Exhibit 5.1

January 24, 2017

Patterson-UTI Energy, Inc.

10713 West Sam Houston Parkway North, Suite 800

Houston, TX 77064

Ladies and Gentlemen:

We have acted as counsel for Patterson-UTI Energy, Inc., a Delaware corporation (the “Company”), with respect to the preparation of the Registration Statement on Form S-3 (the “Registration Statement”) filed on or about the date hereof with the Securities and Exchange Commission (the “Commission”) in connection with the registration by the Company under the Securities Act of 1933, as amended (the “Securities Act”), of the offer and sale by the Company from time to time, pursuant to Rule 415 under the Securities Act, of shares of common stock, par value $0.01 per share, of the Company (the “Common Stock”);

We have also participated in the preparation of a Prospectus relating to the Common Stock (the “Prospectus”), which is contained in the Registration Statement to which this opinion is an exhibit.

In connection with the opinions expressed herein, we have examined, among other things, (i) the Restated Certificate of Incorporation, as amended, and the Second Amended and Restated Bylaws of the Company, (ii) the Registration Statement, (iii) the Prospectus, and (iv) the records of corporate proceedings that have occurred prior to the date hereof with respect to the Registration Statement. We have also reviewed such questions of law as we have deemed necessary or appropriate. As to matters of fact relevant to the opinions expressed herein, and as to factual matters arising in connection with our examination of corporate documents, records and other documents and writings, we relied upon certificates and other communications of corporate officers of the Company, without further investigation as to the facts set forth therein.

In connection with rendering the opinions set forth below, we have assumed that (i) all information contained in all documents reviewed by us is true, correct and complete; (ii) all signatures on all documents examined by us are genuine; (iii) all documents submitted to us as originals are authentic and all documents submitted to us as copies conform to the originals of those documents; (iv) each natural person signing any document reviewed by us had the legal capacity to do so; (v) each person signing in a representative capacity any document reviewed by us had authority to sign in such capacity; (vi) the Registration

Vinson & Elkins LLP Attorneys at Law Austin Beijing Dallas Dubai Hong Kong Houston London Moscow New York Palo Alto Richmond Riyadh San Francisco Taipei Tokyo Washington 1001 Fannin Street, Suite 2500 Houston, TX 77002-6760 Tel +1.713.758.2222 Fax +1.713.758.2346 www.velaw.com

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Statement and any subsequent amendments (including additional post-effective amendments), will be effective and comply with all applicable laws; (vii) the Common Stock will be issued and sold in compliance with applicable federal and state securities laws and in the manner specified in the Registration Statement and the applicable prospectus supplement to the Prospectus; (viii) one or more prospectus supplements to the Prospectus will have been prepared and filed with the Commission describing the Common Stock offered thereby; and (ix) a definitive purchase, underwriting or similar agreement with respect to any Common Stock offered will have been duly authorized and validly executed and delivered by the Company and the other parties thereto.

Based on the foregoing, and subject to the assumptions, qualifications, limitations, and exceptions set forth herein, we are of the opinion that:

1.    With respect to shares of Common Stock, when both (A) the Board has taken all necessary corporate action to approve the issuance of and the terms of the offering of the shares of Common Stock and related matters and (B) certificates representing the shares of Common Stock have been duly executed, countersigned, registered, and delivered (or non-certificated shares of Common Stock shall have been properly issued) in accordance with the applicable definitive purchase, underwriting, or similar agreement approved by the Board or such officers upon payment of the consideration therefor (not less than the par value of the Common Stock) provided for therein for the consideration approved by the Board (not less than the par value of the Common Stock), then the shares of Common Stock will be legally issued, fully paid, and nonassessable.

We express no opinions concerning the enforceability of indemnification provisions to the extent they purport to relate to liabilities resulting from or based upon negligence or any violation of federal or state securities or blue sky laws.

This opinion is limited in all respects to the laws of the Delaware General Corporation Law (including the applicable provisions of the Delaware Constitution and the reported judicial decisions interpreting these laws) and the federal laws of the United States of America, as in effect on the date hereof, and we undertake no duty to update or supplement the foregoing opinion to reflect any facts or circumstances that may hereafter come to our attention or to reflect any changes in any law that may hereafter occur or become effective. We do not express any opinion as to the laws of any other jurisdiction.

We hereby consent to the filing of this opinion of counsel as an exhibit to the Registration Statement and to the reference to our Firm under the heading “Legal Matters” in the Prospectus forming a part of the Registration Statement. In giving this consent, we do not thereby admit that we come within the category of persons whose consent is required under Section 7 of the Securities Act or the rules and regulations of the Commission promulgated thereunder.

Very truly yours,

/s/ Vinson & Elkins L.L.P.