Capillary trapping of CO2 in heterogeneous reservoirs during the injection period

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Capillary trapping plays an important role in CO2 storage. Two capillary trapping mechanisms have been extensively investigated, i.e., snap-off trapping and capillary pinning. The latter mechanism only occurs in heterogeneous reservoirs. The majority of studies related to CO2 trapping have focused on snap-off trapping during the post-injection period, when imbibition of the wetting fluid (e.g. brine) is prevalent at the tail of a buoyantly rising CO2 plume. The main reason that the injection period has been ignored is that snap-off trapping is absent in homogeneous reservoirs; in homogeneous reservoirs there is no imbibition during injection. Here, we investigate capillary trapping in heterogeneous fluvial-type reservoirs during the injection period. We show that snap-off trapping exists in heterogeneous reservoirs even during the injection period; imbibition occurs because the relative permeability characteristics are different for different facies. More trapping occurs by capillary pinning during the injection period. The amount of snap-off and capillary pinned CO2 critically depends on the contrast in rock properties, such as (1) intrinsic permeability, (2) irreducible water saturation, (3) maximum residual CO2 saturation, and (4) capillary entry pressure barrier. The pinned amount is proportional to the fraction of the rocks with low capillary pressure. Pinning is larger in gravity-dominated flow than in viscous-dominated flow.



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