Objection 1. It would seem that infusion of grace is necessary for the remission of venial sins. Because an effect is not produced without its proper cause. Now the proper cause of the remission of sins is grace; for man's sins are not forgiven through his own merits; wherefore it is written (Ephesians 2:4-5): "God, Who is rich in mercy, for His exceeding charity, wherewith He loved us, even when we were dead in sins, hath quickened us together in Christ, by Whose grace you are saved." Therefore venial sins are not forgiven without infusion of grace.
Objection 2. Further, venial sins are not forgiven without Penance. Now grace is infused, in Penance as in the other sacraments of the New Law. Therefore venial sins are not forgiven without infusion of grace.
Objection 3. Further, venial sin produces a stain on the soul. Now a stain is not removed save by grace which is the spiritual beauty of the soul. Therefore it seems that venial sins are not forgiven without infusion of grace.
On the contrary, The advent of venial sin neither destroys nor diminishes grace, as stated in the II-II, 24, 10. Therefore, in like manner, an infusion of grace is not necessary in order to remove venial sin.
I answer that, Each thing is removed by its contrary. But venial sin is not contrary to habitual grace or charity, but hampers its act, through man being too much attached to a created good, albeit not in opposition to God, as stated in I-II, 88, 1; II-II, 24, 10. Therefore, in order that venial sin be removed, it is not necessary that habitual grace be infused, but a movement of grace or charity suffices for its forgiveness.
Nevertheless, since in those who have the use of free-will (in whom alone can there be venial sins), there can be no infusion of grace without an actual movement of the free-will towards God and against sin, consequently whenever grace is infused anew, venial sins are forgiven.
Reply to Objection 1. Even the forgiveness of venial sins is an effect of grace, in virtue of the act which grace produces anew, but not through any habit infused anew into the soul.
Reply to Objection 2. Venial sin is never forgiven without some act, explicit or implicit, of the virtue of penance, as stated above (Article 1): it can, however, be forgiven without the sacrament of Penance, which is formally perfected by the priestly absolution, as stated above (Question 87, Article 2). Hence it does not follow that infusion of grace is required for the forgiveness of venial sin, for although this infusion takes place in every sacrament, it does not occur in every act of virtue.
Reply to Objection 3. Just as there are two kinds of bodily stain, one consisting in the privation of something required for beauty, e.g. the right color or the due proportion of members, and another by the introduction of some hindrance to beauty, e.g. mud or dust; so too, a stain is put on the soul, in one way, by the privation of the beauty of grace through mortal sin, in another, by the inordinate inclination of the affections to some temporal thing, and this is the result of venial sin. Consequently, an infusion of grace is necessary for the removal of mortal sin, but in order to remove venial sin, it is necessary to have a movement proceeding from grace, removing the inordinate attachment to the temporal thing.
Source: Summa Theologica (New Advent)