Exhibit 5.1

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Menlo Park	Madrid
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Davis Polk & Wardwell LLP	212 450 4000 tel
450 Lexington Avenue	212 701 5800 fax
New York, NY 10017

July 29, 2014

Quicksilver Resources Inc.

801 Cherry Street, Suite 3700, Unit 19

Fort Worth, TX 76102

Ladies and Gentlemen:

Quicksilver Resources Inc., a Delaware company (the “Company”), is filing with the Securities and Exchange Commission a Registration Statement on Form S-3 (the “Registration Statement”) for the purpose of registering under the Securities Act of 1933, as amended (the “Securities Act”), (a) shares of common stock, par value $0.01 per share (the “Common Stock”) of the Company and associated preferred stock purchase rights of the Company which may be issued to holders of the Common Stock (the “Rights”); (b) shares of preferred stock, par value $0.01 per share (the “Preferred Stock”) of the Company; (c) the Company’s senior debt securities and subordinated debt securities (collectively, the “Debt Securities”), to be issued pursuant to a debt indenture, between the Company and The Bank of New York Mellon Trust Company, N.A., as successor trustee (the “Trustee”) (the “Indenture”) which may be fully and unconditionally guaranteed by each of Cowtown Pipeline Funding, Inc., Barnett Shale Operating LLC, Silver Stream Pipeline Company LLC, QPP Parent LLC, and QPP Holdings LLC (collectively, the “Delaware Guarantors”) and Cowtown Gas Processing L.P., Cowtown Pipeline Management, Inc. and Cowtown Pipeline L.P. (collectively, the “Non-Delaware Guarantors,” and together with the Delaware Guarantors, the “Guarantors”); (d) warrants of the Company (the “Warrants”), which may be issued pursuant to a warrant agreement between the Company and the warrant agent to be named therein (the “Warrant Agreement”); (e) purchase contracts (the “Purchase Contracts”) which may be issued under one or more purchase contract agreements (each, a “Purchase Contract Agreement”) to be entered into between the Company and the purchase contract agent to be named therein (the “Purchase Contract Agent”); (f) units (the “Units”) to be issued under one or more unit agreements to be entered into among the Company, a bank or trust company, as unit agent (the “Unit Agent”), and the holders from time to time of the Units (each such unit agreement, a “Unit Agreement”); and (g) guarantees (the “Guarantees”) of the Debt Securities by the Guarantors. The Company may offer Depositary Shares (the “Depositary Shares”) representing interests in Preferred Stock deposited with a Depositary and evidenced by Depositary Receipts, and such Depositary Shares are also covered by the Registration Statement. The Common Stock may be issued with associated rights under the Rights Agreement, dated as of March 11, 2003, between the Company and Computershare Shareowner Services (f/k/a Mellon Investor Services LLC), as rights agent, as amended and restated December 20, 2005, and as further amended on February 23, 2011 and March 8, 2013.

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We, as your counsel, have examined originals or copies of such documents, corporate records, certificates of public officials and other instruments as we have deemed necessary or advisable for the purpose of rendering this opinion.

In rendering the opinions expressed herein, we have, without independent inquiry or investigation, assumed that (i) all documents submitted to us as originals are authentic and complete, (ii) all documents submitted to us as copies conform to authentic, complete originals, (iii) all documents filed as exhibits to the Registration Statement that have not been executed will conform to the forms thereof, (iv) all signatures on all documents that we reviewed are genuine, (v) all natural persons executing documents had and have the legal capacity to do so, (vi) all statements in certificates of public officials and officers of the Company and the Guarantors that we reviewed were and are accurate and (vii) all representations made by the Company and the Guarantors as to matters of fact in the documents that we reviewed were and are accurate.

Based upon the foregoing, and subject to the additional assumptions and qualifications set forth below, we advise you that, in our opinion:

1.When the necessary corporate action on the part of the Company has been taken to authorize the issuance and sale of such shares of Common Stock proposed to be sold by the Company, and when such shares of Common Stock are issued and delivered in accordance with the applicable underwriting or other agreement against payment therefor (in excess of par value thereof) or upon conversion or exercise of any security offered under the Registration Statement (the “Offered Security”), in accordance with terms of such Offered Security or the instrument governing such Offered Security providing for such conversion or exercise as approved by the Board of Directors of the Company, for the consideration approved by such Board of Directors (which consideration is not less than the par value of the Common Stock), such shares of Common Stock will be validly issued, fully-paid and non-assessable.

2.Upon designation of the relative rights, preferences and limitations of any series of Preferred Stock by the Board of Directors of the Company and the proper filing with the Secretary of State of the State of Delaware of a Certificate of Designation relating to such series of Preferred Stock, all necessary corporate action on the part of the Company will have been taken to authorize the issuance and sale of such series of Preferred Stock proposed to be sold by the Company, and when such shares of Preferred Stock are issued and delivered in accordance with the applicable underwriting or other agreement against payment therefor (in excess of par value thereof), such shares of Preferred Stock will be validly issued, fully paid and non-assessable.

3.Assuming the Indenture has been duly authorized, executed and delivered by the Trustee, the Company and the Guarantors, and when any supplemental indenture to be entered into in connection with the issuance of any Debt Securities has been duly authorized, executed and delivered by the Trustee, the Company and the Guarantors; the specific terms of a particular series of Debt Securities and the related Guarantees have been duly authorized and established in accordance with the Indenture; and such Debt Securities and the related Guarantees have been duly authorized, executed, authenticated, issued and delivered in accordance with the Indenture and the applicable underwriting or other agreement against payment therefor, such Debt Securities will constitute valid and binding obligations of the Company and each of the related Guarantees will constitute valid and binding obligations of each respective Guarantor, enforceable in accordance with their terms, subject to applicable bankruptcy, insolvency and similar laws affecting creditors’ rights generally, concepts of reasonableness and equitable principles of general applicability, provided that we express no opinion as to (w) the enforceability of any waiver of rights under any usury or stay law, (x) (i) the effect of fraudulent conveyance, fraudulent transfer or similar provision of applicable law on the conclusions expressed above or (ii) any provision of the Indenture that purports to avoid the effect of fraudulent conveyance, fraudulent transfer or similar provision of applicable law by limiting the amount of any Guarantor’s obligation, and (y) the validity, legally binding effect or enforceability of any provision that permits holders to collect any portion of stated

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principal amount upon acceleration of the Debt Securities to the extent determined to constitute unearned interest.

4.When the Warrant Agreement to be entered into in connection with the issuance of any Warrants has been duly authorized, executed and delivered by the Warrant Agent and the Company; the specific terms of the Warrants have been duly authorized and established in accordance with the Warrant Agreement; and such Warrants have been duly authorized, executed, issued and delivered in accordance with the Warrant Agreement and the applicable underwriting or other agreement against payment therefor, such Warrants will constitute valid and binding obligations of the Company, enforceable in accordance with their terms, subject to applicable bankruptcy, insolvency and similar laws affecting creditors’ rights generally, concepts of reasonableness and equitable principles of general applicability.

5.When the Purchase Contract Agreement to be entered into in connection with the issuance of any Purchase Contracts has been duly authorized, executed and delivered by the Purchase Contract Agent and the Company; the specific terms of the Purchase Contracts have been duly authorized and established in accordance with the Purchase Contract Agreement; and such Purchase Contracts have been duly authorized, executed, issued and delivered in accordance with the Purchase Contract Agreement and the applicable underwriting or other agreement against payment therefor, such Purchase Contracts will constitute valid and binding obligations of the Company, enforceable in accordance with their terms, subject to applicable bankruptcy, insolvency and similar laws affecting creditors’ rights generally, concepts of reasonableness and equitable principles of general applicability.

6.When the Unit Agreement to be entered into in connection with the issuance of any Units has been duly authorized, executed and delivered by the Unit Agent and the Company; the specific terms of the Units have been duly authorized and established in accordance with the Unit Agreement; and such Units have been duly authorized, executed, issued and delivered in accordance with the Unit Agreement and the applicable underwriting or other agreement against payment therefor, such Units will constitute valid and binding obligations of the Company, enforceable in accordance with their terms, subject to applicable bankruptcy, insolvency and similar laws affecting creditors’ rights generally, concepts of reasonableness and equitable principles of general applicability.

In connection with the opinions expressed above, we have assumed that, at or prior to the time of the delivery of any such security, (i) the Board of Directors of the Company and the Board of Directors of the Guarantors, as the case may be, shall have duly established the terms of such security and duly authorized the issuance and sale of such security and such authorization shall not have been modified or rescinded; (ii) each of the Company and the Delaware Guarantors shall remain, and each of the Non-Delaware Guarantors is and shall remain, validly existing as a corporation in good standing under the laws of its respective jurisdiction of incorporation; (iii) the Registration Statement shall have been declared effective and such effectiveness shall not have been terminated or rescinded; (iv) the Indenture and the Debt Securities are each valid, binding and enforceable agreements of the Trustee; and (v) there shall not have occurred any change in law affecting the validity or enforceability of such security. We have also assumed that (i) the execution, delivery and performance by the Company or any Guarantor of any security whose terms are established subsequent to the date hereof (a) require no action by or in respect of, or filing with, any governmental body, agency or official and (b) do not contravene, or constitute a default under, any public policy, any provision of applicable law or regulation or any judgment, injunction, order or decree or any agreement or other instrument binding upon the Company or any Guarantor, and (ii) the execution, delivery and performance by any Non-Delaware Guarantor of any security whose terms are established subsequent to the date hereof (x) are within its corporate powers, and (y) do not contravene, or constitute a default under, the certificate of incorporation or bylaws or other constitutive documents of such Non-Delaware Guarantor. We are members of the Bar of the State of New York and the foregoing opinion is limited to the laws of the State of New York and the General Corporation Law of the State of Delaware.

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We hereby consent to the filing of this opinion as an exhibit to the Registration Statement referred to above and further consent to the reference to our name under the caption “Legal Matters” in the prospectus, which is a part of the Registration Statement. In giving this consent, we do not admit that we are in the category of persons whose consent is required under Section 7 of the Securities Act.

Very truly yours,

/s/ Davis Polk & Wardwell LLP