funded using existing cash or by issuing debt or additional shares of the Company. Such capital expenditures and acquisitions will depend on our view of market conditions and other factors.
See our discussion of guarantees issued by Nabors that could have a potential impact on our financial position, results of operations or cash flows in future periods included below under “Off-Balance Sheet Arrangements (Including Guarantees).”
There have been no material changes to the contractual cash obligations table that was included in our 2018 Annual Report.
On August 25, 2015, our Board of Directors authorized a share repurchase program (the “program”) under which we may repurchase, from time to time, up to $400.0 million of our common shares by various means, including in the open market or in privately negotiated transactions. Authorization for the program, which was renewed in February 2019, does not have an expiration date and does not obligate us to repurchase any of our common shares. Since establishing the program, we have repurchased 14.0 million of our common shares for an aggregate purchase price of approximately $119.4 million under this program. The repurchased shares, which are held by our subsidiaries, are registered and tradable subject to applicable securities law limitations and have the same voting and other rights as other outstanding shares. As of September 30, 2019, the remaining amount authorized under the program that may be used to purchase shares was $280.6 million. As of September 30, 2019, our subsidiaries held 52.8 million of our common shares.
On May 23, 2019, our Board of Directors authorized a share repurchase program under which we may repurchase, from time to time, up to $10.0 million of our mandatory convertible preferred shares by various means, including in the open market or in privately negotiated transactions. This authorization does not have an expiration date and does not obligate us to repurchase any of our mandatory convertible preferred shares. Through September 30, 2019, we repurchased and canceled 4,000 mandatory convertible preferred shares for an aggregate purchase price of approximately $.08 million.
We may from time to time seek to retire or purchase our outstanding debt through cash purchases and/or exchanges for equity securities, both in open-market purchases, privately negotiated transactions or otherwise. Such repurchases or exchanges, if any, will depend on prevailing market conditions, our liquidity requirements, contractual restrictions and other factors and may involve material amounts.
Cash Flows
Our cash flows depend, to a large degree, on the level of spending by oil and gas companies for exploration, development and production activities. Sustained decreases in the price of oil or natural gas could have a material impact on these activities, and could also materially affect our cash flows. Certain sources and uses of cash, such as the level of discretionary capital expenditures or acquisitions, purchases and sales of investments, dividends, loans, issuances and repurchases of debt and of our common shares are within our control and are adjusted as necessary based on market conditions. We discuss our cash flows for the nine months ended September 30, 2019 and 2018 below.
Operating Activities. Net cash provided by operating activities totaled $430.8 million during the nine months ended September 30, 2019, compared to net cash provided of $76.9 million during the corresponding 2018 period. Operating cash flows are our primary source of capital and liquidity. The increase in cash flows from operations is primarily attributable to increases in activity and margins in our U.S. Drilling operating segment. Additionally, changes in working capital items such as collection of receivables, other deferred revenue arrangements and payments of operating payables and interest payments are significant factors affecting operating cash flows. Changes in working capital items provided $37.3 million and used $300.7 million in cash during the nine months ended September 30, 2019 and 2018, respectively.
Investing Activities. Net cash used for investing activities totaled $333.7 million during the nine months ended September 30, 2019 compared to net cash used of $250.0 million during the corresponding 2018 period. Our primary use of cash for investing activities is capital expenditures for rig-related enhancements, new construction and equipment, as well as sustaining capital expenditures. During the nine months ended September 30, 2019 and 2018, we used cash for capital expenditures totaling $366.6 million and $339.0 million, respectively.