Reduction In Horizontal Stress Research Articles

The influence of the disturbing effect of roadways through faults on the faults' stability and slip characteristics

AbstractIn order to mitigate the risk of geological disasters induced by fault activation when roadways intersect reverse faults in coal mining, this paper uses a combination of mechanical models with PFC2D software. A mechanical model is introduced to represent various fault angles, followed by a series of PFC2D loading and unloading tests to validate the model and investigate fault instability and crack propagation under different excavation rates and angles. The results show that (1) the theoretical fault model, impacted by roadway advancing, shows a linear reduction in horizontal stress at a rate of −2.01 MPa/m, while vertical stress increases linearly at 4.02 MPa/m. (2) At field excavation speeds of 2.4, 4.8, 7.2, and 9.6 m/day, the vertical loading rates for the model are 2.23, 4.47, 6.70, and 8.93 Pa/s, respectively. (3) Roadway advancement primarily causes tensile‐compressive failures in front of the roadway, with a decrease in tensile cracks as the stress rate increases. (4) An increase in the fault angle leads to denser cracking on the fault plane, with negligible cracking near the fault itself. The dominant crack orientation is approximately 90°, aligned with the vertical stress.

Influence of fracture pattern between Hot Sedimentary Aquifer (HSA) and Enhanced Geothermal Systems (EGS) on thermal production and permeability evolution

Thermal energy extraction relies on the fluid circulation and heat transfer between fluids and rock matrix. The conductive fracture channel provides effective conduits for fluid circulation, while absorbing heat energy from adjacent blocks depending on heat transfer length. The vertical depth variation not only leads to the quality of heat reservoir, but also brings substantial influence on fracture pattern due to stress state and chronological tectonic evolution. Consequently, it is vital to explore the influence of fracture pattern on the model of thermal energy production and subsequent permeability evolution. The induced thermal stress plays an important role in the stress state and fractures and therefore permeability development. It is hypothesized that the two-stage evolution of permeability and porosity: shrinkage effect from thermal cooling is strong sufficient with the highest drawdown gradient, which leads to significant minimum horizontal stress reduction and permeability enhancement. The second stage when rock temperature completely drained allows permeability and porosity reduce to the initial magnitude, with the thermal stress diminishes. The shallow geothermal extraction with less consolidation induces less thermal stress to change rock permeability and porosity. Therefore, the thermal stress needs to be taken into account in evaluating the formation properties evolution and stress state.

Experimental study on stress and permeability response with gas depletion in coal seams

The gas extraction environment in coal seam exhibits uniaxial strain condition with constant overlying strata stress and horizontal strain. Simulating this environment in laboratory often ignores true triaxial stress state, so the difference in horizontal stresses reduction and the accompanying permeability evolution remain ambiguous. Therefore, this study conducted the true triaxial stress and permeability response tests simulating gas extraction environment under shallow and deep in-situ stress conditions. To quantify gas adsorption effect, the adsorbed (CO2) and non-adsorbed (He) gases were also used. The results indicated that the intermediate and minimum principal stresses, i.e., σ2 and σ3, exhibited a linear decreasing trend during gas depletion, but showed more decreases in stress when the intermediate and minimum principal strains, i.e., ε2 and ε3, recover under high gas pressure depletion. High true triaxial stress enhanced the compressibility of pores and fractures in coal, resulting in low horizontal deformation and stress reduction gradient during gas depletion. Similarly, the reduction gradient of σ2, mσ2, was less than that of σ3. This suggested that the difference between horizontal stresses also increased during coalbed methane (CBM) extraction, which exacerbated the risk of coal body damage. For different gas depletion, the stress reduction gradient exhibited mHe < 1 and mCO2 > 1, which was related to the relative affinity of different gas species for the adsorption medium. A significant matrix shrinkage effect resulted in a more pronounced stress reduction. For permeability, the permeability increased exponentially during CO2 depletion, while the permeability of helium exhibited a decreasing followed by an increase with decreasing gas pressure. This is related to the competing mechanism and synergistic effect of the adsorptive gas desorption, effective stress effect, and slippage effect. We quantified the contribution and mechanism of the three to the permeability separately. The permeability anisotropy ratio (Ar) decreased exponentially during gas depletion.

Wellbore injectivity response to step-rate CO2 injection: Coupled thermo-poro-elastic analysis in a vertically heterogeneous formation

Safe and permanent carbon geological storages require an elaborate analysis of geomechanical stability. Subsurface injection of CO2 changes the local temperature and pore pressure and, further, alters the stress due to thermo-poro-elastic responses.A permanent CO2 storage testing was conducted in the water leg of Cranfield reservoir in Mississippi, USA, where hosted CO2 enhanced-oil recovery activities. During CO2 injection, the injection rate was ramped up twice but the bottom-hole pressure did not increase with the imposed injection rates as expected. This unexpected field observation suggests the possibility of an open-mode fracture development at the injector. However, the injector response and potential fracture development have not been rigorously interpreted with detailed near-wellbore temperature and pore pressure change upon the CO2 injection.In this study, we performed history matching of CO2 injection using coupled thermo-poro-elastic reservoir simulation to examine the possibility of fracturing. We built a reservoir model including vertical heterogeneity in both petrophysical and geomechanical properties estimated from well-logging analysis and laboratory experiments.The simulation results show that CO2 injection changes stresses and support the hypothesis of development of an open-mode fracture at the injector during the Cranfield test. The near-injector region exhibits a large temperature reduction up to 55 °C with ensuing effective horizontal stress reduction up to 9.1 MPa. However, a caprock integrity issue is unlikely because of the horizontal stress contrast within layers and low hydraulic communication with the injection zone.This study indicates that the injectant temperature should be considered in the design of high-rate CO2 injector.

Comparison of stresses in 3D v. 2D geomechanical modelling of salt structures in the Tarfaya Basin, West African coast

We predict stresses and strains in the Tarfaya salt basin on the West African coast using a 3D static geomechanical model and compare the results against a simplified 2D plane-strain model. Both models are based on present-day basin geometries, are drained, and use a poroelastic description for the sediments and visco-plastic description for salt. We focus on a salt diapir, where an exploratory well has been drilled crossing a major fault. The 3D model shows a significant horizontal stress reduction in sediments at the top of the diapir, validated with measured data later obtained from the well. The 2D model predicts comparable stress reduction in sediments at the crest of the diapir. However, it shows a broader area affected by the stress reduction, overestimating its magnitude by as much as 1.5 MPa. Both models predict a similar pattern of differential displacement in sediments along both sides of the major fault, above the diapir. These displacements are the main cause of horizontal stress reduction detected at the crest of the diapir. Sensitivity analysis in both models shows that the elastic parameters of the sediments have a minimal effect on the stress–strain behaviour. In addition, the 2D sensitivity analysis concludes that the main factors controlling stress and strain changes are the geometry of the salt and the difference in rock properties between encasing sediments and salt. Overall, our study demonstrates that carefully built 2D models at the exploration stage can provide stress information and useful insights comparable to those from more complex 3D geometries. Thematic collection: This article is part of the Mechanics of salt systems: state of the field in numerical methods collection available at: https://www.lyellcollection.org/cc/mechanics-of-salt-systems

An experimental study of stress changes induced by reservoir depletion under true triaxial stress loading conditions

The reduction of pore pressure during reservoir production alters the initial stress state within the reservoir and the surrounding rocks. This stress variation is responsible for many problems encountered during production (e.g. fault reactivation, casing deformation). In this investigation, a newly designed True Triaxial Stress Loading and Pore Pressure Applying Apparatus (TTSL-PPAA) was applied to study the coupling of pore pressure drawdown and reservoir stress changes under actual field conditions. The objective was to overcome the shortcomings of commonly employed depletion testing under conventional triaxial conditions, which assumes that both horizontal stress components are equal. The depletion simulations were performed by decreasing the pore pressure and under applying constant vertical and keeping zero horizontal strains, i.e. uniaxial strain boundary conditions. The results showed that both horizontal stresses decrease with a linear trend with pore pressure depletion. The reduction rates of minimum and maximum horizontal stresses, however, are not uniform. The rate of maximum horizontal stress reduction is higher than the minimum horizontal stress component. Moreover, the pore pressure/stress coupling intensity is dependent on the initial, pre-production, stress state. By increasing the initial vertical stress, and therefore, decreasing the ratios of horizontal to vertical stress, (referred to as KH = σH/σV and Kh = σh/σV), the effect of pore pressure depletion on in situ stress components was decreased. The obtained test results imply that, in addition to stress magnitude changes, the stress regime might be locally changed by production due to pore pressure/stress coupling. Furthermore, the rock sample failure in some depletion tests indicates that the pore pressure/stress coupling can be used to describe the production induced seismicity reported in hydrocarbon reservoirs worldwide.

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.

Effects of construction sequence and cover depth on crossing-tunnel interaction

An appropriate construction sequence for crossing tunnels can help minimize the adverse impact on the tunnel that is constructed first (considered as the existing tunnel). However, the influence of construction sequence on crossing-tunnel interaction is complex. Two pairs of three-dimensional centrifuge tests were carried out to investigate the effects of construction sequence on crossing-tunnel interaction. In the first pair of tests, the new tunnel was excavated beneath the existing tunnel in a reference test, while in the other test the new tunnel advanced above the existing tunnel. To study the effects of cover depth on the construction sequence, the depths of the existing and new tunnels were increased in the second pair of tests. An advanced hypoplasticity constitutive model with small-strain stiffness was adopted to back-analyze the tests. The existing tunnel was found to be vertically compressed when the new tunnel was excavated underneath, but vertically elongated when the new tunnel advanced above. This is because the reduction of stress acting on the existing tunnel in the horizontal direction was larger than in the vertical direction when the new tunnel was constructed beneath. On the other hand, the decrease in vertical stress on the existing tunnel was larger than the horizontal stress reduction when the new tunnel was excavated above. This behavior was observed in both pairs of tests, irrespective of the cover depths of the tunnels. As the cover depths of the existing and new tunnels increased, settlement of the existing tunnel due to the new tunnel construction beneath decreased. This is because with the larger cover depths of the tunnels, the increase in mobilized shear stiffness of the soil dominated the increase in stress relief caused by the tunnel excavation.

Effects from diaphragm wall installation to surrounding soil and adjacent buildings

A new approach for simulating the excavation and construction of subsequent panels is proposed to investigate the effects from the installation of diaphragm walls on the surrounding and adjacent buildings. The method has been combined with a 3-D nonlinear analysis and a constitutive law providing bulk and shear modulus variation, depending on the stress path (loading, unloading, reloading). From the application of the method in a normally to slightly over-consolidated clayey soil it was found that the panel length is the most affecting factor of ground movements and lateral stress reduction during panel installation. Moreover, from the evaluation of horizontal stress reduction and the variation of horizontal displacements arises that the effects from the construction of a panel are mainly limited to a zone within a distance of the order of the panel length. The effects on an adjacent building have also been investigated by applying a full soil–structure interaction including the whole building. Settlement profiles and settlements are given at specific points as increasing with subsequent installation of panels, providing the ability of specific monitoring guidelines for the upcoming construction of the diaphragm wall in front of the building. Contrary to lateral movements, which mostly take place at the panel under construction, it was found that the effect of settlements covers a larger area leading to a progressive settlement increase. The effect highly depends on the distance from the panel under construction.

High-resolution seismic stratigraphy, sedimentary processes and the origin of seabed cracks and pockmarks at Nyegga, mid-Norwegian margin

Densely spaced high-resolution TOPAS seismic profiles and EM1002 bathymetric data reveal the presence of numerous pockmarks, mound-like structures and elongated seabed cracks at Nyegga, offshore mid-Norway. The seabed cracks are located adjacent to the northern escarpment of the Storegga Slide, appearing as graben-like structures in the TOPAS data. Unlike the cracks, pockmarks and mound-structures are largely associated with vertical zones of acoustic blanking at depth, interpreted as pathways for vertically migrating gaseous fluids. Based on the TOPAS data, a new seismostratigraphic framework has been established and correlated to previously published age models of IMAGES cores MD99-2291 and MD99-2289. Seismic facies interpretation suggests repeated and rapid deposition of up to 40 m thick glacigenic wedges in the eastern part of the study area around 18.2 14C ka BP (21.8 cal. ka), 17.5 14C ka BP (20.8 cal. ka) and 16.9 14C ka BP (20 cal. ka). Towards the west, glacimarine deposition has prevailed, characterized by progressively increasing sedimentation rates with peak values of 30 m/ka during the period from 15.0 14C ka BP (18.2 cal. ka) to 15.8 14C ka BP (19 cal. ka). As the distribution of the Nyegga pockmarks closely coincides with the main Late Weichselian sediment depocenters, we suggest a relation between rapid and repeated sedimentation and periodic overpressure generation at depth, ultimately leading to fluid expulsion at the seabed and the formation of the Nyegga pockmark field. In contrast, seabed cracks at Nyegga appear to have formed due to local extension which we relate to horizontal stress reduction as a consequence of the Storegga Slide event. Potentially, this event has been accompanied by renewed vertical fluid migration and the most recent stage of pockmark development.

Parametric Analysis of Stress Reduction in the Caprock Above Compacting Reservoirs

Abstract It is well known that the fracture gradient is significantly reduced when drilling through depleted reservoirs; hence, mud density must be adjusted or a casing must be set before drilling through depleted reservoirs. However, the magnitude and extent of fracture-gradient reduction in the caprock above depleted reservoirs have not been well characterized. Several casing-shoe tests performed in the cap-rocks showed that fracture gradients in the caprock above depleted reservoirs also declined significantly with reservoir depletion. This gives rise to the problem of determining a safe casing-shoe set position without inducing lost-circulation problems. To address these issues, parametric analysis of stress reduction in the caprock above compacting reservoirs was performed using a finite-element structural model with fluid flow from caprocks. It was found that two factors dominate the in-situ stress change in caprocks. One factor is pore-pressure change caused by dehydration from shale to reservoir sections as reservoir pressure is depleted, and the other factor is the horizontal-stress reduction because of the roof effect of caprocks. Although caprock permeability is generally very low, shale pore fluid can drain over time into reservoir sections because of large pressure differences after reservoir-pressure depletion. The loss of pore fluid causes a fracture-gradient reduction in shale sections. By contrast, if the shear modulus in the shale section is high compared with the shear modulus of the reservoir formation, the horizontal stress in the caprock reduces because of the roof effect. Using the correlations of shale elastic modulus and permeability with respect to shale porosity, a practical method to estimate the extent and magnitudes of stress reduction in the caprock above depleted reservoirs is proposed in this paper.

Stress Transfer and Deformation Mechanisms around a Diaphragm Wall Panel

Practicing engineers and researchers are becoming aware of the importance of ground movements and stress changes in the ground that may occur during the construction of a diaphragm wall type of retaining walls. In this paper, the construction sequence of a typical diaphragm wall panel in stiff clay is fully simulated using a three-dimensional finite difference program. Computed results are compared with some reported results of centrifuge model tests and relevant case histories. It has been found that the influence zone due to the diaphragm wall panel installation falls within a normal distance of approximately one panel depth, D, from the face of the panel, 1/3D below from the toe and 1/3L from the edge and along the length of the panel (L). Significant horizontal stress reduction behind the center of the panel is attributed to both the downward load transfer and the horizontal arching mechanisms. A settlement bowl appears behind the panel. The maximum settlement occurs at a distance of about 0.2D behind...

Dynamic uplift capacity of helical anchors in sand : Clemence, S P; Smithling, A P Proc 4th Australia-New Zealand Conference on Geomechanics, Perth, Western Australia, 14–18 May 1984V1, P88–93. Publ Barton: Inst of Engineers, 1984

Earth anchors are becoming a very useful technique for securing temporary and permanent foundation systems subjected to uplift loads. In recent years, helical anchors have become more widely used because of their ease of installation and low cost. Past research has concentrated on static loading. Recent investigations into the cyclic capacity of anchors have increased rapidly due to increased construction in the ocean and the importance of anchors in advancing offshore technology. Laboratory tests were performed with one-quarter scale single-helix model anchors in dry sand at a constant relative density and embedment depth. The two parameters investigated were the displacement amplitude and prestress load. Appropriate equipment and instrumentation were used to monitor anchor deflection, dynamic load, and horizontal soil stresses during the cycle tests. Static tests were performed to determine the ultimate pullout capacity of dead anchors and the postcyclic failure capapcity of anchors. It was concluded that screw-in anchor installation technique and the application of a prestress load both produced an increase in horizontal soil stresses and soil densification in the vicinity of the single-helix anchor. Cyclic loading caused a reduction in horizontal stress and an upward cyclic creep of the anchor. Reduction of horizontal stress occurred until the active failure stress was approached. At this point, the sand had loosened and the anchor began to pullout rapidly. The post-cyclic static capacity was found to be lower than the ultimate static capacity of a dead anchor. In the prestress range investigated, a critical ratio of dynamic load to effective static capacity exists. Above this critical ratio, a failure of a prestressed anchor will occur earlier than a dead anchor failed by cyclic loading. Prestressed anchors below this critical ratio will experience an increase in anchor life.

A study of ground movement and progressive failure caused by a deep excavation in Oxford Clay

A study of a 29 m deep brickpit which was progressively excavated in overconsolidated Oxford Clay has shown that the surrounding ground within a distance of 60 m was affected by movement. The release of in situ stress caused inward displacement of the sides of up to 200 mm and settlement of about 100 mm. The heave of the base of the pit amounted to some 100 mm. The Oxford Clay moved inward as a block, displacement at the base being almost the same as at ground level. Overthrusting relative to lower beds occurred by sliding on a shear band which developed by progressive failure as the maximum shear stress exceeded the peak shear strength along bedding planes near pit base level. Horizontal extension was concentrated in the ground overlying an ‘end region’ of the shear band, 20–30 m from the face, where, following initiation of displacement, the shear strength dropped from peak to residual causing a rapid reduction in horizontal stress. The observed behaviour is in broad agreement with the mechanism of progressive failure discussed by Bjerrum (1967) and recently published theoretical work on initiation and propagation of shear bands (Palmer and Rice, 1973). Details are given of the precise survey techniques and instrumentation installed to measure the ground movements. The geotechnical properties and structural fabric of the Oxford Clay are described and their influence on ground movement discussed. L'étude d'un puits maçonné en briques d'une profondeur de 29 m qui a été creusé progressivement dans de l'argile d'Oxford surconsolidée a montré que le terrain environnant dans un rayon de 60 m fut affecté par le mouvement. Le relâchement des contraintes in situ causa un déplacement interne des côtés de 200 mm environ et un tassement de l'ordre de 100 mm. Le soulèvement de la base du puits a atteint une valeur de 100 mm. L'argile d'oxford se déplaça intérieurement en bloc, le déplacement à la base étant presque le même qu'au niveau du sol. Le surenfoncement relatif par rapport aux couches plus basses se produisit par glissement sur une bande de cisaillement qui se développa par rupture progressive lorsque la contrainte de cisaillement maximale dépassa la résistance de pic à la rupture au cisaillement le long de plans de stratification près du niveau du fond du puits. L'extension horizontale était concentrée dans le sol surmontant une ‘région d'extrémité’ de la bande de cisaillement. 20 à 30 m de la face où, suivant l'initiation de déplacement, la contrainte de cisaillement tomba de la valeur de pic à la valeur résiduelle causant une réduction rapide des contraintes horizontales. Le comportement observé est en plein accord avec le mécanisme de rupture progressive proposé par Bjerrum (1967) et avec un ouvrage théorique publié récemment sur l'initiation et la propagation de bandes de cisaillement (Palmer et Rice, 1973) Des détails sont donnés sur les techniques précises de nivellement et l'instrumentation, installées pour mesurer les mouvements du sol. Les propriétés géotechniques et la structure de l'argile d'Oxford sont décrites et leur influence sur le mouvement du sol est examinée.