NMR Investigation Of Eor Mechanism Of N₂-CO₂ Huff-n-puff In Tight Sand

CO₂ HnP?huff-n-puff?,despite its promised potential and societal support for potential carbon negativity,suffered from the elevated cost;its application is limited in scale at the time of this writing.Practitioners had explored a mixture of CO₂ with N₂ as the treatment gas to reduce the cost and improve recovery.However,these efforts are currently thwarted by a lack of understanding of the fluids displacement mechanism on the pore-scale that can only be addressed through laboratory experiments.In this study,we investigate why sequential N₂-CO₂ HnP recovers more oil than pure CO₂ HnP.Successful designs of the enhanced oil recovery operation and production forecasts require careful characterization of the reservoir rocks'pore structures and fluids within.For carrying an efficient characterization of pore system in tight sandstones,demands for a time-saving and efficient model to categorize pore fluid types and pore sizes to investigate their co-effect in the HnP process are current.A series of laboratory procedures were designed to remove one type of fluid once as to categorize the fluid types with the T₂ distribution of FF?free fluid?,CAF?capillary bound fluid?and CBF?clay bound fluid?.The heat-treatment method for deriving CBF adopts a linearly increasing heating mode divided to equal heating intervals and is effective below 140?-160?in this study.Since most of the FF reside in large pores and that CBF will be largely contained small pores,T₂ cutoff(T_2C)values are proposed to categorize the pore sizes as an efficient way.The calculated T_2C2C2 for micropores is between 0.68 ms and 1.92 ms for the samples.The HnP process in tight sand was researched within the 9 categories that was subdivided with the derived DCS model?dual classification system of pore fluid types and pore sizes?.We compare these curves with the T₂ curves measured during cycles of a laboratory-scale CO₂ HnP and N₂-CO₂ HnP experiments to assess the recovery rate against fluid types and pore sizes.Through these experimental procedures,we found most of FF,including those stored in medium/small-sized pores,are recovered within the first two cycles along with a significant portion of the CAF in small to medium pores;no displacement of CBF is observed.N₂ and CO₂ perform similarly in the displacement of FF,but N₂ displaces little CAF.However,replacing N₂ with CO₂ after a few N₂ HnP cycles results in significant improvement in the recovery of CAF in the medium-sized pores than the HnP using exclusively CO₂.The injected N₂ displaces pore fluids directly by squeezing their volumes but can only work on highly mobile FF.The displacement of fluids by CO₂ relies more on creating a fluids/CO₂ mixture and the subsequent swelling effect that can also displace CAF.Nevertheless,the injected N₂ does not just‘push'the fluids out but also enlarged the pore throttles and thus resulted in higher CAF recovery in the later cycles of HnP using CO₂.More optimized HnP procedures may further boost the recovery of fluids stored in medium-sized pores,where a significant amount of hydrocarbon would remain unrecovered.