Near Wellbore Two-Phase Effect of a Light Oil Field

Abstract

This work describes a case of a light oil field in Sub-Saharan Africa. Soon after the start up, the production wells showed a productivity index different from expectation. In particular, well test analysis showed different stabilizations between build-up and drawdown. Since the well is flowing with pressure below the bubble point, the impact of oil permeability reduction due to presence of free gas around the wellbore was considered as a possible explanation and was investigated. We applied well test analysis on synthetic data generated by numerical simulation. The analysis confirms the different stabilizations between pressure build-up and drawdown, as observed from the real field case, when the flowing bottomhole pressure is below the bubble point pressure. The different flow capacity is caused by gas saturation around the wellbore, which reduces the effective permeability to oil, hence the productivity index. Besides this, we experienced that a full field model with large grid size is not able to represent this phenomenon. Due to the grid size, the full field model shows less pressure drop than the radial sector model, hence different gas saturation calculation. We showed that in order to simulate the effect without impacting on simulation time, applying higher well skin instead of local grid refinement can be a way forward.