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...In the context of risk, abject failure is a ...failure mode that can cause the cancellation of or immediate halt to a project or event. Generally expresse...
In the context of risk, abject failure is a failure mode that can cause the cancellation of or immediate halt to a project or event. Generally expressed as a percent probability. The glossary is a living growing list of important E&P terms and require continual enhancements. If you would like to contribute to the glossary send an email to petrowiki(at)spe.org.
...Understanding rock failure relationships is important because under reservoir pressure and stress conditions, production can i...nduce rock failure, sometime with catastrophic effects. By applying strength criteria, within reservoir simulators we ...c behavior of rocks, which was largely reversible. Here we deal with permanent deformation. By rock failure, we mean the formation of faults and fracture planes, crushing, and relative motion of individual m...
Understanding rock failure relationships is important because under reservoir pressure and stress conditions, production can induce rock failure, sometime with catastrophic effects. By applying strength criteria, within reservoir simulators we can predict when problems might occur. Stress strain relationships in rocks examined the elastic behavior of rocks, which was largely reversible. Here we deal with permanent deformation. By rock failure, we mean the formation of faults and fracture planes, crushing, and relative motion of individual mineral grains and cements.
...An early failure; often related to poor design, candidate selection or installation problems...
...each of these issues and how they affect wellbore stability analysis. We start with a discussion of failure caused by anisotropic rock strength, which is a characteristic of consolidated shales that can caus...akouts that occur along oblique bedding planes intersecting a well, demonstrating that this mode of failure does occur. In fact, in this well ...failure associated with weak bedding caused severe instabilities, necessitating a sidetrack. Fig. 1--Examp...
In many cases, wellbore stability analysis can be carried out with very simple models that are time-independent and relate stress and pore pressure, only through the effective stress law. These do not account for the fact that stress changes induce pore pressure changes, and vice versa. Nor do these models account for thermal and chemical effects and their relationships to pore pressure and stress. In this section, we briefly discuss each of these issues and how they affect wellbore stability analysis. We start with a discussion of failure caused by anisotropic rock strength, which is a characteristic of consolidated shales that can cause considerable problems in wells drilled at oblique angles to bedding. While the examples shown here demonstrate that it is possible to quantify uncertainties in the minimum safe mud weight, it is also possible to quantify uncertainties in the maximum safe mud weight.
...of the structure. While based on experience, these factors give no indication of the probability of failure of a given structure, as they do not explicitly consider the randomness of the design variables and... the safety factors tend to be rather conservative, and most limits of design are established using failure criteria based on elastic theory. Reliability-based approaches are probabilistic in nature and exp...o the design of tubulars, rather than elasticity-based initial yield criteria to predict structural failure. Such probabilistic design methodologies allow either the computation of the probability of ...
Oilfield tubulars have been traditionally designed using a deterministic working stress design (WSD) approach, which is based on multipliers called safety factors (SFs). The primary role of a safety factor is to account for uncertainties in the design variables and parameters, primarily the load effect and the strength or resistance of the structure. While based on experience, these factors give no indication of the probability of failure of a given structure, as they do not explicitly consider the randomness of the design variables and parameters. Moreover, the safety factors tend to be rather conservative, and most limits of design are established using failure criteria based on elastic theory. Reliability-based approaches are probabilistic in nature and explicitly identify all the design variables and parameters that determine the load effect and strength of the structure.
...ith or without corrosion. This article discusses each cause and possible preventive measures. Pipe failure as a result of twistoff occurs when the induced shearing stress caused by high torque exceeds the p...e common and easily can cause twistoff to improperly selected drillstring components. Pipe-parting failure occurs when the induced tensile stress exceeds the pipe-material ultimate tensile stress. This cond...lied in addition to the effective weight of suspended pipe in the hole above the stuck point. Pipe failure as a result of collapse or burst is rare; however, under extreme conditions of high mud weight and ...
Drillpipe failures is a prevalent drilling problem. It can be put into one of the following categories: twistoff caused by excessive torque; parting because of excessive tension; burst or collapse because of excessive internal pressure or external pressure, respectively; or fatigue as a result of mechanical cyclic loads with or without corrosion. Pipe failure as a result of twistoff occurs when the induced shearing stress caused by high torque exceeds the pipe-material ultimate shear stress. In vertical-well drilling, excessive torques are not generally encountered under normal drilling practices. In directional and extended-reach drilling, however, torques in excess of 80,000 lbf-ft are common and easily can cause twistoff to improperly selected drillstring components.
...e at the azimuth of the greatest horizontal far-field principal stress, possibly leading to tensile failure at the wellbore wall 90 from the orientation of the breakouts (courtesy GeoMechanics Intl. Inc.). ...s smaller in a normal or reverse-faulting stress state than for a strike-slip stress state, tensile failure is less likely in these faulting regimes unless a wellbore is inclined. To consider the potential ...for wellbore failure when a wellbore is inclined to the principal stresses, it is necessary to take into account the mag...
Stress concentration around the wellbore can create breakouts, fractures, or failures. Understanding the stresses on rocks around wellbores is important to well design. For a vertical well drilled in a homogeneous and isotropic elastic rock in which one principal stress (the overburden stress, Sv) is parallel to the wellbore axis, the effective hoop stress, σθθ, at the wall of a cylindrical wellbore is given by Eq. 1. Here, θ is measured from the azimuth of the maximum horizontal stress, SHmax SHmin is the minimum horizontal stress; Pp is the pore pressure; ΔP is the difference between the wellbore pressure (mud weight) and the pore pressure, and σΔT is the thermal stress induced by cooling of the wellbore by ΔT. At the point of minimum compression around the wellbore (i.e., at θ 0, parallel to SHmax), Eq. 1 reduces to The equations for σθθ; and σzz are illustrated in Figure 1 for a strike-slip/normal faulting stress regime (SHmax Sv SHmin) at a depth of 5 km, where the pore pressure is hydrostatic and both ΔP and σΔT are assumed to be zero for simplicity.
...end-caps move to the right. The dots are laboratory data that can be used to define (1) the brittle failure line that follows a nonlinear Coulomb-style ...failure law for shear localization, and (2) curved end caps that indicate the porosity for which the streng...of distributed deformation or will fail because of shear localization and the creation of a brittle failure surface (a fault). In Fig. 2, the data at the left edge of the plot lie along a limit in the ratio ...
Understanding rock properties and how they react under various types of stress is important to development of a geomechanical model before drilling. Some major geomechanical rock properties are described below. To first order, most rocks obey the laws of linear elasticity. In other words, the stress required to cause a given strain, or normalized length change (Δlk /ll), is linearly related to the magnitude of the deformation and proportional to the stiffnesses (or moduli), Mijkl. Furthermore, the strain response occurs instantaneously as soon as the stress is applied, and it is reversible--that is, after removal of a load, the material will be in the same state as it was before the load was applied.
...f uncertainty in the input data needed to run the analysis. Such data include: Mechanical borehole failure occurs when the stresses acting on the rock exceed the compressive or the tensile strength of the r...ock. Compressive failure is caused by shear stresses as a result of low mud weight, while tensile ...failure is caused by normal stresses as a result of excessive mud weight. The ...
Borehole instability is the undesirable condition of an openhole interval that does not maintain its gauge size and shape and/or its structural integrity. Figure 1 illustrates hole-instability problems. Hole closure is a narrowing time-dependent process of borehole instability. It sometimes is referred to as creep under the overburden pressure, and it generally occurs in plastic-flowing shale and salt sections. Hole enlargements are commonly called washouts because the hole becomes undesirably larger than intended.