We recognize an estimated liability for future costs associated with the abandonment of our crude oil and natural gas properties, including facilities requiring decommissioning. A liability for the fair value of an asset retirement obligation and corresponding increase to the carrying value of the related long-lived asset are recorded at the time a well is drilled or acquired, or a facility is constructed. The increase in carrying value is included in proved properties in the accompanying consolidated balance sheets. We deplete the amount added to proved properties and recognize expense in connection with the accretion of the discounted liability over the remaining estimated economic lives of the respective long-lived assets. Cash paid to settle asset retirement obligations is included in the cash flows from operating activities section of our accompanying consolidated statements of cash flows.

Our estimated asset retirement obligation liability is based on historical experience plugging and abandoning wells, estimated plugging and abandonment cost, estimated economic lives, and regulatory requirements. The liability is discounted using the credit-adjusted risk-free rate estimated at the time the liability is incurred or revised.

A roll-forward of our asset retirement obligation is as follows (in thousands):


	Year Ended December 31,
	2024		2023
Balance, beginning of year	$	336,832	$	291,026
Additional liabilities incurred with development activities and other	8,556		7,516
Additional liabilities incurred with acquisitions	37,251		40,373
Liabilities settled	(46,526)		(19,136)
Accretion expense	23,327		17,053
Revisions to estimate(1)	126,174		—
Obligations discharged with divestitures	(27,976)		—
Balance, end of year	$	457,638	$	336,832
Current portion(2)	58,636		31,116
Long-term portion	399,002		$	305,716

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(1)Revisions to estimates for the year ended December 31, 2024 were primarily a result of (a) increases in our estimated plugging and abandonment cost driven by increased regulatory burden, service costs, complexity of plugging activities, and reclamation and environmental obligations that arose from normal operation of the assets, as evidenced through our plugging program activities during 2024, particularly in the DJ Basin, and (b) the acceleration of the estimated settlement date for certain wells.