772 research outputs found
Ships Observing Marine Climate: a catalogue of the VOS participating in the VSOP-NA
Full text linkOur present knowledge of the marine climate, as represented by data sets such as COADS (Woodruff et al., 1987), is based on meteorological observations from the Voluntary Observing Ships (VOS). Because the VOS are merchant ships, rather than specially designed meteorological platforms, errors and biases exist in the data. However there is little information readily available to the climatologist either on the nature of the VOS fleet or on the observing practises which are used. This report, describing the forty-six ships that participated in the Voluntary Observing Ships' Special Observing Project - North Atlantic (VSOP-NA), therefore serves two purposes:(i) it provides a reference document to aid analysis of the VSOP-NA data set,(ii) it gives a detailed description of a subset of the VOS, which will be of value in the interpretation of marine climate data sets.This report is in two parts, Part 1 is an overall summary of the ship characteristics, Part 2 is a ship by ship description. The next section will briefly describe the VSOP-NA project, followed by a summary of the characteristics of the VSOP-NA ships (Section 3). Since these ships were specially selected (Section 2.2), the degree to which they are representative of the whole VOS fleet will be carefully considered. The meteorological instrumentation used by the VOS varies depending on which meteorological agency recruited the ships. That used on the chosen VSOP-NA ships is typical of VOS recruited by the countries bordering the North Atlantic, and will be described in Section 4. Section 5 is a summary of Part 1 of the report.Part 2 presents the VSOP-NA ship catalogue. This includes, for each ship, diagrams of the layout (indicating in particular the exposure of the sensors), a summary of the geographical positions at which observations were obtained, and details of the instrumentation used.<br/
Menjawab Serangan Pemikiran Liberalisme Terhadap Islam menerusi Novel Daun Karya Malim Ghozali P.K.
No full textThis study aims to analyse the liberal critiques of Islamic ideology as depicted in the novel Daun by Malim Ghozali P.K. The main issues highlighted in this article are: [i] the historical context of the development of liberalism in Malaysia as presented in both within and outside the creative work; [ii] explaining how liberal groups attempt to shape an interpretation of Islam to fit their respective agendas. This study is conducted based on a literature review that finds the attitudes of postmodernist scholars, who are evidently unsatisfied and continually deconstructing new theories that ohlm.se the true values of Islam. Therefore, this study employs the concept of New Historicism proposed by Stephen Greenblatt (1988) as an analytical framework, utilizing the concepts of constraint and mobility to view texts as constructed from sociocultural references, specifically the social values that permeate the author's identity through the societal crises the author experienced. The findings of the study indicate that Malim Ghozali P.K. strives to provide a new meaning in order to reject the liberalism ideology prevailing in society, while also critising the government's attitude at the time the novel was written, which use religious issues as a political agenda to gain support.
Full text: PDFKajian ini bertujuan untuk menganalisis serangan pemikiran liberalisme terhadap pemikiran Islam tradisional menerusi novel Daun karya Malim Ghozali P.K. Isu utama yang diketengahkan dalam artikel ini ialah: [i] perhubungan konteks sejarah perkembangan liberalisme di Malaysia yang wujud dalam karya dan luar karya; [ii] menjelaskan bagaimana golongan liberalis cuba membentuk acuan Islam yang ditakrifkan sesuai berdasarkan agenda masing-masing. Kajian ini dijalankan berdasarkan sorotan literatur yang mendapati sikap ahli pascamodenis yang ternyata tidak menyenangi dan sentiasa mendekonstruksi teori-teori baharu yang menentang nilai Islam sebenar. Kajian ini memanfaatkan gagasan teori Historisisme Baru yang dikemukakan oleh Stephen Greenblatt (1988) sebagai kerangka analisis dengan memanfaatkan konsep kekangan dan mobiliti yang melihat teks dibina daripada rujukan sosiobudaya, iaitu nilai sosial yang meresap dalam diri pengarang melalui kemelut yang melanda masyarakat dalam situasi pengarang. Dapatan kajian menunjukkan bahawa Malim Ghozali P.K. berusaha memberikan pemaknaan baharu demi menolak gagasan pemikiran liberalisme yang sedang berlaku dalam masyarakat seterusnya mengkritik sikap pemerintah ketika novel ini ditulis yang menjadikan isu agama sebagai agenda politik untuk meraih sokongan.
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Hydraulic Fracture Containment in Sand
No full textThe mechanism of hydraulic fracturing in soft, high permeability material is considered fundamentally different from that in hard, low permeability rock, where a tensile fracture is created and conventional linear elastic fracture mechanics (LEFM) applies. The fracturing and associated modeling work is then a relatively new area. Particularly, the fracture containment in layered formations remains unknown. This research is aiming to capture the basic physics of the process of hydraulic fracture initiation and propagation in such materials, and further the fracture containment in layered samples. It consists of experimental and simulation studies with application in the petroleum industry. Laboratory tests are performed on lightly cohesive/cohesionless sands. They are pure sand, sand with silt and sand with cement. The mechanical behavior is independently determined in triaxial tests at different confining stress (up to 60 MPa) and porosities. The material is described within the framework of elasto-plasticity. Material parameters are then derived from the simulations, which are performed to match stress-strain behavior of uniform deformation in triaxial tests. In addition, particle breakage and boundary induced deformation localization in large strain tests are also investigated. Hydraulic fracturing injection tests are first performed using different fluids to select an appropriate fracturing fluid. They are viscous Newtonian fluid, bentonite slurry, linear gel, crosslinked gel, and BXLG. Fluid rheology and leak-off have a strong influence on the tendency to fracturing. The test observation shows that BXLG is a fairly efficient fracturing fluid at high stress, so that the following injection tests are performed with BXLG. Injection tests are carried out on sand at different confining stress (up to 20 MPa). Utilizing an X-ray CT scanner provides real-time visualization of the fracture geometry during injection. This technique helps reveal the mechanism of the fracturing. Based of injection tests and associated simulations, the considered fracturing behavior involves leak-off, initiation pressure, propagation behavior, effect of material parameters, and fracture closure. Leak-off in high permeability material is characterized by two-dimensional whole gel leak-off. Both external and internal filtercakes are observed. This results in the fracture tip lagging behind the fluid leak-off front; also, a significant part of the pressure drop occurs across the internal filtercake. Pressurization of the borehole is intrinsically related to the fracture initiation. The onset of shear bands of a pressurized borehole can be considered as the upper bound of fracture initiation. The observed high pressure is then determined by the borehole instability due to shearing. The propagation behavior is related to the leak-off and the associated change in pore pressure. Shear failure occurs at the fracture tip within the internal filtercake. BXLG that builds a relatively efficient filtercake results in smooth closure of the fracture tip. The fracture initiates and propagates at an oblique angle. This is consistent with the mechanism that it propagates in shear. Simulation of fracture propagation shows that besides the confining stress, all the considered constitutive parameters have influence on the predicted pressure and geometry. A higher pressure is required for material of larger Young’s modulus, smaller Poisson ratio, smaller friction angle and larger dilation angle. Furthermore, dilatancy plays a more significant role for fracturing in soft, high permeability material than in hard rock. Fracture recession is an important phenomenon during closure, which can be explained by more intensive leak-off at the tip. Also, CT scans show that the fracture closes mechanically far below the confining stress. The most important deviation from elastic prediction is a larger injection pressure, larger width, and closure pressure much lower than the confining stress. In addition, injection tests on cemented sands of different strength show that the threshold value of soft material and hard rock is about 0.5 MPa in tensile strength, below which the dominant fracturing mechanism appears to be failure in shear. The fracture initiation and propagation across layers are tested on layered samples, which have a difference in permeability or strength. A uniform confining stress is applied over the entire sample so that any containment would be determined by material properties. In tests with a permeability contrast, the sand layer has permeability some 3-4 times larger than the sand+silt layer; and, they both have no tensile strength. The tests show that fractures may be strongly contained by the high permeability layer. The simulation of a two-dimensional layered model qualitatively explains the mechanism. When the tip penetrates the high permeability layer, the pressure must increase to open the fracture. Also, the fluid needs to leak-off to build enough effective stress. This requires a relatively long time. In a three-dimensional situation it may explain the larger propagation rate in the other direction, within the low permeability layer. That means that the fracture will propagate much further into low permeability layers. In another kind of layered test, the two layers are sand with cement and sand with silt. They differ in tensile strength and shear resistance but they have the same permeability. The fracture develops better in the cemented sand. The simulation shows that this can be explained by the combined influence of the constitutive parameters. Comparing qualitatively strength and permeability contrast, most fracture containment is observed in the low permeability layer. Under the conditions of the present study, permeability is more important than strength for containment.GeotechnologyCivil Engineering and Geoscience
The effect of residual oil on deep bed filtration and formation damage
No full textAbstract not availableCivil Engineering and Geoscience
Transport of polymer in soils and solute in gels for contaminant remediation and containment
No full textAbstract not availableCivil Engineering and Geoscience
Initiation of Hydraulic Fractures in Natural Sandstones
No full textHydraulic fracturing is a stimulation technique commonly used for the enhancement of hydrocarbon reservoir recovery. Controlling the initiation of a hydraulic fracture from the open-hole section of a well without zone isolation requires an in-depth understanding of the factors which have a decisive effect on the onset and early growth of a fluid-driven crack. The objective of this research is the identification of the key aspects of hydraulic fracture initiation in a natural sandstone. It also aims at understanding how these different aspects combine and result in specific propagation regimes, depending on the experimental conditions. We propose a conceptual model for the initiation of a fluid-driven crack in a permeable, elastic quasi-brittle material exhibiting post failure tension softening in a cohesive zone. The influence of the finite borehole radius is introduced with appropriate kernels in the integral equation of elasticity. The model accounts for viscous fluid flow in the fracture according to the Lubrication equation, allows for the existence of a potential fluid lag, and fluid losses from the fracture and the borehole injection zone. Compressibility effects are introduced with a coupling between the fluid injection rate at the fracture inlet and the fluid pressure variations in the borehole. A complete set of scaling laws is established for the problem of a radial fracture driven by a viscous fluid in a permeable elastic semi-brittle material, in the case of non negligible compressibility effects influencing the effective fluid flow rate at the fracture inlet. Propagation regimes are identified for the simplified case of a fracture for which the borehole radius, the fluid lag and the process zone can be neglected. The scaling laws developed for this simplified problem highlight the nature of the mechanisms that control the fracture response during the fluid injection. The effect of the finite borehole radius can be introduced in the scaling through one single parameter with the meaning of a dimensionless borehole radius. If the negligible fluid lag and process zone are valid assumptions, the scaling laws provide a powerful tool for the design of experiments at laboratory scale. The introduction of a fluid lag and of a process zone for the description of semi-brittle failure and the creation of new fracture faces considerably increase the complexity of the scaling operations. Independent considerations about the scaling of the lag and the process zone are developed from the conclusions of independent studies and are mentioned in this thesis, but a unified formulation for the complete problem is yet to be achieved. We conducted experiments in model blocks of natural sandstones with various viscous fluids and injection rates. The experimental setup allows for the real time collection of observations concerning the treatment pressure, the fracture inlet opening and the deformation of the test block. A state of the art acoustic monitoring system is used for the active imaging of the crack growth during the fracturing experiment. The signature of the fracture plane is monitored continuously and the fracture opening assessed for several points of the fracture plane. Experimental evidence suggests that a non-negligible process zone has already developed at breakdown. The breakdown of a small scale fluid-driven fracture must be described as a complex process that depends on the fluid rheology, material microstructure and the fluid diffusion from the fracture toward the adjacent pore space. The injection parameters also have a major impact upon the early phase of the fracture growth and the post-breakdown fracture response. The appearance of multiple pressure peaks associated with a step-by-step fracture growth during the injection of the fluid is related to compressibility effects and high leakoff rates in the permeable sandstone. The comparison of the modeling results with experimental evidence collected in the laboratory indicates that some of the experimental observations cannot be understood within the framework of elastic brittle material failure. The existence of a sizeable process zone with specific hydraulic properties has to be invoked to reproduce the experimental fracture response. The impact of the process zone upon the viscous fluid flow and the mechanical response of the fracture need to be addressed as fundamental aspects of the problem of fracture initiation in natural sandstones.Civil Engineering and Geoscience
Internal filtration and external filter cake build-up in sandstones
No full textWater injection is an integral constituent of most field development scenarios. This injection may take place for secondary recovery and pressure maintenance such as sea water injection. Alternatively, it may take place as a waste water disposal method as in the case of produced water re-injection. Associated with most injection schemes is injectivity decline; where the rate of injection decreases over time at a given constant injection pressure gradient. The phenomenon of injectivity decline is comprised of multiple phenomena such as internal filtration, external filter cake build-up, fracture propagation, relative phase changes within the matrix rock and the associated permeability decline due to each of the described phenomena. The study presented in this thesis addresses some of the key phenomena associated with injectivity decline. Two sub-categories were studied both experimentally and theoretically: internal filtration and external filter cake build-up.Civil Engineering and Geoscience
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