Microseismic Monitoring Results Research Articles

Development and field application of strongly resilient temporary plugging diversion agent for fracturing

AbstractTemporary plugging diversion fracturing in multistage horizontal well is normally used to improve stimulation efficiency and increase production in unconventional reservoirs. Temporary plugging agent plays an important role in diversion fracturing. A strongly resilient temporary plugging diversion agent can improve the effectiveness of fracturing diversion. Therefore, a novel organosilicon temporary plugging diversion agent (QBZU) was developed through micellar copolymerization method. Self-synthesized strongly resilient temporary plugging diversion agent (QBZU gel) was prepared by using acrylamide, N, N′-methylene bisacrylamide, surfactants, organosilicon, ammonium persulfate and sodium bisulfite as chemical raw materials. The micellar copolymerization is investigated to overcome the incompatibility of hydrophobic organosilicon and water-soluble monomer by adding some appropriate surfactants. The experimental results indicated that the combination of sodium dodecyl sulphonate and Tween 80 provided excellent copolymerization results. The characteristics of compressive resistance, salt resistance, shearing resistance, resilience and degradation are superior compared with traditional rigid granular temporary plugging agent widely used in oilfields (QG hydrogel). According to the experimental evaluation results of QBZU, its pressure-bearing capacity can reach 56.3MPa, shearing strength can reach 410N, elastic modulus can reach 80MPa, and Poisson ratio can reach 0.48. Meanwhile, the main synthetic factors affecting the resilient performance of QBZU were investigated, including polymer concentration, organosilicon concentration and the types of surfactant. Based on the fracturing pressure curve and microseismic monitoring results, the plugging and fracture diversion effectiveness was further confirmed.

Characterization and Quantitative Assessment of Shale Fracture Characteristics and Fracability Based on a Three-Dimensional Digital Core

At present, assessment techniques for the fracability of shale reservoirs, which rely on the formation of an effective fracture network, are scarce. Hence, in order to assess the fracability, it is critical to establish a quantitative correlation between the pattern of fracture distribution after fracture and fracability. The present investigation utilizes three-dimensional digital core technology and triaxial compression experiments to simulate the fracturing process in typical domestic shale reservoir cores. In addition to utilizing the maximum ball algorithm to extract fracture images, a number of other techniques are employed to compute the spatial quantitative parameters of the fractures, including least squares fitting, image tracking algorithms, and three-dimensional image topology algorithms. The introduction of the notion of three-dimensional fracture complexity serves to delineate the degree of successful fracture network formation subsequent to fracturing. A quantitative fracability characterization model is developed by integrating the constraints of fracture network formation potential and fragmentation potential. The results of this study show that the quantitative characterization of the characteristic parameters of cracks can be achieved by establishing a method for extracting crack information as well as parameters after core compression and completing the construction of a three-dimensional complexity characterization model. Meanwhile, the three-dimensional post-compression fracture image validation shows that the core fracturability index can better reflect the actual fracturing situation, which is in line with the microseismic monitoring results, and significantly improves the accuracy of fracturability characterization, which is an important guideline for the fracturing design of shale gas reservoirs.

Mechanical performance and failure mechanism of U-steel support structure under blast loading

The U-steel support structures of underground caverns are prone to instability and failure under blast loads. The purpose of the underground cavern reinforcement is to mobilise the self-supporting capacity of the surrounding rock to resist the blast. To better understand the mechanical performance and failure mechanism of the U-steel support, the fracture process and vibration behaviour of the support structure under blast loading are investigated by the microseismic monitoring experiment. The dynamic responses of the cavern support structures under blast loading are investigated, and the potentially hazardous sections of the U-steel support structure are revealed by the theoretical analysis. The microseismic monitoring results show that the blast induced microseismic events are concentrated in the arch shoulder of the small chainage, correspondingly the U-steel structures in this region have been partially extruded and deformed. The failure mechanism of the supporting structure is presented. In order to effectively inhibit the internal fracture evolution or macroscopic failure of the rock mass, the synergetic reinforcement scheme of the structures is proposed. The results of the research can be used as a reference for the design and control method of the U-steel support in similar projects.

Property parameter determination in individual layers for separately fractured wells with commingled production in multi-layered reservoirs

At present, without any separate rate test for each layer, there is no way to determinate the properties of individual layers for separately fractured wells with commingled production in multi-layered reservoirs. In order to address this issue, much research work was performed and elucidated in this article. To begin with, we illustrated a basic physical model for a separately fractured well in a multi-layered reservoir. Next, we stated the common determination method that can only be used to gain the average properties of multi-layered reservoirs. Then, according to the physical model, we newly established a mathematical model and plotted standard well-test type curves; additionally, we also discussed why we cannot determinate the properties of individual layers by using the new well-test model. What’s more, we presented a new method to determinate the properties of individual layers. Moreover, we compared the advantages and disadvantages among the three methods. In addition, by using the new determination method, we particularly took two field wells as examples to demonstrate how to determine the properties of individual layers. The proposed new method was validated by use of the common method, the new well-test model and the microseismic monitoring results. At the end, we summarized the research conclusions and indicated that the new method was a good tool to determinate the properties of individual layers in multi-layered reservoirs.

A feasibility study on time-lapse controlled-source electromagnetic method for hydraulic fracturing monitoring of Well Eyangye-2HF in Yichang, Hubei Province, China

Abstract Hydraulic fracturing plays a crucial role in enhancing reserves and production of unconventional oil and gas resources. Injecting fracturing fluids into the ground to improve reservoirs also introduces the risk of inducing earthquakes; thus, monitoring the migration of these fluids is crucial. The microseismic positioning method determines the fracturing fluid by locating microseismic events generated by the fractured rock strata; however, this method is susceptible to errors. Low-resistivity subsurface fluids can directly change electromagnetic field signals, making the electromagnetic method a technically advantageous approach for monitoring the migration of hydraulic fracturing fluids. The monitoring test data of Well Eyangye-2HF show that the time-lapse controlled-source electromagnetic (CSEM) method is suitable for hydraulic fracturing and has good monitoring effects. The results of CSEM method can also compensate for deficiencies in microseismic monitoring. The electric field (Ex) observed using the CSEM method can directly predict the distribution edge of the fracturing fluid, and the anomalous zone of the Exrate of change is consistent with the fracturing-fluid injection parameters and microseismic monitoring results. The analysis of field data and forward simulation, based on electrical logging results, led to the conclusion that hydraulic fracturing operations can cause changes in the resistivity of the target layer and surrounding strata. These changes are attributed to the synergistic effects of formation stress, temperature, and the fracturing fluid. The electric-field changes observed using the CSEM method may be caused by multiple factors; however, use of time-lapse CSEM for monitoring hydraulic fracturing is still feasible.

Prediction of mining-induced seismicity and damage assessment of induced surface buildings in thick and hard key stratum working face: a case study of Liuhuanggou coal mine in China

Thick and hard key stratum working faces are characterized by frequent mine tremors and significant ground tremors during mining which seriously threaten the safety production of the mine. With working face (4-5) 06 of Xinjiang Liuhuanggou Coal Mine as the engineering background, using field investigation, microseismic monitoring, and theoretical analysis, a mining-induced seismicity prediction method and damage assessment of surface buildings for thick and hard key stratum working faces is proposed, which is based on the evolution characteristics of overlying strata spatial structure and the motion state of the key stratum. The results of the study are as follows: 1) The movement law of overlying strata is the basis of mining-induced seismicity prediction for working faces. The magnitude of the risk of mining-induced seismicity occurrence is mainly related to the boundary conditions of the working face, the thickness of the key stratum, the distance from the coal seam to the key stratum, the height of the overlying strata spatial structure, and the fracture step of the key stratum. 2) The mining-induced seismicity energy contains the original accumulation elastic energy of the key stratum, the transfer elastic energy of low rock strata, and the accumulation elastic energy of gravity work. Based on this, a mechanical model of surface building damage induced by the release of mining-induced seismicity energy was established. A ground vibration damage boundary and vibration induction boundary under the action of strong mining-induced seismicity were proposed, and the service life of buildings when they reach the critical damage value under the action of frequent mining-induced seismicity was obtained. 3) The temporal and spatial distribution law of mining-induced seismicity activities in thick and hard key stratum working faces was revealed. According to the results of micro-seismic monitoring, the “zonality” characteristics of the time series and the “transition” law of spatial distribution of mining-induced seismicity verified the reliability of the mining-induced seismicity prediction method. The research results provide a theoretical basis for predicting mining-induced seismicity and assessing the risk of induced disasters during the mining process of thick and hard key stratum working faces, and can provide technical support for mining-induced seismicity prevention and control and safety production in mines with similar conditions.

Microseismic monitoring and experimental study on rockburst in water-rich area of tunnel

During the excavation of tunnels in the Hanjiang-to-Weihe River Diversion Project, many rockbursts occurred in water-rich (WR) areas. To study the mechanism of rockburst under such conditions, the features of microseismic events and rockbursts in WR tunnel sections excavated by both a tunnel boring machine (TBM) and “drilling and blasting” methods were statistically analysed and compared with those of adjacent water-poor (WP) areas. The results show that in both WR and WP areas, rockbursts are related to the classification of surrounding rock and are also affected by the excavation method. The rockbursts in most WP areas are more active than those in the nearby WR areas in sections excavated by TBM. However, the opposite is true under high in situ stress conditions. To explain the microseismic monitoring results, the mechanical behaviour of rock, including short- and long-term mechanical behaviour under different water and loading conditions, was also investigated, and the results show that under high stress conditions, water will accelerate the occurrence of rockburst. Therefore, the appropriate method to control rockburst is stress release, which can be achieved by advanced pilot tunnel excavation, hydraulic fracturing, and presplitting blasting, rather than high-pressure water spraying in the surrounding rock.

Investigation on Mining-Induced Floor Water Inrush from Column and Its Control Based on Microseismic Monitoring

Water inrush is the biggest threat for safe mining in the Ruifeng coalmine, located in North China. In the study mining area, the floor water inrush is mainly caused by mining activities and collapse column. In this article, the mechanical criteria of floor water inrush are obtained based on cusp catastrophe theory, which is used to assess floor water inrush risk in the Ruifeng coalmine. Theoretical analysis shows that floor water inrush is very likely to occur during coal mining without the influence of geological structures. Additionally, FLAC3D was used to simulate the damage of floor strata during the mining face advances. Numerical results show that the water inrush channel occurs in front of the mining face due to the influence of stress concentration. Therefore, a microseismic monitoring system was applied to monitor the formation of water inrush pathway. Field monitoring results show that two water inrush pathways were accurately predicted and positioned. Based on the microseismic monitoring results, target grouting was adopted to prevent water inrush. This study provides significant guidance for the prevention of floor water inrush.

Microseismic Monitoring Technology Developments and Prospects in CCUS Injection Engineering

CO2 geological storage projects are an essential tool for China to achieve the double carbon target of energy savings and emission reductions. In order to safely and effectively control the implementation of injection projects and monitor the dynamics of CO2 injection, multi-dimensional and multi-disciplinary monitoring tools are required. Among them, microseismic monitoring is a key technology for predicting reservoir dynamics and reflecting reservoir geomechanical behavior. Such monitoring has been carried out previously for reservoirs in other countries, but experimental projects are also gradually being developed in China. In this paper, we focus on the research and analysis results of microseismic monitoring of carbon storage projects in various work areas. For different reservoir conditions, we explore combinations of the monitoring implementation methods in China, comparing the differences in each work area. We propose a joint well and ground microseismic monitoring method and a multi-spatial and multi-physical field coupling research system for use in the implementation of domestic demo projects for the future research and development of microseismic monitoring of carbon storage projects. The monitoring program can meet the requirements for certain periodic repeated or continuous observations and can intelligently assess the risk and handle the alert behavior. The foundation is laid for the development of the future microseismic monitoring technology to achieve the goal of developing cost-controllable, permanent, and real-time monitoring equipment. The application of the monitoring system in China has been effective, and this experience can contribute to the development of injection engineering in the future.

An Improved Microseismic Signal Denoising Method of Rock Failure for Deeply Buried Energy Exploration

Microseismic monitoring has become a well-known technique for predicting the mechanisms of rock failure in deeply buried energy exploration, in which noise has a great influence on microseismic monitoring results. We proposed an improved microseismic denoising method based on different wavelet coefficients of useful signal and noise components. First, according to the selection of an appropriate wavelet threshold and threshold function, the useful signal part of original microseismic signal was decomposed many times and reconstructed to achieve denoising. Subsequently, synthetic signals of different types (microseismic noise, microseismic current, microseismic noise current) and with various signal-to-noise ratios (SNRs, −10~10) were used as test data. Evaluation indicators (mean absolute error μ and standard deviation error σ) were established to compare the denoising effect of different denoising methods and verify that the improved method is more effective than the traditional denoising methods (wavelet global threshold, empirical mode decomposition and wavelet transform–empirical mode decomposition). Finally, the proposed method was applied to actual field microseismic data. The results showed that the microseismic signal (with different types of noise) could be fully denoised (car honk, knock, current and construction noise, etc.) without losing useful signals (pure microseismic), suggesting that the proposed approach provides a good basis for the subsequent evaluation and classification of rock burst disasters.

Mechanism and Practice of Multifracturing Using Dynamic Loads in a Low-Permeability Coal Reservoir.

The geological conditions of coal reservoirs in China are complex, and the reservoir permeability is generally lower. Multifracturing is an effective method of improving reservoir permeability and coalbed methane (CBM) production. In this study, two types of dynamic loads, CO2 blasting and a pulse fracturing gun (PF-GUN), were used to conduct multifracturing engineering tests in nine surface CBM wells in the Lu'an mining area in the central and eastern parts of the Qinshui Basin. The curves of pressure versus time of the two dynamic loads were obtained in the laboratory. The prepeak pressurization time of the PF-GUN was 200 ms, and that of the CO2 blasting was 2.05 ms, which just falls in the optimum pressurization time of multifracturing. The microseismic monitoring results revealed that, in terms of the fracture morphology, both the CO2 blasting and PF-GUN loads produced multiple sets of fractures in the near-well zone. In the six wells used for the CO2 blasting tests, an average of three branch fractures were produced outside of the main fracture, and the average angle between the main fracture and the branch fractures exceeded 60°. In the three wells stimulated by PF-GUN, an average of two branch fractures were produced outside of the main fracture, and the average angle between the main fracture and the branch fractures was 25-35°. The multifracture characteristics of the fractures formed via CO2 blasting were more obvious. However, a coal seam is a multifracture reservoir with a large filtration coefficient; the fracture will not extend after reaching the maximum scale under a certain gas displacement condition. Compared with the traditional hydraulic fracturing technique, the nine wells used in the multifracturing tests exhibited an obvious stimulation effect with an average increase of 51.4% in daily production. The results of this study provide an important technical reference for the efficient development of CBM in low- and ultralow-permeability reservoirs.

A new approach to reducing the risk of large-scale man-induced earthquakes based on the results of microseismic monitoring

A new approach to reducing the risk of large-scale man-induced earthquakes based on the results of microseismic monitoring

Diagnosis model of shale gas fracture network fracturing operation pressure curves

Affected by reservoir heterogeneity, developed natural fractures, and bedding fractures, the fracturing pressure curves in fracturing of shale gas horizontal wells present complex shapes. A large amount of information contained in the fracturing curves is still not fully excavated. Based on the theory of shale gas fracture network fracturing, the calculation model of bottom hole net pressure is established by integrating the real-time data such as casing pressure, pump rate, and proppant concentration. Net pressure slope and net pressure index are constructed as key parameters, and the net pressure curve is divided dynamically to describe the mechanical conditions corresponding to the fracture propagation behavior during the fracturing process. Six fracture propagation modes were identified, including fracture network propagation, fracture propagation blockage, normal fracture propagation, fracture propagation long bedding, fracture height growth, and rapid fluid filtration, and then the operation pressure curve diagnosis and identification method were formed for shale gas fracture network fracturing in horizontal wells. The shortcomings of conventional operation curve diagnosis and identification methods are abandoned and the fracture network complexity index is presented. The higher index indicates more time of fracture network propagation and fracture propagation along bedding and the better reservoir stimulation effect. The model is applied to shale gas wells in the southeastern margin of Sichuan Basin, and the average fracture network complexity index of a single well is 0.3, which is in good agreement with the microseismic monitoring results. This proves the good reliability of the method developed. The method is helpful to improve the potential and level of fracturing stimulation of shale reservoirs and is of great significance for improving the post-fracturing evaluation technology of fracture network and guiding the real-time dynamic adjustment of field fracturing operations.

Mechanism and Safety Mining Technology of Overall Instability-Induced Rockbursts of Multi-Coal Seam Spatially Isolated Working Face

The isolated working face has always been the hardest hit area for the occurrence of rockburst, and the spatially isolated working face with multiple coal seams is often easily overlooked due to its hidden nature, which leads to the occurrence of rockburst accidents. This paper takes the Liuhuanggou coal mine (9–15) 08 isolated working face as the engineering background, used field research, theoretical analysis, numerical simulation, and field monitoring, investigated the coal body stress evolution law during mining, and revealed the overall instability rockburst mechanism of the working face and deduced the limit advance distance when the working face was in a state of boundary instability. The research results show the following: (1) with the increase of advancement, the overlying strata on the working face are dynamically transformed from the “C” spatial structure with hollowing on both sides to the “θ” spatial structure with coal body support in the middle; (2) when the dynamic load stress on the coal body of the working face exceeds 1.5 times its integrated compressive strength, there is a risk of overall instability rockburst; (3) when the working face advances to 430 m from the open cutting, the superimposed stress on the coal body exceeds its comprehensive compressive strength and reaches a critical instability state, which is verified by numerical simulation. Based on the analysis of the mechanics and mechanism of the overall instability rockburst of the working face, this paper proposes a set of targeted safe mining solutions: (1) The joint monitoring system of microseism-stress-drilling cutting is adopted to monitor and warn the dangerous rockburst areas. (2) The “rockburst-gas” drilling technique is used to unload the pressure on both sides of the working face and the coal wall. (3) The mining speed shall not exceed 3.2 m/d when the working face is mined before 350 m, and shall not exceed 2.4 m/d when the working face enters the influence area of mined-out LW(4-5)02 (350∼550 m). The microseismic monitoring results and field practice confirmed the reasonableness and effectiveness of the safe mining technology plan.

Mechanism analysis of tunnel collapse in a soft-hard interbedded surrounding rock mass: A case study of the Yangshan Tunnel in China

Tunnels are unavoidably constructed in layered rock masses, and tunnel collapses are often associated with such layering. Two violent collapses at the Yangshan tunnel were analysed in this study to investigate the mechanism of tunnel collapse in a soft-hard interbedded surrounding rock mass. The in situ stress field was obtained by combining hydraulic fracturing tests and multiple regression analysis. The first collapse occurred in the area where extremely high in situ stress and high in situ stress transformed. The extremely high in situ stress state during construction had not received timely attention, and the support measure under the condition of a high-grade surrounding rock mass was still used. Insufficient support strength was the primary cause of the first collapse. The excavation process of the Yangshan tunnel was simulated by a numerical method. The surrounding rock mass of the second collapse, interbedded with soft mudstone and hard sandstone, was dominated by tensile stress. The soft mudstone could not resist the large tensile stress, and the displacement of the tunnel vault increased sharply. Microseismic monitoring was used to highlight the dangerous areas and the types of rock mass failure during the excavation. The microseismic monitoring results were consistent with the numerical simulation results, which were used to describe the mechanism of the second collapse of the Yangshan tunnel. The excavation of a soft-hard interbedded rock mass would lead to tensile stress in the tunnel vault. If the soft rock layer was located in the tunnel vault, tensile failure of the rock mass would occur due to its lower strength.

Simulation of Fracturing and Well Pattern Optimization of Fractured Tight Sandstone Reservoirs

The fracturing and well pattern optimization of fractured reservoirs is a hot spot in petroleum engineering. In this study, taking the tight oil reservoirs of the Yanchang Formation in the Ordos Basin as an example, an optimized plan of hydraulic fracturing and well pattern deployment of the fractured tight reservoir were systematically studied using a fluid-solid coupling model. The results show that the Chang 6 Member of Yanchang Formation mainly develop feldspar sandstone. A large number of vertical fractures are developed in Chang 6 Member, and they have significant shearing properties. The target sandstone reservoir is in the mid-diagenetic A stage. Natural fractures have a significant effect on the extension of hydraulic fractures. When the fracturing direction is parallel to the main strike of natural fractures, the extension distance of the hydraulic fractures is long and the fracturing scale is large; however, when the fracturing direction is orthogonal or oblique to the main strike of natural fractures, the extension distance of the hydraulic fractures is limited. Statistics show that the average length of hydraulic fractures when fracturing parallel to natural fractures is 1.3 times of that when fracturing perpendicular to natural fractures. In addition, after optimization of fracturing parameters, we found that the displacement should be controlled at 2–3 cubic meters/min, and the sand ratio should be controlled at 25%. The smaller the horizontal minimum principal stress difference between the fracturing layer and the shielding layer, the lower the elastic modulus, and the easier it is to control the fracture height. Microseismic monitoring results show that the half-fracture length and fracture height are mainly distributed in 80–140 m and 15–25 m, respectively. The optimized fracture half-length was 120 m and the half-fracture height was 20 m. Finally, the well pattern of the target layer is optimized considering natural fractures. Through this study, we found that the reverse five-spot well pattern is suitable for the efficient development of tight oil reservoirs of the Chang 6 Member in this area.

Stability Analysis of Surrounding Rock in Multi-Discontinuous Hydraulic Tunnel Based on Microseismic Monitoring

The diversion tunnel of a hydropower station is characterized by low quality surrounding rock and weak structural planes. During excavation, rock mass spalling and cracking frequently occur. To evaluate the stability of a rock mass during tunnel excavation, high-precision microseismic monitoring technology was introduced to carry out real-time monitoring. Based on the temporal and spatial distribution characteristics of microseismic events, the main damage areas and their influencing factors of tunnel rock mass were studied. By analyzing the source characteristic parameters of the concentration area of microseismic activities, the rock fracture mechanism of the concentration area was revealed. The 3D numerical model of diversion tunnel was established, and the deformation characteristics of the rock mass under the control of different combination types of weak structural planes were obtained. The results showed that the microseismic event was active between 29 October 2020 and 6 November 2020, and the energy release increased sharply. The main damage areas of the rock mass were located at Stakes K0 + 500–K0 + 600 m. Microseismic source parameters revealed that shear failure or fault-slip failure induced by geological structures had an important influence on the stability of the surrounding rock. The numerical simulation results were consistent with the microseismic monitoring results and indicated that among the three kinds of structural plane combination types, including “upright triangle”, “inverted triangle” and “nearly parallel”, the “upright triangle” structure had the most significant influence on the stability of the surrounding rock. In addition, the maximum displacement of the surrounding rock had a trend of lateral migration to the larger dip angle in the three combined structural plane types. The research results will provide significant references for the safety evaluation and construction design of similar tunnels.

Damage mechanism and stability analysis of rock mass in the high geo-stress tunnel subjected to excavation

High geo-stresses and complicated geological conditions are two typical characteristics in the traffic tunnel of the tailrace surge chamber at the Shuangjiangkou hydropower station, Southwest China. Rock failures such as spalling and rockbursts induced by excavation were encountered repeatedly in this tunnel. To evaluate the stability of rock mass of the tunnel during excavation, the high-precision microseismic (MS) monitoring technology was introduced. Through analysing the tempo-spatial distribution characteristics of MS events, the main damage areas and their influencing factors were studied. The rock fracturing mechanisms of MS clusters were revealed based on the source parameters of MS events. A three-dimensional model was established to obtain the stress and deformation characteristics of rock mass. Results indicated that the MS events mainly occurred between July 10 and 26, 2017 and the energy release was higher during this period. The main damage areas of rock mass were located at the Stake K0 + 110m–K0 + 170m. Weak structures and multi-cavern effect aggravated rock mass damage subjected to excavation-unloading. Tensile failure was the main fracturing type of rock mass. Rock mass damage influenced by weak structures was especially prominent. The numerical simulation results are consistent with the MS monitoring results. The results could provide significant references for safety evaluation and construction design of similar tunnels.

Deformation and Stability Analysis of Surrounding Rock Mass in Deep Underground Cavern Based on Microseismic monitoring and Numerical Modelling

In the excavation and unloading process of large-scale underground engineering, surrounding rock mass damages such as splitting, rock burst and collapse will often occur, affecting the progress and safety of the project. Correctly identifying and delineating the potential instability and failure area of surrounding rock mass and forecasting in advance can effectively reduce the hidden dangers of engineering safety. In this paper, a comprehensive analysis method combining microseismic (MS) monitoring and numerical simulation is proposed to analyse the effect of structure planes on the stability of surrounding rock mass of underground cavern and predict the deformation. First, numerical simulation is applied to analyse and predict the deformation and stress evolution characteristics of surrounding rock mass. Second, based on the MS monitoring data of the underground powerhouse and field investigation, the temporal and spatial distribution characteristics of MS activity were analysed, and the failure region was revealed and delineated. Finally, the effectiveness of the proposed method is verified by the comparison of MS monitoring results and numerical simulation. The results show that MS events are obviously concentrated in the surrounding rock mass damage area. The failure of spalling and rock-burst in underground powerhouse area is the typical brittle failure under high in-situ stress and low strength-to-stress ratio. In essence, the collapse at the top arch of auxiliary powerhouse is caused by the combined action of high in-situ stress and unfavourable structural plane. The results of MS monitoring and numerical simulation have a good corresponding relationship, and the conclusions provide technical guidance for subsequent construction schemes and support measures.

Monitoring the Geometry Morphology of Complex Hydraulic Fracture Network by Using a Multiobjective Inversion Algorithm Based on Decomposition

The fracturing technique is widely used in many fields. Fracture has a greater impact on the movement of fluids in formations. Knowing information about a fracture is key to judging its effect, but detailed information about complex fracture networks is difficult to obtain. In this paper, we propose a new method to describe the shape of a complex fracture network. This method is based on microseismic results and uses the L-system to establish a method for characterizing a complex fracture network. The method also combines with decomposition to construct a new method called the multiobjective fracture network inversion algorithm based on decomposition (MOFNIAD). The coverage of microseismic monitoring results and the degree of fitting of production data are the two objective functions of the inversion fracture network. The multiobjective fracture network inversion algorithm can be optimized to obtain multiple optimal solutions that meet different target weights. Therefore, this paper established a multischeme decision method that approached the ideal solution, sorting technology and AHP to provide theoretical guidance for finding a more ideal fracture network. According to the error of microseismic monitoring results, we established two cases of fracture to verify the proposed method. Judging from the results of the examples, the fracture network finally obtained was similar to actual fractures.