Oil recovery and compositional change of CO_2 huff-n-puff and continuous injection modes in a variety of dual-permeability tight matrix-fracture models

摘要

CO2 offers potential for improving oil recovery of unconventional reservoirs. There is, however, a lack of understanding of mass transport in matrix-facture systems during CO2 injection to simulate hydraulically fractured formations. To bridge the gaps, three dual-permeability tight matrix-fracture models were designed and employed to mimic CO2 huff-n-puff (HnP) and continuous injection (CI). Fracture aperture (h) and fracture permeability (K-f) of the models with confining pressure were firstly correlated, after which CO2 HnP and CI were performed to observe the variations of system pressure (P-sys) and oil recovery factor (ORF) over time. Our results indicated that either CO2 HnP or CI could substantially recover the matrix oil, and an ORF up to 53.28% OOIP (original oil in place) was produced by the CI mode. However, the ORF was found to rapidly decline with cycle number or time. The mass transport of these two modes was dictated by both diffusion and advection. Light hydrocarbons (HCs) extraction/vaporization promoted the mass exchange between fracture and matrix, which, therefore, increased the ultimate ORF. Surface area exposure was the decisive factor to the ORF of CO2 injection and other similar methods followed by fluid conductivity. The results of this study supplement earlier findings and can provide insights into the oil recovery mechanisms of CO2 injection in tight reservoirs.