Well test analysis in volatile oil reservoirs

• Main Author: Sanni, Moshood Olajide
• Published: Imperial College London 2008
• Subjects: 622
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.495439

This thesis discusses characterization of volatile oil reservoirs using well test analysis. For this purpose, typical well test behaviours were simulated with a one-dimensional single well compositional reservoir model, for different production rates; fluid composition and relative permeability curves, with bottomhole pressures above and below the bubble point pressure. It was found that, when the bottomhole pressure falls below the bubble point pressure during a drawdown, a high gas saturation zone is created around the wellbore with two-phase (oil and gas) flow, whereas single phase (oil) with the initial gas saturation remains away from the wellbore. During the subsequent build up, the gas created around the wellbore during the preceding drawdown condenses into the oil and the saturation in the near-wellbore region returns to the initial gas saturation. The log-log pressure-derivative behaviours below the bubble point correspond to a twozone radial composite model, with decreasing mobility during drawdowns and increasing mobilities during build ups. The log-log pressure derivative plot of the build up reflects oil mobility distribution of the reservoir at the end of the preceding drawdown. Knowledge obtained from the study was applied to the analysis of a well test in an actual volatile oil reservoir. Analysis results were validated with compositional reservoir simulation that included the effect of capillary number and non-Darcy flow. Finally, factors affecting well deliverability in volatile oil reservoirs producing at flowing bottomhole pressure below bubble point pressure were studied. The result shows that end point relative permeability of oil phase and oil fluid composition are the most important factors affecting productivity of volatile oil reservoirs producing below bubble point pressure. This study suggests in volatile oil reservoirs, both vertical hydraulic fractures and horizontal wells are best implemented early in the wells life to delay the time when the flowing bottomhole pressure drops below the bubble point pressure, hence improving ultimate recovery.