A glut of supply from shale fields has ensured relatively stable pricing for natural gas distributors such as Piedmont.
This has prompted a number of U.S. power producers to boost their natural gas infrastructure and lower dependence on power generation at time when demand for electricity is weakening due to increased energy efficiency.
U.S. power producer Southern Co. said in August it would buy AGL Resources for about $8 billion in cash.
Duke and Piedmont are also partners in the $5 billion 550-mile (885-km) Atlantic Coast pipeline, which moves gas from Pennsylvania's Marcellus shale field to North Carolina and Virginia.
Duke will also assume about $1.8 billion of Piedmont's net debt, giving the company an enterprise value of $6.7 billion.