If you have been invited to an intimate chapel wedding, first of all, that means you are one of the people who really matters to this couple. Small weddings only include the closest people, so consider it a compliment. Now, about what to wear. Here is the simple, friendly version so you can stop staring at your closet.
An intimate chapel wedding is usually a touch dressier than a backyard party but softer than a big ballroom affair. Think garden-party elegant. A nice dress, a jumpsuit, or slacks with a pretty top for the women, and a sport coat or a button-up with nice pants for the men. You really cannot go wrong with "Sunday best, slightly elevated."
There is gravel and grass on the grounds, so stilettos will fight you. A block heel, a wedge, or a cute flat will keep you comfortable and upright through the outdoor moments and photos.
The usual rules still apply, even at a small wedding. Skip white, ivory and champagne, those belong to the bride. Avoid anything you would wear to a nightclub. And unless the invitation says otherwise, leave the jeans at home, a chapel ceremony is a dress-up occasion, just a relaxed one.
If you are truly unsure, a simple dress in a soft color, or a sport coat over a nice shirt, is never wrong for a chapel wedding. And honestly, the couple invited you because they want you there, not because they are grading your outfit. Show up, be happy for them, and you are dressed correctly.