Borehole Stability in Shale

This thesis is concerned with the stability of inclined boreholes. It addresses the mechanical stability of a wellbore as a function of time while also considering the impacts of chemical osmosis, temperature differences between the drilling mud and formation, and the introduction of an impermeable filter cake on the state-of-stress near the wellbore. The stress and pore pressure expressions are derived for the general case of an arbitrarily inclined wellbore subjected to three, unequal, in-situ stresses. It is assumed that the material is the linearly poroelastic and the stress and pore pressure expressions are found by solving the three-dimensional problem using the concept of generalized plane strain. This allows the problem to be decomposed into a poroelastic plane strain, elastic antiplane, and elastic uniaxial problem. There are three sets of equations developed; namely the poroelastic, chemo-poroelastic, and poro-thermoelastic equations. The poroelastic equations are representative of drilling in a fluid-saturated, porous, chemically-inert rock under isothermal conditions. The fully-coupled chemo-poroelastic expressions are also valid under isothermal conditions but take into account the differences in the activities of the water phases of the formation and drilling mud. The poro-thermoelastic expressions are developed by coupling the effects of pore fluid expansion (contraction) to rock deformation. The poro-thermoelastic expressions are applicable when there is a temperature difference between the rock and drilling mud and when the effects of thermal osmosis can be neglected. The stress and pore pressure expressions are derived (for both a permeable and impermeable wall) and are used to develop a wellbore stability design code. The model evaluates for tensile and shear failure by applying either the poroelastic, chemo-poroelastic, or poro-thermoelastic solution (chosen by user) at a specific borehole orientation. The model enables the user to choose either the Mohr-Coulomb or Drucker-Prager shear failure criteria and is capable of analyzing for failure inside the rock. The output generated by the model represents a safe operating zone, which corresponds to the range of mud weights that can be used to avoid shear and tensile failure at a given wellbore trajectory. Several numerical examples are used to examine the effects of time, chemical osmosis, and thermal loading on wellbore failure at the wall and inside the rock. The results indicate that, in general, as time increases the potential for shear and tensile failure also increase. In addition, radial spalling is initiated inside the rock (near the wellbore) at low mud weights that requires a minimum mud weight to overcome. The numerical examples further indicate that chemical osmosis and thermal loading significantly alter the near wellbore pore pressure and stresses which may lead to time-delayed failure. A parametric analysis is also conducted to ascertain the sensitivity of the near wellbore stress and pore pressure to various input coefficients. The parametric study reveals that determining the Poisson’s ratio, undrained Poisson’s ratio, Biot’s constant, Skempton’s pore pressure constant, intrinsic formation permeability, water activity of both the drilling mud and pore fluid, and the thermal diffusivity are critical to determining the mud weight window. The remaining input parameters did not affect the safe operating zone

Exploring stroke survivors' and physiotherapists' views of self-management after stroke : a qualitative study in the UK

Objectives Stroke is a sudden-onset condition with long-term consequences. Self-management could help address long-term consequences of stroke. Stroke survivors' and health professionals' views of self-management may vary, limiting the successful introduction of self-management strategies. This paper explores stroke survivors' and physiotherapists' views of self-management, focusing on what self-management means, and factors perceived to enable and hinder self-management after stroke, to draw out implications for policy, practice and future research.Design Qualitative study using semistructured interviews and a thematic analysis approach.Setting Stroke unit and community stroke-rehabilitation services in London, UK.Participants 13 stroke survivors (8 men and 5 women; aged 53–89 years) admitted to a London stroke unit. 13 physiotherapists: 8 working in an inpatient stroke unit and 5 in community rehabilitation.Results Key differences were evident in how self-management was understood between these groups. Stroke survivors were unfamiliar with the term self-management, but most could provide their own definition and relate to the term, and understood it as care of the self: ‘doing things for yourself’ and ‘looking after yourself’. They did not recognise self-management as part of their care, but valued therapists as encouraging experts in supporting their recovery after stroke. Physiotherapists commonly understood self-management as a process in which stroke survivors were expected to take an active role in their rehabilitation and manage their recovery and health, with different understandings of self-management among physiotherapists shaped by the context in which they worked. They reported that individual, social and organisational factors enable and hinder self-management after stroke, with individual and organisational barriers particularly evident in the early stages.Conclusions If self-management support approaches are to be used, further work is required to explore the language and strategies used by professionals to support self-management, and the barriers to supporting self-management at different time points after stroke

Comparison of Provision of Stroke Care in Younger and Older Patients: Findings from the South London Stroke Register

Background. Evidence-based stroke care should be available to all patients. However, evidence exists of inequalities according to age. This study compared access to care for younger adults to that for over 65s. Methods. Using population-based data from 4229 patients with first-ever stroke between 1995 and 2010, associations between age and 21 care indicators were investigated using multivariable logistic regression. Results. Age was not associated with stroke unit admission for ischaemic stroke (P = 0.666). Younger PICH patients were least likely to be admitted to stroke units (P = 0.001), instead treated on neurosurgical or ICU wards. Younger age was also associated with admission to neurosurgery or ICU after SAH (P = 0.006), increased occupational or physiotherapy at 1 year (P = 0.043), and contact with a GP 3 months after stroke (P < 0.001). Conclusion. Younger patients have equal or greater access to evidence-based care. However, there is a need to ensure that services meet the needs of this group

The UCSD Radio-Selected Quasar Survey for Damped Lyman alpha System

As large optical quasar surveys for damped Lya become a reality and the study of star forming gas in the early Universe achieves statistical robustness, it is now vital to identify and quantify the sources of systematic error. Because the nature of optically-selected quasar surveys makes them vulnerable to dust obscuration, we have undertaken a radio-selected quasar survey for damped Lya systems to address this bias. We present the definition and results of this survey. We then combine our sample with the CORALS dataset to investigate the HI column density distribution function f(N) of damped Lya systems toward radio-selected quasars. We find that f(N) is well fit by a power-law f(N) = k_1 N^alpha_1, with log k_1 = 22.90 and alpha_1 = -2.18. This power-law is in excellent agreement with that of optically-selected samples at low N(HI), an important yet expected result given that obscuration should have negligible effect at these gas columns. However, because of the relatively small size of the radio-selected sample, 26 damped Lya systems in 119 quasars, f(N) is not well constrained at large N(HI) and the first moment of the HI distribution function, Omega_g, is, strictly speaking, a lower limit. The power-law is steep enough, however, that extrapolating it to higher column densities implies only a modest, logarithmic increase in Omega_g. The radio-selected value of Omega_g = 1.15 x 10^-3, agrees well with the results of optically-selected surveys. While our results indicate that dust obscuration is likely not a major issue for surveys of damped Lya systems, we estimate that a radio-selected sample of approximately 100 damped Lya systems will be required to obtain the precision necessary to absolutely confirm an absence of dust bias.Comment: 12 pages, 9 Figures. Accepted to ApJ April 11, 200

Socioeconomic deprivation and provision of acute and long-term care after stroke: the South London Stroke Register cohort study

Background and aims Socioeconomic deprivation (SED) is associated with increased mortality after stroke, however, its associations with stroke care remains uncertain. We assessed the SED impacts on acute and long-term stroke care, and examined their ethnic differences and secular trends. Methods We used data from 4202 patients with first-ever stroke (mean age 70.1 years, 50.4% male, 20.4% black), collected by a population-based stroke register in South London, England from 1995 to 2010. Carstairs deprivation score was measured for each patient, taking the 1st as the least deprived and the 2nd to 5th quintiles as SED, and was related to 20 indicators of care in multivariate logistic regression models. Results Patients with SED had 29% and 35% statistically significant reductions in odds of being admitted to hospital and having swallow tests, respectively. The multivariate adjusted odds ratio (OR) for receiving five indicators of acute stroke care was 0.81 (95% CI 0.72 to 0.92). It was 0.76 (0.58 to 0.99) in black patients and 0.82 (0.71 to 0.96) in white patients; and 0.70 (0.58 to 0.84) in patients with stroke occurring before 2001 and 0.89 (0.75 to 1.05) since 2001. SED was further associated with receipt of some stroke care during 5 years of follow-up, including atrial fibrillation medication (0.63, 0.48 to 0.83), and in black patients physiotherapy and occupational therapy (0.32, 0.11 to 0.92). Conclusions Stroke healthcare inequalities in England exist for some important indicators, although overall it has improved over time. The impact of SED may be stronger in black patients than in white patients. Further efforts are required to achieve stroke care equality.National Institute for Health Research Programme Grant (RP-PG-0407-10184

Potential Aboriginal-Occupation-Induced Dune Activity, Elbow Sand Hills, Northern Great Plains, Canada

Geomorphological and archeological evidence indicates potential linkages between Plains aboriginal occupation and dune activity in the Elbow Sand Hills of southern Saskatchewan, Canada. Vegetation encroachment has rapidly outpaced migration of an active dune complex over the last 65 years. Optical ages of stabilized dune remnants indicate that dune activity predates Euro-Canadian settlement (ca. AD 1900). Early Euro-Canadian explorers observed local occupation and exploitation of the sand hills by aboriginal groups for herding and impounding bison. Mapping of archeological sites in relation to physiography reveals that sand dunes, in close proximity to permanent water resources, were preferred areas of occupation. Collectively, these results support the hypothesis that aboriginal occupation disturbance may have perpetuated dune activity in the Elbow Sand Hills until the late 19th century, and that Euro-Canadian settlement and land use emphasizing conservation may have encouraged recent stabilization. We propose that similar aboriginal occupation disturbances may have been responsible for perpetuating dune activity in other dune fields in the Great Plains. To this end, climatic variability should not be considered exclusive of other drivers of dune activity in semivegetated inland dune fields of the Great Plains

Provision of acute stroke care and associated factors in a multiethnic population: prospective study with the South London Stroke Register

Objectives To investigate time trends in receipt of effective acute stroke care and to determine the factors associated with provision of care

Magnetic and Transport Properties of Co2MnSnxSb1−x Heusler Alloys

We present the magnetic, structural, and transport properties of the quaternary Heusler alloys Co2MnSnxSb1−x (x = 0, 0.25, 0.50, 0.75, and 1.0), which have been theoretically predicted to be half-metallic. Magnetization measurements as a function of applied field show that the saturation moment for x = 1 (Co2MnSn) is near the Slater–Pauling value of 5μB; however, the moment for x = 0 (Co2MnSb) falls far short of its predicted value of 6μB. Resistivity as a function of temperature was measured from 5 to 400 K, and a phase transition from a half-metallic ferromagnetic phase to a normal ferromagnetic phase was observed between 50 and 80 K for all of the alloys. At low temperature (10 K\u3cT\u3c40 K), the resistivity ratio was found to vary as R(T)/R(T = 5 K) = A+BT2+CT9/2, where the T2 term results from electron-electron scattering, whereas the T9/2 term is a consequence of double magnon scattering