Imperial College London

DrJi-QuanShi

Faculty of EngineeringDepartment of Earth Science & Engineering

Research Fellow
 
 
 
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Contact

 

+44 (0)20 7594 7374j.q.shi

 
 
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Location

 

4.86Aston WebbSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

113 results found

Cao W, Durucan S, Cai W, Shi J-Q, Korre A, Ratouis T, Hjörleifsdóttir V, Sigfússon Bet al., 2023, Probabilistic evaluation of susceptibility to fluid injection-induced seismicity based on statistics of fracture criticality, Rock Mechanics and Rock Engineering, Vol: 56, Pages: 7003-7025, ISSN: 0723-2632

Fault reactivation and associated microseismicity pose a potential threat to industrial processes involving fluid injection into the subsurface. In this research, fracture criticality, defined as the gradient of critical fluid pressure change to trigger seismicity (Δpc/h), is proposed as a novel reservoir depth-independent metric of fault slip susceptibility. Based on statistics of the fracture criticality, a probabilistic evaluation framework for susceptibility to injection-induced seismicity was developed by integrating seismic observations and hydrogeological modelling of fluid injection operations for faulted reservoirs. The proposed seismic susceptibility evaluation method considers the injection-driven fluid pressure increase, the variability of fracture criticality, and regional fracture density. Utilising this methodology, the probabilistic distribution of fracture criticality was obtained to evaluate the potential for injection-induced seismicity in both fault and off-fault zones at the Hellisheiði geothermal site, Iceland. It has been found that the fracture criticality within both fault and off-fault zones shows natural variability (mostly ranging between 0.001-2.0 bar/km), and that fault zones tend to be characterised by larger fracture criticality values than the off-faut zones. Fracture criticality values estimated within each zone roughly follow a Gaussian distribution. Fault zones around five geothermal fluid re-injection wells at the site were estimated to have relatively high probability of seismic event occurrence, and these regions experienced high levels of induced seismicity over the microseismic monitoring period. The seismotectonic state estimated for each zone is generally consistent with the forecasted susceptibility to seismicity based on statistics of fracture criticality.

Journal article

Agrawal H, Durucan S, Cao W, Korre A, Shi J-Qet al., 2023, Rockburst and gas outburst forecasting using a probabilistic risk assessment framework in longwall top coal caving faces, Rock Mechanics and Rock Engineering, Vol: 56, Pages: 6929-6958, ISSN: 0723-2632

A probabilistic risk assessment framework was developed to mathematically represent the complex engineering phenomena of rock bursts and gas outbursts for a heterogeneous coal seam. An innovative object-based non-conditional simulation approach was used to distribute lithological heterogeneity present in the coal seam to respect their geological origin. The changing mining conditions during longwall top coal caving mining (LTCC) were extracted from a coupled numerical model to provide statistically sufficient data for probabilistic analysis. The complex interdependencies among abutment stress, pore pressure, the volume of total gas emission and incremental energy release rate, their stochastic variations and uncertainty were realistically implemented in the GoldSim software, and 100,000 equally likely scenarios were simulated using the Monte Carlo method to determine the probability of rock bursts and gas outbursts. The results obtained from the analysis incorporate the variability in mechanical, elastic and reservoir properties of coal due to lithological heterogeneity and result in the probability of the occurrence of rock bursts, coal and gas outbursts, and safe mining conditions. The framework realistically represents the complex mining environment, is resilient and results are reliable. The framework is generic and can be suitably modified to be used in different underground mining scenarios, overcoming the limitations of earlier empirical indices used.

Journal article

Cao W, Durucan S, Shi JQ, Korre Aet al., 2023, Slip tendency evaluation of fracture systems associated with seismicity at the Hellisheiði geothermal field, Iceland

An understanding of fracture slip susceptibility in geothermal reservoirs is central to the control of fluid injection induced seismicity. To investigate the role of regional fracture systems on induced seismicity, a coupled thermo-hydro-mechanical (THM) model containing fracture networks, which features direct coupling between different physics for the rock matrix, fractures, and their interactions, as well as indirect coupling through changes of material properties, such as stress-dependent rock and fracture permeabilities, was developed. The model was applied to simulate the geothermal fluid extraction and re-injection over a 10-year period (2011-2021) at the Hellisheiði geothermal field, utilising field recorded monthly production and re-injection rates. Based on the model results, the slip tendency of regional fracture systems was examined under reservoir conditions before and after the start of fluid re-injection. Results have shown that fracture networks act as preferential fluid flow paths that influence fluid pressure and stress distribution and fracture slip tendency in geothermal reservoirs. NE-SW and N-S trending fractures are susceptible to slippage before the start of fluid re-injection, and the distribution of fractures with enhanced slip tendency shifts from surrounding the re-injection region at the onset of fluid re-injection, to a two-lobed pattern in the fault-normal direction around the re-injection region in the long term.

Conference paper

Cao W, Durucan S, Shi J-Q, Cai W, Korre A, Ratouis Tet al., 2022, Induced seismicity associated with geothermal fluids re-injection: Poroelastic stressing, thermoelastic stressing, or transient cooling-induced permeability enhancement?, Geothermics, Vol: 102, Pages: 1-18, ISSN: 0375-6505

Both field injectivity and induced seismicity were reported to be inversely correlated with the temperature of re-injected fluids at the Hellisheiði geothermal field in Iceland. This observation has led to a hypothesis that transient cooling-induced permeability enhancement is a novel mechanism for induced seismicity, in addition to elevated fluid pressure, poroelastic stressing, and thermoelastic stressing in geothermal environments. In this work, a 3D calibrated coupled THM model was developed to model the colder fluids re-injection process over a 1-year period and evaluate the potential for induced seismicity in terms of Coulomb stress changes at the Hellisheiði geothermal field. Three modelling scenarios taking into account respectively the poroelastic effect, thermoporoelastic effect, and thermoporoelastic effect with permeability enhancement, were examined and compared to identify the dominant mechanism for the recorded seismicity and examine the contribution from each individual mechanism. Results have shown that, under normal fluid re-injection pressure and temperature conditions, the permeability enhancement effect is the dominant mechanism for induced seismicity at the Hellisheiði geothermal field. Specifically, the contribution to Coulomb stress changes from the permeability enhancement effect is almost twice of that from the thermoelastic stressing, which is in turn two orders of magnitude larger than that from the poroelastic stressing. It has also been noted that, when reducing temperature of re-injected fluids from 120°C to 20°C, the temperature change is increased by 2.1 times at 1,000 m depth, while the amount of mass flow by around 4 times. Thus, the amount of heat transferred can be increased 8.4 times by lowering temperature of the injected fluids, which explains the high sensitivity of induced seismicity to temperature. Outcomes of this work suggest temperature control of injected fluids as a feasible regulation method to mit

Journal article

Cao W, Durucan S, Shi JQ, Korre A, Ratouis Tet al., 2022, Coupled THM modelling of induced seismicity associated with geothermal fluids re-injection: the role of transient cooling-induced permeability enhancement

Seismicity induced by injection of energy depleted geothermal fluid, as well as field injectivity, has been observed to be inversely correlated with the temperature of re-injected fluids at the Hellisheiði geothermal field, Iceland. Using the Hellisheiði field injection data, transient cooling-induced permeability enhancement as a distinctive mechanism for induced seismicity in geothermal reservoirs was investigated. A 3D coupled THM model was developed to model the colder fluids re-injection process over a 1-year period and evaluate the potential for induced seismicity at Hellisheiði. Three modelling scenarios considering respectively the poroelastic effect, thermoporoelastic effect, and thermoporoelastic effect with permeability enhancement, were examined and compared to identify the dominant mechanism for induced seismicity and examine the contribution from each individual mechanism. Results have shown that, under normal fluid re-injection pressure and temperature conditions, the permeability enhancement effect is the dominant mechanism for induced seismicity at Hellisheiði. In addition, by reducing temperature of re-injected fluids from 120 °C to 20 °C, the amount of heat transferred can be increased by 8.4 times, which explains the high sensitivity of induced seismicity to temperature. The findings of this work suggest that temperature control of injected fluids can be a feasible regulation method to mitigate injection-induced seismic risk.

Conference paper

Cao W, Shi J-Q, Durucan S, Korre Aet al., 2021, Evaluation of shear slip stress transfer mechanism for induced microseismicity at In Salah CO2 storage site, International Journal of Greenhouse Gas Control, Vol: 107, Pages: 1-20, ISSN: 1750-5836

Stress transfer caused by injection-induced fault reactivation plays a significant role in triggering induced seismicity. This work aims to investigate to which extent the shear slip stress transfer mechanism might have contributed to a 4-month period of heightened microseismicity around one of the horizontal injection wells (KB-502) at the In Salah CO2 storage site. Building upon previous reservoir modelling and history matching work by the authors, coupled geomechanical and reservoir modelling of CO2 injection at KB-502 was carried out, featuring the explicit simulation of injection-induced fault reactivation and stress transfer, and the implementation of a strain-dependent permeability model to represent the fault hydrological behaviour. This approach allows a much-improved overall match to the field bottomhole pressures at KB-502 over the previous results, where fault zone reactivation and associated dynamic permeability behaviour were not considered, especially over the 4-month period of interest. Based upon the coupled modelling results, Coulomb stress changes were used to evaluate the potential for enhanced microseismicity related to CO2 injection-induced fault reactivation at KB-502. Analyses on the potential for microseismicity have shown that seismic events are likely to take place in both hydraulically connected regions and stress transfer influenced regions. The variation of computed Coulomb stress changes in near-fault areas compares favourably with the heightened field recorded seismicity during the period modelled. The integrated interpretation of microseismic monitoring and coupled geomechanics and reservoir modelling have suggested that the shear slip stress transfer mechanism was active and contributed to the occurrence of induced seismicity at In Salah.

Journal article

Cao W, Durucan S, Cai W, Shi JQ, Korre A, Ratouis T, Hjörleifsdóttir V, Sigfússon Bet al., 2021, Combining Microseismic Observations and Reservoir Simulation to Interpret Fracture Criticality in Faults at the Hellisheiði Geothermal Field, Iceland, Pages: 418-427

Fault reactivation and associated microseismicity induced by fluid injection into the subsurface pose a potential threat in geothermal power generation. In this research, the gradient of critical pore pressure change to trigger seismicity (Δpc/h), referred to as the fracture criticality, has been proposed to represent the critical state of subsurface fractures. The fracture criticality is subjected to variability due to heterogeneous fracture attributes and rock properties. The statistics of fracture criticality could be applied to the probabilistic evaluation of fluid injection-induced seismic risk, which considers the injection-driven pore pressure increase, the variability of fracture criticality, and local fracture density. The seismic risk evaluation based on statistics of fracture criticality was applied to the Hellisheiði geothermal site, where microseismic observations and reservoir simulation over a half-year fluid injection period were integrated to achieve the probabilistic distribution of fracture criticality and evaluate the injection-induced seismic risk in both fault and off-fault zones. It has been found that the fracture criticality within both fault and off-fault zones shows natural variability (mostly ranging between 0-1.0 bar/km), and the values estimated roughly follow Gaussian distributions. Relatively high probability of seismic event occurrence was estimated for fault zones around five geothermal fluid re-injection wells at the site, which were consistent with seismically-active areas over the microseismic monitoring period.

Conference paper

Cao W, Shi JQ, Durucan S, Korre Aet al., 2021, Investigation into the hydro-geomechanical mechanisms related to induced microseismicity at In Salah storage site

It has been recognised that stress transferred from injection-induced fault reactivation plays a significant role in triggering induced seismicity. In this study, the mechanism which triggered a 4-month long period of heightened microseismicity around one of the horizontal injection wells (KB-502) at the In Salah CO2 storage site has been investigated. Building upon previous reservoir modelling and history matching work by the authors, a coupled geomechanical and reservoir modelling study of CO2 injection at KB-502 was carried out, featuring the implementation of a strain-dependent permeability model developed to describe hydromechanical response of the fault zone to CO2 injection. This approach allows a much improved overall match to the field bottomhole pressures at KB-502 over the previous results where fault zone reactivation and associated dynamic permeability behaviour were not considered, especially over the 4-month period of interest. Based upon the coupled modelling results, Coulomb stress changes were used to evaluate the potential for enhanced microseismicity related to CO2 injection-induced fault reactivation at KB-502, and to ascertain, to which extent stress shear slip stress transfer mechanism might have contributed to the elevated microseismicity recorded around KB-502. Analyses on the potential for microseismicity have shown that seismic events are likely to take place in both hydraulically connected regions and stress transfer influenced regions. The integrated interpretation of microseismic monitoring and coupled geomechanics and reservoir modelling have suggested that the shear slip stress transfer mechanism was active and contributed to the occurrence of induced seismicity at the In Salah site.

Conference paper

Cai W, Durucan S, Cao W, Shi JQ, Korre A, Hjörleifsdóttir V, Ratouis T, Sigfússon Bet al., 2021, Seismic Response to Fluid Injection in Faulted Geothermal Reservoirs: A Case Example from Iceland

Seismic response to fluid injection at the Húsmúli area within the Hellisheiði geothermal field in Iceland was investigated by correlating induced seismicity with fault structures and fluid injection rates. Induced seismicity recorded around five injection wells over a half-year fluid injection period was first analysed using the k-means clustering model, which identified two clusters associated with the injection wells studied. Based on the clustered seismic events, the geometry of the injected fluid flow was estimated using the spatial density of seismic events. A statistical approach, using locations of three seismic events to identify one fracture plane, was utilised to identify the dominant fault structures, which spatially correlate well with local fault structures dominating the fluid flow. In the temporal domain, the fluid injection rate and wellhead pressure for the five wells were separately analysed against the seismic response, which shows that the seismic frequency and the b value increased along with local fluctuations of injection rate and pressure in the controlling system. It can be concluded that the integrated analysis of induced seismicity, fault structures and fluid injection in spatial-temporal domains is crucial to understand the seismic response to fluid injection in faulted/fractured geothermal reservoirs.

Conference paper

Cao W, Durucan S, Cai W, Shi J-Q, Korre Aet al., 2020, A physics-based probabilistic forecasting methodology for hazardous microseismicity associated with longwall coal mining, International Journal of Coal Geology, Vol: 232, Pages: 1-14, ISSN: 0166-5162

Mining-induced microseismicity is widely considered as a result of slippage of pre-existing critically stressed fractures caused by stress perturbations around an advancing face. An in-depth analysis of the recorded microseismicity associated with longwall top coal caving mining at Coal Mine Velenje in Slovenia has been previously carried out and reported by the authors. It has been concluded that while microseismic event rate is affected by mining intensity (longwall face daily advance rate) as well as local abundance of pre-existing fractures, spatial and magnitude characteristics of microseismicity are predominantly influenced by the latter. Based upon this improved understanding of fracture-slip seismic-generation mechanism, the current work aimed at establishing a data-driven yet physics-based probabilistic forecasting methodology for hazardous microseismicity using microseismic monitoring data with concurrent face advance records. Through performing statistical analyses and probability distribution fitting for temporal, magnitude and spatial characteristics of microseismicity within a time window, a short-term forecasting model is developed to estimate the probability of potentially hazardous microseismicity over the next time interval in the form of a joint probability. The real time forecasting of hazardous microseismicity during longwall coal mining is realised through regularly updating the statistical model using the most recent microseismic sequence datasets and face advance records. This forecasting methodology is featured by the physical basis which provides a good explicability of forecasting results, and the probabilistic perspective which accounts for the stochastic nature of mining-induced microseismicity. This model has been employed to make time-varying forecasts of hazardous microseismicity around two longwall panels over a one-year coal production period at Coal Mine Velenje, and satisfactory results at both panels were achieved. In addition, t

Journal article

Cao W, Durucan S, Cai W, Shi J-Q, Korre A, Jamnikar S, Rošer J, Lurka A, Siata Ret al., 2020, The role of mining intensity and pre-existing fracture attributes on spatial, temporal and magnitude characteristics of microseismicity in longwall coal mining, Rock Mechanics and Rock Engineering, Vol: 53, Pages: 4139-4162, ISSN: 0723-2632

Knowledge regarding microseismic characteristics associated with longwall coal mining is crucial in evaluating the potential for underground mining hazards. Although microseismicity is induced by mining activities, it still remains uncertain as to what extent mining activities influence the spatial, temporal, and magnitude characteristics of microseismicity. To establish a thorough understanding of the relationship between microseismic characteristics and mining activities, a 27-month long microseismic monitoring campaign was conducted around a highly stressed coal zone and eight producing longwall panels at Coal Mine Velenje in Slovenia. Each microseismic event was classified to be associated with the producing longwall panel that triggered it, and the microseismic response to multi-panel longwall top coal caving face advance was analysed. Monitoring data have shown that locations of microseismic events coincided with stress concentrated regions. It was established that both seismic count and energy-intensive regions associated with coal mining in different panels are spatially connected, but they do not fully overlap with mined-out or stress concentrated areas. In addition, microseismic event counts frequency was found to be well correlated with mining intensity, while seismic energy magnitude and spatial distribution are poorly correlated with the same. Therefore, microseismic characteristics could not be explained solely by the mining-induced stress transfer and mining intensity, but are believed to be dominated by pre-existing natural fractures throughout the coal seam. Analyses of these observations helped the development of a conceptual seismic-generation model, which provides new insights into the causes of microseismicity in coal mining.

Journal article

Cao W, Shi J-Q, Durucan S, Si G, Korre Aet al., 2020, Gas-driven rapid fracture propagation under unloading conditions in coal and gas outbursts, International Journal of Rock Mechanics and Mining Sciences, Vol: 130, ISSN: 1365-1609

Coal and gas outbursts have long posed a serious risk to safe and efficient production in coal mines. It is recognised that coal and gas outbursts are triggered by excavation unloading followed by gas-driven rapid propagation of a system of pre-existing or mining-induced fractures. Gas-filled fractures parallel to a working face are likely to experience opening first, then expansion and rapid propagation stages under unloading conditions. The fracture opening is driven by the effective stress inside the fracture, while the fracture expansion and rapid propagation is propelled by the pressure build-up of desorbed gas in the vicinity of the fracture. Based upon this understanding, this research aimed to identify the key factors affecting outburst initiation and its temporal evolution during roadway developments. Specifically, the response of pre-set fractures in a thin coal seam sandwiched between rock layers to roadway development is simulated using a geomechanical model coupled with fracture mechanics for fracture opening and propagation. In addition, kinetic gas desorption and its migration into open fractures is considered. During simulations outburst is deemed to occur when the fracture length exceeds the dimension of a host element. The findings of this research suggest that the simulated coal and gas outburst caused by roadway development may be considered as a dynamic gas desorption-driven fracture propagation process. The occurrence of coal and gas outbursts is found to be influenced mainly by the coal properties, fracture attributes, and initial gas pressure and the in situ stress conditions. Furthermore, the model predictions in terms of dome-shaped erupted-zone and layer-by-layer coal breakage are consistent with the field reports. In addition, the model results suggest that delayed occurrence of coal and gas outbursts, especially after sudden exposure of a coal seam or after blasting disturbance, reported in the literature may be related to the gas desorp

Journal article

Cao W, Shi J-Q, Durucan S, Korre A, Jamnikar Set al., 2019, Numerical modelling of anomalous microseismicity influenced by lithological heterogeneity in longwall top coal caving mining, International Journal of Coal Geology, Vol: 216, ISSN: 0166-5162

Mining-induced microseismicity has been extensively used to evaluate the potential for rock bursts and coal and gas outbursts in underground coal mines. In a research project completed a few years ago, it was observed that characteristics of microseismicity around a working longwall panel were fairly consistent over the monitoring period until a lithological heterogeneity zone with a relatively high coal strength was reached. The current research presented in this paper aims at achieving a better understanding of the effect of lithological heterogeneity on microseismic activity in longwall coal mining. The heterogeneous zone inferred from the tomography measurements was first digitalised and implemented into a 3D geomechanical model. A microseismicity modelling approach which combines deterministic stress and failure analysis together with a stochastic fracture slip evaluation was used to simulate the evolution of microseismicity induced by the progressive face advance passing through the heterogeneous zone. The heterogeneity was taken into account by varying the material strength and the fracture attributes of the elements within the high strength zone. Results have shown that both the high rock strength of coal lithotype and low power law scaling exponent of fractures within this zone contribute to the reduction in fitted b values from frequency-magnitude distribution of microseismicity and the increase in fitted Gaussian distribution parameters to the logarithmic event energy. These deviations are believed to result from the combined effects of increased stress drops and slipped fracture sizes when the heterogeneous zone is approached.

Journal article

Shi JQ, Durucan S, Korre A, Ringrose P, Mathieson Aet al., 2019, History matching and pressure analysis with stress-dependent permeability using the In Salah CO2 storage case study, International Journal of Greenhouse Gas Control, Vol: 91, ISSN: 1750-5836

Using the In Salah CO2 storage case study, this study demonstrates how reservoir simulation history-matching and pressure analysis can be used to improve conformance assurance. By adopting a holistic approach to reservoir simulation and history matching, in conjunction with the injection pressure analysis and use of microseismic monitoring data, an improved understanding of the injection processes at the In Salah storage site was gained, revealing distinctively different responses to CO2 injection at each of the three injection wells. It has been shown that injection well performance at wells KB-501 and KB-503 was characterised by periods of matrix and fracture flow, the latter being due to shear reactivation of existing fractures in the vicinity of the wellbore. In contrast, the analysis at KB-502 revealed that CO2 injection has resulted in fracture reactivation in both shear and tensile modes with propagation both laterally and vertically.The findings of this study have shown that the injection-induced microseismic events recorded close to the KB-502 well is mainly caused by injecting CO2 into a 4-kilometer long fracture-zone intersecting the borehole. This fracture zone experienced tensile opening of fractures during injection and then subsequent partial closure after the injection was suspended. It is furthermore clear that the rather unique combination of microseismic and InSAR monitoring data and reservoir history matching around the injection well KB-502 provide highly valuable insights into injection-induced seismicity and fracture flow behaviour for CO2 storage projects. Episodes of flow in natural fractures can be separated from flow in induced fractures and the need for a dynamic, stress-dependent, permeability variable have been demonstrated.

Journal article

Yildirim B, Durucan S, Cao W, Cai W, Shi J, Korre A, Wolf KHet al., 2019, Experimental and numerical investigation into hydraulic fracture and natural fracture interaction in shale formations, 53rd U.S. Rock Mechanics/Geomechanics Symposium, Publisher: American Rock Mechanics Association

Two 0.3 m × 0.3 m × 0.3 m shale blocks, one representing a homogeneous sample while the other representing a naturally fractured sample, are modelled using the lattice based DEM code, XSite. The synthetic rock mass approach (SRM), which assigns the smooth joint contacts (SJM) to the weakness planes, is used to represent the natural fractures in shale block-2. Firstly, the developed models are compared with the findings of previously conducted true-triaxial hydraulic fracturing experiments with acoustic measurements, and their subsequent computed tomography (CT) and seismic velocity tomography results. The 3D model results confirmed the curved shape hydraulic fractures, which propagated perpendicular to the minimum stress directions in both shale blocks. Model results also captured the natural fracture (NF) and hydraulic fracture (HF) interaction, particularly the arrest, the dilation of major NFs, followed by crossing with offset mechanism, in shale block-2. Secondly, the parametric studies are carried out to investigate the role of fluid flow rate (q), and fluid viscosity (µ) on different NF/HF interaction mechanisms. The effects of q and µ are discussed based on the total stimulated area including the tensile and shear microcracks, the pipe apertures, and the pressure evolutions within NFs.

Conference paper

Cao W, Durucan S, Cai W, Shi JQ, Korre Aet al., 2019, Multiple-panel longwall top coal caving induced microseismicity: Monitoring and development of a statistical forecasting model for hazardous microseismicity, 53rd U.S. Rock Mechanics/Geomechanics Symposium, Publisher: American Rock Mechanics Association

Continuous microseismic monitoring was carried out around 9 producing longwall top coal caving (LTCC) panels with concurrently recorded daily face advance rates at Coal Mine Velenje in Slovenia over a 27-month monitoring period. The monitoring results suggested that spatial and magnitude characteristics of microseismicity are dominated by those of underlying fractures, while microseismic event rate is under the combined effects of local natural fracture abundance and mining intensity. On this basis, a data-driven yet physics-based forecasting methodology was established for LTCC induced hazardous microseismicity, which is above a given threshold of energy magnitude and within a certain distance to the longwall face. Statistical analyses were first conducted to characterise temporal, magnitude and spatial characteristics of long-term recorded microseismicity, based on which a short-term forecasting model was developed to calculate the probability of hazardous microseismicity considering the three characteristics. The model developed was employed to forecast the likelihood of hazardous microseismicity at one of these LTCC panels, and the forecasted results were supported by the monitoring. This statistical model has important implications in the evaluation of mining-induced hazards, and it can be used to optimise longwall face advance rates to minimise the risk of hazardous microseismicity in burst-prone deep-level mining sites.

Conference paper

Cai W, Durucan S, Shi JQ, Cao W, Agrawal H, Korre A, Jamnikar S, Rošer Jet al., 2019, Development of fractal-fuzzy evaluation methodology and its application for seismic hazards assessment using microseismic monitoring in coal mining, 53rd U.S. Rock Mechanics/Geomechanics Symposium, Publisher: American Rock Mechanics Association

Seismic hazards have become one of the common risks in underground coal mining and their assessment is an important component of the safety management. In this study, a methodology, involving nine fractal dimension-based indices and a fuzzy comprehensive evaluation model, has been developed based on the processed real time microseismic data from an underground coal mine, which allows for a better and quantitative evaluation of the likelihood for the seismic hazards. In the fuzzy model, the membership function was built using a Gaussian shape and the weight of each index was determined using the performance metric F score derived from the confusion matrix. The assessment results were initially characterised as a probability belonging to each of four risk levels (none, weak, moderate and strong). The comprehensive result was then evaluated by integrating the maximum membership degree principle (MMDP) and the variable fuzzy pattern recognition (VFPR). The model parameters of this methodology were first calibrated using historical microseismic data over a period of seven months at Coal Mine Velenje in Slovenia, and then applied to analyse more recent microseismic monitoring data. The results indicate that the calibrated model was able to assess seismic hazards in the mine.

Conference paper

Si G, Durucan S, Shi JQ, Korre A, Cao Wet al., 2019, Parametric analysis of slotting operation induced failure zones to stimulate low permeability coal seams, Rock Mechanics and Rock Engineering, Vol: 52, Pages: 163-182, ISSN: 0723-2632

The main constrain for effective gas drainage in coal mines is the low permeability nature of coal reservoirs. As coal mining activities are extending to deeper subsurface, the ever-increasing in situ stress conditions is anticipated to result in much lower permeability and more challenges for gas emission control in coal mines. In recent years, hydraulic slotting using high-pressure waterjet along underground gas drainage boreholes, as a general solution to stimulate low permeability coal seams, has become increasingly favourable. This paper presents a systematic investigation into the sensitivity of borehole slotting performance to a number of field and operational parameters. A wide range of geomechanical properties, in situ stress conditions, slot geometry and spacing of multiple slots were considered in a series of numerical simulations. The relations between these key parameters and the failure zone size/volume induced by slotting were quantified. The effect of different parameters in improving slotting performance has also been ranked, which provides theoretical base for mine operators to optimise slotting operations.

Journal article

Cao W, Durucan S, Cai W, Shi JQ, Korre Aet al., 2019, Multiple-panel longwall top coal caving induced microseismicity: Monitoring and development of a statistical forecasting model for hazardous microseismicity

Continuous microseismic monitoring was carried out around 9 producing longwall top coal caving (LTCC) panels with concurrently recorded daily face advance rates at Coal Mine Velenje in Slovenia over a 27-month monitoring period. The monitoring results suggested that spatial and magnitude characteristics of microseismicity are dominated by those of underlying fractures, while microseismic event rate is under the combined effects of local natural fracture abundance and mining intensity. On this basis, a data-driven yet physics-based forecasting methodology was established for LTCC induced hazardous microseismicity, which is above a given threshold of energy magnitude and within a certain distance to the longwall face. Statistical analyses were first conducted to characterise temporal, magnitude and spatial characteristics of long-term recorded microseismicity, based on which a short-term forecasting model was developed to calculate the probability of hazardous microseismicity considering the three characteristics. The model developed was employed to forecast the likelihood of hazardous microseismicity at one of these LTCC panels, and the forecasted results were supported by the monitoring. This statistical model has important implications in the evaluation of mining-induced hazards, and it can be used to optimise longwall face advance rates to minimise the risk of hazardous microseismicity in burst-prone deep-level mining sites.

Conference paper

Durucan S, Cao W, Cai W, Shi JQ, Korre A, Si G, Jamnikar S, Roser Jet al., 2019, Monitoring, assessment and mitigation of rock burst and gas outburst induced seismicity in longwall top coal caving mining, Pages: 11-20

Underground coal extractions lead to continuous stress and pressure redistribution around mine openings which, in some cases, may lead to coal and gas outburst and rock bursts. This paper presents seismic monitoring research, which aimed at characterising the dynamic behaviour of the coal seam in response to gateroad development and longwall top coal caving mining at Coal Mine Velenje. The early campaigns involved time-lapse active seismic tomography and microseismic monitoring at the two selected longwall panels. Based on the findings of this early work, a more comprehensive microseismic monitoring programme was set up. Seismic monitoring data were processed and used for model development and rock burst/gas outburst risk assessment purposes, which also led to the development of measures to mitigate against seismic hazards.

Conference paper

Yildirim B, Durucan S, Cao W, Cai W, Shi J, Korre A, Wolf KHet al., 2019, Experimental and numerical investigation into hydraulic fracture and natural fracture interaction in shale formations

Copyright 2019 ARMA, American Rock Mechanics Association. Two 0.3 m × 0.3 m × 0.3 m shale blocks, one representing a homogeneous sample while the other representing a naturally fractured sample, are modelled using the lattice based DEM code, XSite. The synthetic rock mass approach (SRM), which assigns the smooth joint contacts (SJM) to the weakness planes, is used to represent the natural fractures in shale block-2. Firstly, the developed models are compared with the findings of previously conducted true-triaxial hydraulic fracturing experiments with acoustic measurements, and their subsequent computed tomography (CT) and seismic velocity tomography results. The 3D model results confirmed the curved shape hydraulic fractures, which propagated perpendicular to the minimum stress directions in both shale blocks. Model results also captured the natural fracture (NF) and hydraulic fracture (HF) interaction, particularly the arrest, the dilation of major NFs, followed by crossing with offset mechanism, in shale block-2. Secondly, the parametric studies are carried out to investigate the role of fluid flow rate (q), and fluid viscosity (µ) on different NF/HF interaction mechanisms. The effects of q and µ are discussed based on the total stimulated area including the tensile and shear microcracks, the pipe apertures, and the pressure evolutions within NFs.

Conference paper

Cai W, Durucan S, Shi JQ, Cao W, Agrawal H, Korre A, Jamnikar S, Rošer Jet al., 2019, Development of fractal-fuzzy evaluation methodology and its application for seismic hazards assessment using microseismic monitoring in coal mining

Copyright 2019 ARMA, American Rock Mechanics Association. Seismic hazards have become one of the common risks in underground coal mining and their assessment is an important component of the safety management. In this study, a methodology, involving nine fractal dimension-based indices and a fuzzy comprehensive evaluation model, has been developed based on the processed real time microseismic data from an underground coal mine, which allows for a better and quantitative evaluation of the likelihood for the seismic hazards. In the fuzzy model, the membership function was built using a Gaussian shape and the weight of each index was determined using the performance metric F score derived from the confusion matrix. The assessment results were initially characterised as a probability belonging to each of four risk levels (none, weak, moderate and strong). The comprehensive result was then evaluated by integrating the maximum membership degree principle (MMDP) and the variable fuzzy pattern recognition (VFPR). The model parameters of this methodology were first calibrated using historical microseismic data over a period of seven months at Coal Mine Velenje in Slovenia, and then applied to analyse more recent microseismic monitoring data. The results indicate that the calibrated model was able to assess seismic hazards in the mine.

Conference paper

Yildirim B, Durucan S, Cao W, Cai W, Shi J, Korre A, Wolf KHet al., 2019, Experimental and numerical investigation into hydraulic fracture and natural fracture interaction in shale formations

Two 0.3 m × 0.3 m × 0.3 m shale blocks, one representing a homogeneous sample while the other representing a naturally fractured sample, are modelled using the lattice based DEM code, XSite. The synthetic rock mass approach (SRM), which assigns the smooth joint contacts (SJM) to the weakness planes, is used to represent the natural fractures in shale block-2. Firstly, the developed models are compared with the findings of previously conducted true-triaxial hydraulic fracturing experiments with acoustic measurements, and their subsequent computed tomography (CT) and seismic velocity tomography results. The 3D model results confirmed the curved shape hydraulic fractures, which propagated perpendicular to the minimum stress directions in both shale blocks. Model results also captured the natural fracture (NF) and hydraulic fracture (HF) interaction, particularly the arrest, the dilation of major NFs, followed by crossing with offset mechanism, in shale block-2. Secondly, the parametric studies are carried out to investigate the role of fluid flow rate (q), and fluid viscosity (µ) on different NF/HF interaction mechanisms. The effects of q and µ are discussed based on the total stimulated area including the tensile and shear microcracks, the pipe apertures, and the pressure evolutions within NFs.

Conference paper

Cai W, Durucan S, Shi JQ, Cao W, Agrawal H, Korre A, Jamnikar S, Rošer Jet al., 2019, Development of fractal-fuzzy evaluation methodology and its application for seismic hazards assessment using microseismic monitoring in coal mining

Seismic hazards have become one of the common risks in underground coal mining and their assessment is an important component of the safety management. In this study, a methodology, involving nine fractal dimension-based indices and a fuzzy comprehensive evaluation model, has been developed based on the processed real time microseismic data from an underground coal mine, which allows for a better and quantitative evaluation of the likelihood for the seismic hazards. In the fuzzy model, the membership function was built using a Gaussian shape and the weight of each index was determined using the performance metric F score derived from the confusion matrix. The assessment results were initially characterised as a probability belonging to each of four risk levels (none, weak, moderate and strong). The comprehensive result was then evaluated by integrating the maximum membership degree principle (MMDP) and the variable fuzzy pattern recognition (VFPR). The model parameters of this methodology were first calibrated using historical microseismic data over a period of seven months at Coal Mine Velenje in Slovenia, and then applied to analyse more recent microseismic monitoring data. The results indicate that the calibrated model was able to assess seismic hazards in the mine.

Conference paper

Zhang Y, Xue Z, Park H, Shi J-Q, Kiyama T, Lei X, Sun Y, Liang Yet al., 2019, Tracking CO2 Plumes in Clay-Rich Rock by Distributed Fiber Optic Strain Sensing (DFOSS): A Laboratory Demonstration, WATER RESOURCES RESEARCH, Vol: 55, Pages: 856-867, ISSN: 0043-1397

Journal article

Xue Z, Shi JQ, Yamauchi Y, Durucan Set al., 2018, Fiber optic sensing for geomechanical monitoring: (1)-distributed strain measurements of two sandstones under hydrostatic confining and pore pressure conditions, Applied Sciences (Switzerland), Vol: 8

In this study distributed optic fiber has been used to measure both the Rayleigh and Brillouin frequency shift of two different sandstone core samples under controlled hydrostatic confining and pore pressure in the laboratory. The Berea sandstone core is relatively homogeneous, whereas the Tako sandstone core is visibly heterogeneous with a coarse-grain and a fine-grain region. Rayleigh frequency has been found to have a superior performance over Brillouin frequency in terms of better consistency (less scattering) in the tests carried out. The strain gauge readings reveal considerable anisotropy in the stiffness of the Berea core between perpendicular (vertical) and parallel to the bedding (hoop) directions. The strains converted from Rayleigh frequency shift measurements agree reasonably well with readings by one of the four hoop strain gauge channels under increasing confining/pore pressure. For the Tako sandstone core, the contrast in the grain-size, and thus rock elastic properties, is clearly reflected in the hoop strain measurement by both strain gauges and distributed optic fiber. The outcomes of the test have demonstrated successfully the use of a single optic fiber for measuring rock strain response at different regions of a heterogeneous core sample along a continuous trajectory.

Journal article

Cai W, Dou L, Zhang M, Cao W, Shi JQ, Feng Let al., 2018, A fuzzy comprehensive evaluation methodology for rock burst forecasting using microseismic monitoring, Tunnelling and Underground Space Technology, Vol: 80, Pages: 232-245, ISSN: 0886-7798

© 2018 Elsevier Ltd Rock bursts have become one of the most severe risks in underground coal mining and its forecasting is an important component in the safety management. Subsurface microseismic (MS) monitoring is considered potentially as a powerful tool for rock burst forecasting. In this study, a methodology for rock burst forecasting involving the use of a fuzzy comprehensive evaluation model was developed, which allows for a more quantitative evaluation of the likelihood for the occurrence of a rock burst incident. In the fuzzy model, the membership function was built using Gaussian shape combined with the exponential distribution function from the reliability theory. The weight of each index was determined utilising the performance metric F score from the confusion matrix. The comprehensive forecasting result was obtained by integrating the maximum membership degree principle (MMDP) and the variable fuzzy pattern recognition (VFPR). This methodology has been applied to a coal mine in China to forecast rock bursts. To select MS indices for rock burst forecasting using the fuzzy evaluation model, laboratory acoustic emission (AE) measurements of coal samples collected from the mine were performed. The model parameters were first calibrated using historical MS data over a period of four months, during which six rock burst incidents were observed. This calibrated model was able to forecast the occurrence of a subsequent rock burst incident in the mine.

Journal article

Yildirim B, Cao W, Shi JQ, Durucan S, Korre Aet al., 2018, Discrete element modelling of hydraulic fracture interaction with natural fractures in shale formations, 2nd International Discrete Fracture Network Engineering Conference, Publisher: American Rock Mechanics Association

Research presented in this paper aimed at establishing a better understanding of natural fracture (NF)/hydraulic fracture (HF) interaction mechanisms and fracture network development in naturally fractured and nonhomogeneous shale formations through numerical modelling using the two-dimensional particle flow code (PFC2D). Hydraulic fracture propagation was firstly modelled in a 30 m x 30 m model representing intact rock by bonded particle method (BPM), which served as a base case in the research. Then a single, deterministic natural fracture was embedded into the same model by a smooth joint contact model (SJM) to investigate different NF/HF interaction mechanisms under a range of different conditions by varying the angle of approach, differential horizontal stress, and the mechanical properties of a fracture within the model. Based on the parametric research findings, number and diversity of natural fractures in the model were increased both deterministically and stochastically, and the results are compared and discussed.

Conference paper

Cao W, Shi JQ, Durucan S, Si G, Korre Aet al., 2018, Gas-driven rapid fracture propagation and gas outbursts under unloading conditions in coal seams, 52nd U.S. Rock Mechanics/Geomechanics Symposium, Publisher: American Rock Mechanics Association

Coal and gas outbursts have long posed a serious risk to safe and efficient production in coal mines. It is recognised that the coal and gas outbursts are triggered by excavation unloading followed by gas-driven rapid propagation of a system of preexisting or mining-induced fractures. Gas-filled fractures parallel to a working face are likely to experience opening first, then expansion and rapid propagation stages under unloading conditions. This research aimed to identify the key factors affecting outburst initiation and its temporal evolution during roadway developments. Specifically, the response of pre-set fractures in a coal seam sandwiched between rock layers to roadway development is simulated using a geomechanical model coupled with fracture mechanics for fracture opening and propagation. In addition, kinetic gas desorption and its migration into open fractures is considered. During simulations outburst is deemed to occur when the fracture length exceeds the dimension of a host element. The findings of this research suggests that the simulated coal and gas outburst may be considered as a dynamic gas desorption-driven fracture propagation process. The occurrence of coal and gas outbursts is found to be influenced mainly by the coal properties, fracture attributes, and initial gas pressure and the in situ stress conditions.

Conference paper

Cao W, Shi J, Si G, Durucan S, Korre Aet al., 2018, Numerical modelling of microseismicity associated with longwall coal mining, International Journal of Coal Geology, Vol: 193, Pages: 30-45, ISSN: 0166-5162

Microseismicity has long been a precursor for underground mining hazards such as rockbursts and coal and gas outbursts. In this research, a methodology combining deterministic stress and failure analysis and stochastic fracture slip evaluation, based upon the widely accepted fracture slip seismicity-generation mechanism, has been developed to simulate microseismic events induced by longwall mining. Using the built-in DFN facility in FLAC3D, discrete fractures following a power law size distribution are distributed throughout a 3D continuum model in a probabilistic way to account for the stochastic nature of microseismicity. The DFN-based modelling approach developed was adopted to simulate the evolution of microseismicity induced by the progressive face advance in a longwall top coal caving (LTCC) panel at Coal Mine Velenje, Slovenia. At each excavation step, global stress and failure analysis with reference to the strain-softening post-failure behaviour characteristic of coal, and fracture slip evaluation for microseismicity are conducted sequentially. The model findings are compared to the microseismic event data recorded during a long-term field monitoring campaign conducted at the same LTCC panel. It was found that the released energy and frequency-magnitude distribution of microseismicity are associated with the slipped fracture sizes and fracture size distribution. These features for recorded microseismic events were fairly constant until a xylite rich heterogeneous zone ahead of the working face was approached, which indicates that fractures within the extracted coal seam follow the same size distribution. The features obtained from modelled microseismic events were consistent over the production period, and matched well the field observations. Furthermore, the model results indicate that the power law fracture size distribution can be used to model longwall-mining-induced microseismicity. This model provides a unique prospective to understand longwall coal m

Journal article

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