Study on Dynamic Behavior Law and Microseismic Monitoring in Stoping Process of Roadway with High Gas and Wide Coal Pillar

In order to study the dynamic characteristics and microseismic distribution in the mining process of roadway with high gas and wide coal pillar, combined with the two dynamic events of N2105 working face in Yuwu Coal Industry, theoretical analysis and field measurement research were carried out. According to the theory of structural mechanics and geomechanics, the causes of dynamic appearance are analyzed. Combined with the specific situation, the influence of mining depth, coal pillar width, gas pressure, and content on the dynamic performance is analyzed. Stress monitoring and microseismic monitoring are carried out on one side of coal seam. The results show that, with the increase of the mining distance, the backside roof of the goaf is prone to unbalanced fracture due to the lack of lateral stress, and the impact pressure generated is used for the reserved protective coal pillar behind the goaf, causing the floor heave of coal seam. The combined stress generated by the anticlinal structure below the working face interacts with the abutment pressure of the working face to produce superposition effect, which promotes the occurrence of dynamic appearance. The critical depth of rock burst in Yuwu Coal Industry is about 600m. The increase of coal elastic energy caused by roof subsidence is more uniform with the increase of coal pillar width. The decrease of gas pressure in coal seam promotes the rock burst disaster. The vertical stress of coal seam at one side of the working face shows different evolution characteristics along the trend and strike. The vertical stress of coal seam in the lateral range of 53 m is adjusted to different degrees and tends to be stable until 300 m behind the working face. The active microseismic area in the middle of the working face was located 50 m in front of the working face, and the microseismic activity continued to 30–50 m behind the working face. The active microseismic area at the side of the roadway was located 30 m in front of the working face, and the microseismic activity continued to 100–180 m behind the working face. The inflection point, where the stress in the elastic area of coal pillar increases sharply, corresponds to the active microseismic area, which indicates that the dynamic characteristics in the mining process of roadway with high gas and wide coal pillar are related to the distribution law of microseismic. This study has a certain guiding significance for optimizing the width of reserved coal pillar, monitoring the coal seam stress/microseismic, and understanding the dynamic disaster of coal and rock under complex conditions.

Monitoring for seismological and geochemical groundwater effects of high-volume pumping of natural gas at the Stenlille underground gas storage facility, Denmark

The large natural gas storage facility at Stenlille, Denmark, has been monitored to investigate the effect of pumping large amounts of gas into the subsurface. Here, we present a new dataset of microseismicity at Stenlille since 2018. We compare these data with methane in groundwater, which has been monitored since gas storage was established in 1989. Further, we conducted a controlled 172 day microcosm experiment of methane oxidation on an isolated microbial community under both aerobic and anaerobic conditions. For this experiment, water was filtered from a well at Stenlille with elevated levels of thermogenic methane and ethane. No microseismic activity was detected in the gas storage area above an estimated detection level of ML 0.0 for the established network. The long-term monitoring for methane in groundwater has still only detected one leak, in 1995, related to a technical problem during injection. The microcosm experiment revealed that oxidation of methane occurred only under aerobic conditions during the experiment, as compared to anaerobic conditions, even though the filtered water was anoxic

Aseismic strain episodes at Campi Flegrei Caldera, Italy

Since 2004 a research project has been developed to monitor subsurface deformation of Italian volcanoes using borehole strainmeters and long-baseline tiltmeters. Six Sacks-Evertson dilatometers were installed around Campi Flegrei caldera and Vesuvius during 2004–2005 (Scarpa et al., 2007), and in 2008 these instruments were supplemented by two arrays of 28–280 m long water-tube tiltmeters in underground tunnels. Relevant strainmeter and tiltmeter data have been collected and analysed from the instruments installed near Campi Flegrei caldera during the recent unrest episodes. In the period 2004–2005 strain, tilt and GPS data from Campi Flegrei indicate the onset of surface deformation that accompanied a low rate of vertical displacement that continued to 2006, corresponding to an increase of CO2 emission. This strain episode preceded caldera microseismic activity by a few months, as was observed also during a significant inflation episode in 1982. Other transient strain episodes occurred in October 2006, which were accompanied by a swarm of VT (Volcano-Tectonic) and LP (Long Period) events, in 2009, at the time of renewed gas emission activity at Solfatara, and again in March 2010, several minutes before a seismic swarm. The time scale of these transient strain events ranges from some hours to several days, putting tight constraints on the origin of ground uplifts at Campi Flegrei. Their location is compatible with a source inferred from long term deformation signals, located about 4 km beneath Pozzuoli. A proposed mechanism for these aseismic strain episodes is that they are associated with magma growth in reservoirs with occasional pressure relief associated with the leakage of gas.

Seismic Magnitudes, Corner Frequencies, and Microseismicity: Using Ambient Noise to Correct for High-Frequency Attenuation

ABSTRACT Over recent years, a greater importance has been attached to low-magnitude events, with increasing use of the subsurface for industrial activities such as hydraulic fracturing and enhanced geothermal schemes. Magnitude distributions and earthquake source properties are critical inputs when managing the associated seismic risk of these activities, yet inconsistencies and discrepancies are commonly observed with microseismic activity (M<2). This, in part, is due to their impulse response being controlled by the medium, as opposed to the source. Here, an approach for estimating the high-frequency amplitude decay parameter from the spectral decay of ambient seismic noise (κ0_noise) is developed. The estimate does not require a pre-existing seismic catalog and is independent of the source properties, so avoids some of the main limitations of earthquake-based methods. We then incorporate κ0_noise into the Brune (1970) source model and calculate source properties and magnitude relationships for coal-mining-related microseismic events, recorded near New Ollerton, United Kingdom. This generates rupture radii ranging approximately between 10 and 100 m, which agrees with the findings of Verdon et al. (2018), and results in stress-drop values between 0.1 and 10 MPa. Calculating these properties without κ0_noise produces much higher rupture radii of between 100 and 500 m and significantly lower stress drops (∼1×10−2 MPa). Finally, we find that the combined κ0-Brune model parameterized with these source property estimates accurately capture the ML–Mw relationship at New Ollerton, and that stress drop heavily influences the gradient of this relationship.

Diffusivity-dependent fracturing processes and microseismic activity in granite

Enhanced Geothermal Systems have relied on hydraulic fracturing to increase the permeability of rock reservoirs. The permeability enhancement depends on the connectivity between new and existing fractures. This, in turn, depends to a large extent on the interaction between the rock and the fracturing fluid, which not only pressurizes existing and new fractures but also diffuses into the rock matrix. In this research, the effect of the diffusivity of hydraulic oil on the fracturing processes and microseismicity of unconfined prismatic granite specimens was experimentally evaluated using visual and acoustic emission monitoring. The tests consisted of injecting hydraulic oil into two pre-fabricated flaws at two rates (2 ml/min and 20 ml/min), kept constant in each test. The fluid pressure inside the flaws was increased until hydraulic fractures propagated and the fluid front growing from the pre-fabricated flaws was visually monitored throughout the tests. It was observed that the fracturing pressures and patterns were injection-rate-dependent, which shows that diffusivity and poro-elastic effects play an important role in the hydraulic fracturing processes of granite. A smaller fluid front was observed for the 20 ml/min injection rate, associated to a lower volume injected and to a higher fracturing pressure when compared to the 2 ml/min injection rate. This was interpreted to be caused by the different pore pressures that developed inside of the rock matrix, which are function of the fluid front size. Microseismic activity was observed throughout the tests, becoming more intense and localized near the flaws as one approached the end of the test (i.e. visible crack propagation). While microseismic events were observed outside the fluid front region, their density was significantly larger within this area, showing that fluid diffusivity may contribute to an intensification of the microseismic activity.

The Impact of USArray on Earthquake Monitoring in Alaska

Seismic network coverage in Alaska has fundamentally changed with the presence of the USArray Transportable Array (TA) stations. These new stations provided an unprecedented opportunity to expand earthquake reporting in areas of Alaska that have not previously been instrumented. The Alaska Earthquake Center (AEC) has been incorporating all TA data into its standard earthquake analysis. The TA network is currently the second largest contributor of phase picks in the Alaska earthquake catalog, after the AK network operated by the AEC. Recent increases in reported earthquakes (about 45,000 in 2017 and 55,000 in 2018) are directly attributable to the additional TA stations, especially in the northern and western mainland Alaska. In some regions, the earthquake detection threshold decreased by as much as two units of magnitude. With the TA installation complete in 2017, the detection threshold over the entire mainland Alaska region is M∼1.5. The new stations have also led to a decrease in hypocentral location errors, which are now more uniform over the entire mainland Alaska region. The uniformity of the TA network made it possible, for the first time, to make quantitatively valid comparisons of microseismic activity in different parts of the state. Among other observations, this uniform coverage helped demonstrate that the quiescence that has long been inferred in the central and western Arctic Slope region appears to be real, and not just an artifact of network coverage. This combined network should, with time, provide vastly better data for seismic hazard assessments in an area of increasing national interest.

Inferring microseismic source mechanisms and in situ stresses during triaxial deformation of a North-Sea-analogue sandstone

Monitoring microseismic activity provides a window through which to observe reservoir deformation during hydrocarbon and geothermal energy production, or CO2 injection and storage. Specifically, microseismic monitoring may help constrain geomechanical models through an improved understanding of the location and geometry of faults, and the stress conditions local to them. Such techniques can be assessed in the laboratory, where fault geometries and stress conditions are well constrained. We carried out a triaxial test on a sample of Red Wildmoor sandstone, an analogue to a weak North Sea reservoir sandstone. The sample was coupled with an array of piezo-transducers, to measure ultrasonic wave velocities and monitor acoustic emissions (AE) – sample-scale microseismic activity associated with micro-cracking. We calculated the rate of AE, localised the AE events, and inferred their moment tensor from P-wave first motion polarities and amplitudes. We applied a biaxial decomposition to the resulting moment tensors of the high signal-to-noise ratio events, to provide nodal planes, slip vectors, and displacement vectors for each event. These attributes were then used to infer local stress directions and their relative magnitudes. Both the AE fracture mechanisms and the inferred stress conditions correspond to the sample-scale fracturing and applied stresses. This workflow, which considers fracture models relevant to the subsurface, can be applied to large-scale geoengineering applications to obtain fracture mechanisms and in-situ stresses from recorded microseismic data.

Recovering the microseismic response from a geomechanical simulation

The analysis of microseismic events is one of a few tools available for characterizing processes that occur within the subsurface. We have developed a method for modeling microseismic activity in the subsurface that allows us to model microseismic events at the reservoir scale. By embedding this method in a fully coupled hydromechanical numerical code, we simulate the development of hydraulic fractures in an unconventional reservoir with an explicitly represented discrete fracture network for a relatively simple synthetic model and a more complicated model based upon field observations. The results demonstrate that the model can be effectively calibrated against measured microseismic activity, and it can be used to make predictions regarding the timing, location, and amplitude of events in the subsurface.