Shear Failure Criterion Research Articles

Determination of a safe drilling mud window and optimal drilling mud pressure by shear and tensile failure criteria

Wellbore drilling is the main activity to access oil reservoirs for oil production. This study aimed at presenting a safe drilling mud window using two criteria for estimating the minimum and maximum drilling mud pressure in order to prevent tensile and shear failure in oil wells. To this end, data on oil wells was first collected. To calculate the lower limit of the mud window, the oil well was modeled at different drilling mud pressures with the help of the finite difference software FLAC2D. By plotting the normalized yield zone area (NYZA) versus the mud pressure, NYZA = 1 was considered as the shear failure threshold, the corresponding pressure to which was considered as the lower limit for the drilling mud weight. To calculate the upper limit of the mud window, the wellbore was modeled with the help of the finite element software, Abaqus. To this end, the drilling mud pressure was gradually increased, and according to the increase in the crack length in the borehole, the pressure causing tensile failure was considered as the upper limit of mud weight. Ultimately, two linear relationships were proposed using the multivariate linear regression analysis in SPSS. In the first relationship, the minimum drilling mud pressure to prevent a shear failure is estimated using pore pressure, minimum and maximum horizontal stresses, cohesion, and internal friction angle. The second relationship estimates the maximum drilling mud pressure to prevent a tensile failure using pore pressure, minimum and maximum horizontal stresses, and maximum and minimum tensile strengths. Combining these two relationships, a new correlation was obtained to estimate the optimal drilling mud pressure. As its main innovative aspect, this study aimed at providing a safe window for drilling mud using two criteria for estimating the minimum and maximum drilling mud pressures in order to prevent shear and tensile failures in oil wells in south Iran oilfields.

An Approach for Stability Analysis of Gas/Fluid-Filled Cylindrical Hole in Laminated Materials

An explicit analytical workflow for cylindrical hole stability analyses in general laminated materials that possess transversely isotropic (TI) anisotropy is presented. In this approach, the calculation of the distribution of the stresses around a cylindrical hole and the failure evaluation at the hole wall consider the effects of both material elasticity anisotropy and strength anisotropy caused by material laminated structures. Material strength anisotropy is assumed to be caused by the sliding of preexisting weakness planes oriented parallel to the isotropic plane of the material. The effect of anisotropy on strength is modeled by combining a shear failure criterion for the intact matrix and a weak plane failure criterion for the planes of weakness. We derive critical pressure solutions for the stability of the intact matrix around a hole filled with gas or fluid based on the Mohr–Coulomb failure criterion and Drucker–Prager failure criterion; either one of them can be combined with the weak plane failure criterion to give the solution for hole wall shear failure pressure. The solution for hole wall fracture initiation pressure is derived based on the tensile failure criterion. This approach can be applied to holes of arbitrary orientation in general laminated materials.

An analytical solution for analysis of toppling-slumping failure in rock slopes

When a slope consists of a horizontal weak rock or soil overlaid by some vertical strong slender rock columns, the slope is prone to a secondary toppling failure which is known as “toppling-slumping” failure. In the slope, rock columns impose pressure on the lower continuous weak rock or soil mass which leads to a differential settlement at the base of each rock column or a circular shear failure in the continuous mass. If the differential settlements reach a special threshold, rock column overturn and toppling-slumping will happen. However, the circular shear failure of the soil or weak rock mass which is a well-recognized failure mechanism will occur when shear stress in the slope exceeds its shear strength. In this paper, an attempt has been made to better understanding of the “toppling-slumping” failure mechanism through developing an analytical solution. For this purpose, first an Airy stress function is acquired for the lower continuous soil mass and its stress distribution and displacement are accordingly determined analytically due to the overlaid rock columns loads. Then, differential settlements at the base of all rock columns and shear stresses in the soil or weak rock mass are computed and compared with the toppling and shear failure criteria to assess slope failure. Finally, several design-charts are developed that determine the stability of a slope against toppling-slumping failure mechanism based on geometric characteristics of the slope and shear strength of its constituent material. Also, a Mathematica package code was developed to evaluate the stress distribution within the slope, and the safety factor. The results show that the vertical stress distribution plays a significant role in the settlement of the rock columns. Results also manifested that at a certain distance from the slope crest, the set of rock columns will act like a single column on a half-plane. The shear or toppling failure of the slope not only depends on the material's strength but also on the geometry and the normalized column distance.

New Interface for Assessing Wellbore Stability at Critical Mud Pressures and Various Failure Criteria: Including Stress Trajectories and Deviatoric Stress Distributions

This study presents a new interface for wellbore stability analysis, which visualizes and quantifies the stress condition around a wellbore at shear and tensile failure. In the first part of this study, the Mohr–Coulomb, Mogi–Coulomb, modified Lade and Drucker–Prager shear failure criteria, and a tensile failure criterion, are applied to compare the differences in the critical wellbore pressure for three basin types with Andersonian stress states. Using traditional wellbore stability window plots, the Mohr–Coulomb criterion consistently gives the narrowest safe mud weight window, while the Drucker–Prager criterion yields the widest window. In the second part of this study, a new type of plot is introduced where the safe drilling window specifies the local magnitude and trajectories of the principal deviatoric stresses for the shear and tensile wellbore failure bounds, as determined by dimensionless variables, the Frac number (F) and the Bi-axial Stress scalar (χ), in combination with failure criteria. The influence of both stress and fracture cages increases with the magnitude of the F values, but reduces with depth. The extensional basin case is more prone to potential wellbore instability induced by circumferential fracture propagation, because fracture cages persists at greater depths than for the compressional and strike-slip basin cases.

Impact of an underlying soft soil layer on tunnel lining in seismic conditions

The presence of a soft soil layer can influence the propagation of incoming wave from the bedrock, undoubtedly modifying the ground motions and soil dynamic behavior. Despite its importance, the effect of an underlying soft soil layer on the seismic behavior of tunnels is not well known. In this paper, the analytical study, as well as numerical analysis is used to characterize the underlying soft soil layer-tunnel interaction problem. The analytical solution assumes the plane SH wave vertically propagating through an elastic soft interlayer. The soil plasticity is considered in the numerical study with a visco-elasto-plastic constitutive model following the Mohr-Coulomb shear failure criterion combined with the stiffness degradation. The effects of the input motion intensity, relative thickness, relative stiffness and buried depth of the soft layer are taken into account. The analyses highlight the possible beneficial effects on the site seismic response and tunnel seismic behavior due to the presence of the soft soil layer. The soft soil layer generally amplifies the long periods whereas de-amplifies the short periods of the input ground motion, while the responses in terms of acceleration, strain and Arias intensity at the tunnel position are reduced. In addition, the quantitative relationships between the modification ratios of the dynamic internal forces and soft soil layer parameters are presented.

Evaluation of the Performance of Unreinforced Stone Masonry Greek “Basilica” Churches When Subjected to Seismic Forces and Foundation Settlement

Unreinforced stone masonry made of low strength mortar has been used for centuries in forming old type stone masonry churches of the “Basilica” typology. The seismic performance of such stone masonry structures damaged during recent strong seismic activity in Greece, combined with long term effects from foundation settlement, is presented and discussed. A simplified numerical process is presented for evaluating the performance of such damaged stone masonry structures, making use of linear and non-linear numerical tools and assumed limit-state failure criteria. In order to obtain a quantification of the in-plane sliding shear failure criterion, a number of stone masonry wallets were built with weak mortar and were tested in the laboratory. Through the comparison of the obtained numerical predictions with the observed structural behaviour for selected cases of stone masonry “Basilica” churches, the validity of the applied simplified numerical process is demonstrated. It is shown that reasonable approximation of the observed performance of such structures can be obtained when the assumed failure criteria are realistic.

Modelling Leakage Risk Along a Fault Using Modified Discrete Elements

Capacity estimates for chosen CO2 storage reservoirs are limited by the prognosed maximum pore pressure, itself predicted to cause slippage of mapped faults and thus increase the risk of leakage from the site. In most simulation tools, shear failure criteria are checked for predicted pore pressure increased on inclined lines or surfaces representing faults; if the chosen criterion is met, the simulation stops and the pressure and stress distributions in the entire domain are recorded and assigned as critical values not to exceed. Here, we try a different approach, where instead of a line in 2D, a surface is delimited on both sides of a single fault, representing a fault process zone. An in-house model is used, Modified Discrete Element tool, capable of explicitly creating and propagating fractures. The model is coupled to the commercial flow simulator TOUGH2, thus enabling flow through the created fractures to be evaluated. A simple case is simulated, where a sandstone storage reservoir is bounded by a fault, continuing up through the caprock to the next sandstone layer. In the chosen case, the storage site undergoes depletion through gas production, before CO2 is stored in the reservoir, taking the pore pressure back to its initial level. Results show that during depletion, shear stresses may develop such that fractures run alongside the fault all the way up to the upper aquifer. By assigning a permeability value to the open fractured elements, a leakage rate along the fault can be computed. Thus, geological knowledge about the process zone of faults is translated in our tool into quantitative estimates of leakage rates as a function of stress path, historic and future, corresponding to injection plans.

A workflow for infill well design: Wellbore stability analysis through a coupled geomechanics and reservoir simulator

The traditional way of analyzing wellbore stability in depleted reservoirs is by assuming homogenous depletion and uniaxial strain. Then the analytical solution of stress path is obtained. Pore pressure depletion, however, is location and time dependent. The objective of this study is to develop a workflow for infill well design. More specifically, a wellbore stability analysis is developed for infill wells using a coupled geomechanics and reservoir simulator (CGRS) which considers lateral displacement and inhomogeneous depletion. Different shear failure criteria are utilized in a new CGRS wellbore stability model. The upper bounds of shear failure are given by Drucker-Prager Inscribes and Griffith Theory, while the lower bound is given by Drucker-Prager Circumscribe. CGRS wellbore stability model is compared with conventional wellbore stability model (based on the analytical solution of stress path). CGRS wellbore stability can quantify the influence of azimuth on minimum and maximum mud weight during the depletion when initial maximum horizontal stress equals minimum horizontal stress. In addition, CGRS can give the output of a location-dependent mud weight map for the entire reservoir. Neither of the above two functions can be realized by conventional wellbore stability model. Therefore, CGRS can provide dynamic information concerning where to drill, when to drill, and how to drill for infill wells.

Cracking behaviour of fine-grained soils: from laboratory testing to numerical modelling

Many experimental evidences suggest that desiccation cracks in clay initiate as a result of the mobilization of soil tensile strength. However this mechanical approach disregards the cohesionless and effective stress-dependent behaviour of fine-grained soil. On the other hand recent findings in the literature suggest that effective stress-dependent shear failure criteria would be appropriate to explain the mechanisms of desiccation cracking for tensile total stress states. This work aims at assessing the validity of a shear failure criterion to predict the onset of cracking in clay forms exposed to air drying. Clay forms of various geometries were experimentally subjected to non-uniform hydraulic and mechanical boundary conditions. Time and location for crack initiation are monitored using a digital camera. Cracking experiments are then modelled in a hydro-mechanical framework using an effective-stress shear failure criterion. The comparison of simulations with experimental results for both the time and the location of cracking allows assuming that cracking occurs due to failure in shearing.

Destructive and minor destructive tests on masonry buildings: Experimental results and comparison between shear failure criteria

In this paper, an extensive experimental campaign conducted on existing masonry buildings is presented. The main objective of this activity was to study the shear behaviour of masonries constituted by clay bricks and lime-based mortar evaluating the most suitable techniques applicable to get reliable information on the material mechanical properties. Minor destructive tests (MDT) and destructive tests (DT) were performed on eight masonry buildings, damaged by the 2012 Emilia earthquake (Italy). The tests results are here presented and discussed, focusing on the strength parameters obtainable from each test type and on the definition of appropriate failure domains. The strength predictions are finally compared with the outcomes of full-scale destructive tests.

Effect of rock strength variation on the estimated borehole breakout using shear failure criteria

One of the primary goals of wellbore stability analysis is the estimation of the shear failure onset or borehole breakout. Estimation of borehole shear failure requires selecting an appropriate failure criterion. Numerous failure criteria have been used for rock failure analysis, but there is no common agreement of which criterion to select for wellbore stability analysis. In general, rock failure criteria mainly depend on rock mechanical properties and in situ stresses. This paper investigates the effect of rock strength variation and strength anisotropy on the estimated borehole shear failure using thirteen different criteria. A rock strength database was created from different US unconventional shale plays. Rock failure criteria were ranked based on the estimated borehole breakout for different shales. According to the results for different level of rock strength, there are some failure criteria that are highly responsive to variation in rock mechanical properties. Circumscribed Drucker–Prager, Modified Griffith, Inscribed Drucker–Prager criteria have shown great response to the change in the internal angle of friction. Murrell, Stassi D’Alia, Hoek–Brown, and Griffith are sensitive to variation of uniaxial compressive strength. Mogi-Coulomb, Modified Wiebols–Cook, Modified Lade, and Mohr-Coulomb failure criteria did not show any significant response to the variation of rock mechanical properties and strength anisotropy.

3D paleotectonic stress field simulations and fracture prediction for marine-continental transitional facies forming a tight-sandstone reservoir in a highly deformed area

Marine-continental transitional facies coal measures that form tight sandstone reservoirs in heavily deformed areas are a new field for natural gas exploration. The degree of fracture development is the core factor in the enrichment of the potential natural gas resources. In this paper, using the Shanxi Formation in southern Qinshui Basin of eastern China as an example, the characteristics of fracture development in tight sandstone reservoirs were studied, and the paleotectonic stress field was reconstructed. On this basis, the fracture development of the area was predicted. The results show that fractures of different scales were developed in the tight sandstone of the Shanxi Formation and that over 90% of these fractures are tectonic. The types of fracture filling in the tight sandstone of the Shanxi Formation are mainly unfilled and half-filled. Fine geological modeling and paleostress simulations were conducted on the II sand group of the Shanxi Formation using the 3D finite element method. The geostress field of the target layer in the Himalayan period was restored. Considering that the tensile and shear fractures of the target layer were both well developed, we constructed a comprehensive rupture rate (IF) to quantitatively predict the level of development of the fractures by combining the Griffith tensile failure criterion and Mohr–Coulomb shear failure criterion. The prediction results are in agreement with the actual geological conditions, which indicates that this method is suitable for characterizing the fractures in marine-continental transitional facies coal measures that form tight sandstone reservoirs in heavily deformed areas.

Experimental and numerical studies of stepped-scarf circular repairs in composite sandwich panels

This paper investigates the static mechanical performance of bonded circular repairs on sandwich panels made with carbon-epoxy composite skins and a Nomex core. First, the mechanical behaviour of pristine, open-hole and repaired sandwich panels under edgewise compressive loading is studied. Next, pristine and repaired sandwich beams are tested under four-point bending with the circular repair loaded in tension. Then, finite element analyses are performed to predict the strength of the repaired sandwich panel. The adhesive film was considered as an elastic-plastic material with a shear failure criterion. The honeycomb core is assumed to behave as a linear elastic material while for the composite skins, a progressive damage model for woven fabric composites is used to predict the skin behavior until rupture. The good agreement between stiffness and strength levels obtained for both experimental measurements and finite element predictions, for pristine, open-hole and repaired sandwich panels, indicates that an effective analysis tool for the mechanical behavior of the repaired panels has been set-up.

Characterization of pozzolanic lime mortars used as filling material in shaped grooves for restoring member connections in ancient monuments

ABSTRACTThis article presents a set of mechanical characteristics for a pozzolanic mortar consisting of hydrated lime and metakaolin, which is used as filler between metallic connectors and marble blocks during restoration activities of ancient monuments in Greece. Mechanical properties include the uniaxial and triaxial compressive strength, the indirect tensile strength, the bending strength, the elastic modulus and Poisson’s ratio, and a shear failure criterion. Cored cylindrical specimens are used for determining the uniaxial and triaxial compressive behavior and the indirect tensile strength of the mortar, thus ensuring the repeatability of the experimental results. The triaxial tests indicated a plastic behavior of the mortars under study, enabling them to perform well in conditions of high shear forces. Calcite and hydraulic components formed in the setting procedure contributed to the plasticity of the final mortar. The mechanical properties that were developed can be utilized to model material behavior and failure under conditions of confined or triaxial loading. The development of a shear failure criterion for a pozzolanic mortar is a substantial accomplishment that has not been previously reported in the international literature.

Prediction and analysis of the stimulated reservoir volume for shale gas reservoirs based on rock failure mechanism

The ultra-low-permeability shale gas reservoir has a lot of well-developed natural fractures. It has been proven that hydraulic fracture growth pattern is usually a complex network fracture rather than conventional single planar fractures by micro-seismic monitoring, which can be explained as the shear and tensile failure of natural fractures or creation of new cracks due to the increase in reservoir pore pressure caused by fluid injection during the process of hydraulic fracturing. In order to simulate the network fracture growth, a mathematical model was established based on full tensor permeability, continuum method and fluid mass conservation equation. Firstly, the governing equation of fluid diffusivity based on permeability tensor was solved to obtain the reservoir pressure distribution. Then Mohr–Coulomb shear failure criterion and tensile failure criterion were used to decide whether the rock failed or not in any block on the basis of the calculated reservoir pressure. The grid-block permeability was modified according to the change of fracture aperture once any type of rock failure criterion was met within a grid block. Finally, the stimulated reservoir volume (SRV) zone was represented by an enhancement permeability zone. After calibrating the numerical solution of the model with the field micro-seismic information, a sensitivity study was performed to analyze the effects of some factors including initial reservoir pressure, injection fluid volume, natural fracture azimuth angle and horizontal stress difference on the SRV (shape, size, bandwidth and length). The results show that the SRV size increases with the increasing initial pore reservoir and injection fluid volume, but decreases with the increase in the horizontal principal stress difference and natural fracture azimuth angle. The SRV shape is always similar for different initial pore reservoir and injection fluid volume. The SRV is observed to become shorter in length and wider in bandwidth with the decrease in natural fracture azimuth angle and horizontal principal stress difference.

Mechanical performance of repaired sandwich panels: Experimental characterization and finite-element modelling

This paper describes the static performance of adhesively bonded repairs on sandwich panels made with carbon-epoxy composite skins and a Nomex core. First, the mechanical behaviour of pristine and repaired panels under tensile loading was studied. All tests were conducted under room temperature conditions. Then, finite-element analyses were performed to predict the behaviour of repaired panels. Two material models were developed for the adhesive joint: one was linear elastic and the second was elastic-plastic with a shear failure criterion. For the composite skins, an orthotropic linear elastic model was used. Numerical model predictions are in good agreement with the experimental results. It was found also that the strength recovery of the repaired structure increases with the decrease of the scarf angle.

Bounds on the calving cliff height of marine terminating glaciers

AbstractIncreased calving and rapid retreat of glaciers can contribute significantly to sea level rise, but the processes controlling glacier retreat remain poorly understood. We seek to improve our understanding of calving by investigating the stress field controlling tensile and shear failure using a 2‐D full‐Stokes finite element model. Using idealized rectangular geometries, we find that when rapidly sliding glaciers thin to near buoyancy, full thickness tensile failure occurs, similar to observations motivating height‐above‐buoyancy calving laws. In contrast, when glaciers are frozen to their beds, basal crevasse penetration is suppressed and calving is minimal. We also find that shear stresses are largest when glaciers are thickest. Together, the tensile and shear failure criteria map out a stable envelope in an ice‐thickness‐water‐depth diagram. The upper and lower bounds on cliff height can be incorporated into numerical ice sheet models as boundary conditions, thus bracketing the magnitude of calving rates in marine‐terminating glaciers.

Direct Shear Tests of Sandstone Under Constant Normal Tensile Stress Condition Using a Simple Auxiliary Device

Tension–shear failure is a typical failure mode in the rock masses in unloading zones induced by excavation or river incision, etc., such as in excavation-disturbed zone of deep underground caverns and superficial rocks of high steep slopes. However, almost all the current shear failure criteria for rock are usually derived on the basis of compression–shear failure. This paper proposes a simple device for use with a servo-controlled compression–shear testing machine to conduct the tension–shear tests of cuboid rock specimens, to test the direct shear behavior of sandstone under different constant normal tensile stress conditions (σ = −1, −1.5, −2, −2.5 and −3 MPa) as well as the uniaxial tension behavior. Generally, the fracture surface roughness decreases and the proportion of comminution areas in fracture surface increases as the change of stress state from tension to tension–shear and to compression–shear. Stepped fracture is a primary fracture pattern in the tension–shear tests. The shear stiffness, shear deformation and normal deformation (except the normal deformation for σ = −1 MPa) decrease during shearing, while the total normal deformation containing the pre-shearing portion increases as the normal tensile stress level (|σ|) goes up. Shear strength is more sensitive to the normal tensile stress than to the normal compressive stress, and the power function failure criterion (or Mohr envelope form of Hoek–Brown criterion) is examined to be the optimal criterion for the tested sandstone in the full region of tested normal stress in this study.

Study on seismic performance enhancement in bridges based on factorial analysis

The seismic performance of bridges depends on the ductile behavior of its column, as the deck and other substructural components except pile foundations are normally designed to be elastic to facilitate bridge retrofitting. Codes such as AASHTO, Caltrans, IRC: 112 etc. give guidelines for the seismic performance enhancement of columns through ductile detailing. In the present study, a methodology for the seismic performance enhancement of bridges is discussed by using a “Parameter-Based Influence Factor” (PIF) developed from factorial analysis. The parameters considered in the factorial analysis are: percentage of longitudinal reinforcement (Pt), compressive strength of concrete (f′c), yield strength of steel (fy), spacing of lateral ties (S) and column height (H). The influence of each parameter and their combination on the limit states considered is estimated. Pushover analysis is used to evaluate the capacity of columns, considering shear failure criteria. A total of 243 (35 combinations) analysis results are compiled to develop ‘PIF’ used in the performance enhancement process. The study also encompasses other sub-objectives such as evaluating the discrepancies in using the Importance Factor (I) in designing bridges of varied functional importance; and estimating the aspect ratio and slenderness ratio values of bridge columns for its initial sizing.

First Hydraulic Stimulation in Fractured Geothermal Reservoir in Pohang PX-2 Well

The first hydraulic stimulation for enhanced geothermal system (EGS) development in Korea had been conducted in the PX-2 well of 4,348 m depth in Pohang EGS site from January 29th to February 20th, 2016. Treatment histories of injection rate, wellhead pressure and corresponding induced microseismicity data were obtained from the stimulation test upon 140 m long open hole section at the well bottom. Wellhead pressure was up to 89MPa and considerable level of flow rate was attempted up to 47 L/sec. Microseismicity observation showed a trend of lager and more frequent seismicity occurrence in shut-in phase than in injection phase. The injectivity index during the stimulation periods had increased as 2.7 times in January 30th at the wellhead pressure of 73MPa. Postulating the existence of a major fracture zone intersecting the open hole section, the transmissivity and the corresponding equivalent aperture of the fracture were evaluated. Required breakdown pressures by hydrofracturing and hydroshearing mechanisms were estimated based on the various scenarios on the in-situ stress condition, major fracture zone orientation and shear failure criteria.