The motion of air bubbles in a yield stress fluid is analyzed numerically using a 2D approach and the finite volume technique. The multiphase flow is simulated using the volume of fluid method (VoF), which solves the conservation equations of mass and momentum coupled to a transport equation for the volume fraction of the fluids. The effects of yield stress, bubble size, number and position of bubbles rising in a viscoplastic fluid confined between vertical parallel plates are analyzed and discussed. The results indicate that the yield stress has great impact on the rising velocity. In the case of multiple bubbles flowing vertically, it is observed that the displacement of one bubble influences the rising velocity of the others, causing them to approach each other. As the distance between the bubbles increases the interference is reduced and the bubbles begin to flow as single ones. When two bubbles are horizontally positioned, they can approach or move away from each other, depending on the initial distance between them. Furthermore, the bubbles shape is analyzed as a function of the governing parameters. It is observed that for lower Reynolds number the bubbles present a circular shape, but as inertia increases the bubble becomes ellipsoidal.

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For relatively low permeability Pre-salt carbonates reservoirs, the question of progressive closure of the stimulated and acidized fractures is raised since their evolution over time could be the main cause of the production decline. Aiming at quantifying the time-dependent behavior and stress sensitivity of the acidized fracture or propped fracture, a series of fracture permeability tests were carried out in laboratory on tight Pre-salt carbonate samples from various facies of a Brazil offshore well. For creating fracture, direct shear tests were performed on initially intact cylindrical carbonate plugs in a conventional triaxial cell. The fresh fractures are then submitted to acid flow over certain duration before permeability measurement under stress. Long term fracture permeability tests lasting over several months have been carried out and were compared with the short-term tests. Correlation with mechanical properties were also investigated. We compare the performances of propped fracture to acid washed one to formalize recommendations for fracture stimulations in the Pre-salt carbonates. The propped fractures clearly performed well, with negligeable stress-sensitivity on fracture conductivity and no noticeable effect of time over 50 days. Mixed results were found for the acid-washed fractures, in which some showed little stress-sensitivity and little time-effect while others the contrary.

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