Quantifying Bypassed Oil in the Vicinity of Discontinuous Shales during Gravity Dominated Flow

Abstract Gravity-stable, miscible gas injection is a common oil recovery technique throughout the world. In homogeneous environments recovery efficiencies may be more than 90%. However the influence of heterogeneity on the sweep efficiency in these recovery schemes is not well understood. For example most clastic reservoirs contain ‘discontinuous’ shales that cannot be correlated between wells. Several numerical studies have suggested that these may cause significant bypassing of oil during waterflooding or gas injection. However flow experiments and detailed simulation of viscous dominated displacements, without gravity, indicate that very little oil is bypassed1. In this paper, we investigate flow patterns around discontinuous shales during vertical, gravity-influenced miscible gas injection in well-characterised bead-pack experiments at three flow-rates corresponding to unstable, partly stable and completely stable flow regimes. We examine the volume of oil bypassed and whether viscous fingering is reduced by gravity or altered by the presence of a discontinuous shale. The results are compared to the predictions of detailed simulation. We find that, during miscible displacements, an isolated shale causes negligible bypassing. However, during adverse mobility ratio displacements, the simulation program erroneously predicts significant bypassing of oil both upstream and downstream of the shale at high and intermediate flow-rates.