sentences:  , the size of XTO Energy's resource portfolio had tripled since it was acquired by ExxonMobil in 2010.In August 2015, Exxon Mobil Corporation signed two agreements to drill on 48,000 acres in the core of the Midland Basin to be operated by XTO Energy.XTO operates 11 horizontal and four vertical rigs across 1.5 million net acres in Texas' Permian Basin.