Horizontal Stress Changes Research Articles

EFFECT OF SEGMENT DISLOCATION ON THE MECHANICAL PROPERTIES OF LINED TUNNELS IN A COMPLEX ENVIRONMENT

In complex environments, tunnels with a lined structure are more affected by the dislocation of a segment. This study uses finite element simulations to further investigate the relationship between environmental changes and segments in lined tunnels. Tunnels with and without a lining are simulated under varying strata resistance, buried depth, and water level conditions. The horizontal and vertical displacements and stress changes in the two tunnels are compared when segment dislocation occurs. The results show that the lateral displacement of the tunnel is relatively stable and less influenced by external loads. However, the longitudinal displacement change is the opposite, and it increases significantly when the displacement exceeds 10 mm. When the lined tunnel is subjected to a large external load and misalignment occurs, the performance of the tube sheet joint bolts deteriorates, resulting in a stress decrease. Furthermore, the lined tunnel exhibits an increased ability to resist deformation and does not exactly follow the law of small displacements from small misalignments. When a large misalignment occurs, the lined tunnel may be damaged by elliptical deformation along the 45° direction, and its internal forces would be much larger than those of the unlined tunnel.

Investigation on the fracture mechanics characteristics and crack initiation of deep dense shale

The mechanical behavior of shale fractures significantly affects the structure and stability of hydraulic fracturing networks in shale gas reservoirs. Currently, there is insufficient research on the mechanical characteristics of fractures, crack initiation, and the prediction of fracture trajectories in deep dense shale. This study involved conducting direct tension, shear, and three-point bending fracture mechanical experiments on shale samples, complemented by numerical calculations and mechanistic analyses. The results show a significant influence of the bedding plane angle (γ) on the initiation properties and locations of shale fractures. With γ ranging from 0° to 90°, shale fractures transition from alignment with the bedding plane to penetration through it. Fracture initiation locations show a significant degree of randomness when fractures propagate along the bedding plane. The horizontal tensile stress changes with increasing γ, initially increasing and then decreasing as it moves away from the center. The evolution of the horizontal tensile stress field primarily causes alterations in shale fracture paths, reaching its peak value at γ = 45°. Shale initiation fractures can be classified into four types: tensile fractures through the bedding plane, tensile fractures along the bedding plane, shear fractures across the bedding plane, and shear fractures along the bedding plane. Established a criterion called the “maximum stress strength ratio” to determine fracture initiation, which can predict the initiation properties, locations, and paths of shale fractures. When γ is small, fractures mainly propagate along the bedding plane, with a higher likelihood of initiation occurring at the center of the specimen. Conversely, when γ is large, fractures mainly propagate through the bedding plane. The spacing of the bedding planes minimally affects the predicted fracture paths of shale. This study is of significant theoretical importance for reservoir stimulation and stability assessment in deep dense shale gas reservoirs.

A technique for enhancing tight oil recovery by multi-field reconstruction and combined displacement and imbibition

A seepage-geomechanical coupled embedded fracture flow model has been established for multi-field coupled simulation in tight oil reservoirs, revealing the patterns of change in pressure field, seepage field, and stress field after long-term water injection in tight oil reservoirs. Based on this, a technique for enhanced oil recovery (EOR) combining multi-field reconstruction and combination of displacement and imbibition in tight oil reservoirs has been proposed. The study shows that after long-term water flooding for tight oil development, the pressure diffusion range is limited, making it difficult to establish an effective displacement system. The variation in geostress exhibits diversity, with the change in horizontal minimum principal stress being greater than that in horizontal maximum principal stress, and the variation around the injection wells being more significant than that around the production wells. The deflection of geostress direction around injection wells is also large. The technology for EOR through multi-field reconstruction and combination of displacement and imbibition employs water injection wells converted to production and large-scale fracturing techniques to restructure the artificial fracture network system. Through a full lifecycle energy replenishment method of pre-fracturing energy supplementation, energy increase during fracturing, well soaking for energy storage, and combination of displacement and imbibition, it effectively addresses the issue of easy channeling of the injection medium and difficult energy replenishment after large-scale fracturing. By intensifying the imbibition effect through the coordination of multiple wells, it reconstructs the combined system of displacement and imbibition under a complex fracture network, transitioning from avoiding fractures to utilizing them, thereby improving microscopic sweep and oil displacement efficiencies. Field application in Block Yuan 284 of the Huaqing Oilfield in the Ordos Basin has demonstrated that this technology increases the recovery factor by 12 percentage points, enabling large scale and efficient development of tight oil.

Study on Fracture Propagation Rules of Shale Refracturing Based on CT Technology

Reactivating oil and gas wells, increasing oil and gas production, and improving recovery provide more opportunities for energy supply especially in the extraction of unconventional oil and gas reservoirs. Due to changes caused by well completion and production in pore pressure around oil and gas wells, subsequently leading to changes in ground stress, and the presence of natural and induced fractures in the reservoir, the process of refracturing is highly complex. This complexity is particularly pronounced in shale oil reservoirs with developed weak layer structures. Through true triaxial hydraulic fracturing experiments on Jimsar shale and utilizing micro-CT to characterize fractures, this study investigates the mechanisms and patterns of refracturing. The research indicates: (1) natural fractures and the stress states in the rock are the primary influencing factors in the fracture propagation. Because natural fractures are widely developed in Jimsar shale, natural fractures are the main influencing factors of hydraulic fracturing, especially in refracturing, the existing fractures have a greater impact on the propagation of secondary fracturing fractures. (2) Successful sealing of existing fractures using temporary blocking agents is crucial for initiating new fractures in refracturing. Traditional methods of plugging the seam at the root of existing fractures are ineffective, whereas extensive injection of blocking agents, forming large “sheet-like” blocking bodies in old fractures, yields better sealing effects, promoting the initiation of new fractures. (3) Moderately increasing the pumping rate and viscosity of fracturing fluid is advantageous in forming “sheet-like” temporary blocking bodies, enhancing the complexity of the network of new fractures in refracturing. (4) When there is a high horizontal stress difference, after sealing old fractures, the secondary hydraulic fractures initiate parallel to and extend from the old fractures. In cases of low horizontal stress difference, the complexity of secondary hydraulic fractures increases. When the horizontal stress changes direction, the secondary hydraulic fractures also change direction. It is recommended to use high-viscosity fracturing fluid and moderately increase the pumping rate, injecting blocking agents to seal old fractures, thereby enhancing the complexity of the network of refracturing. These findings provide important technical guidance for improving the efficiency of shale oil reservoir development.

Uncertainty of stress path in fault stability assessment during CO2 injection: Comparing smeaheia 3D geomechanics model with analytical approaches

This study investigates the limitation of simplified stress path assumptions, particularly uniaxial strain conditions, in fault stability assessments for CO2 injection sites. We conducted a 3D geomechanics simulation for the Smeaheia fault block in the Norwegian North Sea and compared the results with simplified stress path assumptions. Our results indicate that the uniaxial strain assumption underestimates the change in effective horizontal stress, particularly for bounding faults subjected to a significant change in pore pressure gradient with soft surroundings. Rotations of maximum horizontal stresses parallel to soft surroundings are also observed along the bounding faults due to the directional difference in stiffness contrast along faults. This underestimation results in overestimation of fault stability by up to 60% for an extreme case. Our study thus highlights that the uniaxial strain assumption, which limits to account for lateral deformation on the fault/reservoir boundary, overlooks critical changes in the effective horizontal stress and associated critical scenarios for fault stability assessments. When bounding faults are juxtaposed with low-stiffness shale formation under a normal stress regime, calibrating the fault stability assessment by 30% for a base case and 60% for a conservative assessment can provide a practical way to correct the uncertainties caused by using uniaxial strain assumption.

Monitoring the change in horizontal stress with multi-wave time-lapse seismic response based on nonlinear elasticity theory

Monitoring the change in horizontal stress from the geophysical data is a tough challenge, and it has a crucial impact on broad practical scenarios which involve reservoir exploration and development, carbon dioxide (CO2) injection and storage, shallow surface prospecting and deep-earth structure description. The change in in-situ stress induced by hydrocarbon production and localized tectonic movements causes the changes in rock mechanic properties (e.g. wave velocities, density and anisotropy) and further causes the changes in seismic amplitudes, phases and travel times. In this study, the nonlinear elasticity theory that regards the rock skeleton (solid phase) and pore fluid as an effective whole is used to characterize the effect of horizontal principal stress on rock overall elastic properties and the stress-dependent anisotropy parameters are therefore formulated. Then the approximate P-wave, SV-wave and SH-wave angle-dependent reflection coefficient equations for the horizontal-stress-induced anisotropic media are proposed. It is shown that, on the different reflectors, the stress-induced relative changes in reflectivities (i.e., relative difference) of elastic parameters (i.e., P- and S-wave velocities and density) are much less than the changes in contrasts of anisotropy parameters. Therefore, the effects of stress change on the reflectivities of three elastic parameters are reasonably neglected to further propose an AVO inversion approach incorporating P-, SH- and SV-wave information to estimate the change in horizontal principal stress from the corresponding time-lapse seismic data. Compared with the existing methods, our method eliminates the need for man-made rock-physical or fitting parameters, providing more stable predictive power. 1D test illustrates that the estimated result from time-lapse P-wave reflection data shows the most reasonable agreement with the real model, while the estimated result from SH-wave reflection data shows the largest bias. 2D test illustrates the feasibility of the proposed inversion method for estimating the change in horizontal stress from P-wave time-lapse seismic data.

Analysis of the Influence of Pile-Raft Foundation Reinforcement on an Adjacent Existing Line Foundation

Pile-raft foundation reinforcement will have a certain impact on the adjacent existing line foundation, which will affect the normal service of the subgrade, and even lead to the instability of the subgrade. Until now, there have been few studies on adjacent construction problems, and there are few field experimental data available for us to consult. Therefore, this study relies on the construction project of Shanghai–Nanjing intercity high-speed railway close to the existing line, using in situ monitoring methods, such as stress shovels, horizontal strain gauges, and inclinometers combined with finite-element calculation and rail-inspection vehicle-data analysis. The stress, displacement, and geometric linearity of the adjacent existing line foundation during the reinforcement construction of a pile-raft foundation were studied. Our aim was to measure the impact and optimize the existing roadbed-protection measures employed during the construction period. We found that the cumulative horizontal displacement of the existing line foundation slope toe during the construction period was 24.25 mm, and the lateral displacement rate was less than 0.59 mm/d. The distance between the two lines was 9 m. The horizontal stress of the foundation soil in the depth range varied according to extrusion and retraction with the different construction stages, and the extrusion stress was less than 10 kPa. The horizontal stress changes in different construction stages were not obvious; the track quality index (TQI) in the existing track inspection during the construction period increased by 129.58%, and the existing track geometric linearity fluctuated greatly. According to the test results, the excavation stage of the subgrade foundation pit was the most dangerous stage of the existing subgrade during the construction of the new line pile-raft foundation. Although the change of the horizontal stress in different construction stages was not obvious, the horizontal displacement of the slope toe was sensitive to the construction process. Therefore, it could be used as a key indicator to monitor the deformation and stability of the existing subgrade. The correction coefficient was obtained by combining the detection data of the track-inspection vehicle with the finite element calculation data, based on which the accurate estimation of the horizontal displacement of the slope toe after the excavation of the foundation pit was realized. The monitoring and evaluation method of the stability of the existing line foundation under the influence of the pile-raft foundation reinforcement construction was preliminarily established through field monitoring and the analysis of the track-inspection data. Based on this method, relevant early warning values were proposed. The test results and engineering measures ensured the safe operation of the existing line foundation. This has important theoretical significance for guiding the construction of a new subgrade of adjacent existing lines.

Anisotropic 4D seismic response inferred from ultrasonic laboratory measurements: A direct comparison with the isotropic response

AbstractPore‐pressure depletion causes changes in the triaxial stress state. Pore‐pressure depletion in a flat reservoir, for example, can be reasonably approximated as uniaxial compaction, in which the horizontal effective stress change is smaller than the vertical effective stress. Furthermore, the stress sensitivity of velocities can be angle‐dependent. Therefore, time‐lapse changes in reservoir elastic anisotropy are expected as a consequence of production, which can complicate the interpretation of the 4D seismic response. The anisotropic 4D seismic response caused by pore‐pressure depletion was investigated using existing core velocity measurements. To make a direct comparison between the anisotropic 4D seismic response and the isotropic response based only on vertical velocities, pseudoisotropic elastic properties were utilized, and the two responses were compared in terms of a dynamic rock physics template. A comparison of the dynamic rock physics templates indicates that time‐lapse changes in reservoir elastic anisotropy have a noticeable impact on the interpretation of 4D seismic data. Changes in anisotropy as a result of pore‐pressure depletion cause a time‐lapse amplitude variation with offset response as if there is a reduction in VP/VS (i.e., pseudoisotropic VP/VS decreases), although the vertical VP/VS increases. The impact of time‐lapse changes in anisotropy on the amplitude variation with offset gradient was also investigated, and the time‐lapse anisotropy was found to enhance changes in the amplitude variation with offset gradient for a given case.

Roof Stability and Support Strategies Associated with Longwall-induced Horizontal Stress Changes in Belt Entries

Roof Stability and Support Strategies Associated with Longwall-induced Horizontal Stress Changes in Belt Entries

True triaxial experimental study on fluid flow in single fracture with different dip angles under three-dimensional stress at different depths

The shape of ore body in deep underground engineering is complex, and its fracture development degree is affected by water abundance, fracture zone, fault and other factors. With the deepening of depth, the vertical and horizontal stress changes significantly, which has a great impact on the fracture opening. Therefore, the fracture dip angle and its three-dimensional stress state are the main factors affecting the fluid flow of fractures in a large area of ore body and its hanging and footwall rock mass. In this study, firstly, the three-dimensional fractal dimension of a single-fracture specimen was obtained by using CT (Computer Tomography) scanning technology and the single-fracture specimen with a similar three-dimensional fractal dimension was selected for testing. Then, the self-developed single fracture stress seepage coupling true triaxial test system is applied to test the seepage of single fracture samples with six different fracture dip angles under different stress states of underground depth. The experimental results show that the fluid flow and strain of fractured rock mass are comprehensively affected by the fracture dip angle and the stress state of the underground depth, and the influence degree of the fracture dip angle changes with the deepening of the underground depth. The above conclusions can be mutually verified in the aspects of stress-strain, water pressure and permeability coefficient. Furthermore, the relationship between the minimum injectable water pressure (MIWP) of fractured rock mass with different dip angles and underground depth is obtained, the expression of seepage coefficient of single fracture specimen with different dip angles and underground depth under three-dimensional stress is summarized, and the relationship between eh and em of single fracture specimen with different fracture dip angles is determined. This study provides a basic reference for the design scheme of hydraulic conductivity of fractured zone or fractured rock mass with different underground depths.

Mesozoic‐Cenozoic Regional Stress Field Evolution in Svalbard

AbstractCooling fracture orientations in diabase sills associated with the Cretaceous High Arctic Large Igneous Province and syn‐sedimentary Triassic faults help constrain a model for Svalbard's (NE Barents Shelf) Mesozoic stress field evolution. Fracture data from Edgeøya and adjacent islands in SE Svalbard, from S Spitsbergen, and from literature were used to model preferred orientations and temporal relationships. Orthogonal, roughly E‐W and N‐S, joints and veins in sills from SE Svalbard are interpreted as cooling fractures influenced by the ambient stress field. Aligned preferred orientations within the Triassic host strata are associated with a regional Cretaceous jointing episode driven by sill emplacement and/or erosional unloading. The regional maximum horizontal stress (likely σ1) is inferred to have been parallel to a dominant ≈E‐W set. Spitsbergen's more complex joint patterns are associated with proximity to the Cenozoic West Spitsbergen Fold‐and‐Thrust Belt, but ≈E‐W and ≈N‐S orientations occur and are typically the earlier set. Syn‐sedimentary, ≈NW‐SE striking, Triassic normal faults in SE Svalbard aligned with the maximum horizontal stress indicate a Triassic to Cretaceous counterclockwise stress field shift, with additional counterclockwise shifting during Cenozoic dextral transpression between Svalbard and Greenland. Localized joint preferred orientations consistent with both decoupled and coupled transpression occur. Changes in the regional maximum horizontal stress and deformation regime may reflect timing of which plate margin was crucial in influencing Svalbard's plate interior stress field, starting with Triassic Uralian activity to the E, then Cretaceous Amerasian Basin development to the NW, culminating with Cenozoic dextral transpression and transtension to the SW.

Stress response during in-situ gas depletion and its impact on permeability and stability of CBM reservoir

Coalbed methane (CBM) reservoir is generally believed to be under the uniaxial strain condition. Because of this stress/strain controlled boundary, the horizontal stress is passively changed with gas depletion. This dynamic stress evolution has great impact on permeability and stability of the CBM reservoir. Even though it is generally known that the stress depletion is both poromechanics and desorption controlled process, however, how the gas pressure and the gas type affect the horizontal stress profile and its induced coal failure is still unclear. In this work, we experimentally simulated the uniaxial strain condition in the lab. Both the horizontal stress and permeability were continuously monitored during of different gases (He, N2, CH4 and CO2) depletion. The results show that regardless of gas type, the horizontal stress decreases with the reduction of gas pressure. And gas with a higher adsorption capacity corresponds to a greater horizontal stress loss. During the depletion, the effective vertical stress rises, while effective horizontal stress show various trends for different gases. The effective horizontal stress increases during helium depletion, which leads to a reduction of the permeability. And it decreases during methane and carbon dioxide depletion, which corresponds to a significant increase of the permeability. While for nitrogen, the permeability decreases first and followed by a slight uptick. Both the lab test and modeling results show that a significant non-linearity exists in effective horizontal stress change during adsorptive gas depletion, which is expected to affect the reservoir stability. To evaluate the reservoir stability during depletion, a stability factor (SF) was put forward, and the influencing factors on this parameter were analyzed. With this parameter, the critical reservoir pressure indicating reservoir failure can be determined with appropriate geomechanical parameters input. This research will advance the understanding of the geomechanics change during CBM production.

Horizontal stress increase induced by deep vibratory compaction

Compaction by vertically or horizontal oscillating probes increases not only the soil stiffness but also the horizontal effective stress. An important, but often neglected, consequence is that compaction also causes preloading of the soil. The change in horizontal stress following vibratory compaction can be measured independently by two in situ methods – a cone penetration test (CPT) and a flat dilatometer test (DMT). Values of sleeve resistance (CPT) and horizontal stress index (DMT) can be determined prior and after compaction. In this work, five case histories reported in the literature where different vibratory compaction methods were used (dynamic compaction, vibroflotation, VibroWing, TriStar and resonance compaction) were re-analysed. For each test site, the change in CPT cone resistance and sleeve resistance was determined and compared with the increase in horizontal stress index from DMTs. The preloading effect due to vibratory compaction was estimated using empirical correlations.

Microseismic Focal Mechanisms and Implications for Changes in Stress during the 2014 Newberry EGS Stimulation

Abstract We adapt the relative polarity method from Shelly et al. (2016) to compute focal mechanisms for microearthquakes associated with the 2014 hydroshearing stimulation at the Newberry volcano geothermal site. We focus the analysis on events relocated by Aguiar and Myers (2018), who report that six event clusters predominantly comprise the 2014 sequence. Data quality allows focal mechanism analysis for four of the six event clusters. We use Hardebeck and Shearer (2002, 2003; hereafter HASH) to compute focal mechanisms based on first‐motion polarities and S/P amplitude ratios. We manually determine P‐ and S‐wave polarities for a well‐recorded reference event in each cluster, then use waveform cross correlation to determine whether recordings of other events in the cluster are the same or reversed polarity at each network station. Most waveform polarities are consistent with the affiliated reference event, indicating similar focal mechanisms within each cluster. The deeper clusters are east–west‐striking normal faults, whereas the shallower clusters, close to the top of the open‐hole section of the borehole, are strike slip with east–west motion. Regional studies and prestimulation borehole breakouts find the maximum stress direction is vertical and maximum horizontal stress is approximately north–south. Fault geometry and focal mechanisms of microseismicity during the stimulation suggest that increased pressure from fluid injection predominantly caused changes in horizontal stress, consistent with predictions from numerical studies of stress change caused by fluid injection. At shallow depths, where previous studies suggest the difference between vertical and horizontal stress is lowest, injection appears to have rotated the direction of maximum stress from vertical to horizontal, resulting in strike‐slip motion. At greater depth, vertical stress continued to be the dominant direction during the stimulation, but fault orientation indicates either reactivation of pre‐existing fractures or rotation of the direction of maximum horizontal stress from approximately north–south to east–west.

Drought‐Triggered Magmatic Inflation, Crustal Strain, and Seismicity Near the Long Valley Caldera, Central Walker Lane

AbstractWe use GPS data to show synchronization between the 2011 and 2016 drought cycle in California, accelerated uplift of the Sierra Nevada Mountains, and enhanced magmatic inflation of the Long Valley Caldera (LVC) magmatic system. The drought period coincided with faster uplift rate, changes in gravity seen in the Gravity Recovery and Climate Experiment (GRACE), and changes in standardized relative climate dryness index. These observations together suggest that the Sierra Nevada elevation is sensitive to changes in hydrological loading conditions, which subsequently influences the LVC magmatic system. We use robust imaging of horizontal GPS velocities to derive time‐variable shear and dilatational strain rates in a region with highly variable station distribution. The results show that the highest strain rates are near the eastern margin of the Sierra Nevada and western edge of the Central Walker Lane (CWL) passing directly through LVC. The drought period saw geographic shifts in the distribution in active shear strain in the CWL more than 60 km from the LVC, delineating the minimum extent over which the active magmatic system affects the CWL tectonic environment. We analyze declustered seismicity data to show that locations with higher seismicity rates tend to be (1) areas with higher strain rates and (2) areas in which strain rates increased during drought‐enhanced inflation. We hypothesize that drought conditions reduce vertical surface mass loading, which decreases pressure at depth in the LVC system, in turn enhances magmatic inflation, and drives horizontal elastic stress changes that redistribute active CWL strain and modulate seismicity.

Pore water pressure and total horizontal stress response to EPBM tunnelling in London Clay

The ground response, in terms of surface and subsurface displacements, to twin-bore Crossrail tunnel construction beneath a research monitoring site in Hyde Park, London, using earth-pressure-balance machines (EPBMs) in London Clay, has recently been reported in two companion papers by the authors. This third paper presents and discusses corresponding changes in pore water pressure and total horizontal stress measured using multi-level piezometers and pushed-in spade cells. The three papers together provide a comprehensive and completely unique field monitoring case history of the short-term ground response to EPBM tunnelling in London Clay, making them invaluable for validating future numerical analyses. The fully grouted vibrating-wire piezometers were able to measure the rapid pore water pressure changes around the tunnels as they were constructed. Five distinct immediate pore water pressure responses are identified, induced by different stages of the tunnel drives as the EPBMs approached and passed the instruments. The responses are correlated with tunnel-boring machine operation variables and a postulated arching mechanism, identified for the first time through field measurements. The sense and magnitude of changes in horizontal total stress were reasonable and are correlated with overall pore water pressure changes. Both responses are linked where possible with measured subsurface displacements and generally correlate well, at least qualitatively. Limitations to the measurements and influencing factors are also discussed.

Tectonophysical Paleostress Reconstructions: Interpretation Challenges and Possible Solutions

Abstract —Paleostress inversion may be ambiguous when several markedly different local stress states are inferred for a group of outcrops. Attempts of reconstructing regional stress regimes (compressional, extensional, or strike-slip) by selecting local principal stresses of proximal directions turn out to have poor grounds. Each stress permutation (e.g., extension to compression) attendant with buildup of large irreversible strain (fault slip) requires a 5–6 kbar change in middle-crust horizontal stress and at least 50 Myr stable and uniform loading. Tectonophysical stress reconstructions for present active intracontinental orogens show heterogeneous patterns: Stress directions in uplifts are different from those in large intermontane basins and even in relatively subsided parts of mountain ranges or in adjacent uplifted zones (e.g., a plateau and a range). Paleostresses should be interpreted with reference to present stress fields in the respective areas. It is suggested to reconstruct regional stresses using the approach of L. Sim implying search for “common stress fields”. Another important technique is to trace stress changes in specific structures (large folds etc.) in the course of their evolution. The available data indicate correlation and bipolarity of stress states in large basins and uplifts.

Numerical Modelling of Salt-Related Stress Decoupling in Sedimentary Basins–Motivated by Observational Data from the North German Basin

A three dimensional (3D) finite element model is used to study the conditions leading to mechanical decoupling at a salt layer and vertically varying stress fields in salt-bearing sedimentary basins. The study was inspired by observational data from northern Germany showing stress orientations varying up to 90° between the subsalt and the suprasalt layers. Parameter studies address the role of salt viscosity and salt topology on how the plate boundary forces acting at the basement level affect the stresses in the sedimentary cover above the salt layer. Modelling results indicate that mechanical decoupling occurs for dynamic salt viscosities lower than 1021 Pa·s, albeit this value depends on the assumed model parameters. In this case, two independent stress fields coexist above and below the salt layer, differing in tectonic stress regime and/or stress orientation. Thereby, stresses in the subsalt domain are dominated by the shortening applied, whereas in the suprasalt section they are controlled by the local salt topology. For a salt diapir, the orientation of the maximum horizontal stress changes from a circular pattern above to a radial pattern adjacent to the diapir. The study shows the value of geomechanical models for stress prediction in salt-bearing sedimentary basins providing a continuum mechanics–based explanation for the variable stress orientations observed.

Stress-path-dependent velocities in shales: Impact on 4D seismic interpretation

Overburden stresses and pore pressure are altered by depletion or inflation of a subsurface reservoir, leading to seismic traveltime and reflectivity changes that may be interpreted as footprints of reservoir drainage or injection. Our objective is to contribute to the quantification of expected 4D seismic time shifts and reflectivities by understanding how overburden stresses and strains change and how seismic velocities depend on these stress and strain changes. The stress sensitivity of ultrasonic velocities has been obtained from controlled laboratory experiments in which field shale cores are brought to in situ conditions and then probed with different stress paths, i.e., different ratios between the horizontal and vertical stress change. The tests are performed in undrained conditions, and pore-pressure changes are recorded. The experiments indicate that the velocity and pore-pressure changes depend linearly on the stress path. The latter is a verification of the applicability of Skempton’s law from soil mechanics for shales. Overburden stress paths are, through analytical and numerical geomechanical modeling, seen to depend on the aspect ratio of the depleting or inflating zone, on the elastic contrast between the overburden and the reservoir, and on the reservoir tilt. By combining laboratory data and simulated overburden stress paths, the response of in situ wave velocities to reservoir pore-pressure change can be estimated. The calculated in situ stress dependence of the vertical P-wave velocity shows significant dependence on stress path. The strain sensitivity, expressed by the dilation parameter, or [Formula: see text] factor, increases strongly with the stress path. This expresses the explicit sensitivity of [Formula: see text] to vertical in situ strain. The results also indicate that the time-lapse overburden response may be significantly influenced by pore-pressure changes in the overburden.

Horizontal stress change of energy piles subjected to thermal cycles in sand

Horizontal stress changes of semi-floating energy piles subjected to cyclic thermal loading are investigated through finite element analysis adopting the hypoplastic model. By using the validated finite element model, a parametric study is carried out, considering effects of amplitude of thermal cycles, pile diameter, pile length and relative density of sand. It is revealed that due to volumetric contraction of sand (loose or dense) at the interface under temperature induced cyclic shearing, a reduction of horizontal stress occurs. The degree of reduction in horizontal stress is most affected by the amplitude of thermal cycles and the pile diameter.