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Results
Comparison Between Elasto-plastic And Rigid-plastic Cohesive Surface Elements And Embedded Strong Discontinuity Finite Element Implementation of Rock Fracture
Regueiro, R.A. (Department of Civil, Environmental, and Architectural Engineering, University of Colorado at Boulder) | Yu, S.-K. (Department of Civil, Environmental, and Architectural Engineering, University of Colorado at Boulder) | Rogers, S. (Golder Associates Ltd. ) | Yanske, T. (The Doe Run Company)
ABSTRACT: The paper presents a comparison between embedded strong discontinuity finite element implementation and elastoplastic (EP) and rigid-plastic (RP) cohesive surface finite element (CSE) implementations of cracking/fracture in rock. It is shown that care must be taken when choosing the elastic stiffnesses for the EP CSE model, if they are to act as penalty parameters. The RP CSE and EDE implementations obviate this choice. Formulation and implementation is restricted to small strains and rotations, and numerical examples are conducted for two-dimensional (2D) plane strain. 1. INTRODUCTION For clean rock fractures, or when the fracture aperture thickness is small relative to the boundary value problem spatial domain of interest, the interface between two rock faces (or between a grain and its cement matrix) can be approximated as a strong discontinuity (jump in displacement across a surface of zero measure, i.e., a crack); a weak discontinuity is a jump in strain across a shear or compaction band [6]. Computationally, using the finite element method, it is possible to model this strong discontinuity in various ways, some of which include the following: (1) discrete representation of the fracture surface using contact mechanics [1] or cohesive surface elements (CSE) [2]; or (2) embedded discontinuity approaches such as the extended finite element method (X-FEM) [3, 4] or the assumed enhanced strain (AES) method [5, 6, 7, 8, 9, 10]. This paper will focus on a comparison between a discrete approach (CSE) and an embedded discontinuity approach (AES). For the CSE, we will consider elasto-plastic and rigid-plastic formulations, where the rigid-plastic implementation is handled by an augmented-Lagrange multiplier approach [15, 16]. The elasto-plastic formulation typically chooses large values of cohesive surface elastic stiffnesses (normal and tangential directions) as penalty parameters. We will investigate this aspect as well. The embedded discontinuity approach defaults to a rigid-plastic cohesive surface model. Numerical examples will be presented in two dimensions. The advantage of the CSE approach over the AES one is the ability to model microstructurally the micro-cracking in rock at grain/cement interfaces (two differentmaterials: e.g., quartz silt grain embedded in clay matrix, Fig.1), whereas the current AES formulation is limited to simulating strong discontinuities in a single material. Throughout the paper we assume small deformations and rotations. Symbolic notation is used for clearer presentation, such as the inner product of two second order tensors (a · b)ik = aijbjk, the contraction of two tensors a : b = aijbij , or the dyadic product (a×b)ijkl = aijbkl. Tensor operators are used such as the symmetric gradient ( sv)ij = (vi,j +vj,i)/2, and divergence ( ·a)i = aij,j , where (•),j = (•)/ xj denotes a partial spatial derivative. 2. KINEMATICS AND GOVERNING EQUATIONS FOR STRONG DISCONTINUITIES2.1. Weak form for embedded strong discontinuity The traction continuity condition [s] · n = 0 for a body with strong discontinuities will be used to determine bifurcation. The displacement, and its weighting function , are discretized as compatible fields (see Sect.5.1), while embedding the strong discontinuity jump in displacement [[u]].
ABSTRACT: This paper documents a geomechanical simulation of an oil field in order to devise safe injection pressure boundaries to avoid inducing hydromechanical reservoir issues. Cap rock integrity and fault reactivation are the main concerns. The reservoir rock is an unconsolidated sandstone over an area of 80km2 at an average depth of 2700 to 2900m under water depths ranging from 1000 to 1200m. The reservoir production began with nine horizontal producer wells and the supplementary oil recovery project is being implemented with eight water injection wells. A review on methods and techniques to acquire rock deformability, strength and insitu state of stress data from well log correlations and special well operations is presented here while devising information for this specific oil field. Like any other physical measures, these information and consequent judgments present a high level of uncertainties and the amount of data gathered also plays a role in identifying the global data trends, attempting to achieve more realistic results. The pore pressure changes along time were calculated by a reservoir fluid flow simulator and an elastoplastic nonassociative finite element code was used to perform the structural analysis. The simulations showed that the analyzed faults remain stable in the choosen production project. The over-pressurized area induced by water injection increased the minimum horizontal total stress, reducing the stress contrast between the reservoir and cap rock. Fluid injection above fracture propagation pressures should be avoided. 1.INTRODUCTION Fluid flow simulation in petroleum engineering has been a mature and routine technology in the development projects for the past few decades. As these projects become more expensive and challenging, the need of more sophisticated analyses including coupled hydromechanical behavior increases. These coupled analyzes attempt to answer to some particular geomechanical issues, allowing a geomechanics analyst to devise safe operation boundaries to ensure cap rock integrity and to avoid reservoir fracturing or mechanical fault reactivation problems. The fluid flow simulators include multiphase description of the fluids present on reservoir and they consider large non-linearity on thermodynamic behavior and fluid-rock interaction phenomena. However, the deformability of the pore space is commonly treated in simplified way with constant compressibility as well as permeability changes due to stresses. More recently oil industry started to invest more intensively in geomechanical studies and the main reasons for this are the subsurface and surface problems related to reservoir compaction [1] and to understand the influence of state of stress evolution on reservoir permeability. Another recent focus of geomechanical studies is the cap rock seal integrity. Many works [2] investigated fault reactivation during reservoir production, i.e. how the reservoir exploitation can induce deformation and stress and how it can compromise the cap rock seal integrity. Compaction and subsidence are extensively studied on North Sea chalk reservoirs. In these cases the high depletion due to production and high deformability of the reservoir rock mass produced a great amount of deformation on surrounded rocks. On literature [1] [3] there are many descriptions of well casing damage related to these phenomena.
- North America > United States (1.00)
- Europe > Norway > North Sea (0.34)
- Europe > United Kingdom > North Sea (0.24)
- (2 more...)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Sandstone (0.72)
- Geophysics > Seismic Surveying (1.00)
- Geophysics > Borehole Geophysics (1.00)
- Europe > Norway > North Sea > Central North Sea > Central Graben > PL 018 > Block 2/4 > Greater Ekofisk Field > Ekofisk Field > Tor Formation (0.99)
- Europe > Norway > North Sea > Central North Sea > Central Graben > PL 018 > Block 2/4 > Greater Ekofisk Field > Ekofisk Field > Ekofisk Formation (0.99)
- Asia > Russia > Far Eastern Federal District > Sakhalin Island > Sea of Okhotsk > East Sakhalin - Central Sea of Okhotsk Basin > Lunskoye Field (0.99)
- Reservoir Description and Dynamics > Reservoir Fluid Dynamics > Flow in porous media (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Reservoir geomechanics (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Exploration, development, structural geology (1.00)
ABSTRACT: The paper presents the results of an elastoplastic FLAC modeling study of the rock support performance due to drift excavation and nearby mining activities at the Garson mine of Vale Inco in Sudbury, Canada. The study employs a typical haulage drift section that is 5 m x 5 m and is approximately 1.5 km below surface. The primary support system consists of 2 m long rebars, whereas the enhanced support system includes modified cone bolts (MCB) and a shotcrete liner. The case study is carried out with three analyses. In the first analysis, the primary support system is simulated and the rebar loads are calculated as a result of the drift excavation itself and subsequent extraction of nearby stopes. The second analysis, corresponding to the current practice at the mine, involves the simulation of both primary and enhanced support system together from the beginning of drift development to the end of stope sequencing. In an effort to assess the effect of timing of enhanced support installation, a third analysis is carried out, whereby enhanced support system is installed after the extraction of the first stope. The results are presented in terms of bolt axial loads and stresses in the shotcrete layer. It is shown that the currently adopted support system is sufficient to sustain the effect of mining activities. It is also shown that the delayed installation of enhanced support (after one stope extraction) could reduce bolt yielding of the support system. 1. INTRODUCTION Haulage drifts are used for the transportation of blasted ore from the draw point to nearby ore pass or dumping point in sublevel mining systems. During production, haulage drifts are occupied by mine operators and haulage equipment. Therefore the stability of haulage drifts is important to the safe and uninterrupted production of a mining operation. It would be advantageous to know a priori how drift stability is influenced by mining activities in the proximity of the drift. This paper presents a case study of a haulage drift from the deep workings of the Garson mine of Vale Inco, located in Sudbury, Canada. The case study level is situated 1.5 km below surface and the haulage drift is 5 m by 5 m. Primary support system consists of rebars, whereas secondary or enhanced support system includes modified cone bolts and shotcrete liner. Elastoplastic finite difference modeling is carried out in the first place to assess the state of stress and deformations around the drift, and the response of primary support system to drift development. 2. FLAC MODEL The standard support pattern from Garson mine, Vale Inco was selected for the case study to analyze the variation of axial loads in the bolts and stresses in the shotcrete liner with respect of mining sequences. Figure 1 shows the layout of the problem to be modeled, whereby three lower level stopes and three same-level stopes are mined and filled in the sequence shown in the vicinity of the haulage drift located on the 5000 ft level.
ABSTRACT: a mobilized dilation angle model considering the influence of both confining stress and plastic shear strain is proposed in this paper. The model is used to predict the volumetric-axial strain relationships of a few rocks samples and the results are found to be in good agreement with experimental results. Realistic post-failure dilation behavior of rocks can be captured using the proposed model in combination with Mohr-Coulomb strain-softening models. The model is then used to study the excavation induced displacement around tunnels located in different rock masses. It is illustrated from a few examples that displacement distributions obtained from the dilation angle model are more reasonable, when compared with the general trend measured underground. 1. INTRODUCTION1.1 Rock dilation The mechanical behavior of rocks and rock masses has been extensively investigated in the fields of civil and mining engineering. Experimental and field observations of rock failure show that the failure process is closely associated with rock dilation which is a phenomenon associated with micro-crack initiation and propagation, and increase in void space when the rock is loaded beyond a certain threshold. It is important to understand nonlinear characteristics of rocks before and after the peak stress and subsequently test the behavior under various loading conditions in numerical modeling with a constitutive model which represents the complete stress-strain behavior of rocks adequately, especially for the nonlinear response such as dilation. Cook [1] proved that dilation during compression to failure was a pervasive volumetric property of rocks and not a superficial phenomenon. Dilation represents the true volumetric behavior of rocks, and it is closely related to the process of rock failure. Based on studies by many researches [2-7], the failure process of brittle rocks can be divided into the following stages: (1) crack closure; (2) linear elastic deformation; (3) crack initiation; (4) stable crack growth; (5) crack coalescence and damage; (6) unstable crack growth; (7) failure; (8) post peak behavior. A detailed illustration of the dilation process of rocks can be found in [8]. In continuum mechanics, the parameter most widely used to measure dilation is the dilation angle (?), which can be obtained from triaxial compression tests by calculating plastic axial and volumetric strain increments [9]. For a joint, the dilation angle is determined, from direct shear tests, as the ratio of normal to tangential displacements along a joint [10]. However, in rock engineering, when the dilation angle is taken into consideration, especially for numerical modeling studies, the approach by most researches is often simplistic; it is generally assumed as either one of the two constants-zero in a non-associated flow rule and the same as the friction angle in an associated flow rule. In most popular failure criteria, such as linear Mohr-Coulomb failure criterion and non-linear Hoek brown failure criterion, the rock dilation is assumed to remain as a constant when the rock mass is deformed. In many numerical analysis tools [11-14], the default value for dilation angle is often zero for all the nonlinear constitutive models.
- Asia > Japan (0.46)
- Oceania > Australia (0.28)
- North America > United States (0.28)
- North America > Canada > Ontario (0.28)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock (0.30)
ABSTRACT: Dissolution and precipitation of mineral constituents may have a significant influence on the evolution of the mechanical and transport properties of granular aggregates pushed far from equilibrium. They are major porosity-altering processes that operate in many sedimentary rocks. In addition, they may control the build-up and release of fluid pressures in sedimentary basins and along fault zones. Understanding these physicochemical processes is critical in determining the diagenetic and deformational history of rocks and their potential as hydrocarbon reservoirs. Grain intergrowths accelerated by chemical and stress effects increase compaction, reduce porosity and permeability and may augment strength and stiffness. Precipitated mineral matter may similarly occlude pore throats and alter the capillary and permeability of the aggregate. In this work, we explore the magnitude of these effects by representing grain-grain bonding and intergrowth during compaction and fluid circulation using a granular mechanics model (PFC). Grain intergrowth is accommodated by effective dissolution at grain contact points: temperature and local stress control dissolution rate relative to a critical stress that initiates dissolution. Grain-grain contacts are represented by contact stiffness in parallel with a variable rate damping connection to represent creep intergrowth effects. The redistribution of mineral matter is accommodated by diffusion and subsequent precipitation. Diffusion transports dissolved mater from the interface to the pore space, and then precipitates mineral at the less-stressed surface of the grains. Precipitation rate is indexed through aqueous concentration relative to equilibrium concentration through a rate constant. 1. INTRODUCTION During the past 70 years, porosity and permeability evolution during diagenesis has been the subject of numerous investigations. Porosity and permeability information is important when the study focuses on the safe entombment of radioactive wastes, the recovery of hydrocarbons, geothermal fluids and potable water and the understanding of fluid cycling within the crust. All these endeavors require knowledge of the evolution of the mechanical and transport properties of rocks [1,2,3]. It is commonly accepted that porosity and permeability reduction in rock is caused by the combined effects of mechanical compaction and chemical evolution [1,7,8]. Purely mechanical processes which are concentrated at a finite number of contact points control the behavior in the early stages of burial [9,10]. Subsequently, the pressure solution may dominate the behavior of the system. In deformable systems pushed far from chemical equilibrium the role of mineral dissolution, redistribution and reprecipitation may exert important controls in weakening or healing and in sealing or in developing penetrating flow conduits. Material dissolves along grain-grain contacts under high stress, diffuses along the contacts, and precipitates in pore spaces [1, 10, 11, 13]. The fundamental mechanisms and driving forces for grain-contact dissolution include intergranular pressure solution [1,11,13] and microcracking and microgranulation at grain contacts [13,14]. Both mechanical compaction and grain-contact dissolution cause the reduction of bulk volume. Cementation or precipitation of minerals in the pore space results in the reduction of intergranular porosity. The studies of porosity and permeability evolution during compaction are mainly focused on three aspects: lab experiments, natural observations and theoretical studies.
- Geology > Mineral (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
ABSTRACT: The propagation of fracture in rock is associated with a process zone in the form of a localized region of damage. Digital image correlation was employed to observe the process zone based on the measurements of the surface displacement field. A Berea sandstone beam with a center notch was tested in three-point bending and a charge-coupled device camera was used to acquire digital images. The images were concentrated on the area surrounding the notch and were processed using a crosscorrelation algorithm based on the Fast Fourier Transform approach. The process zone was identified from the detailed measurements of the displacements related to the region surrounding the tip of the notch. In particular, the initiation of fracture was characterized by a region of localized damage. Furthermore, a traction-free part of the fracture was found from the measured displacement profile after sufficient propagation. 1. INTRODUCTION It is observed that a nonlinear process zone is developed near the fracture front for so-called quasi-brittle materials such as rock and concrete, which are materials that exhibit significant microcracking. The development of the nonlinear zone has a fundamental influence on the mechanical response of a structure at failure, such that the global behavior is related to characteristics of the zone within the structure [1, 2]. Thus, the investigation of the fracture process zone (FPZ) is key to understanding the failure response [3]. The FPZ in quasi-brittle materials is characterized by progressive damage with material softening, which is caused by microcracking, crack deflection, void formation, interface breakage, and other phenomena. The FPZ is sometimes separated into two parts: (1) a bridging zone and (2) a microcracking zone, but this separation is somewhat artificial. Crack bridging (ligament connection) is known to form at the crack tip, such that bridging is a result of the weak interface between unbroken crystals, aggregates, etc. The region of microcracks that are not connected following the bridging zone is considered the microcracking zone. The initiation and propagation of fracture in quasi-brittle materials are associated with the development of the FPZ, and the size is generally large enough such that it has to be considered in the study of structural response. Fig. 1 illustrates a conceptual representation of a fracture with a traction-free portion and the FPZ represented by a cohesive zone where the restraining traction acts on the separating surfaces [4]. The traction is due to the influence of crack tortuosity or unbroken ligaments, which may be viewed as a function of separation distance. 2. DIGITAL IMAGE CORRELATION (DIC)2.1. Introduction Conceptually, DIC is a particle tracking method that can be used to determine displacements of speckles in a digital image. One of the early papers on the use of a numerical based analysis of digital images to estimate the displacement field was published in 1982 [5]. The digital images of speckle patterns were related to reference and current configurations. The concept of small regions, referred to as subsets, was introduced in the comparison of the images.
- North America > United States > West Virginia (0.25)
- North America > United States > Pennsylvania (0.25)
- North America > United States > Ohio (0.25)
- North America > United States > Kentucky (0.25)
ABSTRACT: This paper discusses about the effect of dilatancy angle in the ground response curve in homogenous and isotropic rock masses. A numerical method proposed by Brown et al. (1983) was implemented in this study by including the effects of elastic strain increments and variable dilatancy angle within a plastic region. Rock parameters change gradually in a stepwise method to the residual values. Hoek-Brown yield criteria was used in this study. The numerical method based on finite difference was implemented here. The dilatancy angle used in this study was modified by Alonso (2003) in linear, Alonso and Anjelano (2007) with an exponential function of variation. The results from these two aspects are compared. It was attempted to find appropriate constant dilatancy angles for simulation of rock masses from bad-good quality by considering Hoek and brown work (1997). The results show a good agreement between real and simulated models in the ground response curve. 1. INTRODUCTION The importance of a thorough knowledge of the complete stress-strain curve in rock and rock masses has frequently been underlined in the rock mechanics, and most particularly, in earlier breakthroughs in the discipline. In the last 30 years, significant success has been obtained in terms of methodologies for estimating reasonably good elastic parameters and failure criteria in rocks, joints and rock masses. However, the difficulties associated with defining a model that adequately reflects observed complete stress-strain curves have affected possibilities of developing suitably valid approaches for handling post-failure strength behavior and dilatancy. This problem consistently attracted the attention of early researchers in rock mechanics and continues to do so in more recent years. The process of computing is implemented in a stepwise procedure proposed by Brown et al. (1983). In routine engineering applications, however, dilatancy seems to receive a great deal less attention, which is hardly surprising since, first of all, many problems in rock mechanics are solved by avoiding failure, and secondly, because of the inherent difficulties in estimating dilatancy. In routine engineering application, the dilatancy of the rock of the rock is assumed to be constant. Ogawa and Lo (1987) investigated the effect of dilatancy on the elastoplastic stresses and displacements around a circular tunnel by using closed-form analytical solutions and a constant dilatancy angle. Hoek and Brown (1997) suggested the elastic-brittle-plastic, elastic-strain softening and elastic-perfectly plastic behaviors for very good, average and very poor quality rock masses, respectively, and recommended the use of constant dilatancy angle (?) values related to the friction angle (f), such as ? = f/4, f/8 and 0 for very good, average and very poor quality rock masses, respectively. However, Detournay (1986) pointed out the possible calculation errors with the use of constant dilatancy angle in elastic-perfectly plastic M-C media and suggested the use of variable dilatancy. Anjelano and Alonso (2005) proposed the use of dilatancy factor with the peak dilatancy angle in elastic-perfectly plastic and elastic-strain softening M-C media. [6] Afterward it is attempted to find an equivalent media for softening zone with constant dilatancy angle.
ABSTRACT: Selection of appropriate failure criteria is very important in the analysis since it affects on plastic zone and on the resulted displacement and stress field around the opening. Some closed-form solutions have suggested for the ground reaction curve, although they are driven based on elastic-perfectly plastic or elastic-brittle-plastic models of rock mass behavior. Brown et al(1983) proposed a stepwise procedure based on Hoek-Brown criterion to solve stress and displacement around the circular opening for elastic-strain softening model of rock mass behavior. A similar stepwise procedure was extracted in this study for Mohr-Coulomb criterion. Finally a sensitive analysis was implemented for Mohr-Coulomb and Hoek-Brown criteria in respect to their parameters. By comparison of the relative displacement caused by changing strength parameters in Hoek-Brown and Mohr-Coulomb criteria, it can be concluded that Mohr-Coulomb criterion is more sensitive in respect of variation of strength parameters of rock mass than Hoek-Brown. 1. INTRODUCTION Analysis of stresses and displacements around circular opening that excavated in isotropic rock masses has been one of the fundamental problems in geotechnical engineering. Provided that the initial stress field is hydrostatic, the problem may be regarded as axisymmetric and an analytical solution can be found. This solution is useful in various situations that include the validation of constitutive models, the stability assessments of circular openings such as borehole and TBM excavated tunnel, the verification of numerical codes, the construction of ground-support reaction curves, etc . In order to obtain ground response curves for circular tunnels, a number of analytical solutions have been presented by considering the elastic-perfectly plastic and elastic-brittle- plastic models of material behavior with the linear Mohr-Coulomb (M-C) and nonlinear Hoek-Brown (H- B) criteria (Brown et al., 1983; Detournay, 1986; Wang, 1996; Carranza-Torres and Fairhurst, 1999; Sharan, 2003, 2005; Carranza- Torres, 2004; Park and Kim, 2006). For an elastic-strain softening model, Brown et al. (1983) presented a numerical stepwise procedure for the stresses and displacements in the H-B media by assuming the constant value of elastic strain in the plastic region such as that at the elastic-plastic interface and the constant dilatancy angle in the strain softening zone. Alonso et al. (2003) proposed the self-similarity solution by solving the system of ordinary differential equations of equilibrium, persistence, and radial displacement velocity and flow rule. The aim of the present study is to apply a sensitive analysis to strength parameters of Hoek-Brown and Mohr-Coulomb criteria and investigating their effects on the ground reaction curve. 2. DEFINITION OF THE PROBLEM Figure 1 shows a circular tunnel excavated in a continuous, homogeneous, isotropic, initially elastic rock mass subjected to a hydrostatic stress p?. The tunnel surface is subjected to an internal pressure p?. As p? gradually reduces, the radial displacement occurs and a plastic region develops around the tunnel when p? is less than the initial yield stress. The material behaviour of elastic-strain softening model used in this study is shown in Figure 2.
ABSTRACT: Rock slope monitoring is an important hazard management technique for dealing with unstable rock slopes in open pit mines. Data is often used to predict locations of rock slope hazards (areas of increased rock slope deformation) and the likelihood of failure (by reviewing the magnitude of deformation and any accelerating trends). This paper reviews an analysis of rock slope deformation in multiple coal mines, using data from ground based radar interferometry systems. The data is aggregated into a database which has allowed the identification of a number of typical ground response patterns, assessment of the deformation rate levels and examined whether slope instability develops into a collapse situation. Rock slope collapse involves a rapid acceleration and gross movement of the slope often involving dilation or fragmentation of the failed slope mass, which can be high consequence events. Management of rock slope collapse using geotechnical monitoring often involves the use of deformation rate alarms, and the analysis has allowed a study into the alarm thresholds used in the coal mining industry. 1. INTRODUCTION Rock slope monitoring is one of the most important operational hazard management techniques for dealing with unstable rock slopes in open pit mines. As is common to most risk management activities, where hazards have been identified, the hierarchy of hazard management suggests elimination of hazards will be your initial focus. For example, if hazards associated with slope instability have been identified in the design stage of a coal mine, then the opportunity to design slopes to eliminate these hazards should be considered first. However, either through uncertainty in the geological model (unknown hazards) or the requirement for slopes to satisfy economic criteria, there will usually be a level of risk that is not fully realized until the mine is operational. When a mine is operational, the observational method can be employed to validate the design of a mine with the actual observed deformation compared to predicted design levels. Specific focus in the study is spent on treatment of hazards, in particular looking at: • what precursor triggers might be able to foretell whether a slope failure progresses to collapse and; • what deformation rate levels are reached during failure prior to collapse that could be used to determine thresholds for early warning and evacuation of a mine prior to rock slope collapse. 2. RISK MANAGEMENT AND SLOPE STABILITY RADAR (SSR) OVERVIEW2.1. Slope Instability Hazard Management Rock slope monitoring data is often used to predict locations of rock slope hazards (areas of increased rock slope deformation) and the likelihood of failure (by reviewing the magnitude of deformation and any accelerating trends). A framework for slope instability hazard management using the Slope Stability Radar (SSR) has been developed by Harries & Roberts [1]. This framework is equally valid for other forms of rock slope monitoring, although the real-time and large area coverage provided by terrestrial radar scanning makes the technique particularly useful in operational hazard management. The framework has been reproduced in Figure 1.
- Oceania > Australia (0.68)
- Africa (0.46)
- North America (0.46)
ABSTRACT: The development plan of a hydrocarbon field includes the design of all the wells forecasted for each productive scenario considered. From a mechanical point of view, the standard design of the completion system is usually done considering those actions expected to develop during the completion phases. However, strains developing in the rock formation in the near wellbore area due to hydrocarbon production, can induce additional mechanical actions on the well structure. The resulting stress regime may cause damage in the well with a consequent reduction of the production rate and, eventually, the loss of it. Since this phenomenon has been identified as a critical issue for compacting reservoirs, this paper presents a simplified one dimensional semianalytical method for the evaluation of the stresses arising in the completion system. Transfer functions have been used to reproduce the relationship between the relative displacement of the structure with respect to the rock formation and shear stresses at the interface. As a final result, the method predicts vertical profiles of displacements and axial loads arising along the well column at different time stages, in accordance with the expected rock compaction, allowing for the assessment of the long term well integrity. 1. INTRODUCTION The stability of rock formations surrounding hydrocarbon wells is often critical in unsupported conditions [e.g. 1], so that it has to be ensured by metal casings sustaining the walls of the wells. A typical cased hole completion consists of a series of steel pipes with decreasing diameters disposed along the well. The pipes can be cemented, or not, to the rock formation with a cement annulus (see Fig. 1). Although casings are often designed accounting only for the mechanical actions developing during the completion phases, the load acting on them can undergo significant changes during the lifetime of a well because of thermal, hydraulic or mechanical effects. In particular, additional mechanical actions developing during the production can induce severe damages to the well structures [2]. An example is the casing instability due to axial buckling, occurring when a steel pipe is subjected to high compressive stresses and the cement does not provide a good lateral confinement. Practical consequences may include a reduced production rate, the need of regular workovers and, eventually, the loss of the well; therefore claiming for the long-term evaluation of the well integrity. Here the mutual interaction between the deforming reservoir rock and the casing can significantly increase compression on the well structure. In the technical literature, the computation of the load acting on the casing due to the rock compaction has often been tackled by imposing that the casing and the rock formation undergo the same vertical strain, that is to say a 'no slip' condition between rock and completion [e.g. 3, 4]. Nevertheless, field collar logs usually indicate that the vertical casing strain is less than the one measured on the formation [5], and in extreme instances the wellheads have been left protruding in the air after the surface subsided [6].
- Europe (0.46)
- North America > United States (0.46)
- Well Completion > Completion Installation and Operations (1.00)
- Reservoir Description and Dynamics > Reservoir Fluid Dynamics > Integration of geomechanics in models (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Reservoir geomechanics (1.00)
- Well Completion > Completion Selection and Design > Completion equipment (0.88)