In reply to Hooo: Bostik 2402 adhesive will work, but it will stiffen the material somewhat.
I'd glue the patch with 2402, then stitch the patch to the bag with high strength nylon thread (or thin fishing line), using a needle intended for jeans and a thimble (or some plasters and profuse swearing) will probably be necessary.
The closer to the point of failure you stitch the patch to the sack, the less any force on the patch will be able to pull it away from the material from the middle.