Analysis of close contact melting
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AbstractWith the depletion of conventional crude oil resources, a shift towards heavy and offshore oil production is inevitable. One of the major production problems that heavy oil and offshore petroleum producers face is asphaltene deposition. Asphaltene deposits can reduce production by precipitating in the reservoir or on subsurface and surface production flow paths. Although operators try to avoid asphaltene deposition or alternatively partially remove deposits in case of precipitation, accumulation of deposits is often unavoidable. Therefore, understanding the mechanism of asphaltene deposition and the effect of pressure, temperature, composition and flow conditions is essential. The potential steps in asphaltene deposition are precipitation, flocculation, direct adsorption and adhesion. Asphaltenes are also known to self-associate on a molecular level and this self-association must be accounted for when investigating the steps in asphaltene deposition. There are three goals for the present research project: 1) to investigate self-association of asphaltenes and develop methods to extract asphaltenes with consistent physical properties, 2) to examine and possibly model the steps in the mechanism of asphaltene deposition, and 3) to design and construct a prototype test facility (pipe-loop) for detecting asphaltene deposition in flowing systems. Note, only the deposition steps relevant to the pipe-loop design (precipitation and adsorption) were considered. Asphaltene self-association was investigated using molar mass and interfacial tension measurements of asphaltenes in aromatic solvents. The extent of self-association was found to increase with increasing asphaltene concentration and decreasing temperature. Although asphaltenes were found to be surface active, there was no evidence of reverse micelle formation. The self-association data was used to elucidate asphaltene extraction, precipitation and adsorption experimental data. Asphaltene self-association was found to influence asphaltene extraction and a method was developed to obtain asphaltenes with consistent physical properties such as molar mass, density and solubility. The precipitation of asphaltenes from several W estem Canadian heavy oils and bitumens was measured in a high pressure PVT cell. The PVT cell was modified so that the contents could be displaced to a centrifuge tube at nearly constant pressure and temperature. The precipitated asphaltenes could then be separated and quantitative precipitation measurements made. Asphaltene solubility increased with temperature but appeared to be largely insensitive to pressure. A thermodynamic model using the self associated asphaltene molar mass distribution as an input was developed and successfully fit the precipitation data at ambient conditions. It was shown that asphaltene precipitation is reversible and this reversibility proved crucial for the pipe loop design. The precipitation data was also used to determine the operating conditions for the pipe loop. The adsorption of asphaltenes on typically encountered metal surfaces was examined with a UV-Vis spectrophotometer. The adsorption was found to follow Langmuir (Type I) isotherms, indicating that the metal surfaces were saturated with asphaltenes. The adsorption appeared to be site limited. The saturation surface coverages were negligibly small when compared with the expected amount of asphaltene deposition. Hence, asphaltene adsorption is not expected to affect the pipe loop operation. A pilot scale pipe-loop apparatus was designed, constructed and tested to investigate the deposition of asphaltenes under flowing conditions. This pipe-loop and the associated detection equipment will be the prototype of a field test facility. The pipe-loop detected deposition in situ and can be used to screen chemical additives. This apparatus is unique and involves state-of-the-art detection techniques such as x-ray tomography and inline floe size distribution measurement. Hence, it has considerable research and commercial potential.
Bibliography: p. 121-134
CitationAlboudwarej, H. (1998). Analysis of close contact melting (Unpublished master's thesis). University of Calgary, Calgary, AB. doi:10.11575/PRISM/22898
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