A Case Study Using Integrated Multi Disciplinary Approach to Model Microseismic Events During Stimulation

For tight reservoirs where hydraulic fracturing is required to enable sufficient fluid mobility for economic production, it is critical to understand the placement of induced fractures, their connectivity, extent, and interaction with natural fractures within the system. Hydraulic fracture initiation and propagation mechanisms are greatly influenced by the effect of the stress state, rock fabric and pre-existing features (e.g. natural fractures, faults, weak bedding/laminations). A pre-existing natural fracture system can dictate the mode, orientation and size of the hydraulic fracture network. A better understanding of the fracture growth phenomena will enhance productivity and also reduce the environmental footprint as less fractures can be created in a much more efficient way. Assessing the role of natural fractures and their interaction with hydraulic fractures in order to account for them in the hydraulic fracture model is achieved by leveraging microseismicity. In this study, we have used a combination of borehole and surface microseismic monitoring to get high vertical resolution locations and source mechanisms. 3D numerical modelling of hydraulic fracturing in complex geological conditions to predict fracture propagation is essential. 3D hydraulic fracturing simulation includes modelling capabilities of stimulation parameters, true 3D fracture propagation with near wellbore 3D complexity including a coupled DFN and the associated microseismic event generation capability. A 3D hydraulic fracture model was developed and validated by matching model predictions to microseismic observations. Microseismic source mechanisms are leveraged to determine the location and geometry of pre-existing features. In this study, we simulate a DFN based on the recorded seismicity of multi stage hydraulic fractures in a horizontal well. The advanced 3D hydraulic fracture modelling software can integrate effectively and efficiently data from a variety of multi-disciplinary sources and scales to create a subsurface characterization of the unconventional reservoir. By incorporating data from 3D seismic, LWD/wireline, core, completion/stimulation monitoring, and production, the software generates a holistic reservoir model embedded in a modular, multi-physics software platform of coupled numerical solvers that capture the fundamental physics of the processes being modelled. This study illustrates the importance of a powerful software tool that captures the necessary physics of stimulation to predict the effects of various completion designs and thereby ensure the most accurate representation of an unconventional reservoir response to a stimulation treatment.

The Characteristics of Seismic Rotations in VTI Medium

Under the assumptions of linear elasticity and small deformation in traditional elastodynamics, the anisotropy of the medium has a significant effect on rotations observed during earthquakes. Based on the basic theory of the first-order velocity-stress elastic wave equation, this paper simulates the seismic wave propagation of the translational and rotational motions in two-dimensional isotropic and VTI (transverse isotropic media with a vertical axis of symmetry) media under different source mechanisms with the staggered-grid finite-difference method with respect to nine different seismological models. Through comparing the similarities and differences between the translational and rotational components of the wave fields, this paper focuses on the influence of anisotropic parameters on the amplitude and phase characteristics of the rotations. We verify that the energy of S waves in the rotational components is significantly stronger than that of P waves, and the response of rotations to the anisotropic parameters is more sensitive. There is more abundant information in the high-frequency band of the rotational components. With the increase of Thomsen anisotropic parameters ε and δ, the energy of the rotations increases gradually, which means that the rotational component observation may be helpful to the study of anisotropic parameters.

Persistent shallow micro-seismicity at Llaima volcano, Chile, with implications for long-term monitoring

Identifying the source mechanisms of low-frequency earthquakes at ice-covered volcanoes can be challenging due to overlapping characteristics of glacially and magmatically derived seismicity. Here we present an analysis of two months of seismic data from Llaima volcano, Chile, recorded by the permanent monitoring network in 2019. We find over 2,000 repeating low-frequency events split across 82 families, the largest of which contains over 200 events. Estimated locations for the largest families indicate shallow sources directly beneath or near the edge of glaciers around the summit vent. These low-frequency earthquakes are part of an annual cycle in activity at the volcano that is strongly correlated with variations in atmospheric temperature, leading us to conclude that meltwater from ice and snow strongly affects the seismic source mechanisms which is likely dominated by basal slip beneath the glaciers. The results presented here should inform future assessments of eruptive potential at Llaima volcano, as well as other ice-covered volcanoes in Chile and worldwide.

Investigating exospheric Na distributions at different spatial scales to disentangle between single and double peaked global patterns

<p>Since mid ‘80s the Na exosphere of Mercury has been investigated by means of both ground-based observations and spacecraft measurements, showing a wide range of variability from tens of minutes up to seasonal variations along the planetary orbit. It has been shown that the most common Na distribution is characterized by a high latitude double peak probably related to solar wind ion precipitation through the polar cusps. However, the existence of a single peaked equatorial Na emission has been frequently observed too. Generally, it is not straightforward to recognize the contributions due to different surface release processes that produces the observed Na exospheric global image.</p> <p>Here we apply the Multivariate Empirical Mode Decomposition (MEMD) to a dataset of images of the exospheric Na emission collected by the THEMIS ground-based telescope with the goal to disentangle the different contributions operating at different scales that are expected to be responsible of the occurrence of single vs. double peaked emissions or exospheric asymmetries. In particular, we found the existence of a wide range of scales characterizing both type of spatial patterns, ranging from small scales (less than 0.5 Mercury radii) up to large scales (about 1-2 Mercury radii). These scale-dependent patterns can be linked to different source mechanisms as the variability of solar wind magnetic field, different surface release mechanisms (thermal desorption, photon-stimulated desorption, micrometeoroid impact vaporization and ion-sputtering), as well as, to the whole Na exosphere surrounding the Hermean environment. Our conclusions are double checked by applying the MEMD both on Na exospheric measurements and on simulations of the Na exosphere as created by the different source mechanisms. The positive results show the great potential of the MEMD technique to study the complex environment of planetary exospheres and recognize the different components/processes that create it.</p>

Focal Mechanisms of West Bohemia, Central Europe, Earthquakes—End of May 2014: Evidence of Volume Changes

West Bohemia is a region with a lot of mineral springs and gas outflows, which seems to be related to the remains of Quaternary volcanism in Central Europe. Earthquake swarms in shallow depths (less than 15 km) are very frequent there. We focused on the strongest earthquake over the past 30 yr (31 May, 2014 Mw∼3.8) and on two smaller ones (Mw∼2.9 and 2.5) from the same day. Seismograms from local and regional seismic stations were used to calculate the full and deviatoric moment tensors using low-frequency full-waveform inversion. The studied events have similar source mechanisms. The aforementioned earthquake sequence was selected to observe the isotropic part (negative value = implosion) of full moment tensors. It could relate to the motion and phase transition of fluids, especially water, and CO2. The main goal of this study is to contribute to clarification of the nature of earthquake swarms in the western edge of the Bohemian Massif. Negative value of the isotropic part of full moment tensor could be related to the closing of cracks and fissures during a rupture process.