Goodrich Corporation, an aerospace manufacturer and existing member, has announced plans to merge with United Technologies Corporation, already a member of the index, S&P Dow Jones Indices said in a statement.
That leaves an opening for Ensco, which will be included on the list after the close of US trading Monday.
Shares were trading up more than 5% at midday Friday following the news.
With a market capitalisation of $12.5 billion, the contractor will join top integrated E&P companies, service companies and fellow drillers Noble Corporation, Diamond Offshore, Helmerich & Payne and Nabors Industries on the list.
The S&P 500 is made up of leading large companies that make roughly three-quarters of US stock-trading activity.