10. Intangible Assets, Net

A summary of intangible assets, net is presented in the table below:



	December 31, 2016							December 31, 2015							Weighted Average Amortization Period (Years)
	Gross Carrying Value		Accumulated Amortization			Net Carrying Value		Gross Carrying Value		Accumulated Amortization			Net Carrying Value		Weighted Average Amortization Period (Years)
	(In thousands)
Customer relationships	$	7,172	$	(4,234	)	$	2,938	$	7,551	$	(3,921	)	$	3,630	5.5
Pipeline rights-of-way	6,527		(2,950		)	3,577		6,414		(2,287		)	4,127		4.9
	$	13,699	$	(7,184	)	$	6,515	$	13,965	$	(6,208	)	$	7,757

Intangible asset amortization expense was $1.2 million, $1.2 million and $1.3 million for the years ended December 31, 2016, 2015 and 2014, respectively, based upon estimates of useful lives ranging from 1 to 35 years.

Estimated amortization expense for the next five fiscal years is as follows (in thousands):



2017	$	1,087
2018	1,087
2019	1,087
2020	792
2021	639