Investigation of cyclic CO2 huff-and-puff recovery in shale oil reservoirs using reservoir simulation and sensitivity analysis

Optimization of cyclic gas huff-and-puff recovery is challenging because there is a myriad of variables involved in the system, most of which have nonlinear and opposite effects. This study conducted a sensitivity analysis in order to find a set of primary depletion period, injection period, and production period which maximized the final oil recovery factor after a fixed length of production. Input data were based on the actual Bakken formation properties and field data. On the basis of numerical simulations, we found that short injection and production periods were favorable because they increased the overall huff-and-puff cycle number and adequately took advantage of the high initial injection rate and production rate during the huff-and-puff stage. When the injection period (production period) was fixed, a too short or too long production period (injection period) was not optimal for final oil recovery because of the interdependence between these two variables; a balance between them is needed. In addition, a too short or too long primary depletion period was not optimal for final oil recovery, and the optimal primary depletion period depends significantly on the combination of injection and production periods. If cyclic CO2 huff-and-puff recovery starts too early, the recovery rate of primary depletion is higher than huff-and-puff; in this scenario the potential of reservoir pressure drive is not fully utilized. Conversely, if cyclic CO2 huff-and-puff process starts too late, the recovery rate of primary depletion is already lower than huff-and-puff; in this scenario the remaining time is insufficient to take full advantage of cyclic CO2 huff-and-puff process. This study aims to use a comprehensive sensitivity analysis to better demonstrate the interdependence and relationships between primary depletion period, injection period, and production period, as well as the influence on the final oil recovery. The outcome of this study has the potential to advance our understanding of the fundamental mechanisms underlying CO2 huff-and-puff, which will benefit unconventional hydrocarbon energy recovery within shale reservoirs.