The 24-year-old was signed by the West Country club as part of their quest to return to the upper reaches of the Aviva Premiership.
"Bath have a great coaching team and are clearly building towards the future," said Shiells.
"There are some very powerful players in the squad, and I'm looking forward to getting involved and developing myself as a player."
Neal Hatley, the Bath forwards coach, added: "Grant is a talented young guy who already has a good amount of Premiership experience.
"We see him fitting in well with the club and our plans. He's got a lot of potential in him, and we're looking forward to seeing him push on."