Results of the downhole microseismic monitoring at a pilot hydraulic fracturing site in Poland — Part 1: Event location and stimulation performance

2018 ◽  
Vol 6 (3) ◽  
pp. SH39-SH48 ◽  
Author(s):  
Wojciech Gajek ◽  
Jacek Trojanowski ◽  
Michał Malinowski ◽  
Marek Jarosiński ◽  
Marko Riedel
Keyword(s):  
Hydraulic Fracturing ◽  
Seismic Anisotropy ◽  
Transverse Isotropy ◽  
Velocity Model ◽  
Microseismic Monitoring ◽  
Baltic Basin ◽  
Hydraulic Stimulation ◽  
Lower Paleozoic ◽  
Event Location ◽  
Microseismic Events

A precise velocity model is necessary to obtain reliable locations of microseismic events, which provide information about the effectiveness of the hydraulic stimulation. Seismic anisotropy plays an important role in microseismic event location by imposing the dependency between wave velocities and its propagation direction. Building an anisotropic velocity model that accounts for that effect allows for more accurate location of microseismic events. We have used downhole microseismic records from a pilot hydraulic fracturing experiment in Lower-Paleozoic shale gas play in the Baltic Basin, Northern Poland, to obtain accurate microseismic events locations. We have developed a workflow for a vertical transverse isotropy velocity model construction when facing a challenging absence of horizontally polarized S-waves in perforation shot data, which carry information about Thomsen’s [Formula: see text] parameter and provide valuable constraints for locating microseismic events. We extract effective [Formula: see text], [Formula: see text] and [Formula: see text], [Formula: see text] for each layer from the P- and SV-wave arrivals of perforation shots, whereas the unresolved [Formula: see text] is retrieved afterward from the SH-SV-wave delay time of selected microseismic events. An inverted velocity model provides more reliable location of microseismic events, which then becomes an essential input for evaluating the hydraulic stimulation job effectiveness in the geomechanical context. We evaluate the influence of the preexisting fracture sets and obliquity between the borehole trajectory and principal horizontal stress direction on the hydraulic treatment performance. The fracturing fluid migrates to previously fractured zones, while the growth of the microseismic volume in consecutive stages is caused by increased penetration of the above-lying lithologic formations.

Microseismic Monitoring of Hydraulic Fracturing – Data Interpretation Methodology with an Example from Pomerania

2017 ◽  
Author(s):  
Michał Antoszkiewicz ◽  
Mateusz Kmieć ◽  
Paweł Szewczuk ◽  
Marek Szkodo ◽  
Robert Jankowski
Keyword(s):  
Hydraulic Fracturing ◽  
Signal To Noise Ratio ◽  
Velocity Model ◽  
Data Interpretation ◽  
Microseismic Monitoring ◽  
Grid Search ◽  
Moment Tensors ◽  
Spatial Image ◽  
Microseismic Events ◽  
Breakage Mechanism

Microseismic monitoring is a method for localizing fractures induced by hydraulic fracturing in search for shale gas. The aim of this paper is to conduct the data interpretation of the microseismic monitoring based on the results from Pom-erania region of Poland. The data has been collected from an array of geophones deployed on the surface. Ground vibrations have been recorded and analyzed for fracture location, magnitude and breakage mechanism. A velocity model of underlying formations has been used for successful microseismic monitoring. The model has been further tuned with signal from perfora-tion shots of known location. Imaging of events has been done using software MicSeis, which utilizes diffraction stacking of waveforms from multiple stations to image microseismic events with low signal-to-noise ratio. The imaging of microseismic events in MicSeis uses a grid search over all possible origin times and locations in the selected rock volume. The seismic moment tensors are automatically determined from the amplitudes from the grid search procedure and are used to model po-larities of events which then enhance constructive interference. Function characterizing a maximum stack per time sample have been calculated over whole volume and analyzed using the STA/LTA algorithm. Once the event has been detected in time, location has been determined through analysis of the 3D spatial image function. The procedure has been used to detect five events during hydraulic fracturing in Pomerania.

Transdimensional simultaneous inversion of velocity structure and event locations in downhole microseismic monitoring

Geophysics ◽  
2021 ◽  
pp. 1-92
Author(s):  
Xingda Jiang ◽  
Wei Zhang ◽  
Hui Yang ◽  
Chaofeng Zhao ◽  
Zixuan Wang
Keyword(s):  
Hydraulic Fracturing ◽  
Velocity Structure ◽  
Velocity Model ◽  
Microseismic Monitoring ◽  
Location Accuracy ◽  
Simultaneous Inversion ◽  
Velocity Models ◽  
Effective Velocity ◽  
Number Of Layers ◽  
Event Location

In downhole microseismic monitoring, the velocity model plays a vital role in accurate mapping of the hydraulic fracturing image. For velocity model uncertainties in the number of layers or interface depths, the conventional velocity calibration method has been shown to effectively locate the perforation shots; however, it introduces non-negligible location errors for microseismic events, especially for complex geological formations with inclinations. To improve the event location accuracy, we exploit the advantages of the reversible jump Markov chain Monte Carlo (rjMCMC) approach in generating different dimensions of velocity models and propose a transdimensional Bayesian simultaneous inversion framework for obtaining the effective velocity structure and event locations simultaneously. The transdimensional inversion changes the number of layers during the inversion process and selects the optimal interface depths and velocity values to improve the event location accuracy. The confidence intervals of the simultaneous inversion event locations estimated by Bayesian inference enable us to evaluate the location uncertainties in the horizontal and vertical directions. Two synthetic examples and a field test are presented to illustrate the performance of our methodology, and the event location accuracy is shown to be higher than that obtained using the conventional methods. With less dependence on prior information, the proposed transdimensional simultaneous inversion method can be used to obtain an effective velocity structure for facilitating highly accurate hydraulic fracturing mapping.

scholarly journals Tomographic Experiments for Defining the 3D Velocity Model of an Unstable Rock Slope to Support Microseismic Event Interpretation

Geosciences ◽  
2020 ◽  
Vol 10 (9) ◽  
pp. 327
Author(s):  
Zhiyong Zhang ◽  
Diego Arosio ◽  
Azadeh Hojat ◽  
Luigi Zanzi
Keyword(s):  
Rock Mass ◽  
Velocity Distribution ◽  
Velocity Model ◽  
Microseismic Monitoring ◽  
Time Inversion ◽  
Basic Step ◽  
Rock Face ◽  
3D Velocity Model ◽  
Microseismic Events ◽  
The Stability

To monitor the stability of a mountain slope in northern Italy, microseismic monitoring technique has been used since 2013. Locating microseismic events is a basic step of this technique. We performed a seismic tomographic survey on the mountain surface above the rock face to obtain a reliable velocity distribution in the rock mass for the localization procedure. Seismic travel-time inversion showed high heterogeneity of the rock mass with strong contrast in velocity distribution. Low velocities were found at shallow depth on the top of the rock cliff and intermediate velocities were observed in the most critical area of the rock face corresponding to a partially detached pillar. Using the 3D velocity model obtained from inversion, localization tests were performed based on the Equal Differential Time (EDT) localization method. The results showed hypocenter misfits to be around 15 m for the five geophones of the microseismic network and the error was significantly decreased compared to the results produced by a constant velocity model. Although the localization errors are relatively large, the accuracy is sufficient to distinguish microseismic events occurring in the most critical zone of the monitored rock mass from microseismic events generated far away. Thus, the 3D velocity model will be used in future studies to improve the classification of the recorded events.

Locating microseismic events using both head and direct waves traveling in vertical transverse isotropic media

2020 ◽  
Author(s):  
Dowan Kim ◽  
Joongmoo Byun ◽  
Soon Jee Seol
Keyword(s):  
Transverse Isotropy ◽  
Microseismic Monitoring ◽  
Head Wave ◽  
Hydraulic Fractures ◽  
Path Diversity ◽  
Isotropic Media ◽  
Transverse Isotropic ◽  
Microseismic Events ◽  
Vertical Transverse Isotropy ◽  
The Waves

Summary Microseismic monitoring is widely used to detect hydraulic fractures. Accurate mapping of microseismic events is essential to detect such fractures enhancing productivity. The eikonal solver is an efficient forward-modeling method used to map microseismic events. However, traditional eikonal solvers do not distinguish between head and direct waves, computing only the traveltimes of the waves that arrive first. We developed a new eikonal solver that computes the traveltimes of direct waves by imposing new constraints on the conventional, vertical transverse isotropy (VTI) solver. We then performed numerical experiments exploiting the traveltimes of direct waves. We used the traveltimes of only the first arrivals, and those of both first and direct arrivals, when performing inverted event mapping. The results showed that the uncertainties of event locations were minimized when both head and direct waves were analyzed due to the increased both the number of available data and the traveling path diversity. Also, we found that the use of only direct-arrival traveltimes was valuable when head-wave first arrivals were difficult to detect because the signal-to-noise (S/N) ratio was low.

scholarly journals Results of downhole microseismic monitoring at a pilot hydraulic fracturing site in Poland — Part 2: S-wave splitting analysis

2018 ◽  
Vol 6 (3) ◽  
pp. SH49-SH58 ◽  
Author(s):  
Wojciech Gajek ◽  
Michał Malinowski ◽  
James P. Verdon
Keyword(s):  
Hydraulic Fracturing ◽  
Seismic Anisotropy ◽  
Transverse Isotropy ◽  
Geophysical Methods ◽  
Rock Physics ◽  
Azimuthal Anisotropy ◽  
Natural Fracture ◽  
S Wave ◽  
Model Inversion ◽  
Wave Splitting

Observations of azimuthal seismic anisotropy provide useful information, notably on stress orientation and the presence of preexisting natural fracture systems, during hydraulic fracturing operations. Seismic anisotropy can be observed through the measurement of S-wave splitting (SWS) on waveforms generated by microseismic events and recorded on downhole geophone arrays. We have developed measurements of azimuthal anisotropy from a Lower Paleozoic shale play in northern Poland. The observed orthorhombic anisotropic symmetry system is dominated by a vertically transverse isotropy (VTI) fabric, produced by the alignment of anisotropic platy clay minerals and by thin horizontal layering and overprinted by a weak azimuthal anisotropy. Despite the dominating VTI fabric, we successfully identified a weaker horizontal-transverse isotropy fabric striking east–southeast. We do this by constraining the rock-physics model inversion with VTI background parameters incorporated from other geophysical methods: microseismic velocity model inversion, 3D reflection seismic, and borehole cross-dipole sonic logs. The obtained orientation is consistent with a preexisting natural fracture set that has been observed using X-ray micro-imaging (XRMI) image logs from a nearby vertical well. The present-day regional maximum horizontal stress direction differs from the observed fracture strike by approximately 45°. This implies that the SWS measurements recorded during hydraulic stimulation of a shale-gas reservoir are imaging the preexisting natural fracture set, which influences the treatment efficiency, instead of the present-day stress.

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