Capillary trapping of COsub2/sub in heterogeneous reservoirs during the injection period

Capillary trapping plays an important role in CO ₂ 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 CO ₂ 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 CO ₂ 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 CO ₂ critically depends on the contrast in rock properties, such as (1) intrinsic permeability, (2) irreducible water saturation, (3) maximum residual CO ₂ 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.