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UBC Theses and Dissertations

Elasto-plastic deformation and flow analysis in oil sand masses Srithar, Thillaikanagasabai


Prediction of stresses, deformations and fluid flow in oil sand layers are important in the design of an oil recovery process. In this study, an analytical formulation is developed to predict these responses, and implemented in both 2-dimensional and 3-dimensional finite element programs. Modelling of the deformation behaviour of the oil sand skeleton and modelling of the three-phase pore fluid behaviour are the key issues in developing the analytical procedure. The dilative nature of the dense oil sand matrix, stress paths that involve decrease in mean normal stress under constant shear stress, and loading-unloading sequences are some of the important aspects to be considered in modelling the stress-strain behaviour of the sand skeleton. Linear and nonlinear elastic models have been found incapable of handling these aspects, and an elasto-plastic model is postulated to capture the above aspects realistically. The elasto-plastic model is a double-hardening type and consists of cone and cap-type yield surfaces. The model has been verified by comparison with laboratory test results on oil sand samples under various stress paths and found to be in very good agreement. The pore fluid in oil sand comprises three phases namely, water, bitumen and gas. The effects of the individual phase components are considered and modelled through an equivalent fluid that has compressibility and hydraulic conductivity characteristics representative of the components. Compressibility of the gas phase is obtained using gas laws and the equivalent compressibility is derived by considering the individual contributions of the phase components. Equivalent hydraulic conductivity is derived from the knowledge of relative permeabilities and viscosities of the phase components. Effects of temperature changes due to steam injection are also included directly in the stress-strain relation and in the flow continuity equations. The analytical equations for the coupled stress, deformation and flow problem are solved by a finite element procedure. The finite element programs have been verified by comparing the program results with closed form solutions and laboratory test results. The finite element program has been applied to predict the responses of a hor izontal well pair in the underground test facility of Alberta Oil Sand Technology and Research Authority (AOSTRA). The results are discussed and compared with the measured responses wherever possible, and indicate the analysis gives insights into the likely behaviour in terms of stresses, deformations and flow and would be important in the successful design and operation of an oil recovery scheme.

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