Effective Normal Stress Research Articles

Semi-constant source radius and variable stress drop of repeating earthquakes: A fluid-invasion hypothesis for earthquake generation

Summary Earthquakes with very similar waveforms have been recognized as repeating earthquakes, or repeaters, which are interpreted as repeated ruptures of an isolated, frictionally locked asperity patch that are caused by stress loading due to aseismic slip in the surrounding stable regime of the fault. Ideally, the repeated ruptures of the same asperity result in the same source dimension and stress drop. However, significant variations in the stress drop have been observed during fluid-injection experiments even for a single repeater sequence. This study focuses repeater sequences in three different tectonic regimes beneath the Japanese Islands and investigates variations in the source dimension and stress drop in the same repeater sequences. The obtained results show that the variation in the source radius is mostly within 20–30% in individual repeater sequences but the stress drop is largely variable by one order of magnitude or more. We propose a fluid-invasion hypothesis to explain our observations. When pore-fluid pressures gradually invade into an asperity, the edge of the asperity tends to behave as aseismic because of the reduction of the effective normal stress to values below the critical value of effective normal stress, which makes the source radius somewhat smaller. Simultaneously, the amount of the fluid invasion into the asperity fluctuates the shear strength of the asperity, with the stress drop being smaller for higher pore-fluid pressures. The reduced source radius and stress drop expected from the fluid-invasion hypothesis can explain positive correlations between the source radius and seismic moment. We propose a conceptual model for earthquake generation by involving the triggering of aseismic slip by the enhancement of pore-fluid pressures and subsequent rupture of asperity by the stress loading due to the aseismic slip, which can be potentially applied to the genesis of repeaters and non-repeaters.

Shale Fracture Static Stability Evaluation Method Based on 3D Discrete Fracture Model: A Case Study on the Luzhou Area in Southern Sichuan Basin, SW China

Abstract The stress interference in deep shale may lead to shale fracturing and instability and consequently results in engineering risks such as casing deformation in horizontal wells. Therefore, in the early stages of shale gas exploration and development, it is of great significance to evaluate the stability of shale fractures in advance. The stability evaluation of shale reservoir faults can be mainly divided into two categories at present: quantitative evaluation of single faults and qualitative evaluation of wide-range faults. The quantitative evaluation of single faults primarily relies on numerical simulation techniques, while the qualitative evaluation of wide-range faults primarily employs geophysical methods. Both methods are difficult to meet the needs of shale gas exploration and development. The shale fracture stability evaluation method based on 3D discrete fracture model which is developed in this article can quantitatively evaluate the static stability of wide-range shale fractures in the early stage of exploration and development. In this article, seismic geometric attributes were calculated, and fracture seismic facies was established by means of the Bayesian probabilistic cluster analysis method. Then, 3D discrete fracture modeling under the constraint of fracture seismic facies was performed to establish a discrete fracture model. Finally, according to the Mohr–Coulomb criterion, the shear stress and effective normal stress of discrete fracture were calculated by using the formation pressure data in other exploration and development results and the regional in situ stress obtained through seismic prestack inversion. And thus, the stability evaluation of the fractures in shale reservoirs was eventually completed. In addition, this method was verified by using the casing deformation points in some actual shale-gas horizontal wells, the actual small fault points, and microseismic data. It indicates that the method has higher accuracy and is better applicable to the early shale gas exploration and development.

Seasonal slow slip in landslides as a window into the frictional rheology of creeping shear zones.

Whether Earth materials exhibit frictional creep or catastrophic failure is a crucial but unresolved problem in predicting landslide and earthquake hazards. Here, we show that field-scale observations of sliding velocity and pore water pressure at two creeping landslides are explained by velocity-strengthening friction, in close agreement with laboratory measurements on similar materials. This suggests that the rate-strengthening friction commonly measured in clay-rich materials may govern episodic slow slip in landslides, in addition to tectonic faults. Further, our results show more generally that transient slow slip can arise in velocity-strengthening materials from modulation of effective normal stress through pore pressure fluctuations. This challenges the idea that episodic slow slip requires a narrow range of transitional frictional properties near the stability threshold, or pore pressure feedbacks operating on initially unstable frictional slip.

Development of a new soil–structure contact stress sensor for underground construction applications

This paper describes the design, development, calibration and validation of a novel soil–structure contact stress sensor. The new sensor design combines a novel operating principle, fibre Bragg grating (FBG) strain sensing and data-driven mapping techniques to create a multi-axis contact stress sensor that is both economical and suitably robust for deployment in underground construction applications. The instrumentation process is informed using a ‘virtual twin’ of the sensor in which synthetic data is generated by extracting and interpolating virtual FBG strains obtained from a large number of 3D finite-element calculations. A physical prototype is subsequently developed to demonstrate proof of concept. Results from laboratory validation tests give confidence in the sensor's ability to provide accurate contact stress measurements in typical soil–structure interface shear applications. In particular, the novel sensor structure and operating principle was shown to achieve excellent measurement of effective normal stress. The new sensor design harnesses many of the inherent benefits of FBGs including immunity to electromagnetic noise and water ingress, and the use a single lightweight cable and connector, which significantly simplifies installation on site compared to electrical multi-axis sensors.

Frictional Strength and Frictional Instability of Glaucophane Gouges at Blueschist Temperatures Support Diverse Modes of Fault Slip From Slow Slip Events to Moderate‐Sized Earthquakes

AbstractFluid overpressure from the water released by subducted sediments and oceanic crust is an important mechanism for generating earthquakes via brittle failure and frictional instability. If unstable, such fault materials may also host diverse fault reactivation mechanisms from slow slip events to moderate‐sized earthquakes in cold subduction zones. We examine this hypothesis for glaucophane gouge ‐ a key index minerals for blueschist facies ‐ at lower confining stresses where behavior is poorly understood. We measure friction and stability at temperatures of 100°C–500°C and effective normal stresses of 50–200 MPa, to explore the controls of temperature, stresses and excess pore fluid pressures on fault friction. The frictional coefficient of glaucophane at representative temperatures and stresses is ∼0.70, insensitive to temperature but with a slight increase at lower effective stresses. Elevating temperature promotes a transition from velocity‐strengthening to weak velocity‐weakening behavior, indicating the destabilizing effect of high temperature downdip in subduction zones. Reducing effective normal stress or concomitantly elevating pore fluid pressure further strengthens the velocity‐weakening response and would be manifest as moderate‐sized earthquakes. Our results support the potential for enhanced unstable sliding of glaucophane gouges at lower effective stresses and blueschist facies temperatures ‐ with weak to moderate velocity‐weakening responses upon fluid pressurization vital for understanding the abundance of slow slip to moderate‐sized earthquakes apparent in cold subduction zones.

Fault‐Valve Instability: A Mechanism for Slow Slip Events

AbstractGeophysical and geological studies provide evidence for cyclic changes in fault‐zone pore fluid pressure that synchronize with or at least modulate slip events. A hypothesized explanation is fault valving arising from temporal changes in fault zone permeability. In our study, we investigate how the coupled dynamics of rate and state friction, along‐fault fluid flow, and permeability evolution can produce slow slip events. Permeability decreases with time, and increases with slip. Linear stability analysis shows that steady slip with constant fluid flow along the fault zone is unstable to perturbations, even for velocity‐strengthening friction with no state evolution, if the background flow is sufficiently high. We refer to this instability as the “fault valve instability.” The propagation speed of the fluid pressure and slip pulse, which scales with permeability enhancement, can be much higher than expected from linear pressure diffusion. Two‐dimensional simulations with spatially uniform properties show that the fault valve instability develops into slow slip events, in the form of aseismic slip pulses that propagate in the direction of fluid flow. We also perform earthquake sequence simulations on a megathrust fault, taking into account depth‐dependent frictional and hydrological properties. The simulations produce quasi‐periodic slow slip events from the fault valve instability below the seismogenic zone, in both velocity‐weakening and velocity‐strengthening regions, for a wide range of effective normal stresses. A separation of slow slip events from the seismogenic zone, which is observed in some subduction zones, is reproduced when assuming a fluid sink around the mantle wedge corner.

Unstable Sliding of Plagioclase Gouge and Deformation Mechanisms Under Hydrothermal Conditions With Effective Normal Stresses of 100–300 MPa

AbstractPlagioclase feldspar is a major mineral in mafic crustal rocks. To better understand the deformation mechanism of plagioclase feldspar during frictional faulting, we conducted shearing experiments on simulated plagioclase gouge in a wide range of effective normal stress of 100–300 MPa, pore‐water pressure of 30–100 MPa, and temperatures ranging from 100°C to 600°C. The coefficient of friction is found to range from 0.65 to 0.74 across the entire temperature range, showing no significant thermal weakening process. Except for a case at 200°C with an effective normal stress of 300 MPa, the frictional sliding is velocity weakening over the whole temperature range, showing a steady‐state rate dependence (a−b) ranging from −0.5 × 10−3 to −8.6 × 10−3. This property facilitates nucleation of unstable slips in frictional faulting. Above 200°C, the direct rate effect parameter (a) and the evolution effect parameter (b) of friction increase with temperature up to a threshold of 400°C or 500°C, depending on the effective normal stress. This thermal enhancement suggests thermally activated creep at contact junctions governed by intergranular pressure solution, as evidenced by microstructural signatures indicating the prevalence of very fine precipitates formed at the surfaces of gouge particles as a result of pressure solution. In frictional sliding of plagioclase, a low effective normal stress of 100 MPa corresponds to a higher degree of velocity weakening and tends to facilitate seismic slip rather than slow slips, whereas the high effective normal stress of 300 MPa corresponds to a minor velocity weakening which may cause slow‐slip events in faults of limited size.

Rate-and-state friction of epidote gouge under hydrothermal conditions and implications for the stability of subducting faults under greenschist metamorphic conditions

Epidote is a common hydrous mineral present in subduction zones subject to greenschist metamorphic conditions – and potentially an important control on the fault stability-instability transition observed under greenschist facies. We explore controls on this transition through shear experiments on simulated epidote gouge at temperatures of 100–500 °C, effective normal stresses of 100–300 MPa and pore fluid pressures of 30–75 MPa. We use rate-and-state friction to define these controls of temperature, effective stress and pore fluid pressure on gouge stability. Experimental results indicate that the epidote gouge is frictionally strong (μ ∼ 0.73) and the frictional strength is insensitive to variations in temperature or pressure. With increasing temperature, the epidote gouge exhibits a first transition from velocity-strengthening to velocity-weakening at sub-greenschist conditions (T < 100 °C) before transitioning to velocity-strengthening under greenschist metamorphic conditions (T > 300 °C). Elevating the pore fluid pressure or decreasing the effective stress promotes unstable sliding. The transition in gouge rheology at varied temperatures and pressures is explained by the competition between granular flow-induced gouge dilation and pressure solution-induced gouge compaction. Our results demonstrate that the rate-and-state frictional stability of epidote gouges support the potential for a fault stability-instability-stability transition for subduction under greenschist metamorphic conditions.

Characteristics and effectiveness of deep and ultradeep tight sandstone fractures: Insights from geologic and geophysical data analysis

Effective natural fractures are those that contribute to reservoir storage and conductivity. In this study, the effectiveness of fractures in deep and ultradeep reservoirs (depths &gt; 6 km) is evaluated by using geophysical image logs, cores, microstructures, mechanical tests, and geologic evidence of the tectonic context (including current in situ stress) and sedimentary faces and diagenesis. Intraformational open-mode fractures are the most common fracture type in this tectonically active region, and their effectiveness is affected by the coupling between past tectonic activity, in situ stress, and chemical reactions such as mineral precipitation or dissolution (diagenesis). Our results show that the effectiveness of fractures is mainly related to the aperture and filling, and fractures formed by early tectonic movement are more likely to be filled by minerals, reducing their effectiveness. Strata near orogenic belts in strongly cemented, tight diagenetic facies exhibit a greater abundance of mineral-filled fractures, whereas dissolution increases the fracture apertures, produces secondary pores in host rocks, and roughens the fracture walls. The effective normal stress acting on the fracture surface decreases when the fracture is parallel to the maximum horizontal stress component, so the fracture has a large aperture and, therefore, is more effective. The results of our laboratory measurements reveal that the contribution of fractures to the reservoir space is limited, and the reservoir space provided by fractures accounts for only 13% of all types of reservoir space, whereas the presence of fractures increases reservoir permeability by 1–2 orders of magnitude. Fractures improve the effectiveness of the reservoir by connecting the dispersed pores in the reservoir matrix. Fractures with a low angle to the maximum horizontal stress component have greater apertures and permeabilities and are the main channels for fluid flow.

TECTONOPHYSICAL ZONING OF ACTIVE FAULTS OF THE BAIKAL RIFT SYSTEM

Tectonophysical zoning of active faults of the Baikal rift system (BRS) was performed based on the degree of hazard caused by the generation of earthquakes of magnitude 7.0 and higher. The first basis for this procedure was the results of the natural stress state reconstruction, earlier performed from seismological indicators of rupture deformations (earthquake focal mechanisms). The second important element of fault zoning was electronic maps of active faults of Eurasia hosted on the GIN RAS sever. Within the algorithmic framework for Rebetsky’s method of cataclastic analysis, both of these datasets allowed calculating the Coulomb stresses for the segments of the BRS faults. During the study, a development of the cataclastic method has been carried out in using a diagram of brittle fracture as the Mohr–Coulomb model, considering a decrease in the range of positive Coulomb stress values with an increase in effective normal stress levels. Such an approach provides a more reliable identification of the fault segments with the maximum Coulomb stress levels. The performed calculations showed that in the BRS crust there are several up to 50 km long fault segments having critically high (80–100 % of the maximum) and high (40–80 % of the maximum) Coulomb stress levels. It is these corebased hazardous zones that are considered as places where seismogenic ruptures of future М&gt;7.0 earthquakes may start. There have been distinguished three zones that present such hazard: 1) in the western segment of the BRS in the western part of the Tunka Valley in the Tunka, Khamardaban-Mondy and Baikal-Mondy fault systems; 2) in the Selenga River delta in the Proval, Delta, Ust-Selenga and Sakhalin-Enkhauk fault systems; 3) within the northeastern flank of the BRS on the fault system of the Muyakan basin (along the North Muya ridge). It is proposed to perform tectonophysical monitoring of changes in the stress state of these three zones and make observations of their surface motions using remote sensing methods.

Improved method for quantitative fault sealing evaluation in sand-clay sequences: A case study from the west subsag of Bozhong Sag in the Bohai Bay basin, China

This research aims to enhance the assessment of fault sealing capacity and predicting sealing hydrocarbon column height prior to drilling, which is instrumental in enhancing drilling success rates. Traditional methods for fault seal evaluation, which primarily consider the fault rock's clay content, often overlook the influence of effective fault normal stress (σNeff). This research analyzed numerous drilled fault-bounded reservoirs in the west subsag of Bozhong Sag to determine the fault sealing's buoyancy pressure (Pb). The shale gouge ratio (SGR) and σNeff for these faults were computed, and a calibration of the relationship between SGR and Pb across varying σNeff ranges was conducted to define the sealing failure envelope. This calibration reveals distinct fault sealing failure envelopes at different σNeff intervals, underscoring the significant impact of σNeff on fault sealing capacity. Consequently, an improved SGR parameter (ISGR) was proposed to incorporate both the fault rock's clay content and the influence of σNeff on fault sealing. By plotting ISGR against Pb, fault sealing failure envelopes were established, and a relationship between fault sealing capacity and ISGR was formulated. This relationship provides a novel means to evaluate the Pb sealable at specific ISGR values. Comparisons of the ISGR algorithm with other algorithms demonstrate a superior correlation with seal capacity in our samples. The fault sealing capacity of drilled fault-bounded traps was evaluated using this relationship, and the predicted hydrocarbon column heights are in close alignment with actual measurements, validating the reliability and accuracy of the ISGR algorithm in fault sealing evaluation. The consideration of both clay content and σNeff in the ISGR parameter and the associated calibration method makes it particularly suitable for assessing the fault's sealing capacity in diverse sand-clay sequences with varying strike and burial depths.

The role of heterogeneous stress in earthquake cycle models of the Hikurangi-Kermadec subduction zone

Summary Seismic and tsunami hazard modelling and preparedness are challenged by uncertainties in the earthquake source process. Important parameters such as the recurrence interval of earthquakes of a given magnitude at a particular location, the probability of multi-fault rupture, earthquake clustering, rupture directivity and slip distribution are often poorly constrained. Physics-based earthquake simulators, such as RSQSim, offer a means of probing uncertainties in these parameters by generating long-term catalogues of earthquake ruptures on a system of known faults. The fault initial stress state in these simulations is typically prescribed as a single uniform value, which can promote characteristic earthquake behaviours and reduce variability in modelled events. Here, we test the role of spatial heterogeneity in the distribution of the initial stresses and frictional properties on earthquake cycle simulations. We focus on the Hikurangi-Kermadec subduction zone, which may produce Mw &amp;gt; 9.0 earthquakes and likely poses a major hazard and risk to Aotearoa New Zealand. We explore RSQSim simulations of Hikurangi-Kermadec subduction earthquake cycles in which we vary the rate and state coefficients (a and b). The results are compared with the magnitude-frequency distribution (MFD) of the instrumental earthquake catalogue and with empirical slip scaling laws from global earthquakes. Our results suggest stress heterogeneity produces more realistic and less characteristic synthetic catalogues, making them particularly well suited for hazard and risk assessment. We further find that the initial stress effects are dominated by the initial effective normal stresses, since the normal stresses evolve more slowly than the shear stresses. A heterogeneous stress model with a constant pore-fluid pressure ratio and a constant state coefficient (b) of 0.003 produces the best fit to MFDs and empirical scaling laws, while the model with variable frictional properties produces the best fit to earthquake depth distribution and empirical scaling laws. This model is our preferred initial stress state and frictional property settings for earthquake modelling of the Hikurangi-Kermadec subduction interface. Introducing heterogeneity of other parameters within RSQSim (e.g. friction coefficient, reference slip rate, characteristic distance, initial state variable, etc) could further improve the applicability of the synthetic earthquake catalogues to seismic hazard problems and form the focus of future research.

Frictional Properties of Natural Granite Fault Gouge Under Hydrothermal Conditions: A Case Study of Strike‐Slip Fault From Anninghe Fault Zone, Southeastern Tibetan Plateau

AbstractThe Anninghe Fault (ANHF) is a major left‐lateral strike‐slip fault in southwestern China and one of the main seismogenic fault zones with a history of strong earthquakes. To understand the frictional properties of natural granitic gouges from the principal slip zone, we conducted hydrothermal friction experiments using both saw‐cut and ring shear methods. These experiments were performed at temperatures (T) of 25–600°C, pore pressures (Pf) of zero (dry), 30 and 100 MPa, sliding velocities (V) of 0.01–100 μm/s and effective normal stresses () of 68, 100, and 200 MPa. The (apparent) friction coefficient is low (μ &lt; 0.5) at high T (600°C), high Pf (100 MPa) and low V (&lt;1 μm/s); but high (μ &gt; 0.6) under all other T, Pf and V conditions. Under high Pf, the velocity dependence of friction, (a‐b), displays three regimes with increasing temperature, from positive below ∼100°C to negative at 100–300°C (at V = 1–3 μm/s) or else 100–450°C (at V = 30–100 μm/s), becoming positive again above 300–450°C. At low Pf, the negative (a‐b) expands to the range ∼300–600°C. Microstructural observations and microphysical interpretation imply that the frictional weakening and transitions in (a‐b) are related to competition between dilatant granular flow and deformation of the fine‐grained gouge by intergranular pressure solution accompanied by healing phenomena (leading to cavitation‐creep‐like behavior). Our results provide a possible explanation for the distribution of earthquakes at different depths in the continental crust, in particular for the depth range of the seismogenic zone between 4 and 24 km along the ANHF.

Activation of Dissolution‐Precipitation Creep Causes Weakening and Viscous Behavior in Experimentally Deformed Antigorite

AbstractAntigorite occurs at seismogenic depth along plate boundary shear zones, particularly in subduction and oceanic transform settings, and has been suggested to control a low‐strength bulk rheology. To constrain dominant deformation mechanisms, we perform hydrothermal ring‐shear experiments on antigorite and antigorite‐quartz mixtures at temperatures between 20 and 500°C at 150 MPa effective normal stress. Pure antigorite is strain hardening, with frictional coefficient (μ) &gt; 0.5, and developed cataclastic microstructures. In contrast, antigorite‐quartz mixtures (10% quartz) are strain weakening with μ decreasing with temperature from 0.36 at 200°C to 0.22 at 500°C. Antigorite‐quartz mixtures developed foliation similar to natural serpentinite shear zones. Although antigorite‐quartz reactions may form mechanically weak talc, we only find small, localized amounts of talc in our deformed samples, and room temperature friction is higher than expected for talc. Instead, we propose that the observed weakening at temperatures ≥200°C primarily results from silica dissolution leading to a lowered pore‐fluid pH that increases antigorite solubility and dissolution rate and thus the rate of dissolution‐precipitation creep. We suggest that under our experimental conditions, efficient dissolution‐precipitation creep coupled to grain boundary sliding results in a mechanically weak frictional‐viscous rheology. Antigorite with this rheology is much weaker than antigorite deforming frictionally, and strength is sensitive to effective normal stress and strain rate. The activation of dissolution‐precipitation in antigorite may allow steady or transient creep at low driving stress where antigorite solubility and dissolution rate are high relative to strain rate, for example, in faults juxtaposing serpentinite with quartz‐bearing rocks.

Relating Hydro‐Mechanical and Elastodynamic Properties of Dynamically Stressed Tensile‐Fractured Rock in Relation to Applied Normal Stress, Fracture Aperture, and Contact Area

AbstractWe exploit nonlinear elastodynamic properties of fractured rock to probe the micro‐scale mechanics of fractures and understand the relation between fluid transport and fracture aperture under dynamic stressing. Experiments were conducted on rough, tensile‐fractured Westerly granite subject to triaxial stresses. We measure fracture permeability for steady‐state fluid flow with deionized water. Pore pressure oscillations are applied at amplitudes ranging from 0.2 to 1 MPa at 1 Hz frequency. During dynamic stressing we transmit ultrasonic signals through the fracture using an array of piezoelectric transducers (PZTs) to monitor evolution of interface properties. We examine the influence of fracture aperture and contact area by conducting measurements at effective normal stresses of 10–20 MPa. Additionally, the evolution of contact area with stress is characterized using pressure sensitive film. These experiments are conducted separately with the same fracture and map contact area at stresses from 9 to 21 MPa. The measurements are a proxy for “true” contact area for the fracture surface and we relate them to elastic properties using the calculated PZT sensor footprints via numerical modeling of Fresnel zones. We compare the elastodynamic response of the fracture using the stress‐induced changes in ultrasonic wave velocities for transmitter‐receiver pairs to image spatial variations in contact properties. We show that nonlinear elasticity and permeability enhancement decrease with increasing normal stress. Additionally, post‐oscillation wave velocity and permeability exhibit quick recoveries toward pre‐oscillation values. Estimates of fracture contact area (global and local) demonstrate that the elastodynamic and permeability responses are dominated by fracture topology.

The Frictional‐Viscous Transition in Experimentally Deformed Granitoid Fault Gouge

AbstractIn crustal faults dominated by granitoid gouges, the frictional‐viscous transition marks a significant change in strength constraining the lower depth limit of the seismogenic zone. Dissolution‐precipitation creep (DPC) may play an important role in initiating this transition, especially within polymineralic materials. Yet, it remains unclear to what extent DPC contributes to the weakening of granitoid gouge materials at the transition. Here we conducted sliding experiments on wet granitoid gouges to large displacement (15 mm), at an effective normal stress and pore fluid pressure of 100 MPa, at temperatures of 20–650°C, and at sliding velocities of 0.1–100 μm/s, which are relevant for earthquake nucleation. Gouge shear strengths were generally ∼75 MPa even at temperatures up to 650°C and at velocities &gt;1 μm/s. At velocities ≤1 μm/s, strengths decreased at temperatures ≥450°C, reaching a minimum of 37 MPa at the highest temperature and lowest velocity condition. Microstructural observations showed that, as the gouges weakened, the strain localized into thin, dense, and ultrafine‐grained (≤1 μm) principal slip zones, where nanopores were located along grain contacts and contained minute biotite‐quartz‐feldspar precipitates. The stress sensitivity exponent n decreased from a large number at 20°C to ∼2.2 at 650°C at the lowest velocities. These findings suggest that high temperature, slow velocity and small grain sizes promote DPC‐accommodated granular flow over cataclastic frictional granular flow, leading to the observed weakening and strain localization. Field observations together with extrapolation suggest that DPC‐induced weakening occurs at depths of 7–20 km depending on geothermal gradient.

Implication of fast shear wave azimuth and critically stressed fractures in a coalbed methane reservoir

The orientation of the fractures and stresses within coal seams plays a critical role in gas production from coalbed methane reservoirs. In this study, we used resistivity images and sonic logs to investigate these parameters, aiming to (1) establish the relationship between fracture orientations and the polarization angles of fast shear waves and (2) detect active fractures in the coal seam. Shear waves in anisotropic formations are split into fast and slow components, with Alford’s rotation method used to determine the polarization angles of the fast shear wave. We found that the fast shear wave aligns with the direction of higher fracture intensity. Subsequently, we incorporated the poroelastic strain model to estimate the vertical ([Formula: see text]), maximum horizontal ([Formula: see text]), and minimum horizontal ([Formula: see text]) stresses in the wellbore. These stress magnitudes aided in identifying the faulting regime and were corroborated by vertical opening mode fractures. Validation of [Formula: see text] and [Formula: see text] involved comparisons with the breakout width in image logs and the closure pressure observed during hydraulic fracturing treatments. Applying Mohr’s Coulomb criteria, the stress model discerned the state of the fractures, transforming the stress magnitudes into shear and effective normal stress on each fracture plane. Our observations indicated that the identified fractures existed in a noncritically stressed condition, suggesting a lack of interconnectivity among them. These findings correspond to the absence of gas production to date, providing insights into the dynamics of fractures and their impact on production behavior.

Establishing a unified viscoplastic constitutive equation for EA4T steel: Comparative analysis with Arrhenius model

Quasi-static and impact compression experiments were conducted on EA4T steel using a Gleeble-3800 thermal simulation machine. The study aimed to investigate the complex microstructural evolution and thermal deformation behavior of EA4T steel under various experimental conditions, including temperatures ranging from 970 °C to 1170 °C and strain rates ranging from 0.01s−1 to 1s−1. To precisely elucidate these phenomena, we meticulously constructed a unified visco-plastic constitutive model using the internal variable methodology. The model's parameterization was achieved through the effective application of genetic algorithm optimization techniques. Rigorous validation of the model was performed by meticulously comparing its outputs with experimental data, including key metrics such as average grain size, recrystallized fraction, and effective flow stress. In addition,a comparative analysis with the improved Arrhenius model highlights the superior performance of the unified visco-plastic constitutive equation in capturing the intricate microstructural evolution and thermal deformation behavior exhibited by EA4T steel.

Frictional process, AE characteristics, and permeability evolution of rough fractures on Longmaxi shale with specific roughness under water injection

Studying the frictional process, acoustic emission (AE) characteristics and permeability evolution of fractures under water injection is fundamental for proposing theories and methods to control the stability of geological structures. In this work, the shear-flow test was conducted on Longmaxi shale fractures with different specific roughness to analyze the frictional deformation and mechanical properties of shale fractures during the frictional slip process, and the influence of effective normal stress and surface morphology was investigated. AE monitoring technology was also utilized to reveal the AE evolution during the frictional slip process. Based on the AF/RA values, AE signals were subjected to cluster analysis using the Gaussian Mixture Model (GMM) to distinguish and quantify the crack types and proportions during frictional slip. Combining the development of cracks and considering the ratio of worn area, a theoretical analytical expression was established to reasonably describe the friction-damage evolution, classifying the friction-damage during the frictional slip process into tensile and shear damage, respectively. Meanwhile, the permeability evolution of Longmaxi shale fractures was also studied in detail. A theoretical model capable of describing and predicting the permeability evolution was proposed under the condition that the permeability evolution and damage process of Longmaxi shale fractures correspond to each other. Finally, the interaction between asperities on shale fractures with different surface morphologies during the frictional slip process was discussed, and the influence mechanism of surface morphology on permeability evolution was revealed.

Investigation on dynamic mechanism of fault slip and casing deformation during multi-fracturing in shale gas wells

Fracturing horizontal well casing deformation has become very prominent, particularly in tectonic stress-concentrated shale gas fields, limiting the efficient development progress of shale gas. The main failure mode of casing shearing deformation had been attributed to fault slip caused by multi-fracturing. The current research did not provide a clear picture of the dynamic evolution relationship between hydraulic fracturing, fault slip, and casing deformation. In this paper, the dynamic model of fault slip induced by formation pressure change is established, incorporating the effects of stress drop, physical change of friction, and casing and cement-sheath resistance loads. The discontinuous displacement approach and explicit/implicit coupling iteration methods are used to reveal the relationships between the effective normal stress, shear stress, friction coefficient, and sliding velocity during the fault slip process. Furthermore, the microscopic process of casing deformation sheared by fault slip is investigated using static equilibrium theory, and a characterization method for determining the amount casing deformation caused by real-scale fault slip is proposed. The results show that three stages exist in the process of casing deformation sheared by fault slip, including trigger activation stage, accelerated slip stage, and deceleration slip stage. Fault slip is clearly influenced by fault strike. To reduce the amount of fault slip, the fault direction with the maximum in-situ stress should be avoided as much as possible. Serious casing deformation still occurs for large-scale activated faults even though the optimization measure of wellbore structure has been well taken. To fundamentally reduce the possibility of casing shear deformation, it is necessary to prevent fault slip through optimizing the design of hydraulic fracturing. This study lays the theoretical groundwork for the casing deformation control method in shale gas wells.