adjacent horizontal fractured wells. Fluid leakoff during fracture propagation is considered in the model. The effect of asymmetric fractures on production is evaluated with numerical simulations. The newly developed fracture model shows that the fracture can grow asymmetrically if the horizontal well is near where the stress field is different between its two sides. Numerical simulation is used to quantify the productivity reduction caused by asymmetric hydraulic fractures. Our results provide a reason for why asymmetric fractures occur and demonstrate that they do penalize well performance. Our model suggests the importance of fracturing under a balanced-stress distribution that benefits long-term production. Use of this model also suggested that an optimized hydraulic-fracturing-treatment design will improve the overall performance of multiple parallel wells, which minimizes or avoids asymmetric fracture wings. The fracture-propagation model and productivity model provide simple but profound guidelines for well-pad management, including well spacing, stage planning and spacing, and completion and production order.

Focal-time analysis: A new method for stratigraphic depth control of microseismicity and induced seismic events

Focal-time analysis is a straightforward data-driven method to obtain robust stratigraphic depth control for microseismicity or induced seismic events. The method eliminates the necessity to build an explicit, calibrated velocity model for hypocenter depth estimation, although it requires multicomponent 3D seismic data that are colocated with surface or near-surface microseismic observations. Event focal depths are initially expressed in terms of zero-offset focal time (two-way P-P reflection time) to facilitate registration and visualization with 3D seismic data. Application of the focal-time method requires (1) high-quality P- and S-wave time picks, which are extrapolated to zero offset and (2) registration of correlative P-P and P-S reflections to provide [Formula: see text] and [Formula: see text] time-depth control. We determine the utility of this method by applying it to a microseismic and induced-seismicity data set recorded with a shallow-borehole monitoring array in Alberta, Canada, combined with high-quality multicomponent surface seismic data. The calculated depth distribution of events is in good agreement with hypocenter locations obtained independently using a nonlinear global-search method. Our results reveal that individual event clusters have distinct depth distributions that can provide important clues about the mechanisms of fault activation.

Direct and Microseismic Observations of Hydraulic Fracturing in Barre Granite and Opalinus Clayshale

AbstractWhile hydraulic fracturing is a well‐established and widely implemented process, there are still some aspects that can be better understood. Specifically, researchers do not fully understand the relation between the hydraulic fracture network and the microseismic events induced during hydraulic stimulation. In this laboratory study, we address this knowledge gap by conducting hydraulic fracturing experiments where we directly observe microstructural changes using a high‐speed camera attached to a 5X magnification lens and analyze the images using a digital image correlation code. These data are compared to simultaneously acquired acoustic emissions, from which we infer the location and focal mechanisms of induced microseismic events. Experiments were conducted on Opalinus clayshale and Barre granite, at injection rates of 0.019 and 0.39 ml/s for each rock. The simultaneous recording of microseismic and visual observations is unique and provides significant insight into the details of hydraulic fracturing. Our results show several interesting differences between granite and shale, and between injection rates. For example, we find that while the creation of hydraulic fractures is generally considered as tensile at the field scale, evidence of shearing can be observed to varying degrees at the microstructural scale. Specifically, we see that microstructural shearing is more evident in granite in the form of distinct en echelon microcracks. On the effect of the injection rate, we find that a reduced injection rate tends to create a more complex network of microcracks, along with a lower proportion of double‐couple (shear) focal mechanisms in the microseismic data.

Real‐Time Imaging, Forecasting, and Management of Human‐Induced Seismicity at Preston New Road, Lancashire, England

ABSTRACTEarthquakes induced by subsurface fluid injection pose a significant issue across a range of industries. Debate continues as to the most effective methods to mitigate the resulting seismic hazard. Observations of induced seismicity indicate that the rate of seismicity scales with the injection volume and that events follow the Gutenberg–Richter distribution. These two inferences permit us to populate statistical models of the seismicity and extrapolate them to make forecasts of the expected event magnitudes as injection continues. Here, we describe a shale gas site where this approach was used in real time to make operational decisions during hydraulic fracturing operations.Microseismic observations revealed the intersection between hydraulic fracturing and a pre‐existing fault or fracture network that became seismically active. Although “red light” events, requiring a pause to the injection program, occurred on several occasions, the observed event magnitudes fell within expected levels based on the extrapolated statistical models, and the levels of seismicity remained within acceptable limits as defined by the regulator. To date, induced seismicity has typically been regulated using retroactive traffic light schemes. This study shows that the use of high‐quality microseismic observations to populate statistical models that forecast expected event magnitudes can provide a more effective approach.