Hybrid CO2-N2 huff-n-puff strategy in unlocking tight oil reservoirs

Abstract

Increasing energy and environmental demands raise intensive interests on cleaner but challenging resource explorations, where unconventional tight reservoirs attract attentions due to its huge reserves but complex reservoir conditions. As commonly-used gas solvents for enhancing tight oil recovery, CO2 and N2 show their superb capabilities in each single usage, while their combinations, particularly utilizing of huff-n-puff (HnP) strategy, has been rarely investigated. In this paper, novel gas solvents, hybrid CO2-N2 gases with six molar ratios were synthesized and applied for a series of laboratory HnP tests using artificial cores. The proposed technology was specifically evaluated by analysing relevant properties, optimising production strategy, and examining the post-production residual oil saturation by means of nuclear magnetic resonance (NMR). Overall, our experiments validate the first three HnP cycles usually have higher oil recovery, while the subsequently further cycles would only be mildly beneficial. In this case, the hybrid CO2-N2 HnP simultaneously takes advantages of CO2-induced viscosity reduction and N2-induced elastic energy increase. The optimum proportion of CO2-N2 in the hybrid gas solvent is found to be 1:2, with the best HnP oil recovery of 39.0%. The NMR-assisted coreflood tests indicate largely uneven residual oil distributions. The effectivities of CO2 and N2 HnP are determined to be different in variation of pore scale in production tight oil, where the N2 HnP has better oil extraction ability in small pores.