Fault Stability Research Articles

A two-grid simulation framework for fast monitoring of fault stability and ground deformation in multiphase geomechanics

We present a simulation framework for fast monitoring of fault stability and ground deformation in multiphase geomechanics and demonstrate its efficacy for a joint CO2 sequestration and enhanced oil recovery case study. Fault stability is estimated by tracking the time evolution of the change in Coulomb Failure Function at critical points on the fault. The staggered solution algorithm for the coupled problem is augmented with a feature that allows for the flow and geomechanics sub-problems to be solved on two different unstructured tetrahedral grids. We demonstrate the accuracy of the two-grid method on the classical Mandel's problem. For the field scale problem, the geomechanics grid extends to the ground surface while the flow grid is truncated at a depth above which no well activity occurs and the flow field can be assumed to remain unperturbed. This framework reduces the computational burden associated with initialization of pressure and stress fields in the overburden, allows for a study of the critical interaction between overburden and faults, allows for fast renditions of flow-induced ground deformation, and allows choosing flow and geomechanics grid resolutions independently to capture disparate length scales of the underlying physics.

Transient evolution of permeability and friction in a slowly slipping fault activated by fluid pressurization

The mechanisms of permeability and friction evolution in a natural fault are investigated in situ. During three fluid injection experiments at different places in a fault zone, we measured simultaneously the fluid pressure, fault displacements and seismic activity. Changes in fault permeability and friction are then estimated concurrently. Results show that fault permeability increases up to 1.58 order of magnitude as a result of reducing effective normal stress and cumulative dilatant slip, and 19-to-60.8% of the enhancement occurs without seismic emissions. When modeling the fault displacement, we found that a rate-and-state friction and a permeability dependent on both slip and slip velocity together reasonably fit the fault-parallel and fault-normal displacements. This leads to the conclusion that the transient evolution of fault permeability and friction caused by a pressure perturbation exerts a potentially dominant control on fault stability during fluid flow.

Complex in situ stress states in a deep shale gas reservoir in the southern Sichuan Basin, China: From field stress measurements to in situ stress modeling

The deep shale gas reservoir (3,500 ∼ 5,000 m) of the Silurian Longmaxi Formation in the southern Sichuan Basin of China has great potential for exploration and development. Complicated by the fault zones and other geostructures, the in situ stress state of shale gas formations in the southern Sichuan Basin is poorly understood, and field in situ stress measurement data are rare in these deep formations. This study provides vital insights into the in situ stress state based on in situ stress measurements and into the development of a 3D in situ stress model. To accurately model the distributions of in situ stress in the Longmaxi shale gas formation, multiple diagnostic fracturing injection tests (DFIT) were conducted on a 310-km2 shale gas development block. The minimum horizontal principal stress (σhmin) and pore pressure were interpreted from the DFIT data by using pressure transient analysis (PTA) methods. The maximum horizontal principal stress (σhmax) was estimated from multiple sources. The 3D in situ stress model was constructed by using the finite element (FE) method based on a site-specific 3D geological model. The stress model was calibrated and constrained by the results from field DFIT stress measurements.The in situ stress modeling revealed a complex in situ stress state in this deep shale gas formation. The presence of faults changes both the magnitudes and orientations of in situ stress. There is a general decrease in the angle between the σhmax direction and the fault strike in the vicinity of the faults. The stress magnitudes changed significantly near some faults, and the stress state transitioned from a strike-slip faulting stress regime to a reversed faulting stress regime. The reservoir is highly overpressured, with pore pressure gradients ranging from 17 to 20 kPa/m within the reservoir, which causes some faults to have a higher risk of slipping. Based on the 3D stress model, the fault stability and pore pressures that are required to slip the faults were estimated. Because many previous studies about in situ stress modeling have been based on well log-interpreted stresses, this study emphasizes the importance of constructing in situ stress models based on direct field stress measurements and of constraining the stresses through multiple sources and methods.

Study on Improving Fault Stability of Doubly Fed Induction Wind Turbine by Using Active-Power Transient Frequency Characteristics

With the continuous increase in wind power penetration, doubly fed wind turbines can quickly respond to changes in grid frequency, and have particularly important inertia-response characteristics. This article starts with the excitation control principle of a doubly fed induction generator, compares the transient frequency characteristics of the synchronous generator under fault, and proposes that the doubly fed induction generator can control the speed or active power of the generator through excitation. It proves the unique “active power transient frequency characteristics” of doubly fed induction generators. Under different wind speeds, the inertia response capability of the wind turbine is quantified, and the degrees of influence of the inertia time constant and frequency characteristic slope of the doubly fed induction generator on the transient change process are analyzed. Finally, simulation and experiments verify the correctness of the above theory, which provides a basis for significantly improving the transient stability of the power system and realizing the controllability of the transient stability of the power system.

Transformer Fault Diagnosis Method Based on Neural Network and D-S Evidence Theory

Abstract The BP neural network is used to calculate the basic fault probability distribution of the dissolved gas in the transformer oil and the core grounding online monitoring data, and the Dempster-Shafer evidence theory is used to fuse the multi-source information of the basic probability of various types of faults to obtain a transformer fault diagnosis model. Transformer samples are used to verify the model, and the support vector machine and convolutional neural network fault diagnosis models are compared, and it is concluded that the proposed method is better in terms of fault diagnosis accuracy and stability.

Frictional Stability of Metamorphic Epidote in Granitoid Faults Under Hydrothermal Conditions and Implications for Injection‐Induced Seismicity

AbstractThe presence of metamorphic epidote on faults has been implicated in the transition from stable to unstable slip and the nucleation of earthquakes. We present structured laboratory observations of mixed epidote and simulated Pohang granodiorite (analogous to the EGS‐enhanced geothermal system site) gouges to evaluate the impact of heterogeneity and contiguity of epidote‐patch structure on frictional instability. Experiments are at a confining pressure of 110 MPa, pore fluid pressures of 42–63 MPa, temperatures 100–250°C and epidote percentages of 0–100 vol.%. The simulated Pohang granodiorite gouge is frictionally strong (friction coefficient ∼0.71) but transits from velocity‐strengthening to velocity‐weakening at temperatures >150°C. This velocity‐weakening effect is amplified in approximate proportion to increasing epidote content. Modes of epidote precipitation likely control the size and contiguity of the epidote‐only patches and this in turn changes the response of 50:50 epidote‐granodiorite mixed gouges for different geometric configurations. However, 50:50 epidote‐granodiorite mixtures that are variously homogeneously mixed, encapsulated and checkerboarded in their structures are insensitive to their geometries – all reflect the high frictional strength and strong velocity‐weakening response of 100:0 pure epidote. This suggests that the epidote present as thin coatings on fractures/faults can enhance velocity‐weakening behavior, independent of individual patch size and can thereby support the potential seismic reactivation of faults. Considering the frictional and stability properties of epidote at conditions typical of shallow depths, the presence of low‐grade metamorphism exerts a potentially important control on fault stability in granitoids with relevance as a marker mineral for susceptibility to injection‐induced seismicity.

Refined time-shift multiscale diversity entropy: a novel feature extraction algorithm for fault diagnosis of planetary gearbox

Abstract Planetary gearboxes play a critical role in aerospace and heavy industry fields, such as wind turbines, heavy vehicles and construction machines. Intelligent fault diagnosis is significant for safe operation and fault prevention of planetary gearboxes. Recently, multiscale diversity entropy and related entropy methods are proposed to extract features of time series and applied for the fault diagnosis. However, there are still some limitations in fault feature representation and stability for multiscale diversity entropy. To solve the problem, in this paper, a novel planetary gearboxes fault diagnosis method via refined time-shift multiscale diversity entropy (RTSMDE) is proposed. First, a novel entropy algorithm called RTSMDE is proposed to measure the complexity of time series and extract fault features of the vibration signals, which is robust and efficient in performance. Then, the obtained features are utilized to fulfil automatically the fault pattern identifications using support vector machine. To confirm the superiority of the RTSMDE-based fault diagnosis method, simulated signals and experimental studies are constructed and three used widely methods are employed to present a comprehensive comparison. The results indicate that RTSMDE performs best and obtains the highest accuracy.

Changes in Fault Slip Potential Due to Water Injection in the Rongcheng Deep Geothermal Reservoir, Xiong’an New Area, North China

The Xiong’an New Area is abundant in geothermal resources due to its unique geological structure. To address whether large-scale deep geothermal exploitation will induce a fault slip, we first determined the initial in situ stress field using shallow (~4000 m) in situ stress measurements from the North China plain. After characterizing the in situ stress field, we analyzed the initial stability of the main active faults in the sedimentary strata of the Rongcheng deep geothermal reservoir based on the Mohr–Coulomb failure criteria. Assuming that this area will be subjected to forty years of continuous fluid injection, we calculated the excess pore pressure in the deep geothermal reservoir and, subsequently, estimated the fault slip potential of the main active faults in this region from 2021 to 2060. Our results indicated that both the in situ stress field in the shallow crust of the Xiong’an New Area and the Middle-Late Pleistocene active faults will initially maintain a stable state. With constant fluid injection for forty years at six geothermal wells in the Rongcheng deep geothermal reservoir, the maximum superposed excess pore pressure at a single well is 18 MPa; this excess pore pressure value impacts the stress state of faults within 8 km of the well location. These pore pressure perturbations heavily impact the F5-10, F5-11, and F9-2 segments of the Rongcheng uplift boundary fault, with FSP values of 92%, 23%, and 47% in 2060, respectively. Porosity exacts little impact on the fault slip potential on the boundary fault segments of F5-10 and F9-2 in the Rongcheng deep geothermal reservoir, while an enhanced permeability can weaken the FSP values for these faults. The predicted maximum moment magnitude of an induced earthquake due to continuous injection of forty years can be up to Mw 5.0 with a 5% fluid loss in the Rongcheng deep geothermal reservoir. Long-term water injection may increase the ambient thermoelastic stress to the point where faults in a critical (or subcritical) stress state become unstable. The results can provide a reference for geothermal development in terms of injection rate and locations of geothermal wells.

Pre-evaluation of fault stability for underground mining based on geomechanical fault-slip analysis

In underground near-fault mining, fault reactivation is related to many kinds of mine disasters such as mine water inrush, mine earthquakes, gas outburst, rock burst, roof collapse. pre-evaluation of fault stability in the pre-mining phase is of great significance for predicting possible mine fault-related disasters. In this study, based on the geomechanical analysis of fault slip tendency, a technique to the pre-mining evaluation of fault stability under Andersonian stress system was introduced by extending the original Morris’s theory of fault slip-tendency, effects of fault orientation, in-situ stress and pore fluid pressure on fault stability were analyzed. Taking Shilin coal mine as the research background, a pre-evaluation on the stability of all 15 faults distributed in the whole coalfield at the mining depths of 369 and 514 m was conducted, the analysis results demonstrated that all faults in the Shilin coalfield are quite stable at the two mining depths. The work presented in this study is practical and can provide a quantitative geomechanical way in the analysis of pre-mining fault stability and in the risk pre-assessment of fault-related mine disasters.

Fault rock heterogeneity can produce fault weakness and reduce fault stability

Geological heterogeneity is abundant in crustal fault zones; however, its role in controlling the mechanical behaviour of faults is poorly constrained. Here, we present laboratory friction experiments on laterally heterogeneous faults, with patches of strong, rate-weakening quartz gouge and weak, rate-strengthening clay gouge. The experiments show that the heterogeneity leads to a significant reduction in strength and frictional stability in comparison to compositionally identical faults with homogeneously mixed gouges. We identify a combination of weakening effects, including smearing of the weak clay; differential compaction of the two gouges redistributing normal stress; and shear localization producing stress concentrations in the strong quartz patches. The results demonstrate that geological heterogeneity and its evolution can have pronounced effects on fault strength and stability and, by extension, on the occurrence of slow-slip transients versus earthquake ruptures and the characteristics of the resulting events, and should be further studied in lab experiments and earthquake source modelling.

Mantle melting, lithospheric strength and transform fault stability: Insights from the North Atlantic

The offset ridge-transform structure of ocean basins is one of the most prominent expressions of plate tectonics. Yet why this configuration is favored over a continuous divergent boundary has remained unresolved. We examine this issue using mantle Bouguer anomalies (MBAs) from contrasting ridge systems in the North Atlantic. The Reykjanes Ridge north of the Bight transform fault has no transform offsets and is characterized by rapidly propagating melting centers along a linear axis and a continuous MBA low. To the south, the adjoining Mid-Atlantic Ridge has typical ridge segments each underlain by a discrete MBA “bulls-eye” low and offset by transform and non-transform discontinuities. We hypothesize that the pattern of mantle melting, as reflected in the MBAs, creates chemical and rheologic variations in the residual mantle that either favor or hinder transform fault formation. Within ridge segments, mantle melting efficiently extracts water producing dry and strong residual mantle. At segment ends, low extents of melting and inefficient melt extraction preserve damp and weak mantle. On the linear Reykjanes Ridge continuous melting and rapidly propagating melting centers create continuous strong mantle with possibly weak rheologic variations at high angles to the opening direction, not favoring transform faults. In contrast, stable segmented mantle melting on the Mid-Atlantic Ridge forms bands of strong and weak residual mantle aligned in the spreading direction, the latter creating favorable locations for shear deformation and transform faults. Our hypothesis also explains the lack of transform faults at Earth's endmember spreading rates. At ultra-slow ridges, overall melting is limited and irregular and melt extraction is inefficient. At ultra-fast ridges, mantle melting is pervasive and melt extraction is efficient. In both cases, organized spreading-parallel compositional rheological variations do not form and transform faults are not favored. Our model implies that beyond cooling and strengthening with age, the pattern of mantle melting shapes the rheological structure of oceanic lithosphere and the geometry of plate tectonics.

De-risking the energy transition by quantifying the uncertainties in fault stability

Abstract. The operations needed to decarbonize our energy systems increasingly involve faulted rocks in the subsurface. To manage the technical challenges presented by these rocks and the justifiable public concern over induced seismicity, we need to assess the risks. Widely used measures for fault stability, including slip and dilation tendency and fracture susceptibility, can be combined with response surface methodology from engineering and Monte Carlo simulations to produce statistically viable ensembles for the analysis of probability. In this paper, we describe the implementation of this approach using custom-built open-source Python code (pfs – probability of fault slip). The technique is then illustrated using two synthetic examples and two case studies drawn from active or potential sites for geothermal energy in the UK and discussed in the light of induced seismicity focal mechanisms. The analysis of probability highlights key gaps in our knowledge of the stress field, fluid pressures, and rock properties. Scope exists to develop, integrate, and exploit citizen science projects to generate more and better data and simultaneously include the public in the necessary discussions about hazard and risk.

Impact of Far-Field Glacially-Induced Stresses on Fault Stability in the Eastern Paris Basin

Impact of Far-Field Glacially-Induced Stresses on Fault Stability in the Eastern Paris Basin

Uncertainty of Stress Path in Fault Stability Assessment During Co2 Injection: Comparing Smeaheia 3d Geomechanics Model with Analytical Approaches

Uncertainty of Stress Path in Fault Stability Assessment During Co2 Injection: Comparing Smeaheia 3d Geomechanics Model with Analytical Approaches

Mapping CO2 and Pressure Fronts with Optic Fiber for Fault Stability Assessment

Mapping CO2 and Pressure Fronts with Optic Fiber for Fault Stability Assessment

Diagnostic and predictive analysis of production and injection‐induced fault activation

AbstractMitigating the hazard of fault reactivation during CO injection and oil production is important for ensuring environmental sustainability of geologic carbon sequestration and hydrocarbon recovery. Currently, we do not know how the Coulomb Failure Stress (CFS), which is a commonly used metric to assess the potential of inducing slip on a fault, evolves differently for gas injection and oil production scenarios. Using high‐resolution simulations of CO injection‐induced and oil production‐induced fault slip and theories of Coulomb failure and poroelasticity, we address this gap in knowledge. We analyze the fault tractions, slip, and reservoir pressure to understand how a fault is destabilized in successive steps, how hypocenter locations are selected, and how these processes differ between injection and production. We extract and summarize the characteristic features of induced fault activation in terms of the dependence of CFS and the Fault Stress Ratio (FSR) on the dimensionless reservoir pressure perturbation caused by injection or production. We apply the framework to the Farnsworth oilfield in Texas, which is undergoing CO injection and oil production, to disambiguate observations related to fault stability. Finally, we discuss how our framework can inform models that aim to forecast fluid flow‐induced slip and mitigate the induced seismicity hazard in oilfields and gas storage sites.

Nanometric flow and earthquake instability

Fault zones accommodate relative motion between tectonic blocks and control earthquake nucleation. Nanocrystalline fault rocks are ubiquitous in “principal slip zones” indicating that these materials are determining fault stability. However, the rheology of nanocrystalline fault rocks remains poorly constrained. Here, we show that such fault rocks are an order of magnitude weaker than their microcrystalline counterparts when deformed at identical experimental conditions. Weakening of the fault rocks is hence intrinsic, it occurs once nanocrystalline layers form. However, it is difficult to produce “rate weakening” behavior due to the low measured stress exponent, n, of 1.3 ± 0.4 and the low activation energy, Q, of 16,000 ± 14,000 J/mol implying that the material will be strongly “rate strengthening” with a weak temperature sensitivity. Failure of the fault zone nevertheless occurs once these weak layers coalesce in a kinematically favored network. This type of instability is distinct from the frictional instability used to describe crustal earthquakes.

Earthquake Nucleation Along Faults With Heterogeneous Weakening Rate.

The transition from quasistatic slip growth to dynamic rupture propagation constitutes one possible scenario to describe earthquake nucleation. If this transition is rather well understood for homogeneous faults, how the friction properties of multiscale asperities may influence the overall stability of seismogenic faults remains largely unclear. Combining classical nucleation theory and concepts borrowed from condensed matter physics, we propose a comprehensive analytical framework that predicts the influence of heterogeneities of weakening rate on the nucleation length Lc for linearly slip‐dependent friction laws. Model predictions are compared to nucleation lengths measured from 2D dynamic simulations of earthquake nucleation along heterogeneous faults. Our results show that the interplay between frictional properties and the asperity size gives birth to three instability regimes (local, extremal, and homogenized), each related to different nucleation scenarios, and that the influence of heterogeneities at a scale far lower than the nucleation length can be averaged.

Experimental evidence for multiple controls on fault stability and rupture dynamics

The stability of frictional sliding affects the spectrum of fault slip, from slow-slip events to earthquakes. In laboratory experiments, the transition from stable sliding to stick-slip is often explained by the ratio of the stiffness of the loading system to a critical value that depends on effective normal stress and other physical properties. However, theoretical considerations indicate other controls on fault stability that have not been validated experimentally. Here, we exploit the dependence of frictional properties on load-point velocity to explore the dynamics of frictional sliding with gradual variations of frictional properties. We use the period-multiplying and chaotic cycles that appear at the transition between stick-slip and stable sliding as a sensitive indicator of fault stability. In addition to the stiffness ratio, we find that the ratio of the parameters that describe the dependence on velocity and state constitutes another control on the stability of faulting and rupture dynamics. Variations of these two non-dimensional parameters among faults may help explain the wide range of rupture styles and recurrence patterns observed in nature.

Probabilistic-based geomechanical assessment of maximum operating pressure for an underground gas storage reservoir, NW China

One of the most cost-effective ways to increase deliverability and working gas capacity in underground gas storage (UGS) reservoirs is to operate the gas storage at higher pressure (increased delta pressure). Maximum operating pressure (MOP) for a gas storage reservoir depends on several geomechanical factors, including in-situ stresses, stresses induced by local and global pressure changes in the storage reservoir, and the stabilities of the caprock and faults. PetroChina’s Hutubi (HTB) gas storage site is located in the west gate of China’s west–east gas pipeline project. Multiple major faults intersect the gas storage reservoir and caprock, which pose safety concerns for increasing the gas storage pressure. The typical practice in an HTB storage site has been to operate gas storage pressures at levels equal to the original reservoir pressure because of concerns related to caprock integrity and fault stability. To investigate the possibilities of increasing the gas storage pressure, a comprehensive geomechanical study has been conducted. By utilizing the data from rock mechanics laboratory testing, in-situ stress measurements, 3D field-specific geomechanical models, and coupled reservoir-geomechanical simulations, this paper presents probabilistic risk assessment of fault slippage in response to injection-related increases in pore pressure. Uncertainties of relevant geomechanical parameters, such as the frictional strengths of faults, orientation, and magnitudes of in-situ stresses, have been incorporated through Monte Carlo simulations. The authors find that: (1) fault slippage is the controlling factor which determines the limit of MOP at a HTB gas storage site; (2) increasing the gas storage pressure by 12.5% above the initial reservoir pressure (34.5 MPa) leads to approximately 0.6% probability of fault instability that could provide a leakage pathway. Thus, from a geomechanical perspective, operating the storage pressure at initial reservoir pressure at a HTB gas storage field is an overly conservative approach that leads to under-utilization of the existing storage capability. This paper provides the first integrative geomechanical study for increasing MOP for large underground gas storage reservoir in China. The methodology as well as comprehensive data repository are potential sources for future geomechanical studies on similar underground gas storage projects.