Combine water and sugar in a small saucepan over medium heat. Cook 3 minutes or until sugar dissolves. Pour into a freezer-safe container; cool completely. Add tequila, juice, and Triple Sec to sugar mixture; stir to combine. Cover and freeze overnight. Place grapes in a large heavy-duty zip-top plastic bag; freeze overnight.
Place half of tequila mixture, 1 1/2 cups grapes, and 3 1/2 cups ice in a blender; process until desired consistency. Repeat with remaining tequila mixture, grapes, and ice.