A comparative experimental study of gas injection in shale plugs by flooding and huff-n-puff processes

The increase in U.S crude oil over the past decade have been primarily driven by tight and shale oil. With the high production decline rates in shale reservoirs, improved oil recovery techniques must be applied to maintain the oil recovery from existing wells. Gas injection has been investigated and demonstrated as the most effective solution to face such challenge. As gas can be injected into the subsurface by two modes: continuous injection (flooding) and cyclic injection (huff-n-puff), this study aims to compare the recovery efficiencies of such two processes in shale core plugs with ultra-low matrix permeability. Eagle Ford core samples were used in this study and saturated with shale oil. Using N₂as the gas source, gas injection tests were operated on the same plug orderly by both modes under the same operating conditions. The sensitivity of soaking time on the huff-n-puff performance was evaluated. Lab-scale numerical models were built to simulate flooding and huff-n-puff processes and to history match the experimental data. It was found that optimization design of huff-n-puff is important to achieve the maximum oil recovery. The results show that the huff-n-puff possess can achieve a higher oil recovery than the flooding process.