	March 31, 2021			December 31, 2020
2011 Senior Notes	$	150,000		$	150,000
2021 Senior Notes		60,000			—
Credit Facility		—			111,000
Total debt		210,000			261,000
Less: Debt issuance costs		(1,834	)		(1,567	)
Long-term debt, net	$	208,166		$	259,433

We have four series of senior notes outstanding with an aggregate principal amount of $210 million issued through private placement transactions. Two series of the notes were issued in 2011 (“2011 Senior Notes”). Series A of the 2011 Senior Notes consists of $75 million in aggregate principal amount of notes that bear interest at a fixed rate of 4.01% and are due in full on September 30, 2021. Because we intend to draw on our Credit Facility (as defined below) as needed to retire Series A of the 2011 Senior Notes and our Credit Facility is classified as long-term debt, we have continued to classify the 2011 Senior Notes as long-term. Series B of the 2011 Senior Notes consists of $75 million in aggregate principal amount of notes that bear interest at a fixed rate of 4.11% and are due in full on September 30, 2023. Interest on each series of the Senior Notes is payable semi-annually on March 30 and September 30.

On October 16, 2020, we, along with Core Laboratories (U.S.) Interests Holdings, Inc. as issuer, entered into two new series of senior notes with aggregate principal amount of $60 million in a private placement transaction (“2021 Senior Notes”). The 2021 Senior Notes were issued and funded on January 12, 2021. Series A of the 2021 Senior Notes consists of $45 million in aggregate principal amount that bear interest at a fixed rate of 4.09% and are due in full on January 12, 2026. Series B of the 2021 Senior Notes consists of $15 million in aggregate principal amount that bear interest at a fixed rate of 4.38% and are due in full on January 12, 2028. Interest on each series of the 2020 Senior Notes is payable semi-annually on June 30 and December 30, commencing on June 30, 2021.

On June 22, 2020, we entered into Amendment No. 1 (the “Amendment”) to the Seventh Amended and Restated Credit Agreement, dated June 19, 2018 (as amended, the “Credit Facility”). The Amendment increases the maximum leverage ratio permitted under the Credit Facility for certain periods. Pursuant to the terms of the Amendment, the maximum leverage ratio permitted under the Credit Facility is as follows:

Quarter ending	Maximum leverage ratio permitted
June 30, 2020 up to and including June 30, 2021		3.00
September 30, 2021		2.75
December 31, 2021 and thereafter		2.50

Moreover, the Amendment modified the range of variable interest rates that the Credit Facility may bear to be a range from LIBOR plus 1.500% to LIBOR plus 2.875% and included the addition of a LIBOR floor of 0.50%. The Amendment also reduced the aggregate borrowing commitment under the Credit Facility to $225 million and the amount by which we may elect to increase the facility size, known as the “accordion” feature, to $50 million, subject to the satisfaction of certain conditions. Interest payment terms are variable depending upon the specific type of borrowing under the Credit Facility.

Any outstanding balance under the Credit Facility is due on maturity on June 19, 2023. Our available capacity at any point in time is reduced by outstanding borrowings and outstanding letters of credit which totaled $11.1 million at March 31, 2021, resulting in an available borrowing capacity under the Credit Facility of $213.9 million. In addition to indebtedness under the Credit Facility, we had $6.0 million of outstanding letters of credit and performance guarantees and bonds from other sources as at March 31, 2021.

The Credit Facility is unsecured, and contains customary representations, warranties, terms and conditions for similar types of facilities.

The terms of the Credit Facility requires us to meet certain covenants, including, but not limited to, an interest coverage ratio (calculated as consolidated EBITDA divided by interest expense) and a leverage ratio (calculated as consolidated net indebtedness divided by consolidated EBITDA), where consolidated EBITDA (as defined in each agreement) and interest expense are calculated using the most recent four fiscal quarters. The Credit Facility and Senior Notes include a cross-default provision, whereby a default under one agreement may trigger a default in the other agreement. The Credit Facility has more restrictive covenants with a minimum interest coverage ratio of 3.0 to 1.0 and permit a maximum leverage ratio as described above. The Credit Facility allows non-cash charges such as impairment of assets, stock compensation and other non-cash charges to be added back in the calculation of consolidated EBITDA. The terms of our Credit Facility also allow us to negotiate in good faith to amend any ratio or requirement to preserve the original intent of the agreement if any change in accounting

principles would affect the computation of any financial ratio or covenant of the Credit Facility. In accordance with the terms of the Credit Facility, our leverage ratio is 2.33, and our interest coverage ratio is 5.58, each for the period ended March 31, 2021. We believe that we are in compliance with all covenants contained in our Credit Facility. Certain of our material, wholly-owned subsidiaries, are guarantors or co-borrowers under the Credit Facility.

We entered into two interest rate swap agreements for a total notional amount of $50 million, one of which was entered into during 2020. These two interest rate swap agreements were terminated and settled during the three months ended March 31, 2021. See Note 15, Derivative Instruments and Hedging Activities for additional information.

We entered into two forward interest rate swap agreements for a total notional amount of $35 million during 2020. The purpose of these forward interest rate swap agreements are to fix the underlying risk-free rate, that would be associated with the anticipated issuance of new long-term debt by the Company. These two forward interest rate swap agreements were terminated and settled during the three months ended March 31, 2021.

The estimated fair value of total debt at March 31, 2021 and December 31, 2020 approximated the book value of total debt. The fair value was estimated using Level 2 inputs by calculating the sum of the discounted future interest and principal payments through the maturity date.