<PAGE>   1
                                                                  EXHIBIT 10.1.3

                 THIRD AMENDMENT TO LOAN AND SECURITY AGREEMENT

     THIS THIRD AMENDMENT TO LOAN AND SECURITY AGREEMENT (this "Amendment") is
made and entered into as of this 18th day of October, 2000, by and among THE CIT
GROUP/BUSINESS CREDIT, INC., a New York corporation (hereinafter "CITBC"), in
its individual capacity and as Agent for the Lenders hereinafter named
(hereinafter the "AGENT"), Foothill Capital Corporation, a California
corporation ("FCC"), GMAC Business Credit, LLC, a Michigan limited liability
company ("GMAC"), The CIT Group/Equipment Financing, Inc., a Delaware
corporation ("CITEF"), and any other party hereafter becoming a Lender pursuant
to Section 13, Paragraph 9 of the Agreement (as hereinafter defined), each
individually sometimes referred to as a "LENDER" and, collectively, the
"LENDERS"), and UTI Drilling, L.P., a Texas limited partnership ("UTI"), Norton
Drilling, L.P., a Delaware limited partnership, as successor in interest (by
conversion) to Norton Drilling Company, a Delaware corporation ("NDLP"),
Universal Well Services, Inc., a Delaware corporation ("UWSI"), UTI Management
Services, L.P., a Texas limited partnership ("UTIMS"), and Suits Drilling
Company, an Oklahoma corporation ("SDC"), (UTI, NDLP, UWSI, UTIMS and SDC,
together with any additional entities which may become a Company under the
Agreement from time to time, being referred to herein individually as a
"COMPANY" and, collectively, as the "COMPANIES").

                                    RECITALS

     A. WHEREAS, pursuant to the terms and subject to the conditions of that
certain Loan and Security Agreement dated as of November 22, 1999 between the
parties hereto (such Loan and Security Agreement, as the same is hereby amended
and may hereafter be amended from time to time, being hereinafter referred to as
the "Agreement"), the Companies were granted a $75,000,000 revolving line of
credit which included a letter of credit facility;

     B. WHEREAS, the indebtedness of the Companies to the Lenders is currently
evidenced by that certain Revolving Loan Promissory Note dated May 18, 2000 (the
"Existing Revolving Note"), executed by the Companies and payable to CITBC as
Agent for the benefit of the Lenders;

     C. WHEREAS, payment of the Obligations of the Companies was supported by
the guaranties of UTI Energy Corp., a Delaware corporation (the "PARENT"),
UTICO, Inc., a Delaware corporation ("HOLDING"), UTICO Hard Rock Boring, Inc., a
Delaware corporation ("UHRB"), International Petroleum Services Company, a
Pennsylvania corporation ("IPSCO"), Norton Drilling Services, Inc., a Delaware
corporation ("NDS"), Norton Drilling Company Mexico, Inc., a Delaware
corporation ("NDM") and UTI Drilling Canada, Inc., a Delaware corporation ("UTI
CANADA") (Parent, Holding, UHRB, IPSCO, NDS, NDM and UTI Canada are referred to
herein, individually, as a "GUARANTOR" and, collectively, as the "GUARANTORS");

     D. WHEREAS, to secure, in part, the indebtedness under the Agreement and
the Existing Revolving Note (and all renewals, extensions, modifications and/or
rearrangements thereof and in connection therewith) and all other indebtedness,
liabilities and obligations of the Companies to the Agent for the benefit of the
Lenders, then existing or thereafter arising, (i) the Companies have heretofore
executed in favor of the Agent certain Loan Documents (as defined in the
Agreement),

THIRD AMENDMENT TO LOAN & SECURITY AGREEMENT - Page 1
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including, without limitation, the Guaranty, (as defined in the Agreement),
which Loan Documents shall continue as amended in connection herewith in full
force and effect upon the execution of this Amendment, all of the Loan Documents
to continue to secure the payment by the Companies of the Obligations (as
defined in the Agreement) all as more fully set forth therein and herein;

     E. WHEREAS, the Companies have requested and, pursuant to the terms and
subject to the conditions hereof and in connection herewith, the Agent and the
Lenders have agreed to increase the amount of the Line of Credit (as defined in
the Agreement) to $90,000,000, and accept the Revolving Note (as herein defined)
in replacement and substitution (but not extinguishment) of the Existing
Revolving Note;

     F. WHEREAS, in furtherance of the foregoing and to evidence the agreements
of the parties hereto in relation thereto the parties hereto desire to amend the
Agreement as hereinafter provided;

     NOW, THEREFORE, in consideration of the premises herein contained and other
good and valuable consideration, the receipt and sufficiency of which are hereby
acknowledged, the Companies, the Agent and the Lenders, intending to be legally
bound, agree as follows:

                                    AGREEMENT

                                    ARTICLE I
                                   DEFINITIONS

     1.01 Capitalized terms used in this Amendment are defined in the Agreement,
as amended hereby, unless otherwise stated herein.

                                   ARTICLE II
                             AMENDMENTS TO AGREEMENT

     Effective as of the respective date herein indicated, the Agreement is
hereby amended as follows:

     2.01 AMENDMENT AND RESTATEMENT OF DEFINITION OF "ACQUISITION FACILITY
COMMITMENT". Effective as of the date of execution of this Amendment, the
definition of "Acquisition Facility Commitment" set forth in Section 1 of the
Agreement is amended and restated to read in its entirety as follows:

     "`ACQUISITION FACILITY COMMITMENT' shall mean, with respect to any Lender,
     a portion of the Revolving Loans which may be advanced as Acquisition
     Facility Loans, evidencing the amount of its commitment to make Acquisition
     Facility Loans (all such loans being Revolving Loans), as modified from
     time to time pursuant to the terms hereof, not to exceed $70,000,000 in the
     aggregate."

     2.02 AMENDMENT AND RESTATEMENT OF DEFINITION OF "ADMINISTRATIVE MANAGEMENT
FEE". Effective as of the date of execution of this Amendment, the definition of
"Administrative Management Fee" set forth in Section 1 of the Agreement is
amended and restated to read in its entirety as follows:

THIRD AMENDMENT TO LOAN & SECURITY AGREEMENT - Page 2
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     "`ADMINISTRATIVE MANAGEMENT FEE' shall mean the sum of $100,000 per annum
     which shall be paid to the Agent for its own account in accordance with
     Section 8, Paragraph 8 hereof to offset the expenses and costs (excluding
     Out-of-Pocket Expenses) of the Agent in connection with record keeping,
     periodic examinations, analyzing and evaluating the Collateral."

     2.03 AMENDMENT AND RESTATEMENT OF DEFINITION OF "LINE OF CREDIT". Effective
as of the date of execution of this Amendment, the definition of "Line of
Credit" set forth in Section 1 of the Agreement is amended and restated to read
in its entirety as follows:

     "`LINE OF CREDIT' shall mean the commitment of the Lenders in the
     aggregate amount of $90,000,000 to (a) make Revolving Loans pursuant to
     Sections 3 and 4 of this Agreement, and (b) assist the Companies in opening
     Letters of Credit pursuant to Section 5 of this Agreement (up to the Letter
     of Credit Sub-Line)."

     2.04 AMENDMENT AND RESTATEMENT OF SECTION 4, PARAGRAPH 1 OF THE AGREEMENT.
Effective as of the date of execution of this Amendment, Section 4, Subparagraph
1(a)(i) of the Agreement is amended and restated to read in its entirety as
follows:

     "(a) (i) Following the making of such Revolving Loan and consummation of
     the Permitted Acquisition, there is at least $15 million of Availability
     (determined without regard to the Excluded L/Cs) and no more than an
     aggregate of $70 million of Revolving Loans outstanding, and (ii) until the
     Obligors have invested in the aggregate $25 million in cash (whether cash
     on hand or cash provided from Revolving Loans to consummate Permitted
     Acquisitions pursuant to this Section 4) in Permitted Acquisitions that are
     Domestic Acquisitions, the aggregate amount of Revolving Loans outstanding
     for the purpose of consummating Canadian Acquisitions cannot exceed the sum
     of (A) $20 million plus (B) the amount of cash invested in Domestic
     Acquisitions (whether cash on hand or cash provided from Revolving Loans);
     or"

     2.05 AMENDMENT AND RESTATEMENT OF SECTION 7, PARAGRAPH 11 OF THE AGREEMENT.
Effective as of the date of execution of this Amendment, Section 7, Paragraph 11
of the Agreement is amended and restated to read in its entirety as follows:

     "11. Until termination of this Agreement and payment and satisfaction in
     full of all Obligations hereunder, if any Event of Default shall occur and
     be continuing which has not been waived in writing by the Agent or if the
     Availability (determined as of the Excluded L/Cs were not outstanding)
     shall at any time be less than $20,000,000, then the Obligors agree that
     the Parent will, on a consolidated basis:

          (a)  maintain as of the last day of each calendar month a Tangible Net
               Worth of not less than $120,000,000; and

          (b)  maintain as of the last day of each month TTM EBITDA of not less
               than $15,000,000."

THIRD AMENDMENT TO LOAN & SECURITY AGREEMENT - Page 3
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     2.06 AMENDMENT OF SECTION 8, PARAGRAPH 7 OF THE AGREEMENT. Effective as of
the date of execution of this Amendment, Section 8, Paragraph 7 of the Agreement
is amended to add a new Subparagraph 7(c) as follows:

     "(c) Upon the execution of an amendment to this Agreement increasing the
     Line of Credit to $90,000,000, the Companies shall pay to the Agent for the
     pro rata benefit of the Lenders an additional one time Loan Facility Fee in
     the amount One Hundred Fifty Thousand U.S. Dollars ($150,000.00.)"

     2.07 REVOLVING LOAN COMMITMENT. Effective as of the date of execution of
this Amendment, the Revolving Loan Commitment for each Lender will be the amount
set forth under each Lender's name of the signature page hereof.

     2.08 AMENDMENT AND RESTATEMENT OF EXHIBIT A TO THE AGREEMENT. Effective as
of the date of execution of this Amendment, Exhibit A to the Agreement is
amended and restated in its entirety as set forth on Exhibit A attached hereto.

                                   ARTICLE III
                              CONDITIONS PRECEDENT

     3.01 CONDITIONS TO EFFECTIVENESS. The effectiveness of this Amendment is
subject to the satisfaction of the following conditions precedent in a manner
satisfactory to CITBC, unless specifically waived in writing by CITBC:

         (a) CITBC shall have received each of the following, each in form and
substance satisfactory to CITBC, in its sole discretion, and, where applicable,
each duly executed by each party thereto, other than CITBC:

               (i) This Amendment, duly executed by the Companies and the
     Consent, Ratification and Release is executed by the Guarantors;

               (ii) A Revolving Loan Promissory Note in the stated principal
     amount of $90,000,000 in amendment, substitution and replacement of the
     Existing Revolving Note duly signed by the Companies; and

               (iii) certified copies of the resolutions of the Board of
     Directors of each of the Companies and the Guarantors authorizing the
     execution, delivery and performance of the Revolving Loan Promissory Note,
     this Amendment and any and all other Loan Documents executed by any of the
     Companies or the Guarantors in connection therewith, along with a
     certificate of incumbency certified by the secretary of each of the
     Companies and the Guarantors with specimen signatures of the officers of
     the Companies and the Guarantors who are authorized to sign such documents,
     all in form and substance satisfactory to the Agent; and

               (iv) All other documents CITBC may request with respect to any
     matter relevant to this Amendment or the transactions contemplated hereby.

THIRD AMENDMENT TO LOAN & SECURITY AGREEMENT - Page 4
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         (b) The representations and warranties contained herein and in the
Agreement and the other documents executed in connection with the Agreement
(herein referred to as "Loan Documents"), as each is amended hereby, shall be
true and correct as of the date hereof, as if made on the date hereof.

         (c) No Default or Event of Default shall have occurred and be
continuing, unless such Default or Event of Default has been otherwise
specifically waived in writing by CITBC.

         (d) All corporate proceedings taken in connection with the transactions
contemplated by this Amendment and all documents, instruments and other legal
matters incident thereto shall be satisfactory to CITBC.

         (e) CITBC's receipt of the fee described in Section 2.06 of this
Amendment.

                                   ARTICLE IV
                  RATIFICATIONS, REPRESENTATIONS AND WARRANTIES

     4.01 RATIFICATIONS. The terms and provisions set forth in this Amendment
shall modify and supersede all inconsistent terms and provisions set forth in
the Agreement and the other Loan Documents, and, except as expressly modified
and superseded by this Amendment, the terms and provisions of the Agreement and
the other Loan Documents are ratified and confirmed and shall continue in full
force and effect. The Companies and CITBC agree that the Agreement and the other
Loan Documents, as amended hereby, shall continue to be legal, valid, binding
and enforceable in accordance with their respective terms.

     4.02 REPRESENTATIONS AND WARRANTIES. The Companies hereby represent and
warrant to CITBC that (a) the execution, delivery and performance of this
Amendment and any and all other Loan Documents executed and/or delivered in
connection herewith have been authorized by all requisite corporate or limited
partnership action (as applicable) on the part of the Companies and will not
violate the Articles (or Certificates) of Incorporation or Bylaws of the
Companies that are corporations or the limited partnership agreements or
certificates of limited partnership of the Companies that are limited
partnerships; (b) each of the Company's Board of Directors (or the general
partner of the applicable limited partnership) has authorized the execution,
delivery and performance of this Amendment and any and all other Loan Documents
executed and/or delivered in connection herewith; (c) the representations and
warranties contained in the Agreement, as amended hereby, and any other Loan
Document are true and correct on and as of the date hereof and on and as of the
date of execution hereof as though made on and as of each such date; (d) no
Default or Event of Default under the Agreement, as amended hereby, has occurred
and is continuing, unless such Default or Event of Default has been specifically
waived in writing by CITBC; (e) the Companies are in full compliance with all
covenants and agreements contained in the Agreement and the other Loan
Documents, as amended hereby; and (f) the Companies have not amended their
Articles (or Certificates) of Incorporation or their Bylaws since the date of
the Agreement, except as otherwise disclosed to Agent.

THIRD AMENDMENT TO LOAN & SECURITY AGREEMENT - Page 5
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                                    ARTICLE V
                            MISCELLANEOUS PROVISIONS

     5.01 SURVIVAL OF REPRESENTATIONS AND WARRANTIES. All representations and
warranties made in the Agreement or any other Loan Document, including, without
limitation, any document furnished in connection with this Amendment, shall
survive the execution and delivery of this Amendment and the other Loan
Documents, and no investigation by CITBC or any closing shall affect the
representations and warranties or the right of CITBC to rely upon them.

     5.02 REFERENCE TO AGREEMENT. Each of the Agreement and the other Loan
Documents, and any and all other Loan Documents, documents or instruments now or
hereafter executed and delivered pursuant to the terms hereof or pursuant to the
terms of the Agreement, as amended hereby, are hereby amended so that any
reference in the Agreement and such other Loan Documents to the Agreement shall
mean a reference to the Agreement, as amended hereby.

     5.03 EXPENSES OF CITBC. As provided in the Agreement, the Companies agree
to pay on demand all reasonable costs and expenses incurred by CITBC in
connection with the preparation, negotiation, and execution of this Amendment
and the other Loan Documents executed pursuant hereto and any and all
amendments, modifications, and supplements thereto, including, without
limitation, the reasonable costs and fees of CITBC's legal counsel, and all
reasonable costs and expenses incurred by CITBC in connection with the
enforcement or preservation of any rights under the Agreement, as amended
hereby, or any other Loan Documents, including, without limitation, the
reasonable costs and fees of CITBC's legal counsel.

     5.04 SEVERABILITY. Any provision of this Amendment held by a court of
competent jurisdiction to be invalid or unenforceable shall not impair or
invalidate the remainder of this Amendment and the effect thereof shall be
confined to the provision so held to be invalid or unenforceable.

     5.05 SUCCESSORS AND ASSIGNS. This Amendment is binding upon and shall inure
to the benefit of CITBC and the Companies and their respective successors and
assigns, except that the Companies may not assign or transfer any of their
rights or obligations hereunder without the prior written consent of CITBC.

     5.06 COUNTERPARTS. This Amendment may be executed in one or more
counterparts, each of which when so executed shall be deemed to be an original,
but all of which when taken together shall constitute one and the same
instrument.

     5.07 EFFECT OF WAIVER. No consent or waiver, express or implied, by CITBC
to or for any breach of or deviation from any covenant or condition by the
Companies shall be deemed a consent to or waiver of any other breach of the same
or any other covenant, condition or duty.

     5.08 HEADINGS. The headings, captions, and arrangements used in this
Amendment are for convenience only and shall not affect the interpretation of
this Amendment.

     5.09 APPLICABLE LAW. THIS AMENDMENT AND ALL OTHER LOAN DOCUMENTS EXECUTED
PURSUANT HERETO SHALL BE DEEMED TO HAVE BEEN MADE AND TO BE

THIRD AMENDMENT TO LOAN & SECURITY AGREEMENT - Page 6
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PERFORMABLE IN AND SHALL BE GOVERNED BY AND CONSTRUED IN ACCORDANCE WITH THE
LAWS OF THE STATE OF NEW YORK.

     5.10 FINAL AGREEMENT. THE AGREEMENT AND THE OTHER LOAN DOCUMENTS, EACH AS
AMENDED HEREBY, REPRESENT THE ENTIRE EXPRESSION OF THE PARTIES WITH RESPECT TO
THE SUBJECT MATTER HEREOF ON THE DATE THIS AMENDMENT IS EXECUTED. THE AGREEMENT
AND THE OTHER LOAN DOCUMENTS, AS AMENDED HEREBY, MAY NOT BE CONTRADICTED BY
EVIDENCE OF PRIOR, CONTEMPORANEOUS OR SUBSEQUENT ORAL AGREEMENTS OF THE PARTIES.
THERE ARE NO UNWRITTEN ORAL AGREEMENTS BETWEEN THE PARTIES. NO MODIFICATION,
RESCISSION, WAIVER, RELEASE OR AMENDMENT OF ANY PROVISION OF THIS AMENDMENT
SHALL BE MADE, EXCEPT BY A WRITTEN AGREEMENT SIGNED BY COMPANIES AND CITBC.

     5.11 RELEASE. THE COMPANIES HEREBY ACKNOWLEDGE THAT THEY HAVE NO DEFENSE,
COUNTERCLAIM, OFFSET, CROSS-COMPLAINT, CLAIM OR DEMAND OF ANY KIND OR NATURE
WHATSOEVER THAT CAN BE ASSERTED TO REDUCE OR ELIMINATE ALL OR ANY PART OF ITS
LIABILITY TO REPAY THE "OBLIGATIONS" OR TO SEEK AFFIRMATIVE RELIEF OR DAMAGES OF
ANY KIND OR NATURE FROM CITBC. THE COMPANIES HEREBY VOLUNTARILY AND KNOWINGLY
RELEASE AND FOREVER DISCHARGE CITBC, ITS PREDECESSORS, AGENTS, EMPLOYEES,
SUCCESSORS AND ASSIGNS, FROM ALL POSSIBLE CLAIMS, DEMANDS, ACTIONS, CAUSES OF
ACTION, DAMAGES, COSTS, EXPENSES, AND LIABILITIES WHATSOEVER, KNOWN OR UNKNOWN,
ANTICIPATED OR UNANTICIPATED, SUSPECTED OR UNSUSPECTED, FIXED, CONTINGENT, OR
CONDITIONAL, AT LAW OR IN EQUITY, ORIGINATING IN WHOLE OR IN PART ON OR BEFORE
THE DATE THIS AMENDMENT IS EXECUTED, WHICH THE COMPANIES MAY NOW OR HEREAFTER
HAVE AGAINST CITBC, ITS PREDECESSORS, AGENTS, EMPLOYEES, SUCCESSORS AND ASSIGNS,
IF ANY, AND IRRESPECTIVE OF WHETHER ANY SUCH CLAIMS ARISE OUT OF CONTRACT, TORT,
VIOLATION OF LAW OR REGULATIONS, OR OTHERWISE, AND ARISING FROM ANY "LOANS",
INCLUDING, WITHOUT LIMITATION, ANY CONTRACTING FOR, CHARGING, TAKING, RESERVING,
COLLECTING OR RECEIVING INTEREST IN EXCESS OF THE HIGHEST LAWFUL RATE
APPLICABLE, THE EXERCISE OF ANY RIGHTS AND REMEDIES UNDER THE AGREEMENT OR OTHER
LOAN DOCUMENTS, AND NEGOTIATION FOR AND EXECUTION OF THIS AMENDMENT.

              [The Remainder of this Page Intentionally Left Blank]

THIRD AMENDMENT TO LOAN & SECURITY AGREEMENT - Page 7
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     IN WITNESS WHEREOF, this Amendment has been executed and is effective as of
the date first above-written.

                        COMPANIES:

                        UTI DRILLING, L.P.
                        UTI MANAGEMENT SERVICES, L.P.

                        By: Utico Hard Rock  Boring, Inc., the sole general
                        partner of UTI Drilling, L.P. and UTI Management
                        Services, L.P.

                                By:
                                    -------------------------------------------
                                Name: John E. Vollmer, III
                                Title: Vice President

                        NORTON DRILLING, L.P.

                        By: Norton GP, L.L.C., its sole general partner
                            By: Norton Drilling Services, Inc., as Sole Member
                                of Norton GP, L.L.C.

                                By:
                                    ------------------------------------------
                                Name:
                                      ----------------------------------------
                                Title:
                                       ---------------------------------------


                        UNIVERSAL WELL SERVICES, INC.
                        SUITS DRILLING COMPANY

                                By:
                                    ------------------------------------------
                                Name: John E. Vollmer, III
                                Title: Vice President of each of the
                                       foregoing Companies


                        LENDERS:

                        THE CIT GROUP/BUSINESS CREDIT,INC.
                        as Agent and Lender

                        By:
                            ---------------------------------------------------
                        Name:
                              -------------------------------------------------
                        Title:
                               ------------------------------------------------

                        Revolving Loan Commitment:  $30,000,000.00

THIRD AMENDMENT TO LOAN & SECURITY AGREEMENT
<PAGE>   9

                         GMAC BUSINESS CREDIT, LLC
                         as Syndication Agent and Lender

                         By:
                             ---------------------------------------------------
                         Name:
                               -------------------------------------------------
                         Title:
                                ------------------------------------------------

                         Revolving Loan Commitment:  $25,000,000.00


                         FOOTHILL CAPITAL CORPORATION
                         as Documentation Agent and Lender

                         By:
                             ---------------------------------------------------
                         Name:
                               -------------------------------------------------
                         Title:
                                ------------------------------------------------

                         Revolving Loan Commitment:  $25,000,000.00


                         THE CIT GROUP/EQUIPMENT FINANCING, INC.

                         By:
                             ---------------------------------------------------
                         Name:
                               -------------------------------------------------
                         Title:
                                ------------------------------------------------

                         Revolving Loan Commitment:  $10,000,000.00

THIRD AMENDMENT TO LOAN & SECURITY AGREEMENT
<PAGE>   10

                        CONSENT, RATIFICATION AND RELEASE

     The undersigned each hereby consents to the terms of the within and
foregoing Amendment, confirms and ratifies the terms of that certain Guaranty
Agreement dated November 22, 1999 executed (or assumed) by the undersigned for
the benefit of Agent and the other Lenders (the "Guaranty Agreement"), and
acknowledges that the Guaranty Agreement is in full force and effect and
ratifies the same, that the undersigned each has no defense, counterclaim,
set-off or any other claim to diminish the undersigned's liability under such
document, that the undersigned's consent is not required to the effectiveness of
the within and foregoing Amendment, and that no consent by the undersigned is
required for the effectiveness of any future amendment, modification,
forbearance or other action with respect to the Obligations, the Collateral, or
any of the other Loan Agreements. THE UNDERSIGNED EACH HEREBY VOLUNTARILY AND
KNOWINGLY RELEASES AND FOREVER DISCHARGES LENDER, ITS PREDECESSORS, AGENTS,
EMPLOYEES, SUCCESSORS AND ASSIGNS, FROM ALL POSSIBLE CLAIMS, DEMANDS, ACTIONS,
CAUSES OF ACTION, DAMAGES, COSTS, EXPENSES, AND LIABILITIES WHATSOEVER, KNOWN OR
UNKNOWN, ANTICIPATED OR UNANTICIPATED, SUSPECTED OR UNSUSPECTED, FIXED,
CONTINGENT, OR CONDITIONAL, AT LAW OR IN EQUITY, ORIGINATING IN WHOLE OR IN PART
ON OR BEFORE THE DATE THIS AMENDMENT IS EXECUTED, WHICH THE UNDERSIGNED MAY NOW
OR HEREAFTER HAVE AGAINST LENDER, ITS PREDECESSORS, AGENTS, EMPLOYEES,
SUCCESSORS AND ASSIGNS, IF ANY, AND IRRESPECTIVE OF WHETHER ANY SUCH CLAIMS
ARISE OUT OF CONTRACT, TORT, VIOLATION OF LAW OR REGULATIONS, OR OTHERWISE, AND
ARISING FROM ANY "LOANS", INCLUDING, WITHOUT LIMITATION, ANY CONTRACTING FOR,
CHARGING, TAKING, RESERVING, COLLECTING OR RECEIVING INTEREST IN EXCESS OF THE
HIGHEST LAWFUL RATE APPLICABLE, THE EXERCISE OF ANY RIGHTS AND REMEDIES UNDER
THE LOAN AGREEMENT OR OTHER AGREEMENTS, AND NEGOTIATION FOR AND EXECUTION OF
THIS AMENDMENT.

GUARANTORS:                                  UTICO, INC.

UTI DRILLING CANADA, INC.
UTICO HARD ROCK BORING, INC.                 By:
NORTON DRILLING SERVICES, INC.                   -------------------------------
NORTON DRILLING COMPANY                      Kenneth J. Kubacki
         MEXICO, INC.                        Vice President and Treasurer
INTERNATIONAL PETROLEUM
          SERVICES COMPANY                   UTI ENERGY CORP.


By:                                          By:
   -------------------------------------         -------------------------------
John E. Vollmer III, Vice President          John E. Vollmer III
signing as such on behalf of each of the     Senior Vice President
foregoing Obligors

THIRD AMENDMENT TO LOAN & SECURITY AGREEMENT
<PAGE>   11

                                    EXHIBIT A
                         REVOLVING LOAN PROMISSORY NOTE

                                October 18, 2000

$90,000,000

FOR VALUE RECEIVED, the undersigned Companies (each a "COMPANY" and,
collectively, the "COMPANIES"), promise, jointly and severally, to pay to the
order of THE CIT GROUP/BUSINESS CREDIT, INC. (herein "CITBC"), as Agent for the
Lenders under a certain Loan and Security Agreement dated November 22, 1999
between CITBC as Agent and Lender, other Lenders parties thereto and each
Company, as amended from time to time (herein the "AGREEMENT") at its office
located at 1211 Avenue of the Americas, New York, New York 10036, or such other
address as may be designated by the Agent, in lawful money of the United States
of America and in immediately available funds, the principal amount of Ninety
Million and No/100 Dollars ($90,000,000), or such other principal amount
advanced pursuant to Section 3, Paragraph 1 or Section 4 of the Agreement. The
balance of such Revolving Loan will fluctuate as a result of the daily
application of the proceeds of collections of the Accounts and the making of
additional Revolving Loans as described in said Section 3 or Section 4 of the
Agreement. The Revolving Loans may be borrowed, repaid and reborrowed by any
Company, subject to the terms of the Agreement. A final payment in an amount
equal to the outstanding aggregate balance of principal and interest remaining
unpaid, if any, under this Revolving Loan Promissory Notes as shown on the books
and records of the Agent shall be due and payable upon any termination of the
Agreement.

All capitalized terms used herein shall have the meaning provided therefor in
this Agreement, unless otherwise defined herein.

The Companies further promise, jointly and severally, to pay interest at such
office, in like money, on the unpaid principal amount owing hereunder from time
to time from the date hereof on the dates and at the rates specified in Section
8, Paragraph 1 of the Agreement.

If any payment on this Revolving Loan Promissory Note becomes due and payable on
a day other than a Business Day, the maturity thereof shall be extended to the
next succeeding Business Day, and with respect to payments of principal,
interest thereon shall be payable at the then applicable rate during such
extension.

This Revolving Loan Promissory Note is a Revolving Loan Promissory Note referred
to in the Agreement, and is subject to, and entitled to, all provisions and
benefits thereof and is subject to optional and mandatory prepayment, in whole
or in part, as provided therein.

THIRD AMENDMENT TO LOAN & SECURITY AGREEMENT
<PAGE>   12

The date and amount of the advance(s) made hereunder may be recorded on the
schedule which is attached hereto and hereby made part of this Note or the
separate ledgers maintained by the Agent, provided that any failure to record
any such information on such schedule shall not in any manner affect the
obligation of any Company to make payments of principal and interest in
accordance with the terms of this Revolving Loan Promissory Note. The aggregate
unpaid principal amount of all advances made pursuant hereto may be set forth in
the balance column on said schedule or such ledgers maintained by the Agent. All
such advances, whether or not so recorded, shall be due as part of this
Revolving Loan Promissory Note.

Each Company confirms that any amount received by or paid to the Agent in
connection with this Agreement and/or any balances standing to its credit on any
of its accounts on the Agent's books under this Agreement may in accordance with
the terms of this Agreement be applied in reduction of this Revolving Loan
Promissory Note, but no balance or amounts shall be deemed to effect payment in
whole or in part of this Revolving Loan Promissory Note unless the Agent shall
have actually charged such account or accounts for the purposes of such
reduction or payment of this Revolving Loan Promissory Note.

Upon the occurrence and during the continuance of any one or more of the Events
of Default specified in the Agreement or upon termination of this Agreement, all
amounts then remaining unpaid on this Revolving Loan Promissory Note may become,
or be declared to be, immediately due and payable as provided in the Agreement.

Each Company and the Guarantors, sureties and endorsers jointly and severally
waive grace, demand, presentment for payment, notice of dishonor or default,
notice of intent to accelerate, notice of acceleration, protest and diligence in
collecting this Revolving Loan Promissory Note.

This Revolving Loan Promissory Note shall be governed by, and construed in
accordance with, the laws of the State of New York and the applicable federal
laws of the United States.

THIRD AMENDMENT TO LOAN & SECURITY AGREEMENT
<PAGE>   13

This Revolving Loan Promissory Note is given in amendment, replacement and
substitution, but not extinguishment, of all amounts unpaid under that certain
Revolving Loan Promissory Note dated May 18, 2000 payable by the Companies to
the order of CTIBC as Agent for the Lenders in the stated principal amount of
$75,000,000.00.

                       COMPANIES:

                       UTI DRILLING, L.P.

                       By: UTICO HARD ROCK BORING, INC.,
                           as sole General Partner

                           By:
                               -----------------------------------------------
                               John E. Vollmer III, Vice President

                       SUITS DRILLING COMPANY

                       By:
                           ---------------------------------------------------
                           John E. Vollmer III, Vice President

                       UNIVERSAL WELL SERVICES, INC.

                       By:
                           ---------------------------------------------------
                           John E. Vollmer III, Vice President

                       UTI MANAGEMENT SERVICES, L.P.

                       By: UTICO HARD ROCK BORING,
                           as Sole General Partner

                           By:
                               -----------------------------------------------
                               John E. Vollmer III, Vice President

                       NORTON DRILLING, L.P.

                       By: Norton GP, L.L.C., as sole General Partner

                           By: Norton Drilling Services, Inc., its sole Member

                               By:
                                   -------------------------------------------
                               Name:
                                     -----------------------------------------
                               Title:
                                      ----------------------------------------

THIRD AMENDMENT TO LOAN & SECURITY AGREEMENT
<PAGE>   14

                                SCHEDULE TO GRID

<Table>
<Caption>
             Date                           Loan                        Payment                       Balance
<S>                               <C>                          <C>                           <C>

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</Table>

THIRD AMENDMENT TO LOAN & SECURITY AGREEMENT