From the National Rail website:
"From Monday 12 September to Friday 21 October 2016 the Severn Tunnel will be closed for Network Rail to carry out their electrification upgrade. This work, which is a huge part of the Great Western Electrification Programme, will see Network Rail install a solid conductor rail - suspended from brackets in the roof of the tunnel - which will provide the power for the new Super Express Trains.
During the six-week closure, there will be no trains between Patchway and Severn Tunnel Junction and services will be affected in the following ways.
On Weekdays - Trains between London Paddington and South Wales will be diverted between Swindon and Newport adding approximately 30 minutes to the journey time. Train services will also run between London Paddington and Bristol Parkway. Trains between Cardiff Central and Portsmouth Harbour, and between Cardiff Central and Taunton will run from Bristol Parkway instead. Buses will run between Severn Tunnel Junction and Bristol Parkway, and between Bristol Parkway and Newport to connect with these services.
On Weekends of 17/18 September and 15/16 October - The line from Bristol Parkway through to Cardiff Central will be closed. Buses will run between these stations..
On Weekends of 24/25 September, 1/2 October and 8/9 October - The railway will also be closed between Swindon and Bristol Parkway. This will not further impact on South Wales services as they are being diverted away from this section of line anyway"