Searching for "AutoCAD 2015 Portable" typically leads to unofficial or modified versions of the software. It is important to note that Autodesk does not officially support or provide a "portable" version of AutoCAD 2015.
Why AutoCAD "Portable" is technically problematic:
Installing AutoCAD 2015 Portable on Google Drive: A Step-by-Step Guide
Google Drive Desktop App Settings:
AutoCAD Web: Use the AutoCAD Web App to edit, create, and view CAD drawings directly in your browser. This is the closest official "portable" experience available.