Summary A wellbore cement sheath is expected to provide zonal isolation and borehole integrity during well construction and well life. Mechanical interactions of the cement sheath to existing and operationally induced stresses, along with other elements in proximity to the wellbore, have increasingly large technical, economic, and environmental ramifications. Staged-finite-element procedures during well construction consider sequentially the stress states and displacements at and near the wellbore. The model replicates complicated stress states arising from simultaneous action of far-field stresses, overburden pressure, cement hardening and shrinkage, debonding at the interfaces, and plastic flow of cement sheath and rock formation. At present, temperature, flow, and poroelasticity effects are not included. The technique tracks the time-dependent behavior of cement slurry, curing (with or without shrinkage), and hardened cement during the critical period after slurry placement. Material models for casing, cement, and rock formation and failure criteria for cement, formation, and interface bonds were calibrated using published information and experimental data. Calculations were conducted for various loading and unloading scenarios, geometric configurations, properties of rock formations, and cement-slurry formulations. Results are discussed in terms of field implication;, for example:Interface microchannels may or may not develop, depending upon shrinkage magnitudes; andsimplifying modeling assumptions that are often used, such as 2D stresses and/or deformations, may obscure critical casing, cement, and formation behavior in the wellbore region and in the producing horizon. This paper, part of a series quantifying the interacting physical components and processes at and near the wellbore region, initiates useful comparisons of analytical results and field realities. The series illustrates and compares results and practical implications from simple to increasingly complex, but more realistic, assumptions, such as isotropic/directional-stress states and isotropic/anisotropic casing, cement, and formation-material parameters.

fax 01-972-952-9435. AbstractA wellbore cement sheath is expected to provide zonal isolation and borehole integrity during well construction and well life. Cement sheaths mechanically interact with other elements in the wellbore region to stresses from geological processes and operational activities. Quantifying these interacting physical components and processes has technical, economic, and environmental implications of great, and growing, significance.Staged finite element procedures during well construction sequentially consider the stress states and displacements at and near the wellbore. The model replicates complicated stress states arising from simultaneous action of far-field stresses, overburden pressure, cement hardening and shrinkage, debonding at the interfaces, and plastic flow of cement sheath and rock formation. Presently, temperature and flow are not included. The technique tracks the time-dependent behavior of cement slurry, curing (with or without shrinkage), and hardened cement during the critical period after slurry placement.Material models for casing, cement and rock formation, failure criteria for cement, formation and interface bonds were calibrated using published information and experimental data. Calculations were conducted for various loading and unloading scenarios, geometric configurations, properties of rock formations, and cement slurry formulations. Results are discussed in terms of field implications, for example: (1) interface micro-channels may or may not develop, depending upon shrinkage magnitudes; (2) simplifying modeling assumptions that are often used, such as 2-D stresses and/or deformations, may obscure critical casing, cement, and formation behavior in the wellbore region, and in the producing horizon.This paper, part of a series quantifying the interacting physical components and processes at and near the wellbore region, initiates useful comparisons of analytical results and field realities. The series illustrates and compares results and practical implications for simple to increasingly complex, but more realistic assumptions, such as isotropic/directional stress states, and isotropic/anisotropic casing, cement, and formation material parameters.

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Background: There are concerns regarding glenoid erosion with metal shoulder hemiarthroplasty. PyroCarbon may offer an alternative because of favorable wear characteristics and preservation of the glenoid. The purpose of this study was to assess in vitro bone wear characteristics of PyroCarbon relative to cobalt chromium alloy hemiarthroplasty in a shoulder wear simulator. Methods: Wear of PyroCarbon and cobalt chromium prostheses articulating with bone were characterized by means of bone wear penetration rate, changes to surface roughness, and wear particle analysis. Results: PyroCarbon prostheses produced significantly less damage to bone and were less damaged by the bone than cobalt chromium prostheses. Cobalt chromium testing was halted at approximately 320,000 cycles because the bone was consumed. Wear testing of PyroCarbon specimens continued through five million cycles. Linearized bone penetration rate, bone volume loss rate, and surface roughness for cobalt chromium test specimens were 30 times greater than for PyroCarbon. Conclusions: Results demonstrate significantly less damage to bone in simulated shoulder function testing for PyroCarbon hemiarthroplasty implants relative to conventional cobalt chromium implants. Our study supports use of PyroCarbon in humeral head hemiarthroplasty as a viable alternative to conventional metal hemiarthroplasty. Further investigation of PyroCarbon performance in clinical settings is warranted.

In the drilling of deep horizons, the mud-weight window between pore-pressure gradient and fracture gradient often narrows because of rock properties and underground-stress state. Nonproductive-time events such as lost circulation, wellbore instability, kicks, underground crossflow, and pipe sticking are more likely. Such problems greatly increase drilling costs. Bridging pre-existing natural fractures or drilling-induced fractures with lost-circulation materials (LCM) is often performed to increase fracture gradient and widen the mud-weight window. This technique, referred to as wellbore strengthening, includes the stress cage, fracture-closure stress, and fracture-propagation-resistance methods. Although these methods are often used, several aspects of these approaches are still not thoroughly understood.To reduce the risk of lost circulation while drilling in formations with narrow mud-weight windows, such as pressure-depleted reservoirs and deepwater formations, a good understanding of the mechanisms of wellbore strengthening in different downhole scenarios helps engineers to optimize the design of drilling fluids and operational procedures.This paper discusses the mechanism of wellbore strengthening, with a focus on hoop stresses at and near the wellbore, in elastic and poroelastic models, by use of the finite-element method to evaluate wellbore and near-wellbore stresses during fracture creation and propagation, and after plugging fractures with LCM. Factors affecting fracture behavior, such as horizontal-stress anisotropy, LCM-bridging location, initial pore pressure, and fluid leakoff, are investigated. A better understanding of the several interacting events local to the wellbore and near-wellbore regions can result in improved operational practices related to lost-circulation prevention and mitigation. IntroductionThe purpose of wellbore strengthening during the drilling of oil/ gas wells is to widen the mud-weight window. To maintain downhole pressure within the mud-weight window, drilling fluids are designed to provide wellbore-hydrodynamic pressure low enough to prevent downhole lost circulation but high enough to prevent borehole instability or kicking. While drilling in deep horizons or depleted reservoirs, the window between pore-pressure gradient and fracture gradient often narrows because of rock behavior and the near-wellbore-stress state. Nonproductive-time (NPT) events such as lost circulation, wellbore instability, kicks, underground crossflow, and pipe sticking are more likely. Such problems greatly increase drilling costs.To reduce NPT, drilling engineers often use wellbore strengthening to widen the drilling-mud-weight window and increase the fracture gradient in the formations being drilled, thereby preventing lost circulation and its related problems. Although NPT reduction is desirable, it may at times not be the most-important consideration. Sometimes lost-circulation problems determine whether the well can be completed without requiring additional casing strings and associated change...