Cavern Collapse Research Articles

Characteristics of Energy Dissipation in T-Shaped Fractured Rocks under Different Loading Rates

T-shaped fractured rocks in the engineering rock mass with different inclination angles, quantities, and cross patterns will cause slope landslides, cavern collapse, roof fall, and other disasters under the action of external forces. Deformation evolution of the T-shaped fractured rock is also significant for monitoring the stability of rock engineering structures. In this paper, the compression test of T-shaped fracture specimens was carried out under different loading rates. By modulating both the fracture inclination angle and the loading rate, the attributes pertaining to energy dissipation in the T-shaped fractured specimen were scrupulously scrutinized and subsequently expounded upon. The difference in the energy characteristics between fractured rock and intact rock was investigated to understand the deformation evolution of T-shaped fractured rock samples. The results show that when the fracture angle is 45° and 90°, the elastic strain energy and dissipated energy decrease as the secondary fracture angle increases. At the peak point, as the secondary fracture angle increases from 0°, the total absorbed energy, elastic strain energy, and dissipated energy of the T-shaped fractured rock increase, the ratio Ue/U of elastic strain energy to total energy increases, and the ratio Ud/U of dissipated energy to total energy decreases. The increase in loading rate leads to an increase in Ue/U and a decrease in Ud/U at the peak point of the T-shaped fractured rock specimen. The increase in loading rate leads to an increase in the total absorbed energy and elastic energy at the peak point of the T-shaped fractured rock, while the dissipated energy decreases. Investigative endeavors into the mechanics and energetic attributes of T-shaped fractured rocks bestow pragmatic and directive significance upon the safety assessment and stability prognostication of sundry geological undertakings.

Stability Analysis of Cavern Collapse in Fractured-Cavity Oil Reservoirs

Fractured-vuggy oil reservoirs, with the decrease of formation pressure during the exploitation process, lead to the collapse of caverns or the closure of sizeable fractured oil channels, which seriously affects oil well production and the recovery rate of oil reservoirs. True three-dimensional geomechanical model tests were carried out to avoid the impact of cave collapse and fracture closure on oil well production. Taking the Tahe Oilfield in Xinjiang area of China as the engineering background, we researched the collapse failure mechanism of the karstic caves in fractured-cavity oil reservoirs and the evolution of fracture closure through a true three-dimensional geomechanical model test and the numerical simulation software RFPA. The collapse failure modes of caverns with and without prefabricated cracks were revealed, along with the displacement and stress changes during cave collapse and the mechanism of cave collapse failure. Our study revealed the mechanism of the interaction between cracks and the cave. The research results show that prefabricated cracks reduce the roof of the cave’s bearing capacity, making the karst cave collapse with incomplete cracks. The impact of the collapse is much smaller than the cavern without prefabricated cracks. The crack closure extends from the near end to the far end. The research results will provide necessary theoretical support for the large-scale safe extraction of deep petroleum resources, increase oil production in China, and have important theoretical significance and engineering application value.

Evolution of surface deformation related to salt-extraction-caused sinkholes in Solotvyno (Ukraine) revealed by Sentinel-1 radar interferometry

Abstract. Rock salt has remarkable mechanical properties and high economic importance; however, the strength of salt compared to other rocks makes it a rather vulnerable material. Human activities could lead to acceleration of the dissolution of soluble rock salt and collapse of subsurface caverns. Although sinkhole development can be considered a local geological disaster regarding the characteristic size of surface depressions, the deformations can result in catastrophic events. In this study we report the spatiotemporal evolution of surface deformation in the Solotvyno salt mine area in Ukraine based on Sentinel-1 interferometric synthetic aperture radar measurements. Although the mining operations were finished in 2010, several sinkholes have been opened up since then. Our results show that despite the enormous risk management efforts, the sinkholes continue to expand with a maximum line-of-sight deformation rate of 5 cm/yr. The deformation time series show a rather linear feature, and unfortunately no slowdown of the processes can be recognized based on the investigated 4.5-year-long data set. We utilized both ascending and descending satellite passes to discriminate the horizontal and vertical deformations, and our results revealed that vertical deformation is much more pronounced in the area. Analytical source modeling confirmed that the complex deformation pattern observed by Sentinel-1 radar interferometry has a direct connection to the former mining activity and is confined to the mining territory. With the 6 d repetition time of Sentinel-1 observations, the evolution of surface changes can be detected in quasi real time, which can facilitate disaster response and recovery.

Mirco-earthquake source depth detection using machine learning techniques

Discrimination of mirco-earthquake on source depth plays an important role in the field of micro-seismic monitoring. Conventional machine learning methods for data classification rely on carefully hand-engineered features that are vulnerable to low signal-to-noise ratio. Convolutional neural networks (CNNs) demonstrate some merits in dealing with structured data modelling where a set of meaningful features can be automatically extracted from sample learning. This paper explores the use of machine learning techniques for discrimination between deep and shallow mirco-seismic events. A benchmarked dataset including 444 micro-earthquakes from an underground cavern collapse in South Louisiana is employed for performance evaluation in this study, where several feature-based classifiers are compared against the CNN classifier. Empirical results show that the deep learning method outperforms the conventional classification techniques in discriminating the source depth.

Augmentation of Vortex Cavitator Performance by the Use of Co-Directional Swirl of the Flux after Vortex Chamber

Abstract Background: Acoustic cavitation is the creation and collapse of cavitation caverns in liquid in an acoustic field with a frequency of f = 1–3 kHz. The acoustic-cavitation processes manifest themselves during the collapse phase, with high pressure gradient continuum deformation, with a multiple transformation of energy forms. Liquid whistles are widely used to create an acoustic field of high power, but their efficiency only reaches 6–12%. We propose a liquid whistle in the form of a vortex cavitator (analogue of the Ranque vortex tube) with a rotating body in which a reduction in the input power is predicted. Objective: Verification of feasibility of using a rotating body in a vortex cavitator with a rotation co-directional to the operational pump impeller. Method: The method for identifying the feasibility of using a rotating body is to exclude body from the prototype and directly connect vortex chamber outlet with the pump inlet, which ensures the most complete preservation of co-directional vortex component of the flux entering the pump impeller. Results: The results of experimental studies confirmed the validity of the hypothesis to a greater extent, since we achieved an increase in pressure at the outlet of the pump and a decrease in power at the drive relative to the original design. Conclusions: The feasibility of designing the vortex cavitator body with rotation capability has been established, which will provide a reduction in input power of at least 30% by a rotation of the body, co-directional with the impeller.

Study on the mechanism of roof collapse and leakage of horizontal cavern in thinly bedded salt rocks

In the thinly bedded salt formations, due to the limited thickness of the salt strata along with the fast upward dissolution, the collapse of the non-salt roof of the cavern often occurs, leading to the leakage of brine and instability of the cavern as well as other undesirable geological consequences. To investigate the gypsum-salt interbedded roof collapse, the failure mechanism and the control methods of Dainan siltstone, which is the indirect roof of the horizontal salt caverns in Zhaoji Salt Mine, were thoroughly investigated. Through the drilled cores from this mine, a series of contrast tests such as SEM, EDS, XRD, soaking, and nuclear magnetic properties were carried out to study the physical properties of the salt layer and non-salt layer of the cavern roof and to determine the cavern roof collapse and leakage mechanism. Using the Comsol software, the model of a horizontal cavern roof leakage was established, and the leakage range and leakage of brine at different time were analyzed, as well as the safety problems caused by it. Finally, some suggestions have been provided for the leakage control of the cavern roof in bedded salt cavern.

Salt dissolution potential estimated from two-dimensional vertical thermohaline flow and transport modeling along a Transylvanian salt diapir, Romania

The presented study explores the salt dissolution potential of fluids around a salt diapir in the Transylvanian town of Ocna Mures, Romania, which is facing land-collapse hazards related to salt mining activities. A structural three-dimensional (3D) model of the salt diapir, the adjacent basin sediments, and the mining galleries was developed based on existing maps, borehole data, own field observations, and geological publications of the Transylvanian Basin. The salt dissolution potential was estimated from 2D vertical thermohaline flow and transport model scenarios along the southeastern flank of the diapir. Results showed that the following factors increase the salt dissolution capacity along the upper 180 m of the diapir: (1) the presence of more permeable Quaternary alluvial sediments in connection with a fault zone of higher permeability along the diapir, and (2) the presence of more permeable sandstone units within the Miocene sediments in the east of the diapir, which provide freshwater access to the upper parts of the diapir. Thermohaline simulation with viscosity variation of the fluid, instead of a constant viscosity, influences the resulting salt fluxes by up to 50% within studied temperature ranges of 10–60 °C in the model domain. The range of theoretical dissolution rates along the upper 180 m of the diapir supports the hypothesis that cavern collapse is more likely to occur where cavern side walls have already been mined to almost no remaining side walls of rock salt, which is the case in the southeastern part of the diapir.

Geomechanical investigation of roof failure of China's first gas storage salt cavern

JK-A cavern is the first operating cavern of Jintan underground gas storage, which is also the first operating salt cavern gas storage in China. In 2015, the sonar survey carried out under working conditions indicated that a massive roof collapse of the JK-A cavern had taken place. To prevent similar accidents from happening again, finding the causes and the collapse time of JK-A cavern would be valuable. By using the target salt formation information, rock properties, and monitored gas pressure, a 3D geomechanical model of JK-A cavern has been built. The responses of the rock mass around JK-A cavern under leaching, sealing tests, debrining, and gas injection/delivery have been investigated. A safety evaluation system consisting of cavern volume shrinkage, dilatancy safety factor, displacement, vertical stress, and equivalent strain is proposed to find the likely collapse time and the reason for the JK-A cavern roof collapse. Results show that there are two main causes for the cavern roof failure. (1) A large-span flat roof, detrimental for bearing loads. Once local damage takes place, massive collapse may be triggered by the self-weight of the loosened rock. (2) The decrease speed of the internal gas pressure is too fast. The loads applied to the cavern roof cannot be transferred in a timely fashion, which causes a stress concentration zone to form, and local damage develops. The roof collapse of JK-A cavern took place 1.3 years after its gas injection/delivery started. Numerical analysis results show that dilatancy safety factor and vertical stress have high accuracy and sensitivity in the prediction of the cavern roof collapse. There is a high potential for a roof collapse of JK-A cavern to happen again. We propose that the gas pressure and pressure decrease rate are to be controlled strictly in the later operating period to prevent any collapses in the future.

Experimental Study of Ground Subsidence Mechanism Caused by Sewer Pipe Cracks

Simultaneous leakage of soil particles and groundwater into underground vacant spaces caused by sewer pipe defects and cracks can cause underground cavities and even ground collapse on the surface. Numerical studies of this problem are limited, and the mechanism of ground subsidence caused by leakage of soil and groundwater into construction sites has yet to be verified. In this study, the factors affecting the mechanism of ground subsidence are verified via physical modelling of a sewer pipe defect. Through visual inspection and particle image velocimetry analysis, it has been found that the groundwater flow direction, hydraulic gradient around the leakage point, and strength of the ground to support itself are the main factors that dominate the mechanism of ground subsidence. These factors can affect ground deformation and cavity expansion direction, ground subsidence development and collapse speed, and occurrence of sudden ground collapse, respectively. It was also found that pore pressure fluctuates when cavity cavern collapse and cavity expansion occur.

Seismic reflection imaging of karst in the Persian Gulf: Implications for the characterization of carbonate reservoirs

Karstification positively and negatively affects the quality of carbonate reservoirs; for example, dissolution and brecciation can increase porosity and permeability, whereas cavern collapse or cementation driven by postkarstification fluid flow may occlude porosity and reduce permeability. Karst may also pose challenges to drilling because of the unpredictable and highly variable porosity and permeability structure of the rock and the corresponding difficulty in predicting drilling mud weight. When combined, outcrop, petrographic, and geochemical data can constrain the style, distribution, and origin of seismic-scale karst, which may provide an improved understanding of carbonate reservoir architecture and allow development of safer drilling programs. However, relatively few studies have used seismic reflection data to characterize the regional development of seismic-scale karst features. In this study we use time-migrated two-dimensional seismic reflection data to determine the distribution, scale, and genesis of karst in a 3-km-thick (9800-ft-thick), Jurassic–Miocene carbonate-dominated succession in the Persian Gulf. We map 43 seismic-scale karst features, which are expressed as vertical pipe columns of chaotic reflections capped by downward-deflected depressions that are onlapped by overlying strata. The columns are up to 2 km (6500 ft) tall, spanning the Upper Jurassic to Upper Cretaceous succession, and are up to 5.5 km (18,000 ft) in diameter. We interpret these pipes to have formed in response to hypogene karstification by fluids focused along preexisting faults, with hypogene-generated depressions enhanced by epigene processes during key intervals of exposure. Our study indicates that seismic reflection data can and should be used in conjunction with petrographic and geochemical techniques to determine the presence of hypogene karst plays and to help improve the characterization of carbonate reservoirs and associated drilling hazards.

A combination of waveform inversion and reverse-time modelling for microseismic event characterization in complex salt structures

The increased emission of greenhouse gases into the atmosphere, causing climate changes, leads to a strong requirement of renewable energy resources. However, they are intermittent and need buffer storage to bridge the time gap between production and public demands. The injection of gas (e.g. compressed air or hydrogen) in sealed underground structures like salt caverns is one approach to solve this problem. Possible risks related to cavern storage are gas leakages from the injection tube into the surrounding sediments, material failure in salt rock surrounding the cavern during irregular operation and in the most extreme case a partial collapse of the cavern. For the early detection of these problems, a geophysical monitoring strategy is required. The objective of this paper was to map possible leakage paths outside of the salt structures and local failures within the cavern walls by the localization of crack-induced microseismic events. Classical methods require arrival time picking and phase identification. An alternative approach is elastic reverse-time modelling (RTMOD), where the recorded microseismic events are numerically backpropagated from the receiver positions into the elastic underground model. The resulting seismic wavefield focuses at the location of the event, which can be subsequently imaged by estimating the maximum of the seismic energy at each underground point. However, the success of this approach highly depends on the used elastic background model. In case of complex salt bodies, the strong velocity contrast between the salt and the surrounding sediments is a major problem. Therefore, we propose a combined monitoring approach, consisting of a seismic full waveform inversion of active source reflection seismic data to accurately image the background velocity model and subsequent RTMOD for the microseismic event localization. Accuracy and sensitivity with respect to the acquisition geometry and random noise will be demonstrated using a complex benchmark model. Furthermore, the localization accuracy is discussed for three different scenarios covering the detection of a partial cavern collapse, a gas leakage and the occurrence of cracks within the cavern wall due to extreme loading conditions during irregular operation.

Flat-plate roof collapse of shallow caverns and protective measures: a case study of Longyou ancient siltstone caverns

Longyou rock caverns were found and were out of water in June 1992, which located in a small hill near to the north bank of Qujiang River, Zhejiang province, China. A series of flat-plate collapses occurred in the roof of cavern No. 24 from 30 September to 4 October in 2010; and finally, a daylight collapse appeared in the roof with a volume of 250 m3. Site investigation and laboratory tests on the surrounding siltstone indicate that the roof collapse can be attributed to the following adverse factors: (1) large span and shallow depth, (2) rock strength reduction due to water saturation, (3) rock weathering, and (4) dynamic loading caused by the walking of a large number of visitors. In order to protect the complete caverns Nos. 1–5 which are currently open to visitors, it is suggested to adopt integrated protection system, including (1) selecting the best tour routes and monitor system, (2) surface water drainage system, (3) beam–pillar supporting system, and (4) effective measures to slow weathering.

Moment tensor inversion for two micro-earthquakes occurring inside the Háje gas storage facilities, Czech Republic

Broadband data from the Přibram seismological network was used to investigate the source of two earthquakes, with magnitudes M w = 0.2 and 0.4 respectively, occurring in the period of October–November 2009 in the Haje natural gas storage area (Czech Republic). Both events were located inside the limits of the storage area and at depths similar to those of the underground caverns where the gas is stored. We applied an inversion technique using the software ISOLA for moment tensor retrieval in order to assess the source process of both events and recognize whether a significant isotropic component existed that could be interpreted as a possible cavern collapse. We also performed an uncertainty analysis so as to confirm the reliability of the focal mechanism solutions and we controlled the consistency between the inverted focal mechanisms and those calculated using the P-waves first motions. Our results showed that the nodal plane orientation, the centroid depth, and the magnitude remained stable. Furthermore, we calculated synthetic waveforms for collapse-type ruptures and compared them with the original records. The match between the synthetic and the original data was very poor supporting the interpretation of the shear character of the events. The combination of the inversion results, which indicated significant double-couple components and of the synthetic tests, which supported the inexistence of an isotropic component at the source, led to the conclusion that the possibility of rocks falling from the ceiling of the caverns or a cavern collapse is highly unlikely.

Analysis on Earthquake Damage Forms and Affecting Factors of Underground Cavern

According to the information about underground damage in earthquake all over the world, it is summarized that earthquake damage is caused by mountain slope failure, the collapse of cavern’s export, dislocation damage of cavern’s cross-section, great displacement along the fault intersection, spalling of surrounding rock, disturbance or deformation of the support and lining system, et. The earthquake damage factors of underground cavern were analyzed by using numerical simulation techniques. It is shown that when the earthquake intensity value is high, or the initial stress field is small, or lining stiffness is high, the underground will more easily be destroyed by earthquake. Compared with the square tunnel and horseshoe-shaped tunnel, dynamic stability of circular tunnel is better. The export of cavern is vulnerable to earthquake damage.

Evidence for remotely triggered microearthquakes during salt cavern collapse

SUMMARY Microseismicity is a good indicator of spatio-temporal evolution of physical properties of rocks prior to catastrophic events like volcanic eruptions or landslides and may be triggered by a number of causes including dynamic characteristics of processes in play or/and external forces. We show evidence of triggered microseismicity observed in the vicinity of an underground salt cavern prone to collapse by a remote M∼ 7.2 earthquake, which occurred ∼12 000 km away. High-dynamic range broad-band records reveal the strong time-correlation between a dramatic change in the rate of local high-frequency microseismicity and the passage of low-frequency seismic waves, including body, Love and Rayleigh surface waves. Pressure was lowered in the cavern by pumping operations of brine out of the cavern. We demonstrate the near critical state of the cavern before the collapse by means of 2-D axisymmetric elastic finite-element simulations. On this basis, we show that the increment of stress necessary for the failure of the Dolomite layer, which ensures the stability of the whole system, is of the same order of magnitude as the maximum dynamic stress magnitude observed during the passage of the earthquakes waves. This suggests that the stress oscillations due to the seismic waves correlated with the recorded microearthquakes induced damage of the overburden, which eventually led to the collapse of the salt cavern. We show that the contribution of Rayleigh waves is the most efficient to trigger microseismicity at periods close to the natural fundamental frequency of the cavern system found at about 10–20 s by investigating the impulse response of the cavern + overburden + brine system.

Multi-parameter monitoring of a solution mining cavern collapse: First insight of precursors

In order to improve our understanding of the large-scale ground failure phenomena caused by old underground mining works, a solution mine was instrumented in 2004 prior to its collapse as part of the mining scheme. A permanent monitoring system was set up, including a high-resolution microseismic network linked to a surface field-displacement measurement system. The large amount of data transmitted for on-line processing provided daily insight into the evolution of the geological system. First, microseismic activity showed upward progressive failure migration throughout 2008 without any significant surface movement. Second, after two days of intensive brine extraction, a high microseismicity and energy release rate marked the failure of a thin and very rigid bed at a depth of 120 m. This failure occurred 24 hours before the final collapse; it was followed by transient brine pressure signals, and by acceleration of the surface subsidence rate, reaching 1 m/h in the final phase.

Twelve-year monitoring of the idle Etrez salt cavern

In 2001, a paper describing a salt-cavern abandonment test was published in this journal [1]. This test was motivated by concerns raised by the long-term behaviour of abandoned salt caverns, which are leached out from salt formations, having depths ranging from 200 to 2000 m and typical volumes from 10,000 to 1,000,000 m. Thousands of such caverns have been created, and are used for brine production and/or hydrocarbon storage. These caverns eventually will be abandoned: the cavern will be filled with brine, a special plug will be set at the casing seat, and cement will be poured in the well. A large ‘‘bubble’’ of saturated brine will be isolated. The long-term evolution of this brine is a serious concern. After cavern plugging, cavern brine pressure will increase, as has been proved by numerous ‘‘shut-in pressure tests’’ performed worldwide [2,3]. The final value of cavern brine pressure is of utmost importance from the perspective of environmental protection. In some circumstances, brine pressure may reach a figure larger than the geostatic pressure, leading to hydrofracturing: brine will flow upward through fractures to shallow water-bearing strata, leading to water pollution, ground subsidence and possible cavern collapse. Pressure evolution in a closed cavern results from four main factors: cavern creep closure, brine thermal expansion, brine permeation through the cavern walls and brine leaks.

Microseismic Monitoring of a Controlled Collapse in Field II at Ocnele Mari, Romania

Several decades of faulty exploitation of salt through solution mining led to the creation of an underground cavern containing several million cubic meters of brine. To eliminate the huge hazard near a densely inhabited area, a technical solution was implemented to resolve this instability concern through the controlled collapse of the roof while pumping the brine out and filling the cavern with sterile. To supervise this, an area of over 1 km2 was monitored with a staggered array of 36 one-component, 15 Hz geophones installed in 12 boreholes about 160–360 m deep. A total of 2,392 seismic events with Mw −2.6 to 0.2 occurred from July 2005 to March 2006, located within an average accuracy of 18 m. The b-value of the frequency-magnitude distribution exhibited a time variation from 0.5 to 1 and from there to 1.5, suggesting that the collapse initiated as a linear fracture pattern, followed by shear planar fragmentations and finally a 3-D failure process. The brunching ratio of seismicity is indicative of a super-critical process, except for a short period in mid-February when temporary stability existed. Event relocation through the use of a collapsing technique outlines that major clusters of seismicity were associated with the main cavern collapse, whereas smaller clusters were generated by the fracturing of smaller size nearby caverns. It is shown that one-component recordings allow for stable and reliable point source event mechanism solutions through automatic moment tensor inversion using time domain estimates of low frequency amplitudes with first polarities attached. Detailed analysis of failure mechanism components uses 912 solutions with conditional number CN 0.5. The largest pure shear (DC) components characterize the events surrounding the cavern ceiling, which exhibit normal and strike-slip failures. The majority of mechanism solutions include up to 30% explosional failure components, which correspond to roof caving under gravitational collapsing. The largest vertical deformation rate relates closely to the cavern roof and floor, as well as the rest of the salt formation, whereas the horizontal deformation rate is most prominent in areas of detected collapses.

Use of cavitation for cracking hydrocarbons

It was shown that at the beginning of the stage of disappearance of cavitation, the volume fraction of caverns should not exceed the threshold value: ∼10%. When this condition is violated, more energy is expended on interaction of caverns and not on creation of anomalously high pressure values and temperatures in the vicinity of cavern collapse points. It was shown that it is necessary to attain the maximum cavern density in the cavitation medium of the petroleum product in order to intensify cracking.

Unusual cases of mining subsidence from Great Britain, Germany and Colombia

Subsidence involves sinking of the ground surface. The movement generally is localised and may or may not involve some amount of horizontal movement. It may be rapid or take place gradually over a period of time. Subsidence may be brought about by natural causes as, for example, when the roof of a cavern in limestone is weakened to the extent that it is no longer self-supporting and so collapses. Other more exotic examples are associated with volcanism and earthquakes. Perhaps, however, subsidence is more frequently associated with movements caused by mining activities, that is, the removal of mineral deposits, be they in solid, liquid or gaseous form, from within the ground. Mining is one of the earliest activities of man and has taken place in every continent. Subsidence also can result from subsurface excavations such as tunnels, caverns, cellars and sewers. One of the added problems of some forms of subsidence is that it is impossible to predict. This, coupled with the fact that the presence of potentially collapsible voids may be unknown, unrecorded or simply forgotten about further complicates the problem. The objectives of this paper are to document and draw attention to a number of unusual examples of subsidence. These have been generated due to the mining of chalk in southern England, the collapse of slate mines and caverns in Germany, the underground extraction of evaporites in Northern Ireland, fault reactivation in Wales and northern England and the extensive extraction of pumice on the flanks of Galeras volcano in Colombia.