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Asset Retirement Obligations
9 Months Ended
Sep. 30, 2021
Asset Retirement Obligation Disclosure [Abstract]  
Asset Retirement Obligation Disclosure
The following table presents the changes in carrying amounts of the asset retirement obligations associated with our working interests in crude oil and natural gas properties for the nine months ended September 30, 2021:
(in thousands)
Asset retirement obligations at beginning of period$166,570 
Obligations incurred with development activities and other4,069 
Accretion expense9,185 
Revisions in estimated cash flows(783)
Obligations discharged with asset retirements and divestitures(26,805)
Asset retirement obligations at end of period152,236 
Current portion (1)
(33,875)
Long-term portion$118,361 
_____________
(1) The current portion of the asset retirement obligation is included in other accrued expenses on our condensed consolidated balance sheets.

Our estimated asset retirement obligations liability is based on historical experience in plugging and abandoning wells, estimated economic lives and estimated plugging, abandonment and surface reclamation costs considering federal and state regulatory requirements in effect at that time. The liability is discounted using the credit-adjusted risk-free rate estimated at the
time the liability is incurred or revised. To the extent future revisions to these assumptions impact the present value of the existing asset retirement obligations liability, a corresponding adjustment is made to the properties and equipment balance. Changes in the liability due to the passage of time are recognized as an increase in the carrying amount of the liability and as accretion expense.