Document:

<PAGE>   1
                                                                    EXHIBIT 10.8

                                    AGREEMENT

         This Agreement is dated as of May 11, 2001, and is among Hibernia
National Bank, on behalf of itself and each additional lender from time to time
party to the Hibernia Credit Agreement as hereinafter defined (hereinafter
"Hibernia"), EnCap Energy Capital Fund III, L.P. on behalf of itself and its
affiliates (hereinafter "EnCap"), PetroQuest Energy, L.L.C., a Louisiana limited
liability company (the "LLC"), and PetroQuest Energy, Inc., a Delaware
corporation ("PEI"; the LLC and PEI are hereinafter collectively called
"Debtors").

         RECITALS:

         1. Hibernia, EnCap, PEI, PetroQuest Energy One, L.L.C., a Louisiana
limited liability company, and PetroQuest Energy, Inc., a Louisiana corporation,
are the parties to that certain Subordination Agreement dated and effective as
of December 21, 2000 (the "Subordination Agreement"), which Subordination
Agreement is recorded as set forth on the Exhibit A attached hereto.

         2. The LLC is the successor by merger and name change to PetroQuest
Energy One, L.L.C., a Louisiana limited liability company, and PetroQuest
Energy, Inc., a Louisiana corporation.

         3. The Hibernia Credit Agreement (as defined in the Subordination
Agreement) has been amended and restated, and the parties, as a result of such
amendment and restatement, desire to clarify and restate certain aspects of the
Subordination Agreement.

         Now Therefore, in consideration of the premises, the parties agree as
follows:

         A. DEFINITIONS. The following definitions contained in the
Subordination Agreement are restated as follows:

            DEBTORS. The term "Debtors" shall mean collectively (i) PetroQuest
            Energy, L.L.C., a Louisiana limited liability company and the
            successor by merger and name change to PetroQuest Energy One,
            L.L.C., a Louisiana limited liability company, and PetroQuest
            Energy, Inc., a Louisiana corporation, and (ii) PetroQuest Energy,
            Inc. a Delaware corporation.

            HIBERNIA CREDIT AGREEMENT. The term "Hibernia Credit Agreement"
            shall mean that certain Credit Agreement dated as of December 21,
            2000 by and among Hibernia, PetroQuest Energy, Inc., a Louisiana
            corporation, PetroQuest Energy, Inc., a Delaware corporation, and
            PetroQuest Energy One, L.L.C., a Louisiana limited liability
            company, as amended by First

                                       1
<PAGE>   2

                  Amendment thereto dated as of December 31, 2000, as amended
                  and restated by that certain Amended and Restated Credit
                  Agreement dated as of May 11, 2001 among Debtors, Hibernia
                  National Bank, as Administrative Agent, and the financial
                  institutions parties thereto as lenders and which may become
                  parties thereto as lenders, and as the same may be further
                  amended, restated, or supplemented in compliance herewith,
                  from time to time and in effect.

                  HIBERNIA INDEBTEDNESS. The term "Hibernia Indebtedness" means
                  any and all indebtedness and other obligations (whether for
                  principal, interest, fees, indemnifications, expenses, or
                  otherwise) owing by any Debtor to Hibernia National Bank, as
                  Administrative Agent and/or to any financial institution that
                  is a party to or which may become a party to the Hibernia
                  Credit Agreement, under the Hibernia Credit Agreement or the
                  Loan Documents.

         B. RATIFICATION AND CONFIRMATION. The Subordination Agreement, as
modified by this Agreement, is ratified and confirmed.

         IN WITNESS WHEREOF, the parties hereto have executed this Agreement on
the date set forth above.

HIBERNIA:                   HIBERNIA NATIONAL BANK

                            By:  /s/ David Reid
                                 --------------------------------------------
                                 David Reid
                                 Senior Vice President

DEBTORS:                    PETROQUEST ENERGY, INC.
                            a Delaware corporation

                            By:  /s/ Michael O. Aldridge
                                 ---------------------------------------------
                                 Name:  Michael O. Aldridge

                                 Title: Senior Vice President, Chief Financial
                                        Officer and Secretary

                                       2
<PAGE>   3

                           PETROQUEST ENERGY, L.L.C.,
                           a Louisiana limited liability company

                           By:     PETROQUEST ENERGY, INC., a
                                   Delaware corporation, its sole member

                                   By:  /s/ Michael O. Aldridge
                                      ------------------------------------------
                                      Name: Michael O. Aldridge

                                      Title: Senior Vice President, Chief
                                             Financial Officer and Secretary

ENCAP:                     ENCAP ENERGY FUND CAPITAL
                           FUND III, L.P.

                           By:     ENCAP INVESTMENTS L.L.C.,
                                   General Partner

                                   By: /s/ D. Martin Phillips
                                      ------------------------------------------
                                      Name: D. Martin Phillips

                                      Title: Managing Director

                                       3
<PAGE>   4

                                    EXHIBIT A

SUBORDINATION AGREEMENT DATED DECEMBER 21, 2000, BY AND BETWEEN HIBERNIA
NATIONAL BANK, ENCAP ENERGY CAPITAL FUND III, L.P., PETROQUEST ENERGY INC., A
LOUISIANA CORPORATION, PETROQUEST ENERGY ONE, L.L.C., AND PETROQUEST ENERGY
INC., A DELAWARE CORPORATION

Acadia Parish: filed in Mortgage Book 596, Page 7, Entry No. 681620, 1/5/01

Cameron Parish: filed in Mortgage Book 256, Entry No. 268265, 1/5/01

Claiborne Parish: filed in Mortgage Book 398, Page 38, Entry No. 399355, 1/8/01

Lafayette Parish: filed in Mortgage Records, File No. 01-000551, 1/05/01

Lafourche Parish: filed in Misc. Book 100, Folio 808, Entry No. 885283, 12/28/00

Minerals Management Service: Filed on December 29, 2000

St. Charles Parish: filed in Mortgage Book 776, Folio 28, Entry No. 250153,
1/8/01

St. Mary Parish: filed in Mortgage Book 860, Page 516, Entry No. 243322, 1/9/01

Terrebonne Parish: filed in Mortgage Book 1332, Entry No. 1086650, 1/8/01

Vermilion Parish: filed in Mortgage Book, Entry No. 20100188, 1/08/01

                                       4<PAGE>   1
                                                                   EXHIBIT 10.17

                                 LOAN AGREEMENT

         This Loan Agreement (this "Agreement") is made as of January 31, 2001,
by and among Cell Robotics International, Inc., a Colorado corporation (the
"Borrower"), Dipl. Ing. Oton Tisch ("Tisch"), Ronald K. Lohrding, Ph.D., Raymond
Radosevich, Ph.D., HaeMedic LLC and Humagen Fertility Diagnostics, Inc. (each, a
"Lender," and collectively, the "Lenders").

                             PRELIMINARY STATEMENTS

         The Borrower wishes to borrow from the Lenders, and the Lenders are
willing to lend to the Borrower, an aggregate U.S.$1,000,000, on the terms and
subject to the conditions provided in this Agreement.

                                    AGREEMENT

         The parties, intending to be legally bound, agree as follows:

1. DEFINITIONS.

         "Agreement" shall have the meaning provided in the introductory
paragraph of this Agreement.

         "Applicable Law" shall mean all laws, rules and regulations applicable
to the Person, conduct, action, covenant or Loan Documents in question,
including, but not limited to, all applicable common law and equitable
principles, all provisions of all applicable state and federal constitutions,
statutes, rules, regulations and orders of governmental body, and all orders,
judgments and decrees of all courts and arbitrators.

         "Bankruptcy Code" shall mean the United States Bankruptcy Code of 1978,
as amended.

         "Borrower" shall have the meaning provided in the introductory
paragraph of this Agreement.

         "Default Rate" shall mean the rate per annum equal to the lesser of (a)
12% and (b) the Maximum Legal Rate.

         "Event of Default" shall mean the occurrence of any event or the
existence of any condition specified in Section 5.1.

         "Indebtedness" shall mean all indebtedness, liabilities and obligations
owing, arising, due or payable from the Borrower to the Lender under the Lender
Note.

         "Lender" or "Lenders" shall have the meaning provided in the
introductory paragraph of this Agreement.

         "Loan" shall mean the loans provided by the Lenders to Borrower under
Section 2.1.

<PAGE>   2

         "Loan Documents" shall mean this Agreement, the Notes and all other
documents and instruments heretofore, now or hereafter evidencing, describing,
guaranteeing or securing the Loan contemplated hereby or delivered in connection
with this Agreement.

         "Maximum Legal Rate" shall have the meaning ascribed to such term in
Section 6.1.

         "Notes" shall have the meaning ascribed to such term in Section 2.2.

         "Person" shall mean any natural person, corporation, unincorporated
organization, trust, joint-stock company, joint venture, association, company,
limited or general partnership, any government or any agency or political
subdivision of any government.

         "Tisch" shall have the meaning provided in the introductory paragraph
of this Agreement.

         "UCC" means the Uniform Commercial Code as in effect from time to time
in the State of New Mexico or other applicable jurisdiction, as amended at the
time in question.

2. LOAN; NOTES.

         2.1. Loan. On or before March 12, 2001, each Lender agrees to loan the
Borrower in immediately available funds the amount set forth opposite such
Lender's name on Exhibit A attached hereto; provided that Tisch agrees to loan
to Borrower in immediately available funds $500,000 of the amount set forth
opposite his name at any time, upon Borrower's request, between March 15, 2001
and May 31, 2001.

         2.2. Notes. Each Loan shall be evidenced by a promissory note
substantially in the form of Exhibit B attached hereto (as the same may be
amended, modified, supplemented or renewed from time to time, the "Notes"),
which shall be executed by the Borrower and delivered to the applicable Lender,
as applicable, contemporaneously with the making and in the principal amount of
such Loan by such Lender, as applicable.

3. REPRESENTATIONS, WARRANTIES AND COVENANTS OF THE LENDERS.

         Each Lender makes the following representations, warranties and
covenants as of the date hereof:

         3.1. Valid Existence; Power and Authority. Where applicable, each
Lender is duly organized, validly existing and in good standing under the laws
of the jurisdiction of its organization and is duly qualified to transact
business in all places where the failure to be so qualified would have a
material adverse effect on it. The Lender has the power to make and perform the
Loan Documents executed by him or it, and all such Loan Documents will
constitute the legal, valid and binding obligation of the Lender, enforceable in
accordance with their respective terms, subject only to bankruptcy, insolvency,
reorganization, moratorium or similar laws affecting creditors' rights
generally, and by general principles of equity.

                                       2
<PAGE>   3

         3.2. Authority; Non-Contravention. The execution, delivery and
performance of the Loan Documents by the Lender (a) have been duly authorized by
all necessary action of such Lender, as applicable, and (b) do not violate (i)
any provision of Applicable Law, (ii) any provision of the organizational or
governing documents of such Lender, as applicable, or (iii) any provision of any
agreement to which such Lender is bound, except where the violation thereof
could not reasonably be expected to have a material adverse effect on such
Lender.

         3.3. Further Assurances. Until payment in full of the Loan, as
applicable, each Lender shall take such further actions and provide to the
Borrower such further assurances as may be reasonably requested by the Borrower
to ensure compliance with the intent of the Loan Documents.

         3.4 State of Residence. Each Lender is a resident of the state set
forth in their respective addresses set forth beneath their signature on the
signature page hereof.

4. REPRESENTATIONS, WARRANTIES AND COVENANTS OF THE BORROWER.

         The Borrower makes the following representations, warranties and
covenants as of the date hereof:

         4.1. Valid Existence; Power and Authority. The Borrower is a
corporation duly organized, validly existing and in good standing under the laws
of the jurisdiction of its incorporation and is duly qualified to transact
business in all places where the failure to be so qualified would have a
material adverse effect on it. The Borrower has the power to make and perform
the Loan Documents executed by it, and all such Loan Documents will constitute
the legal, valid and binding obligations of the Borrower, enforceable in
accordance with their respective terms, subject only to bankruptcy, insolvency,
reorganization, moratorium or similar laws affecting creditors' rights
generally, and by general principles of equity.

         4.2. Authority; Non-Contravention. The execution, delivery and
performance of the Loan Documents by the Borrower have been duly authorized by
all necessary corporate action of the Borrower, and do not violate any provision
of Applicable Law or any provision of the articles of incorporation or bylaws of
the Borrower or any governing instrument or agreement to which the Borrower is
bound, except where the violation thereof could not reasonably be expected to
have a material adverse effect on the Borrower.

         4.3. Further Assurances. Until payment in full of the Loan, the
Borrower shall take such further actions and provide to the Lenders such further
assurances as may be reasonably requested by the Lenders to ensure compliance
with the intent of the Loan Documents.

         4.4. Use of Proceeds. The proceeds of the Loan shall be used by the
Borrower solely for payment of existing indebtedness, capital expenditures,
working capital and other general corporate needs.

                                       3
<PAGE>   4

5. DEFAULTS AND REMEDIES.

         5.1. Events of Default. The following shall constitute an Event of
Default with respect to the Indebtedness:

                  (a) Any representation or warranty made by the Borrower under
or in connection with this Agreement which shall prove to have been incorrect in
any material respect when made, or any failure by the Borrower to perform or
observe any term, covenant or agreement contained in this Agreement on its part
to be performed or observed, and if capable of being remedied such failure shall
remain unremedied for 30 days after written notice thereof shall have been given
to the Borrower by the Lender;

                  (b) The Borrower shall fail to pay any of the Indebtedness on
the due date thereon (whether due at stated maturity, on demand, upon
acceleration or otherwise) and such failure shall not be remedied within five
days after the occurrence thereof; or

                  (c) The Borrower shall (i) become insolvent within the meaning
of the Bankruptcy Code, (ii) admit in writing its inability to pay or otherwise
fail to pay its debts generally as they become due, (iii) voluntarily seek,
consent to or acquiesce in the benefit or benefits of any Debtor Relief Law
(meaning the Bankruptcy Code and all other applicable liquidation,
conservatorship, bankruptcy, moratorium, rearrangement, receivership,
insolvency, reorganization or similar debtor relief laws from time to time in
effect affecting the rights of creditors generally) or (iv) be made the subject
of an involuntary case for bankruptcy (or a petition for a receiving order)
which is not dismissed within 60 days after commencement of the case.

         5.2. Acceleration of the Indebtedness. Upon or at any time after the
occurrence of an Event of Default, the principal amount of the Indebtedness then
outstanding together with all interest accrued thereon shall, at the option of
the Lender and without notice or demand by the Lender, become at once due and
payable and the Borrower shall forthwith pay to the Lender, in addition to any
and all sums and charges due, the entire principal of and accrued and unpaid
interest on the Indebtedness plus reasonable attorneys' fees and expenses, if
the same is collected by or through an attorney at law. From and after the
occurrence and during the continuance of an Event of Default, the Indebtedness
shall bear interest at the Default Rate.

         5.3. Remedies. Upon or at any time after the occurrence of any Event of
Default, the Lender shall have and may exercise, at its election, from time to
time any and all rights and remedies available to a secured party under the UCC,
in addition to any and all other rights and remedies afforded by this Agreement,
at law, in equity or otherwise, including, without limitation, (a) declaring the
entire unpaid balance of the Indebtedness, or any part thereof, immediately due
and payable, whereupon it shall be due and payable; (b) reducing any claim to
judgment; and (c) bringing suit or other proceedings before any appropriate
court either for specific performance of any covenant or condition contained in
this Agreement or in aid of the exercise of any right granted to the Lender in
this Agreement.

                                       4
<PAGE>   5

6. MISCELLANEOUS.

         6.1. Maximum Rate of Interest. Notwithstanding anything to the contrary
contained herein, no provisions of the Loan Documents shall require the payment
or permit the collection of interest in excess of the maximum rate permitted by
law (the "Maximum Legal Rate"). If any excess of interest in such respect is
herein provided for, or shall be adjudicated to be so provided, in the Loan
Documents or otherwise in connection with this loan transaction, the provisions
of this paragraph shall govern and prevail, and neither the Borrower nor the
sureties, guarantors, successors or assigns of the Borrower shall be obligated
to pay the excess amount of such interest, or any other excess sum paid for the
use, forbearance or detention of sums evidenced hereby. If for any reason
interest in excess of the Maximum Legal Rate shall be deemed charged, required
or permitted by any court of competent jurisdiction, any such excess shall be
applied as a payment and reduction of the principal amount hereof until it has
been paid in full, and any remaining excess shall forthwith be paid to the
Borrower. In determining whether or not the interest paid or payable exceeds the
Maximum Legal Rate, the Borrower and the Lenders shall, to the extent permitted
by Applicable Law, (a) characterize any non-principal payment as an expense, fee
or premium rather than as interest, (b) exclude voluntary prepayments and the
effects thereof and (c) amortize, prorate, allocate and spread in equal or
unequal parts the total amount of interest throughout the entire contemplated
term of the Notes so that the interest for the entire term does not exceed the
Maximum Legal Rate.

         6.2. No Third Party Beneficiaries. This Agreement shall not confer any
rights or remedies upon any Person other than the parties and their respective
successors and permitted assigns.

         6.3. Entire Agreement. This Agreement (including the documents referred
to in this Agreement) and the other Loan Documents constitute the entire
agreement among the parties and supersede any prior understandings, agreements
or representations by or among the parties, written or oral, to the extent they
relate in any way to the subject matter hereof.

         6.4. Succession and Assignment. This Agreement shall be binding upon
and inure to the benefit of the parties named in this Agreement and their
respective successors and permitted assigns. No party may assign either this
Agreement or any of its rights, interests or obligations hereunder without the
prior written approval of the other parties.

         6.5. Counterparts. This Agreement may be executed in one or more
counterparts, each of which shall be deemed an original but all of which
together shall constitute one and the same instrument.

         6.6. Headings. The article and section headings contained in this
Agreement are inserted for convenience only and shall not affect in any way the
meaning or interpretation of this Agreement.

         6.7. Notices. All notices, requests, demands, claims and other
communications under this Agreement and the other Loan Documents shall be in
writing. Any notice, request, demand, claim or other communication hereunder
shall be deemed duly given if (and then two business

                                       5
<PAGE>   6

days after) it is sent by (a) confirmed facsimile; (b) overnight delivery; or
(c) registered or certified mail, return receipt requested, postage prepaid, and
addressed to the intended recipient as set forth below:

        If to the Borrower:            Cell Robotics International, Inc.
                                       2715 Broadbent Parkway N.E.
                                       Albuquerque, New Mexico 87107
                                       Attn: Ronald K. Lohrding, Ph.D.
                                       Telephone:  (505) 343-1131
                                       Facsimile:  (505) 344-8112

        With a copy to, which copy     Baker & McKenzie
        shall not constitute notice:   2001 Ross Avenue, Suite 2300
                                       Dallas, Texas 75201
                                       Attn:  W. Crews Lott
                                       Telephone:  (214) 978-3000
                                       Facsimile:  (214) 978-3099

        If to the Lenders:             To their respective addresses set forth
                                       beneath their signature on the signature
                                       page hereof

Any party may send any notice, request, demand, claim or other communication
hereunder to the intended recipient at the address set forth above using any
other means (including personal delivery, expedited courier, messenger service,
telecopy, telex, ordinary mail or electronic mail), but no such notice, request,
demand, claim or other communication shall be deemed to have been duly given
unless and until it actually is received by the intended recipient. Any party
may change the address to which notices, requests, demands, claims and other
communications hereunder are to be delivered by giving the other parties notice
in the manner provided in this Agreement.

         6.8. GOVERNING LAW. THIS AGREEMENT SHALL BE GOVERNED BY AND CONSTRUED
IN ACCORDANCE WITH THE DOMESTIC LAWS OF THE STATE OF NEW MEXICO WITHOUT GIVING
EFFECT TO ANY CHOICE OR CONFLICT OF LAW PROVISION OR RULE (WHETHER OF THE STATE
OF NEW MEXICO OR ANY OTHER JURISDICTION) THAT WOULD CAUSE THE APPLICATION OF THE
LAWS OF ANY JURISDICTION OTHER THAN THE STATE OF NEW MEXICO. Any legal action or
proceeding with respect to any Loan Document shall be brought in any New Mexico
state or federal court sitting in Bernalillo County, New Mexico, and, by
execution and delivery of this Agreement, each party hereby accepts for itself
and in respect of its property, generally and unconditionally, the jurisdiction
of the aforesaid courts. Each party hereby irrevocably waives any objection,
including, without limitation, any objection to the laying of venue or based on
the grounds of forum non conveniens, which it may now or hereafter have to the
bringing of any such action or proceeding in such respective jurisdictions.

         6.9. Amendments; Waivers; Remedies Cumulative. No amendment of any
provision of this Agreement shall be valid unless the same shall be in writing
and signed by the Borrower, and the Lenders. No failure on the part of the
Lenders to exercise, and no delay by the Lenders in exercising, any right or
remedy hereunder or under any other Loan Document shall operate as

                                       6
<PAGE>   7

a waiver thereof, nor shall any single or partial exercise of any right or
remedy hereunder preclude any other or further exercise thereof or the exercise
of any other right or remedy. The rights and remedies herein provided are
cumulative and are in addition to any other rights and remedies provided by
Applicable Law, any Loan Document or otherwise.

         6.10. Severability. Any term or provision of this Agreement that is
invalid or unenforceable in any situation in any jurisdiction shall not affect
the validity or enforceability of the remaining terms and provisions hereof or
the validity or enforceability of the offending term or provision in any other
situation or in any other jurisdiction.

         6.11. Construction. The parties have participated jointly in the
negotiation and drafting of this Agreement. In the event an ambiguity or
question of intent or interpretation arises, this Agreement shall be construed
as if drafted jointly by the parties and no presumption or burden of proof shall
arise favoring or disfavoring any party by virtue of the authorship of any of
the provisions of this Agreement. Any reference to any federal, state, local or
foreign statute or law shall be deemed also to refer to all rules and
regulations promulgated thereunder, unless the context requires otherwise. The
word "including" shall mean "including, without limitation." The terms "herein,"
"hereof" and "hereunder" and other words of similar import refer to this
Agreement as a whole and not to any particular section, paragraph or
subdivision. Any pronoun used shall be deemed to cover all genders. Terms
defined in the singular have the same meaning when used in the plural and vice
versa. All references to any instruments or agreements, including, without
limitation, references to this Agreement and any of the other Loan Documents,
shall include any and all modifications or amendments thereto and any and all
extensions or renewals thereof.

                                       7
<PAGE>   8

         The parties have executed and delivered this Agreement as of the date
of the first sentence of this Agreement.

THE BORROWER:

CELL ROBOTICS INTERNATIONAL, INC.

By:
    ------------------------------------
    Ronald K. Lohrding, Ph.D.
    President and CEO

                                       8
<PAGE>   9

THE LENDERS:

------------------------------------------
Dipl. Ing. Oton Tisch

Address:
Apartado 76949
Caracas 1070 Venezuela

                                       9
<PAGE>   10

------------------------------------------
Ronald K. Lohrding, Ph.D.

Address:
c/o Cell Robotics International, Inc.
2715 Broadbent Parkway N.E.
Albuquerque, New Mexico  87107

                                       10
<PAGE>   11

------------------------------------------
Raymond Radosevich, Ph.D.

Address:
602 Cedar Hill Road, N.E.
Albuquerque, New Mexico  87122

                                       11
<PAGE>   12

HAEMEDIC LLC

-----------------------------------------
Steven A. Crees, President

Address:
3405 Shadyview Lane N.
Plymouth, Minnesota  55447

                                       12
<PAGE>   13

HUMAGEN FERTILITY DIAGNOSTICS, INC.

------------------------------------------
Debra Bryant, Ph.D., President

Address:
2400 Hunter's Way
Charlottesville, Virginia  22911

                                       13
<PAGE>   14

                                    EXHIBIT A

                                 Amount of Loan

<TABLE>
<CAPTION>
Lender                                      Loan Amount
------                                      ------------
<S>                                         <C>
Oton Tisch                                  U.S.$900,000

Ronald K. Lohrding, Ph.D.                   U.S.$ 25,000

Raymond Radosevich, Ph.D.                   U.S.$ 20,000

HaeMedic LLC                                U.S.$ 10,000

Humagen Fertility Diagnostics, Inc.         U.S.$ 45,000
</TABLE>

<PAGE>   15

                                    EXHIBIT B

                                  Form of Note

                             [Follows on next page]

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