Objection 1. It would seem that venial sin can be forgiven without penance. For, as stated above (84, 10, ad 4), it is essential to true penance that man should not only sorrow for his past sins, but also that he should purpose to avoid them for the future. Now venial sins are forgiven without any such purpose, for it is certain that man cannot lead the present life without committing venial sins. Therefore venial sins can be forgiven without penance.
Objection 2. Further, there is no penance without actual displeasure at one's sins. But venial sins can be taken away without any actual displeasure at them, as would be the case if a man were to be killed in his sleep, for Christ's sake, since he would go to heaven at once, which would not happen if his venial sins remained. Therefore venial sins can be forgiven without penance.
Objection 3. Further, venial sins are contrary to the fervor of charity, as stated in the II-II, 24, 10. Now one contrary is removed by another. Therefore forgiveness of venial sins is caused by the fervor of charity, which may be without actual displeasure at venial sin.
On the contrary, Augustine says in De Poenitentia [De vera et falsa Poenitentia, the authorship of which is unknown, that "there is a penance which is done for venial sins in the Church every day" which would be useless if venial sins could be forgiven without Penance.
I answer that, Forgiveness of sin, as stated above (Question 86, Article 2), is effected by man being united to God from Whom sin separates him in some way. Now this separation is made complete by mortal sin, and incomplete by venial sin: because, by mortal sin, the mind through acting against charity is altogether turned away from God; whereas by venial sin man's affections are clogged, so that they are slow in tending towards God. Consequently both kinds of sin are taken away by penance, because by both of them man's will is disordered through turning inordinately to a created good; for just as mortal sin cannot be forgiven so long as the will is attached to sin, so neither can venial sin, because while the cause remains, the effect remains.
Yet a more perfect penance is requisite for the forgiveness of mortal sin, namely that man should detest actually the mortal sin which he committed, so far as lies in his power, that is to say, he should endeavor to remember each single mortal sin, in order to detest each one. But this is, not required for the forgiveness of venial sins; although it does not suffice to have habitual displeasure, which is included in the habit of charity or of penance as a virtue, since then venial sin would be incompatible with charity, which is evidently untrue. Consequently it is necessary to have a certain virtual displeasure, so that, for instance, a man's affections so tend to God and Divine things, that whatever might happen to him to hamper that tendency would be displeasing to him, and would grieve him, were he to commit it, even though he were not to think of it actually: and this is not sufficient for the remission of mortal sin, except as regards those sins which he fails to remember after a careful examination.
Reply to Objection 1. When man is in a state of grace, he can avoid all mortal sins, and each single one; and he can avoid each single venial sin, but not all, as was explained in I-II, 74, 8, ad 2; I-II, 109, 8. Consequently penance for mortal sins requires man to purpose abstaining from mortal sins, all and each; whereas penance for venial sins requires man to purpose abstaining from each, but not from all, because the weakness of this life does not allow of this. Nevertheless he needs to have the purpose of taking steps to commit fewer venial sins, else he would be in danger of falling back, if he gave up the desire of going forward, or of removing the obstacles to spiritual progress, such as venial sins are.
Reply to Objection 2. Death for Christ's sake, as stated above (Question 66, Article 11), obtains the power of Baptism, wherefore it washes away all sin, both venial and mortal, unless it find the will attached to sin.
Reply to Objection 3. The fervor of charity implies virtual displeasure at venial sins, as stated above (Question 79, Article 4).
Source: Summa Theologica (New Advent)