COMMITMENTS AND CONTINGENCIES We lease office space or yards in Edmond and Oklahoma City, Oklahoma; Houston, Texas; Englewood, Colorado; Pinedale, Wyoming; and Canonsburg, Pennsylvania under the terms of operating leases expiring through December 2021. We own our corporate headquarters in Tulsa, Oklahoma. We also have several compressor rentals, equipment leases, and lease space on short-term commitments to stack excess drilling rig equipment and production inventory. Future minimum rental payments under the terms of the leases are approximately $5.1 million , $2.1 million , $0.6 million , and less than $0.1 million in twelve-month periods beginning October 1, 2018 (and through 2021), respectively. Total rent expense incurred was $7.2 million and $6.4 million for the first nine months of 2018 and 2017 , respectively. In 2014, Superior signed capital lease agreements for 20 compressors with initial terms of seven years. Estimated annual capital lease payments under the terms during the four successive twelve-month periods beginning October 1, 2018 (and through the end of 2021) are $6.2 million , $6.2 million , and $5.3 million . Total maintenance and interest remaining related to these leases are $4.6 million and $0.8 million , respectively at September 30, 2018 . Annual payments, net of maintenance and interest, average $4.2 million annually through 2021 . At the end of the term, Superior has the option to purchase the assets at 10% of their then fair market value. The employee oil and gas limited partnerships require, on the election of a limited partner, that we repurchase the limited partners interest at amounts to be determined by appraisal. In any one year, these repurchases are limited to 20% of the units outstanding. We made repurchases of approximately $1,700 and $2,900 in the first nine months of 2018 and 2017, respectively. We manage our exposure to environmental liabilities on properties to be acquired by identifying existing problems and assessing the potential liability. We also conduct periodic reviews, on a company-wide basis, to identify changes in our environmental risk profile. These reviews evaluate whether there is a probable liability, its amount, and the likelihood that the liability will be incurred. Any potential liability is determined by considering, among other matters, incremental direct costs of any likely remediation and the proportionate cost of employees expected to devote significant time directly to any possible remediation effort. As it relates to evaluations of purchased properties, depending on the extent of an identified environmental problem, we may exclude a property from the acquisition, require the seller to remediate the property to our satisfaction, or agree to assume liability for the remediation of the property. We have not historically experienced any environmental liability while being a contract driller since the greatest portion of risk is borne by the operator. Any liabilities we have incurred have been small and have been resolved while the drilling rig is on the location and the cost has been included in the direct cost of drilling the well. During the second quarter of 2018, as part of the Superior transaction, we entered into a contractual obligation that commits us to spend $150.0 million to drill wells in the Granite Wash/Buffalo Wallow area over three years starting January 1, 2019. This amount is already included in our drilling plan. For each dollar of the $150.0 million that we do not spend (over the three year period), we would forgo receiving $0.58 of future distributions from our 50% ownership interest in our consolidated mid-stream subsidiary. If we elected not to drill or spend any money in the designated area over the three year period, the maximum amount we could forgo from distributions would be $87.0 million. For the next twelve months, we have committed to purchase approximately $10.1 million of new drilling rig components.