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Mert Ozdemirtas Primary supervisor: Dr. Ergun Kuru Co-supervisor: Dr. Tayfun Babadagli "Experimental and Numerical Modeling of Borehole Ballooning - Effect of Fractures Roughness" Borehole ballooning or breathing, is commonly observed during drilling through fractured zones. It refers to the small, partial and continuous mud losses and significant rapid mud gains due to annular pressure fluctuations resulting from mud circulation and non-circulation. Better understanding of the factors controlling borehole ballooning and /or breathing is needed for correct interpretation of the symptoms obtained while drilling and avoid mixing this phenomena with lost circulation and well kicks. Limited studies in this area dealt with one-dimensional (cylindrical) models for different (newtoniao and non-newtonian) fluid types with parallel plate approximation and simplified linear fracture deformation assumptions. In my research, I introduced a two dimensional transient model of borehole ballooning and/or breathing. The model considers the effects of non-Newtonian fluid rheology, and fracture roughness on the fracture volume change as a function of transient wellbore pressure fluctuations inherent in typical drilling operations. The model has been solved numerically to investigate the effects of drilling fluid properties (i.e., drilling fluid density and rheology) and formation properties (i.e., formation pressure, fracture roughness, and rate of change of fracture aperture.) on the cumulative fluid loss and gain rate between the borehole and the fractured formation. Analyses on the importance of fracture roughness and non-linear deformation approximations were provided and situations where the degree of roughness become critical were identified. At the same time, I am also working on the experimental verification for my numerical model.
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