<PAGE>   1
                                                                  EXHIBIT 10.21

                            STOCK PURCHASE AGREEMENT

         THIS STOCK PURCHASE AGREEMENT ("Agreement") is made and entered into as
of the 29th day of December 1999, by and between DENNIS G. HARRISON, SUE ANN
HARRISON, and WILLIE EDWARDS (collectively, "Sellers"), HARRISON WELL SERVICE,
INC., a Texas corporation (the "Company"), and SIERRA WELL SERVICE, INC., a
Delaware corporation ("Purchaser").

                                    RECITALS:

          A.   The Company is based out of Denver City, Texas and is principally
engaged in the well servicing business in the Permian Basin of West Texas (the
"Business").

          B.   Sellers own all of the issued and outstanding capital stock of
the Company (the "Stock").

          C.   Sellers desire to sell the Stock, and Purchaser desires to
purchase the same, upon and subject to the terms and conditions set forth in
this Agreement.

         NOW, THEREFORE, in consideration of the premises, the mutual
representations, warranties, and covenants which are hereinafter set forth, and
other good and valuable consideration, the legal sufficiency of which are hereby
acknowledged, the parties hereto have agreed and do hereby agree as follows:

                                    ARTICLE I

                                PURCHASE AND SALE

         Section 1.01 THE STOCK. Subject to all of the terms and conditions of
this Agreement, Sellers hereby agree to sell, transfer, and deliver, and
Purchaser hereby agrees to purchase, pay for, and accept, at the Closing
(hereinafter defined), all of the Stock.

         Section 1.02 PRE-CLOSING DISTRIBUTION OF CERTAIN ASSETS. At or prior to
Closing, the Company shall distribute to Sellers, without representation,
warranty, or recourse by or upon the Company, and without adverse tax effect
upon the Company, all of its right, title and interest in and to the assets
listed on attached SCHEDULE 1.02 (collectively, the "Excluded Assets"). In
connection with such distribution, upon Closing, Sellers jointly and severally
agree to indemnify and hold the Company and Purchaser harmless from and against
any and all claims, demands, causes of action, liabilities, losses, and/or
expenses (including, without limitation, reasonable attorneys' fees and other
expenses of litigation) arising from or in connection with the acquisition,
ownership, operation, use, or distribution of the Excluded Assets by the
Company, specifically including, without limitation, any adverse tax
consequences occasioned to or suffered by the Company or Purchaser as a
consequence of the distribution of the Excluded Assets to Sellers pursuant to
this Section 1.02.


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         Section 1.03 PURCHASE PRICE. Subject to adjustment as hereinafter
provided in this Agreement, the total purchase price ("Purchase Price") for the
Stock is a sum equal to (A) $1,225,000.00 plus or minus (B) the "net financial
assets" (as defined below) of the Company as of the Closing Date ("NFA"),
payable by Purchaser to Sellers, in proportion to their ownership of the Stock,
as follows:

         (1)      Purchaser shall pay to Sellers in immediately available funds
                  at the Closing a sum equal to (a) $600,000.00 plus or minus
                  (b) NFA, as preliminarily estimated and finally adjusted
                  pursuant to Section 5.08; and

         (2)      Purchaser shall execute and deliver to Dennis G. Harrison,
                  Trustee, for the benefit of Sellers, at Closing its promissory
                  note (the "Note") in the original principal amount of
                  $625,000.00, bearing interest at a floating rate equal to the
                  prime rate of interest quoted in the Wall Street Journal
                  published on the Friday most immediately preceding the
                  applicable quarterly adjustment date, and payable to the order
                  of Sellers on or before one year from date, said Note to be
                  substantially identical in form and substance to that attached
                  hereto as EXHIBIT A, to be convertible into equity of
                  Purchaser under certain circumstances as therein provided, and
                  to be secured by a security interest upon the tangible assets
                  of the Company as of Closing that is junior in priority to any
                  liens upon such assets as of the Closing Date, as created by a
                  properly completed Form UCC-1 executed by the Company, as
                  Grantor, and by security agreement substantially identical in
                  form and substance to that attached hereto as EXHIBIT B
                  (collectively, the "Security Agreement").

         As used in this Agreement, "net financial assets" means the difference
between (a) "Total Current Assets" and (b) "Total Liabilities," as such account
groups are shown on the Balance Sheet (hereinafter defined) and then adjusted or
converted as of the Closing Date pursuant to Section 5.08.

                                   ARTICLE II

                        REPRESENTATIONS AND WARRANTIES OF
                             SELLERS AND THE COMPANY

         Sellers and the Company hereby jointly and severally represent and
warrant to Purchaser as of the date of this Agreement as follows:

         Section 2.01 ORGANIZATION AND QUALIFICATION; NO SUBSIDIARIES. The
Company is a corporation duly organized, validly existing, and in good standing
under the laws of the jurisdiction of its incorporation and has the requisite
power and authority to own, lease, and operate its assets and to carry on its
business as now being conducted. The Company has not received any notice of
proceedings relating to the revocation or modification of any such
authorizations. The Company is duly qualified or licensed to do business and is
in good standing in each jurisdiction where the

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STOCK PURCHASE AGREEMENT                                                  Page 2


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character of the assets owned, leased, or operated by it or the nature of its
activities makes such qualification or licensing necessary. The Company does not
have any subsidiaries. The Company does not directly or indirectly own any
equity or similar interest in, or any interest convertible into or exchangeable
or exercisable for, any equity or similar interest in, any corporation,
partnership, or other business association or entity.

         Section 2.02 GOVERNING DOCUMENTS. The articles of incorporation and the
bylaws of the Company, true and correct copies of which have been furnished to
Purchaser, are in full force and effect and have not been modified, amended, or
rescinded. The Company is not in violation of its articles of incorporation or
bylaws.

         Section 2.03 CAPITALIZATION OF THE COMPANY. The authorized capital
stock of the Company consists of 720 shares of common stock, par value $100 per
share. As of the date hereof, 360 shares of common stock of the Company are
issued and outstanding, all of which are validly issued, fully paid, and
nonassessable (and collectively constitute the Stock). There are no options,
warrants, or other rights, agreements, arrangements or commitments of any
character relating to the issued or unissued capital stock of the Company or
obligating the Company to issue or sell any shares of capital stock of, or other
equity interests in, the Company. There are no obligations, contingent or
otherwise, of the Company to repurchase, redeem, or otherwise acquire any shares
of the Stock or to provide funds to or make any investment (in the form of a
loan, capital contribution, or otherwise) in any other person or entity.

         Section 2.04 AUTHORITY RELATIVE TO THIS AGREEMENT. Sellers and the
Company have full power and authority to execute and deliver this Agreement and
to perform their obligations hereunder and to consummate the transactions
contemplated hereby. The execution and delivery of this Agreement by Sellers and
the Company and their consummation of the transactions contemplated hereby have
been duly and validly authorized by all necessary action and no other
proceedings on the part of Sellers or the Company are necessary to authorize
this Agreement or to consummate the transactions so contemplated. This Agreement
has been duly and validly executed and delivered by Sellers and the Company and
constitutes the legal, valid, and binding obligations of Sellers and the
Company, except as the same may be limited by bankruptcy, insolvency,
reorganization, moratorium or similar laws now or hereafter in effect relating
to or limiting creditors' rights or by legal principles of general applicability
governing availability of equitable remedies.

         Section 2.05 NO CONFLICT; REQUIRED FILINGS AND CONSENTS.

         (a) The execution and delivery of this Agreement by Sellers and the
Company do not, and the performance of this Agreement by Sellers and the Company
will not (i) conflict with or violate the articles of incorporation, bylaws, or
other governing documents of the Company, (ii) conflict with or violate any
laws, statutes, rules, regulations, or pronouncements of any court, tribunal, or
governmental agency, whether federal, state, or local, (collectively, "Laws")
applicable to Sellers or the Company, or by which they or their respective
assets are bound or affected, or (iii) result in any material breach of or
constitute a material default (or an event that with notice or lapse

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STOCK PURCHASE AGREEMENT                                                 Page 3


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of time or both would become a material default) under, or give to others any
rights of termination, amendment, acceleration or cancellation of, or result in
the creation of a lien or encumbrance on any of the assets of Sellers or the
Company pursuant to, any note, bond, mortgage, indenture, contract, agreement,
lease, license, permit, franchise, or other instrument or obligation to which
Sellers or the Company is a party or by which any of them or their respective
assets are bound or affected.

             (b) The execution and delivery of this Agreement by Sellers and the
Company do not, and the performance of this Agreement by Sellers and the Company
will not, require any consent, approval, authorization, or permit of, or filing
with or notification to, any governmental or regulatory authority, domestic or
foreign.

         Section 2.06 COMPLIANCE; PERMITS. The Company is not in conflict with,
or in default under or violation of, (i) any federal, state, or local laws
applicable to the Company or by which the Company or any of the assets of the
Company are bound or affected or (ii) any note, bond, mortgage, indenture,
contract, agreement, lease, license, permit, franchise or other instrument or
obligation to which the Company is a party or by which the Company or its assets
are bound or affected. The Company possesses all permits, licenses, leases,
agreements, and authorizations of governmental authorities and any other
applicable persons or entities necessary to or for the operation of its assets,
properties, and business, all of which permits, licenses, leases, agreements,
and authorizations are in full force and effect.

         Section 2.07 REPORTS AND FINANCIAL STATEMENTS OF THE COMPANY. The
balance sheet of the Company for the period ending as of November 30, 1999, a
copy of which is attached hereto as EXHIBIT C ( the "Balance Sheet"), is true
and complete in all material respects, fairly represents the financial position
of the Company as of the period shown, and was prepared in accordance with
reasonable and acceptable accounting principles and practices, consistently
applied. The Balance Sheet does not contain any untrue statement of a material
fact or omit to state a material fact necessary in order to make the statements
therein not misleading.

         Except as and to the extent of (i) liabilities reflected or reserved
against in the Balance Sheet and (ii) liabilities which have arisen since
November 30, 1999 in the ordinary course of business and which have been fully
disclosed to Purchaser in writing, the Company does not have any liabilities or
obligations (whether accrued, absolute or contingent), and including without
limitation, any liabilities resulting from failure to comply with any laws or
any federal, state, or local tax liabilities due or to become due whether (a)
incurred in respect of or measured by income for any financial statement or
balance sheet period, or (b) arising out of transactions entered into, or any
state of facts existing, prior or subsequent thereto.

         Section 2.08 ABSENCE OF CERTAIN CHANGES OR EVENTS. Since November 30,
1999, except as shown on attached SCHEDULE 2.08, the Company has conducted its
businesses only in the ordinary course and in a manner consistent with past
practice and, since such date, except as shown on SCHEDULE 2.08, there has not
been (a) any material adverse change in the financial condition, results of
operations, or business of the Company, (b) any material damage, destruction, or
loss (whether

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STOCK PURCHASE AGREEMENT                                                 Page 4


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or not covered by insurance) with respect to any assets of the Company, (c) any
change by the Company in its accounting methods, principles, or practices, (d)
any entry by the Company into any commitments or transactions material to the
Company, (e) any declaration, setting aside, or payment of any dividends or
distributions in respect of shares of the Stock or any redemption, purchase, or
other acquisition of any of the capital stock of the Company, (f) any increase
or decrease in or establishment or termination of any bonus, insurance,
severance, deferred compensation, pension, retirement, profit sharing, stock
option (including, without limitation, the granting of stock options, stock
appreciation rights, performance awards, or restricted stock awards), stock
purchase, or other employee benefit plan of the Company, or any other increase
in the compensation payable or to become payable to any officers or key
employees of the Company, (g) any incurrence of any indebtedness or other
obligations or liabilities by the Company, (h) any proposed law or regulation or
any actual event or condition of any character that is known to the Sellers or
the Company that materially adversely affects the business or future prospects
of the Company, (h) any claim, litigation, event, or condition of any character
that materially adversely affects the business or future prospects of the
Company, (i) any issuance or purchase of, or agreement to issue or purchase
shares of the capital stock or other securities of the Company, (j) any
mortgage, pledge, lien, or encumbrance made or agreed to be made on any of the
Company's assets or properties, (k) any sale, transfer, other disposition of, or
agreement to sell, transfer, or dispose of the Company's properties or assets,
tangible or intangible, except as expressly permitted by this Agreement and
except in the ordinary course of business and then only for full and fair value
received, (l) any loans, advances, or agreements with respect to any loans or
advances, other than to customers in the ordinary course of business and that
have been properly reflected as "accounts receivable" on the Company's books;
(m) any transaction outside the ordinary course of business, (n) any capital
expenditure by the Company in excess of $10,000, or (o) any agreement by Sellers
or the Company to do any of the items described in Subparagraphs (a) through (n)
above.

         Section 2.09 ABSENCE OF LITIGATION. There are no claims, actions,
proceedings, or investigations pending or, to the knowledge of Sellers or the
Company, threatened against the Company or any of the assets of the Company
before any court, arbitrator, or administrative, governmental, or regulatory
authority or body, domestic or foreign. As of the date hereof, neither the
Company nor its assets are subject to any order, writ, judgment, injunction,
decree, determination, or award.

         Section 2.10 EMPLOYEE BENEFIT PLANS. Except as set forth on attached
SCHEDULE 2.10 (collectively, the "Plans"), the Company does not have any
employee benefit plans (as defined in Section 3(3) of the Employee Retirement
Income Security Act of 1974, as amended and the regulations thereunder), any
bonus, stock option, stock purchase, restricted stock, incentive, deferred
compensation, medical, accident, life insurance, disability income, retiree
medical or life insurance, supplemental retirement, severance, and other benefit
plans, programs, or arrangements, or any sick leave and vacation plans. The
Company has operated the Plans in compliance with all applicable federal, state,
and local laws and regulations, has received no notice of any claims or
violations of any laws and regulations applicable to the Plans, and, has no
existing obligations or liabilities under the Plans which have not been fully
funded or reserved for on the Balance Sheet. Any and all such

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STOCK PURCHASE AGREEMENT                                                 Page 5


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plans, programs, and arrangements previously adopted and subsequently terminated
by the Company were duly and validly terminated in accordance with all
applicable laws and the Company has no continuing obligations or liabilities and
has received no notice of any claims or violations with respect to such former
plans, programs, and arrangements.

         Section 2.11 TITLE TO STOCK. Sellers own good and marketable title to
all of the Stock, in the proportions among them as set forth on attached
SCHEDULE 2.11, free and clear of liens, claims, and encumbrances of any kind or
character.


         Section 2.12 ASSETS OF THE COMPANY.

                  (a) SCHEDULE 2.12(a) attached hereto contains a complete and
correct description of all real property owned by or leased to the Company or in
which the Company otherwise holds contractual rights, together with all
applicable real property leases and contracts. All such leases and contracts are
valid and in full force and effect and all rents, royalties, and other sums
payable by the Company thereunder have been timely paid. There exists no default
or event or circumstance which, with notice or lapse of time or both, will
constitute a default under any of such lease agreements.

                  (b) SCHEDULE 2.12(b) attached hereto contains a complete and
accurate description of all vehicles, equipment, furniture, fixtures, and other
personal property of any kind or character, tangible or intangible, that is
owned by, in the possession of, or used by the Company in connection with the
Company's business. Except as shown on SCHEDULE 2.12(B), no personal property
owned or used by the Company in connection with its business is held under any
lease, security agreement, conditional sales contract, or other title retention
or financing agreement or is located any place other than in the possession of
the Company.

         Section 2.13 TITLE TO AND CONDITION OF ASSETS. The Company has, or will
have at Closing, good and marketable title to all of its assets and properties,
real and personal, tangible and intangible, that are material to the Company's
business and future prospects, free and clear of liens, claims, charges, and
encumbrances of any kind or nature, save and except (a) the liens and security
interests shown on attached SCHEDULE 2.13, (b) liens for real and personal
property taxes that are not yet due and payable and (c) possible minor matters
that, in the aggregate, are not substantial in amount and do not materially
interfere with or detract from the present or intended use of any of the assets
and properties nor materially impair the business operations of the Company. The
tangible assets of the Company are in good operating condition and repair,
normal wear and tear excepted.

         Section 2.14 TAXES. The Company is and at all times in the past has
been a "C" corporation within the meaning of Section 1361 of the Internal
Revenue Code of 1986, as amended. The Company presently maintains and at all
times in the past has maintained a fiscal year ending as of December 31 for
federal income tax purposes. The Company has filed all state and federal Tax
Returns (defined below) required to be filed by the Company, and the Company has
paid and

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STOCK PURCHASE AGREEMENT                                                 Page 6

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discharged all Taxes (as defined below) shown due thereon and has paid all other
Taxes as are due. The liability for Taxes set forth in each such Tax Return does
not materially understate the Taxes required to be reflected on such Tax Return.
For purposes of this Agreement, "Tax" or "Taxes" means taxes of any kind,
payable to any federal, state, local, or foreign taxing authority, including
(without limitation) (i) income, franchise, profits, gross receipts, ad valorem,
value added, sales, use, service, real or personal property, payroll,
withholding, employment, social security, unemployment compensation, and
production taxes, and (ii) interest, penalties, and additions to tax imposed
with respect thereto, whether disputed or not; and "Tax Returns" means all
returns, reports, declarations, claims for refund, and information statements
with respect to Taxes required to be filed with the Internal Revenue Service or
any other taxing authority, domestic or foreign, through the time of Closing
hereunder, including, without limitation, consolidated, combined, and unitary
tax returns and any schedule or amendment thereto. The Company has not (i)
granted any waiver of any statute of limitations with respect to, any Tax, or
(ii) obtained an extension of time with respect to the filing of any Tax Return
other than Tax Returns that were duly filed within the applicable extension
period. No claim has been made by an authority in a jurisdiction where the
Company does not file Tax Returns that the Company may be subject to taxation by
that jurisdiction. There are no liens or security interests on any of the assets
of the Company that arose in connection with any failure (or alleged failure) to
pay any Tax. The Company has disclosed on all federal income Tax Returns all
positions taken therein that could give rise to a substantial understatement of
federal income Tax within the meaning of Section 6662 of the Code. The Company
has withheld and paid all Taxes required to have been withheld and paid in
connection with amounts paid or owing to any employee, creditor, independent
contractor, or other third-party. There is no dispute or claim concerning any
liability for Tax of the Company (i) claimed or raised by any authority in
writing or (ii) as to which the Company or any officers (and employees
responsible for Tax matters) of the Company have knowledge.

         Section 2.15 ENVIRONMENTAL MATTERS.

               (a)  The Company has conducted all operations and activities in
material compliance with all applicable Environmental Laws (as defined below),
and none of the assets of the Company is being or has been operated in violation
of any Environmental Laws.

               (b)  The Company possesses all permits required under
Environmental Laws for the operation of its assets.

               (c)  The assets of the Company, and the operations conducted
thereon or therewith, are not subject to any existing, unfulfilled
administrative or judicial order requiring remedial action under any
Environmental Law.

               (d)  Neither Sellers nor the Company has received any notice of
any investigation or inquiry regarding failure of the assets of the Company, or
the operations conducted thereon or therewith, to comply with Environmental
Laws.

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STOCK PURCHASE AGREEMENT                                                 Page 7

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         As used in this Section 2.15, "Environmental Laws" shall mean the
federal Comprehensive Environmental Response, Compensation and Liability Act, as
amended, the Superfund Amendments and Reauthorization Act, as amended, Safe
Drinking Water Act, Texas Solid Waste Disposal Act, the Resource Conservation
and Recovery Act, as amended, the Hazardous and Solid Waste Amendments Act, as
amended, the Toxic Substances Control Act, as amended, the Clean Water Act, as
amended, and the Clean Air Act, as amended, and any other applicable federal,
state, or local environmental law and all rules, regulations and administrative
orders related thereto, as such laws, rules, regulations and administrative
orders exist as of the Closing Date.

         Section 2.16 BROKERS. No broker, finder, or investment banker is
entitled to any brokerage, finder's, or other fee or commission in connection
with the transactions contemplated by this Agreement based upon arrangements
made by or on behalf of Sellers or the Company.

         Section 2.17 CONTRACTS. Attached hereto as SCHEDULE 2.17 is a list of
all currently effective agreements respecting property, goods, and/or services
of or binding upon the Company or its assets (the "Contracts"). None of the
Contracts has been modified, amended, supplemented or altered except as
specifically shown on attached SCHEDULE 2.17. The Company has made copies of all
of the Contracts (including all modifications, amendments, and supplements
thereto) available to Purchaser. All of the Contracts are in full force and
effect and the Company is not in default under any of the Contracts.

         Section 2.18 NO UNION CONTRACTS. There is no collective bargaining or
other union agreement to or by which the Company is a party or is bound, nor is
a collective-bargaining agreement currently being negotiated; and all non-exempt
employees have been paid in accordance with the Fair Labor Standards Act of 1938
and the Portal-to-Portal Act of 1947. The Company is in compliance with all
federal, state, or other applicable laws, domestic or foreign, respecting
employment and employment practices, terms and conditions of employment, and
wages and hours, and has not and is not engaged in any unfair labor practice.
The Company has not experienced any material labor difficulty during the last
three years.

         Section 2.19 INSURANCE. SCHEDULE 2.19 attached hereto lists all
insurance policies (the "Insurance Policies") held by the Company and all such
listed policies are in the respective principal amounts set forth therein. The
Company maintains (a) insurance on all of its business, operations, and assets
of a type customarily insured, covering property damage and loss of income by
fire or other casualty and (b) insurance protection against all liabilities,
claims, and risks against which it is customary to insure. All premiums due and
payable under the Insurance Policies have been paid. The Company is not, and but
for a requirement that notice be given or that a period of time elapse or both
would not be, in violation under any such Insurance Policies.

         Schedule 2.20 BANK ACCOUNTS. SCHEDULE 2.20 attached hereto contains a
true and correct list of the names and addresses of all banks, financial
institutions, and other depositories in which the Company has an account,
deposit, or safe deposit box and the names of all persons authorized

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STOCK PURCHASE AGREEMENT                                                 Page 8

<PAGE>   9


to draw on those accounts or deposits or who have access to them and the account
numbers of each account.

         Schedule 2.21 OTHER LIABILITIES AND OBLIGATIONS. SCHEDULE 2.21 attached
hereto contains a true and correct list of all liabilities and obligations of
the Company not disclosed elsewhere in this Agreement of any kind, character,
and description, whether accrued, absolute, contingent, or otherwise, and
whether or not required to be disclosed or accrued in the financial statements
of the Company that exceed $5,000 to any one creditor. In the case of
liabilities that may not be fixed, an estimate of the maximum amount that may be
payable is also included.

         Section 2.22 DISCLOSURE. No representations or warranties made by
Sellers or the Company in this Agreement, and no statements of Sellers or the
Company contained in any document executed or delivered by any of them to
Purchaser pursuant hereto or in connection with the transactions contemplated
hereby, contain any untrue statement of a material fact or omit to state any
material fact necessary to make the statements herein or therein not misleading.

         Section 2.23 TRANSACTIONS WITH AFFILIATES. Except as shown on attached
SCHEDULE 2.23, the Company is not a party to any transaction, contract, or
agreement with any (i) current or former officer or director of the Company or
(ii) any parent, spouse, child, brother, sister or other family relation of any
such officer or director, or (iii) any corporation or partnership of which any
such officer or director or any such family relation is an officer, director,
partner or greater than 10% stockholder (based on percentage ownership of voting
stock) or (iv) any "affiliate", or "associate" of any such persons or entities
(as such terms are defined in the rules and regulations promulgated under the
Securities Act of 1934, including, without limitation, any transaction involving
a contract, agreement, or other arrangement providing for the employment of,
furnishing of materials, products, or services by, rental of real or personal
property from, or otherwise requiring payments to, any such person or entity,
except with respect to services provided on reasonable terms which would not
materially alter the presentation of the Balance Sheet, if such transactions had
been entered into with an unrelated third party.


                                   ARTICLE III

                   REPRESENTATIONS AND WARRANTIES OF PURCHASER


         Purchaser hereby represents and warrants to Sellers as of the date of
this Agreement as follows:

         Section 3.01 ORGANIZATION AND QUALIFICATION. Purchaser is a corporation
duly organized, validly existing and in good standing under the laws of
Delaware. Purchaser is duly qualified or licensed to do business and is in good
standing in each jurisdiction where the character of the assets owned or
operated by Purchaser or the nature of its activities makes such qualification
or licensing necessary.

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STOCK PURCHASE AGREEMENT                                                 Page 9

<PAGE>   10


         Section 3.02 AUTHORITY RELATIVE TO THIS AGREEMENT. Purchaser has the
requisite corporate power and authority to execute and deliver this Agreement
and to perform its obligations hereunder and to consummate the transactions
contemplated hereby. The execution and delivery of this Agreement by Purchaser
and the consummation by Purchaser of the transactions contemplated hereby have
been duly and validly authorized by all necessary corporate action and no other
corporate proceedings on the part of Purchaser are necessary to authorize this
Agreement or to consummate the transactions so contemplated. This Agreement has
been duly and validly executed and delivered by Purchaser and, assuming the due
authorization, execution and delivery by all other parties hereto, constitutes
the legal, valid and binding obligation of Purchaser, except as the same may be
limited by bankruptcy, insolvency, reorganization, moratorium or similar laws
now or hereafter in effect relating to or limiting creditors, rights or by legal
principles of general applicability governing the availability of equitable
remedies.

         Section 3.03 NO CONFLICT; REQUIRED FILINGS AND CONSENTS.

               (a)  The execution and delivery of this Agreement by
Purchaser do not, and the performance of this Agreement by Purchaser will not
(i) conflict with or violate the certificate of incorporation or bylaws of
Purchaser, (ii) to the knowledge of Purchaser, conflict with or violate any law
applicable to Purchaser or by which any of its assets are bound or affected or
(iii) to the knowledge of Purchaser, result in any breach or constitute a
default (or an event that with notice or lapse of time or both would become a
default) under, or give to others any rights of termination, amendment,
acceleration or cancellation of, or result in the creation of a lien or
encumbrance on any of the material assets of Purchaser pursuant to, any note,
bond, mortgage, indenture, or any material contract, agreement, lease, license,
permit, franchise, or other instrument or obligation to which Purchaser is a
party or by which Purchaser or any of its assets are bound or affected.

               (b)  To the knowledge of Purchaser, the execution and delivery of
this Agreement by Purchaser do not, and the performance of this Agreement by
Purchaser will not, require any consent, approval, authorization or permit of,
or filing with or notification to, any governmental or regulatory authority,
domestic or foreign.

         Section 3.04 BROKERS. No broker, finder, or investment banker is
entitled to any brokerage, finder's or other fee or commission in connection
with the transactions contemplated by this Agreement based upon arrangements
made by or on behalf of Purchaser.

                                   ARTICLE IV

                       CONDUCT OF BUSINESS OF THE COMPANY
                            PENDING THE CLOSING DATE

         Pending the Closing Date or earlier termination of this Agreement, and
except as otherwise specifically contemplated in this Agreement or the schedules
hereto or as may be consented to and approved in writing by Purchaser, the
Company covenants and agrees as follows:

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STOCK PURCHASE AGREEMENT                                                Page 10

<PAGE>   11


         Section 4.01 ORDINARY COURSE OF BUSINESS. The Company will carry on its
business substantially in the same manner as heretofore conducted, and will not
engage in any transaction or activity, enter into any agreement or make any
commitment, except in the ordinary course of business. Without limiting the
generality of the foregoing, the Company will:

               (a)  operate and maintain its assets diligently and in a
good and workmanlike manner and comply in all material respects with all
applicable laws and with the terms of any agreements binding upon those assets;

               (b)  maintain and keep in full force and effect all of the
Contracts and all permits, licenses and similar rights and privileges of the
Company and each subsidiary of the Company and comply in all material respects
with all of its material obligations therein and thereunder;

               (c)  maintain all of its tangible assets in at least as
good a condition as they were in at the date hereof, ordinary wear and tear
excepted, and remove no material items therefrom;

               (d)  maintain the Insurance Policies in full force and effect;
and

               (e)  pay, perform, and discharge, on a basis consistent with
the Company's prior practices, all obligations of the Company under the notes,
leases, and other instruments evidencing the indebtedness or other liabilities
of the Company which are shown on the Balance Sheet and/or on the schedules
attached to this Agreement in accordance with their respective terms (but
without right of prepayment);

         Section 4.02 RESTRICTED ACTIONS. Without the prior written consent of
Purchaser, such consent not to be unreasonably withheld, the Company will not
(a) enter into any agreement or commitment, the result of which would be to
incur or expand the existing indebtedness or liabilities of the Company; (b)
incur any additional indebtedness other than trade payables incurred in the
ordinary course of business; (c) sell, transfer, assign, convey or otherwise
dispose of any of the assets of the Company other than dispositions of (i)
equipment or other personal property which is replaced with property and
equipment of comparable or better value and utility in the ordinary and routine
maintenance and operation of its business and (ii) personal property and
equipment no longer used or useful in the ordinary course of business; or (d)
create or permit the creation of any new lien, security interest, or other
encumbrances on any asset of the Company.

         Section 4.03 AMENDMENTS TO GOVERNING DOCUMENTS. No change or amendment
shall be made in the articles of incorporation or bylaws of the Company.

         Section 4.04 ORGANIZATION. The Company will preserve its corporate
existence and will keep its business organization intact and use its reasonable
efforts to preserve its relationships with its suppliers, customers, and others
having business relations with the Company.

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STOCK PURCHASE AGREEMENT                                                Page 11

<PAGE>   12



         Section 4.05 EMPLOYMENT AGREEMENTS. The Company will not enter into any
agreement relating to employment with any person.

         Section 4.06 ISSUANCE OF SHARES; DIVIDENDS. The Company will not issue
shares of capital stock, or grant any options, warrants or other rights to
purchase or acquire the capital stock of the Company. The Company will neither
declare nor pay or set aside for payment any dividend or other distribution on
its outstanding shares of capital stock, nor redeem, purchase or otherwise
acquire any of its capital stock.

         Section 4.07 NO DEFAULT. The Company will not knowingly take any action
or knowingly take any action that causes a material breach of any
representation, warranty, covenant or agreement of the Company under this
Agreement.

         Section 4.08 INVESTMENT. The Company will not make any new capital
investment in, make any loan to, or acquire the securities or assets of any
other person or entity.

         Section 4.09 USES OF CASH. Other than in the ordinary course of
business, the Company will not make any payments during the period between the
date hereof and the Closing Date except for the purpose of paying Taxes, paying
rentals or other sums owing by the Company under the Contracts as and when due,
discharging rentals and trade payables incurred in the ordinary course of
business, and retiring or discharging presently existing indebtedness to
financial institutions or other third parties as and when due in accordance with
the terms of the notes or other instruments evidencing such indebtedness (but
without right of prepayment).


                                    ARTICLE V

                         OTHER AGREEMENTS OF THE PARTIES


         Section 5.01 PURCHASER'S DUE DILIGENCE. Prior to entering into this
Agreement, Purchaser has been afforded the opportunity to conduct such
inspections, reviews, and audits of the assets of the Company and of its books
and records as Purchaser, in its discretion deemed appropriate. Pending Closing,
Purchaser may conduct such further inspections, reviews, and audits as Purchaser
may reasonably require. All expenses incurred by the Purchaser relating to its
inspections, reviews, and audits shall be borne and paid exclusively by the
Purchaser. Sellers and the Company shall cooperate with Purchaser in all
reasonable respects in facilitating such inspections, reviews, and audits.
Without limiting the foregoing, Sellers and the Company agree that, until
Closing, they will (a) provide or cause to be provided to Purchaser and its
counsel, accountants, consultants, and other authorized representatives, during
normal business hours or otherwise, if necessary, full access to all of their,
assets, books, agreements, commitments and records; and (b) furnish Purchaser
and its representatives with such data, records, and other information
concerning any of their operations and affairs as Purchaser may reasonably
request.

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STOCK PURCHASE AGREEMENT                                                Page 12

<PAGE>   13


         Sellers further agree that, upon request by Purchaser following
Closing, they will execute and deliver to Purchaser or its accountants such
audit response letters and further confirmations as Purchaser or its accountants
may reasonably require for purposes of verification of the accuracy, validity,
and completeness of all financial and other information provided or made
available by Sellers and the Company to Purchaser in connection with the
transactions contemplated by this Agreement.

         Section 5.02 EXCLUSIVE DEALING. Unless and until this Agreement shall
have been terminated in accordance with the terms hereof, neither Sellers nor
the Company shall directly or indirectly, solicit, initiate, or participate in
negotiations with, or provide any information to, any corporation, partnership,
person or other entity or group (other than the Purchaser or an affiliate or an
associate of the Purchaser) concerning, or enter into any agreement providing
for any sale of the assets of the Company, any sale of shares of capital stock
in the Company, or any similar transactions involving the Company.

         Section 5.03 CONSENTS, APPROVALS, AND FILINGS. Sellers, the Company,
and Purchaser shall each use reasonable efforts to obtain at the earliest
practicable date and, in any event, prior to the Closing Date, all consents and
approvals, including any third party consents, and to make all filings required
to be obtained or made under any federal, state, or local laws or any agreement
or other instrument (including, without limitation, any consent or approval
necessary to avoid the loss of any rights under any agreement) prior to
consummating the transactions contemplated hereby, whether such consent,
approval or filing is to be obtained from or made with private parties or
applicable governmental authorities.

         Section 5.04 BROKERS AND FINDERS. Purchaser, on the one hand, and
Sellers and the Company, on the other hand, shall indemnify each other and hold
each other harmless from any claim against the other for a broker's or finder's
fee, commission, or other like payment in connection with the transactions
contemplated by this Agreement which arises from or is based upon arrangements
made by or through the indemnifying party.

         Section 5.05 PUBLIC ANNOUNCEMENTS. Pending Closing, except as may be
required by applicable law, no party hereto shall issue any press release or
make any other public pronouncements concerning this Agreement or the
transactions contemplated hereby without the written consent and approval of all
other parties.

         Section 5.06 RESIGNATION OF OFFICERS AND DIRECTORS. Sellers and the
Company will secure the resignation of each then-serving officer and director of
the Company effective as of the Closing Date.

         Section 5.07 NOTICE OF DEVELOPMENTS. Prior to the Closing, Sellers and
the Company will give prompt notice to Purchaser of any material event affecting
the assets, liabilities, business, financial condition, operations, results of
operations, or future prospects of the Company. No disclosure pursuant to this
Section 5.07 shall, however, be deemed to amend or supplement this

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STOCK PURCHASE AGREEMENT                                                Page 13

<PAGE>   14


Agreement or the schedules hereto or to prevent or cure any misrepresentation,
breach of warranty, or breach of covenant under this Agreement.

         Section 5.08 PRELIMINARY AND FINAL COMPUTATIONS OF NFA. As part of its
diligence processes in this transaction, Purchaser intends to have the books and
records of the Company audited by certified professional accountants and to have
audited financial statements prepared for the Company in compliance with
generally accepted accounting principles ("GAAP"), all at the expense of
Purchaser. Sellers, the Company, and Purchaser recognize that the financial data
necessary to permit computation of NFA at the Closing may not be complete or
final at such time. The parties accordingly shall estimate NFA at Closing and
shall then finalize their calculation of NFA on a post-Closing basis within
sixty (60) days following the Closing. At least three (3) business days prior to
the Closing, Purchaser shall deliver to Sellers a preliminary settlement
statement, reflecting its good faith estimate of NFA as of the Closing Date
based upon the best information then available to Purchaser. At or prior to
Closing, Sellers and Purchaser shall attempt to reconcile any differences they
may have regarding the preliminary settlement statement. If there are any items
the parties are unable to reconcile prior to Closing, Purchaser's position shall
prevail to enable Closing to proceed, but without prejudice to the right of
either party to dispute any item of the final settlement statement. Within
forty-five (45) days after the Closing Date, Purchaser shall deliver to Sellers
an audited balance sheet of the Company for the period ended as of the Closing
Date, prepared in accordance with GAAP, and a proposed final settlement
statement including Purchaser's final computation of NFA. Sellers shall have ten
(10) days from its receipt of the such balance sheet and proposed settlement
statement to notify Purchaser of their objections, if any, to the information
set forth therein, failing which Sellers shall be deemed to have irrevocably
accepted the computations and substance of those documents for all purposes of
this Agreement. If Sellers timely and properly contest any items within such
balance sheet or proposed final settlement statement, Sellers and Purchaser
shall promptly meet and utilize their best efforts to resolve their differences,
it being the intention of the parties to finalize all post-Closing adjustments
within sixty (60) days following the Closing Date. If the parties are unable to
reach final agreement on any such post-Closing matters, they shall resolve their
differences by means of the dispute resolution procedures set forth in Section
10.02 and final settlement between the parties shall be deferred pending
conclusion of such procedures. At the conclusion of the post-Closing accounting
contemplated by this Section 5.08, Sellers or Purchaser, as appropriate, shall
immediately remit to the other, in immediately available funds, the net sum
determined owning by such party in the final settlement statement, whether
finalized by agreement of the parties or through dispute resolution processes.
As used in this Agreement, "Final Balance Sheet" and "Final Settlement
Statement" shall mean the final balance sheet and the final settlement statement
proposed by Purchaser, as the same may be modified either by agreement of the
parties or dispute resolution pursuant to the foregoing.

         Section 5.09 ACCOUNTS RECEIVABLE. Upon Closing, Sellers expressly
represent and warrant that any accounts receivable of the Company reflected on
the Final Balance Sheet as of the Closing Date will be collectible in the normal
course of business within 150 days from the Closing Date without resort to
litigation or the retention of collection services. Following Closing, the
Company shall apply partial payments by customers to the respective outstanding
receivables of such

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STOCK PURCHASE AGREEMENT                                                Page 14

<PAGE>   15

customers in the order of their aging (i.e. older accounts paid first). To the
extent any such receivables remain unpaid following such 150-day period, the
Company, without limitation of its other legal rights and remedies, may require
Sellers to immediately purchase the same from the Company at full face value and
without subsequent recourse of any kind upon the Company or Purchaser or may
offset the face amount of such unpaid receivables against its payment
obligations under the Note.

         Section 5.10      TAX RETURNS. Sellers shall be responsible for the
timely preparation and filing (without extension, unless otherwise agreed by
Purchaser in writing) of all federal, state, and local Tax Returns covering or
for (or based upon income received or realized during) all periods prior to and
including the Closing Date, except that, if Closing occurs, Purchaser shall be
responsible for the timely preparation and filing of the federal income tax
returns of the Company for calendar year 2000 and ensuing years and the Texas
state franchise tax returns of the Company due on or before May 15 of 2001 and
ensuing years. Sellers shall prepare all such Tax Returns for which it is
responsible in accordance with applicable law and shall submit such Tax Returns
to the Company and Purchaser for their review and concurrence at least ten (10)
business days prior to filing. If, after the Closing Date, any applicable taxing
authority shall determine there to be a deficiency in the amount of any federal,
state, or local Tax paid or payable by the Company which is not reserved for or
reflected on the Final Balance Sheet and which relates to any period prior to
the Closing Date, Sellers shall be fully responsible for the payment of any such
deficiency. Following Closing, if any Tax Return covering a period of time prior
to the Closing Date shall be audited by an applicable taxing authority, the
Company shall promptly notify Sellers of such audit. Purchaser and the Company
shall have primary authority to conduct all discussions and negotiations with
applicable taxing authorities regarding each such audited Tax Return as it may
relate to periods prior to the Closing Date, but Sellers shall have the right to
participate in all such discussions and negotiations. The Company shall have
exclusive authority to conduct all discussions and negotiations with applicable
taxing authorities regarding Tax Returns relating to periods from and after the
Closing Date and shall be solely responsible for the payment of all Taxes
attributable to such periods.

         Section 5.11      INDEMNIFICATION.

                  (a) Indemnification by Sellers. Sellers jointly and severally
agree to indemnify and hold harmless Purchaser and the Company after the Closing
Date against and in respect of any of the following matters which may be
asserted or established:

                           (i) All liabilities of the Company of any nature,
         whether accrued, absolute, contingent, or otherwise, which existed as
         of the Closing Date and are not provided for or reflected on the Final
         Balance Sheet;

                           (ii) Any and all damages, losses, expenses, or
         deficiencies resulting from any breach of the warranties,
         representations and covenants of Sellers and the Company contained
         herein or in any schedule hereto; and

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STOCK PURCHASE AGREEMENT                                                Page 15

<PAGE>   16


                           (iii) All demands, assessments, judgments, costs, and
         expenses (including reasonable legal fees and other expenses of
         litigation, both at the trial and appellate level) arising from or in
         connection with any action, suit, proceeding, or claim incident to any
         of the foregoing.

                  (b) Indemnification by Purchaser. Purchaser agrees to
indemnify and hold harmless Sellers after the Closing Date against or in respect
of any of the following matters:

                           (i) Any and all damages, losses, expenses, or
         deficiencies resulting from any breach of the warranties,
         representations and covenants of Purchaser contained herein or in any
         schedule hereto; and

                           (ii) All demands, assessments, judgments, costs, and
         expenses (including reasonable legal fees and other expenses of
         litigation, both at the trial and appellate level) arising from or in
         connection with any action, suit, proceeding, or claim incident to any
         of the foregoing.

         Section 5.12 RELEASE AND WAIVER BY SELLERS. Any provisions of this
Agreement to the contrary notwithstanding, following Closing hereunder, Sellers
shall have no right of contribution, indemnity, reimbursement, or other legal or
equitable right of recourse upon the Company based upon or attributable to the
breach or non-performance by Sellers or the Company of any of their jointly made
representations, warranties, and covenants under this Agreement, and Sellers, as
of Closing hereunder, expressly release and waive any and all such rights of
contribution, indemnity, reimbursement or other recourse upon or against the
Company.

         Section 5.13 ACCEPTANCE OF TANGIBLE ASSETS BY PURCHASER AT CLOSING.
Upon and by Closing under this Agreement, Purchaser shall accept, and shall be
deemed to have accepted, all tangible personalty of the Company, including,
without limitation, the items of tangible personalty shown on attached SCHEDULE
2.12(B), on an "as is, where is" basis, with all faults, whether latent or
patent, and without surviving representation or warranty, express or implied, as
to matters of condition, merchantability, or fitness for a particular purpose.

         Section 5.14 INVESTMENT REPRESENTATIONS BY SELLERS. Upon Closing and in
connection with any conversion of the unpaid balance of the Note into common
stock in Purchaser as more particularly provided in the Note, Sellers expressly
represent, warrant, acknowledge, and agree to and with the Company that:

                  (a)      any such stock so acquired by Sellers (the "Stock")
                           may not have been registered under the Securities Act
                           of 1933 or any other state securities laws
                           (collectively, the "Securities Acts") because
                           Purchaser may or will issue the Stock in reliance
                           upon this Section 5.14 and the exemptions from the
                           registration requirements of the Securities Acts
                           providing for issuance of securities not involving a
                           public offering;

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STOCK PURCHASE AGREEMENT                                                Page 16

<PAGE>   17



                  (b)      Sellers will acquire any Stock for Sellers' own
                           account, for investment, and not with a view to the
                           resale or distribution thereof;

                  (c)      Sellers will not transfer, sell or offer for sale any
                           portion of the Stock unless there is an effective
                           registration or other qualification relating thereto
                           under the Securities Act of 1933 and under any
                           applicable state securities laws or unless the holder
                           of the Stock delivers to Purchaser an opinion of
                           counsel, satisfactory to Purchaser, that such
                           registration or other qualification under such Act
                           and applicable state securities laws is not required
                           in connection with such transfer, offer or sale;

                  (d)      Purchaser is under no obligation to register the
                           Stock or to assist Sellers in complying with any
                           exemption from registration under the Acts if Sellers
                           should at a later date wish to dispose of the Stock;

                  (e)      Prior to entering into this Agreement, Sellers have
                           made an investigation of Purchaser and its business
                           and have obtained or had made available to Sellers
                           all information with respect thereto which Sellers
                           needed to make an informed decision to structure the
                           transactions contemplated hereby as provided in this
                           Agreement;

                  (f)      Sellers possess adequate experience and
                           sophistication as investors for the evaluation of the
                           merits and risks of investment in the Stock; and

                  (g)      Sellers are "accredited investors," as defined in
                           Regulation D as promulgated under the Securities Act
                           of 1933, as amended, (the "1933 Act").

                                   ARTICLE VI

                 CONDITIONS TO OBLIGATION OF PURCHASER TO CLOSE

         The obligation of Purchaser to close under this Agreement shall, unless
waived in writing by Purchaser, be subject to the satisfaction on or before the
Closing Date of each of the following conditions, and Sellers and the Company
shall use their reasonable efforts to cause each such condition to be so
satisfied.

         Section 6.01 REPRESENTATIONS AND WARRANTIES TRUE. The representations
and warranties of Sellers and the Company contained in this Agreement shall be
true and correct in all respects as of the date of this Agreement and as of the
Closing Date, as though such representations and warranties were made at and as
of such date, except for changes expressly permitted or contemplated by this
Agreement.

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STOCK PURCHASE AGREEMENT                                                Page 17

<PAGE>   18


         Section 6.02 PERFORMANCE. Sellers and the Company shall have performed
and complied with all covenants, agreements, obligations and conditions required
by this Agreement to be performed or complied with by them on or prior to the
Closing Date.

         Section 6.03 CONSENTS. The consents, approvals, and filings described
in Section 5.04 shall have been obtained or made, as the case may be, without
the imposition of conditions or limitations having a material adverse effect.

         Section 6.04 OPINION OF COUNSEL TO SELLERS AND THE COMPANY. Sellers and
the Company shall have delivered to Purchaser the opinion of their legal
counsel, Crenshaw, Dupree & Milam, L.L.P., in form and substance reasonably
acceptable to Purchaser and its counsel, and favorably addressing the
organization, qualification, good standing, and capitalization of the Company,
the due and proper authorization, execution, and delivery and the enforceability
of this Agreement and all instruments and documents executed by Sellers and the
Company in connection herewith, and such other matters as Purchaser or its
counsel may reasonably require.

         Section 6.05 NO INJUNCTION. There shall not be in effect any
preliminary or permanent injunction or temporary restraining order issued by any
state or federal court that prevents the consummation of the transactions
contemplated hereby.

         Section 6.06 OTHER DOCUMENTS. All documents required to be delivered to
Purchaser by Sellers and the Company on or prior to the Closing Date shall have
been so delivered.

         Section 6.07 MATERIAL ADVERSE CHANGE. Prior to the Closing Date, there
shall not have been any material adverse change in, to, or affecting the
Company, its operations, or its assets.

         Section 6.08 CERTIFICATES. Sellers and the Company shall have delivered
to Purchaser certificates confirming the continuing validity of their
representations and warranties pursuant to Section 6.01 and certifying their
performance hereunder as contemplated by Section 6.02.

                                   ARTICLE VII

                    CONDITIONS TO THE OBLIGATIONS OF SELLERS
                            AND THE COMPANY TO CLOSE

         The obligations of Sellers and the Company to consummate the
transactions contemplated by this Agreement shall, unless waived in writing by
them, be subject to the satisfaction on or before the Closing Date of each of
the following conditions, and Purchaser shall use its reasonable efforts to
cause each such condition to be so satisfied:

         Section 7.01 REPRESENTATIONS AND WARRANTIES TRUE. The representations
and warranties of Purchaser contained in this Agreement shall be true and
correct in all respects as of the date made

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STOCK PURCHASE AGREEMENT                                                Page 18


<PAGE>   19



and as of the Closing Date, as though such representations and warranties were
made at and as of such date, except for changes permitted or contemplated by
this Agreement.

         Section 7.02 PERFORMANCE. Purchaser shall have performed and complied
in all material respects with all covenants, agreements, obligations, and
conditions required by this Agreement to be performed or complied with by
Purchaser on or prior to the Closing Date.

         Section 7.03 CONSENTS AND APPROVALS. The consents, approvals, and
filings described in Section 5.04 shall have been obtained or made, as the case
may be, without the imposition of conditions or limitations having a material
adverse effect.

         Section 7.04 NO INJUNCTION. There shall not be in effect any
preliminary or permanent injunction or temporary restraining order issued by any
state or federal court that prevents the consummation of the transactions
contemplated hereby.

         Section 7.05 OTHER DOCUMENTS. All documents required to be delivered to
Sellers or the Company by Purchaser on or prior to the Closing Date shall have
been so delivered.

         Section 7.06 CERTIFICATES. Purchaser shall have delivered to Sellers
and the Company certificates confirming the continuing validity of its
representations and warranties pursuant to Section 7.01 and certifying its
performance hereunder as contemplated by Section 7.02.

                                  ARTICLE VIII

                                   THE CLOSING

         Section 8.01 CLOSING. The closing ("Closing") of the transaction
contemplated hereby shall take place at the offices of Purchaser at 406 N. Big
Spring, Midland, Texas, on a business date of Purchaser's unilateral selection
within thirty (30) days following any successful completion by Purchaser of an
initial public offering of common stock in the Company through a recognized
national stock exchange (the "Sierra IPO"), but in no event later than March 31,
2000, or at such other place and time as the parties hereto might hereafter
mutually agree in writing. Such date or any alternative date so selected by the
parties is referred to in this Agreement as the "Closing Date." Purchaser shall
afford Sellers and the Company at least ten (10) days prior written notice of
its desired Closing Date pursuant to the foregoing.

         Section 8.02 DELIVERIES. At the Closing, the following shall occur:

               (a)  Sellers shall endorse and deliver the certificate(s)
evidencing the Stock to Purchaser, free and clear of liens, claims, and
encumbrances;

               (b)  Sellers and the Company shall deliver to Purchaser
their closing certificates in compliance with the provisions of Section 6.08;

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STOCK PURCHASE AGREEMENT                                                Page 19


<PAGE>   20



               (c)  Sellers and the Company shall deliver, or cause to be
delivered, to Purchaser the legal opinion of their counsel, as specified in
Section 6.04;

               (d)  Sellers and Purchaser shall execute and deliver a
non-competition agreement which is substantially identical in form and substance
to that attached hereto as EXHIBIT D;

               (e)  The parties shall attempt to agree upon an estimate
of NFA and shall execute a preliminary closing statement as contemplated by
Section 5.08;

               (f)  Purchaser shall pay to Sellers in immediately
available funds their respective parts of the cash portion of the Purchase Price
payable at Closing pursuant to Section 1.03(1);

               (g)  Purchaser shall execute and deliver the Note to
Dennis G. Harrison, Trustee (for the benefit of Sellers);

               (i)  The Company and Dennis G. Harrison, Trustee, shall
execute and deliver the Security Agreement and appropriate financing statements;

               (j)  Purchaser shall deliver to Sellers its closing
certificate in compliance with the provisions of Section 7.06; and

               (k)  Sellers and the Company shall execute such
notifications to depository institutions and such changes of authorized
signatories upon the accounts of the Company as Purchaser may reasonably
request.


                                   ARTICLE IX

                           TERMINATION AND ABANDONMENT

         Section 9.01 Termination and Abandonment. This Agreement may be
terminated at any time prior to the Closing:

               (a)  by mutual agreement of all of the parties hereto;

               (b)  by Sellers if Closing shall not have occurred on or
prior to March 31, 2000, other than due to breach or non-performance by Sellers
or the Company hereunder;

               (c)  by Purchaser, if the conditions set forth in Article
VI shall not have been complied with and performed in any material respect and
such noncompliance or nonperformance shall not have been cured or eliminated (or
by its nature cannot be cured or eliminated) on or before the Closing Date;

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STOCK PURCHASE AGREEMENT                                                Page 20


<PAGE>   21




               (d)  by Sellers and the Company if the conditions set forth in
Article VII have not been complied with and performed in any material respect
and such noncompliance or nonperformance shall not have been cured or eliminated
(or by its nature cannot be cured or eliminated) on or before the Closing Date;
or

               (e)  by either Purchaser or Sellers and the Company, by written
notice to the other, if any action or proceeding shall have been instituted
before any court or other governmental body or, to the knowledge of the party
giving such notice, shall have been threatened formally in writing by any public
authority with requisite jurisdiction, to restrain or prohibit the transactions
contemplated by this Agreement or to subject one or more of the parties or their
directors or their officers to liability on the grounds that it or they have
breached any law or regulation or otherwise acted improperly in connection with
such transactions, and such action or proceeding shall not have been dismissed
or such written threat shall not have been withdrawn or rescinded on or before
the Closing Date.

     Section 9.02 REMEDIES, RIGHTS AND OBLIGATIONS ON TERMINATION. If this
Agreement is terminated and abandoned as provided in this Article IX:

         (a) Redelivery. Each party will redeliver all documents, work papers,
and other materials of any other party relating to the transactions contemplated
by this Agreement, whether obtained before or after the execution of this
Agreement, to the party furnishing the same, and all information received by any
party to this Agreement with respect to the business of any other party shall
not at any time be used for the advantage of, or disclosed to third parties by,
such party to the detriment of the party furnishing such information; provided,
however, that this subsection (a) shall not apply to any documents, work papers,
material, or information which is a matter of public knowledge or which has
heretofore been or is hereafter published in any publication for public
distribution or filed as public information with any governmental authority or
is otherwise in the public domain.

         (b)(1) Default by Purchaser. If Purchaser fails or refuses to close in
accordance with the terms of this Agreement and if Sellers and the Company have
timely satisfied all the conditions to Purchaser's obligation to close hereunder
and are not in default hereunder, then, and as the exclusive remedy of Sellers
and the Company for such breach, Sellers and the Company shall have the right to
declare this Agreement terminated and to receive the total sum of $25,000 from
Purchaser as liquidated damages and not a penalty for the breach hereof by
Purchaser (the same as if Purchaser had deposited such sum with Sellers and the
Company as a down payment or earnest money hereunder). In this regard, the
parties acknowledge and agree that the damages occasioned to Sellers and the
Company by any such breach are difficult of ascertainment or calculation and
that the specified sum of liquidated damages represents a fair and reasonable
estimate of such damages.

         (b)(2) Default by Sellers and/or the Company. If Sellers and/or the
Company fails or refuses to close in accordance with the terms of this Agreement
and if Purchaser has timely satisfied all the conditions to Purchaser's
obligation to close hereunder and is not in default

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STOCK PURCHASE AGREEMENT                                                Page 21

<PAGE>   22

hereunder, Purchaser shall have the right either (i) to declare this Agreement
canceled or (ii) to seek enforcement of this Agreement at law or in equity,
including, without limitation, an action for specific performance of the terms
of this Agreement by Sellers and the Company or for recovery of the actual
damages occasioned to Purchaser by such breach. The provisions of Section 10.02
of this Agreement shall be inapplicable to any action instituted by Purchaser
pursuant to Section 9.02(b)(2)(ii).

                 (c) Continuing Liability. The continuing liability of the
parties to this Agreement with respect to any breach of any representation,
warranty, covenant, or agreement contained therein shall not be affected by such
termination or abandonment, unless this Agreement is terminated or abandoned by
agreement of the parties pursuant to Section 9.01 (a) or (d).

                                    ARTICLE X

                              POST CLOSING REMEDIES

         Section 10.01 OFFSET/WAIVER. After the Closing, Purchaser, without
limitation of its other rights and remedies, shall have a right of offset
against its obligations under the Note for any breach of the representations,
warranties, and covenants of Sellers and the Company under this Agreement.
In any proceedings by the Purchaser or the Company to assert or prosecute any
claims under, or to otherwise enforce, this Agreement, Sellers agree that they
will not assert as a defense, or as a bar to recovery, and hereby waive any
right to so assert such defense or such bar to recovery, that (a) prior to the
Closing, the Company shall have had knowledge of the circumstances giving rise
to the claim being pursued by Purchaser, or (b) prior to the Closing, the
Company engaged in conduct or took action that caused or brought about the
circumstances giving rise to such claim, or otherwise contributed thereto.

         Section  10.02 DISPUTE RESOLUTION.


                 (a) Unless expressly provided otherwise in this Agreement, any
and all claims, disputes, controversies, and other matters in question involving
the parties hereto and arising out of or relating to this Agreement and the
transactions contemplated hereby, any provision hereof, the alleged breach of
such provision, or in any way relating to the subject matter of this Agreement
(collectively, "Disputes"), whether such Disputes sound in contract, tort, or
otherwise, at law or in equity, under State or federal law, whether provided by
statute or common law, for damages or any other relief, shall be resolved in
accordance with this Section 10.02.

                 (b) The parties shall attempt in good faith to resolve any
Dispute promptly by negotiations between representatives who have authority to
settle the controversy. Any party may give the other party written notice of any
Dispute not resolved in the normal course of business, together with a request
that the parties meet and confer ("Notice of Dispute"). Within fifteen (15) days
after delivery of a Notice of Dispute, the parties or their representatives
shall meet at a mutually acceptable time and place, and thereafter as often as
they reasonably deem necessary, to exchange

--------------------------------------------------------------------------------
STOCK PURCHASE AGREEMENT                                                Page 22

<PAGE>   23

relevant information and to attempt to resolve the Dispute. If the matter has
not been resolved within thirty (30) days after delivery of the Notice of
Dispute, or if the parties fail to meet within fifteen (15) days after delivery
of the Notice of Dispute, either party may initiate mediation of the claim or
dispute as provided hereafter. If a party or its representative intends to be
accompanied at a meeting by an attorney, the other parties shall be given
advance notice of such intention and may also be accompanied by an attorney. All
negotiations pursuant to this clause are confidential and shall be treated as
compromise and settlement negotiations for purposes of the Federal Rules of
Evidence and any state's rules of evidence.

               (c)  If a Dispute has not been resolved by negotiation as
provided in Section 10.02 (b), the parties shall endeavor to settle the claim or
dispute by mediation under the Center for Public Resources ("CPR") Model
Procedure for Mediation of Business Disputes. A neutral third party will be
selected from the CPR panel of neutrals. If the parties encounter difficulty in
agreeing on a neutral third party, they will seek the assistance of CPR in the
selection process. Mediation under this Section 10.02 (b) will commence within
sixty (60) days of the Notice of Dispute.


               (d) If the Dispute has not been resolved by non-binding means
pursuant to Sections 10.02(b) or (c) within thirty (30) days of the initiation
of mediation, either party may submit such Dispute for resolution by binding
arbitration as follows:

               (i)  It is the intention of the parties that the arbitration
                    shall be conducted pursuant to the Texas Arbitration Act,
                    Tex. Civ. Prac. & Rem. Code Ann., Sections 171.001 et seq.
                    (Vernon 1997 & Supp. 1999) (the "Act"), as modified by this
                    Agreement. The validity, construction, and interpretation of
                    this Section 10.02 (d), and all procedural aspects of the
                    arbitration conducted pursuant to this Section 10.02 (d),
                    including, but not limited to, the determination of issues
                    that are subject to arbitration (i.e. arbitrability), the
                    scope of the arbitrable issues, allegations of fraud in the
                    inducement to enter into this Agreement, or this arbitration
                    provision, allegations of waiver, laches, delay, or other
                    defenses to arbitrability, and the rules governing the
                    conduct of the arbitration (including the time for filing an
                    answer, the time for filing counterclaims, the times for
                    amending pleadings, the specificity of the pleadings, the
                    extent and scope of discovery, the issuance of subpoenas,
                    the times for designation of experts, whether the
                    arbitration is to be stayed pending resolution of related
                    litigation involving third parties not bound by this
                    Agreement, the receipt of evidence and the like), shall be
                    decided by the arbitrators. The arbitration shall be
                    administered by the American Arbitration Association
                    ("AAA"), and shall be conducted pursuant to the Commercial
                    Arbitration Rules of the AAA, as modified by this Agreement.
                    Notwithstanding any provision of this Agreement to the
                    contrary, the parties expressly agree that the arbitrators
                    shall have

--------------------------------------------------------------------------------
STOCK PURCHASE AGREEMENT                                                Page 23

<PAGE>   24

                    absolutely no authority to award incidental, special,
                    treble, exemplary, or punitive damages of any type under any
                    circumstances regardless of whether such damages may be
                    available under Texas law, the law of any other state, or
                    federal law, or under the Act, or the Commercial Arbitration
                    Rules of the AAA, the parties hereby waiving their right, if
                    any, to recover incidental, special, treble, exemplary, or
                    punitive damages in connection with any such Disputes.

              (ii)  The arbitration proceeding shall be conducted in Midland,
                    Texas before a panel of three (3) arbitrators appointed in
                    accordance with the Commercial Arbitration Rules of the AAA.
                    The arbitrators shall conduct a hearing as soon as
                    reasonably practicable after appointment of the third
                    arbitrator, and a final decision completely disposing of all
                    Disputes that are the subject of the arbitration proceedings
                    shall be rendered by the arbitrators as soon as reasonably
                    practicable after the hearing. There shall be no transcript
                    of the hearing before the arbitrators. The arbitrators'
                    ultimate decision after final hearing shall be in writing,
                    but shall be as brief as possible, and the arbitrators shall
                    not assign reasons for their ultimate decision.

              (iii) The fees and expenses of the arbitrators shall be borne
                    equally by the parties, but the decision of the arbitrators
                    may include such award of the arbitrators' fees and expenses
                    and of other costs and attorneys' fees as the arbitrators
                    determine appropriate.

              (iv)  To the fullest extent permitted by law, the arbitration
                    proceeding and the arbitrators' award shall be maintained in
                    confidence by the parties.

              (v)   The award of the arbitrators shall be binding upon the
                    parties and final and nonappealable to the maximum extent
                    permitted by law, and judgment thereon may be entered by a
                    court of competent jurisdiction and enforced by any party as
                    a final judgment of such court.

          (e) All applicable statutes of limitation and defenses based upon the
passage of time shall be tolled while the procedures specified in Sections
10.02(b) and (c) are pending. The parties will take such actions, if any, as may
be required to effectuate such tolling.

          (f) Each party is required to continue to perform its obligations
under this Agreement pending final resolution of any Dispute covered by this
Article X.
--------------------------------------------------------------------------------
STOCK PURCHASE AGREEMENT                                                Page 24

<PAGE>   25

                                   ARTICLE XI

                            MISCELLANEOUS PROVISIONS

         Section 11.01 AMENDMENT AND MODIFICATION. Subject to applicable law,
this Agreement may be amended, modified, or supplemented only by writing duly
executed by all of the parties hereto.

         Section 11.02 WAIVER OF COMPLIANCE. Any failure of the Sellers or the
Company, on the one hand, or Purchaser, on the other, to comply with any
obligation, covenant, agreement or condition contained herein may be expressly
waived in writing by Purchaser or Sellers and the Companies, respectively;
provided, however, such waiver or failure to insist upon strict compliance shall
not operate as a waiver of, or estoppel with respect to, any subsequent or other
failure.

         Section 11.03 NOTICES. All notices, requests, demands, and other
communications required or permitted hereunder shall be in writing and shall be
deemed to have been duly given if delivered by hand (including, without
limitation, by overnight courier), transmitted by facsimile, or mailed,
certified or registered mail (return receipt requested) with postage prepaid:

                  (a)      if to Sellers or the Company (only until the Closing
                           Date with respect to the Company), to:

                                    Mr. Dennis G. Harrison
                                    Harrison Well Service, Inc.
                                    P.O. Box 1548
                                    Denver City, Texas 79323
                                    Telephone: (806) 592-2433
with a copy to:

                                    Crenshaw, Dupree & Milam, L. L. P.
                                    1500 Broadway St.
                                    Lubbock, Texas 79408
                                    Telephone: (806) 762-5281
                                    Fax: (806) 762-3510
                                    Attention: John Crews

or to such other person or addresses as Sellers or the Company shall furnish
Purchaser in writing in accordance with this Section 11.03; and

--------------------------------------------------------------------------------
STOCK PURCHASE AGREEMENT                                                Page 25

<PAGE>   26

                (b)      if to Purchaser (or to the Company after Closing), to:

                                    Sierra Well Service, Inc.
                                    406 N. Big Spring
                                    Midland, Texas 79701
                                    Telephone: (915) 570-0829
                                    Fax: (915) 570-0598
                                    Attention: Kenneth V. Huseman, President

with a copy to:

                                    Kerr & Ward, L. L. P.
                                    500 W. Texas, Suite 1310
                                    Midland, Texas 79701
                                    Telephone: (915) 684-9990
                                    Fax: (915) 684-9997
                                    Attention: William M. Kerr, Jr.

or to such other persons or addresses as Purchaser shall furnish to Sellers and
the Company in writing in accordance with this Section 11.03.

         Delivery of notices shall be effective only upon actual receipt by the
intended recipient (or, in the case of facsimile transmission, the completion of
such transmission during the recipient's normal business hours).

         Section 11.04 SURVIVAL. This Agreement is intended in part to evidence
the continuing rights and obligations of the parties following Closing, and all
of the representations, warranties, covenants, and obligations of the parties
are hereby expressly made to survive Closing.

         Section 11.05 EXPENSES. Purchaser shall pay its own fees and expenses,
including, without limitation, professional fees and expenses, incurred in
connection with the negotiation, execution, and performance of this Agreement
and the transactions contemplated hereby. All fees and expenses of Seller and
all fees and expenses of the Company for periods to and including the Closing
Date, including, without limitation, professional fees and expenses, incurred in
connection with the negotiation, execution, and performance of this Agreement
and the transactions contemplated hereby, shall be borne and paid exclusively by
Sellers.

         Section 11.06 BINDING NATURE; NO THIRD PARTY BENEFICIARIES. This
Agreement and all of the provisions hereof shall be binding upon and inure to
the benefit of the parties hereto and their respective successors and permitted
assigns. Nothing contained herein, express or implied, is intended to confer on
any person other than the parties hereto or their respective successors and
permitted assigns, any rights, remedies, obligations or liabilities under or by
reason of this Agreement.

--------------------------------------------------------------------------------
STOCK PURCHASE AGREEMENT                                                Page 26

<PAGE>   27

         Section 11.07 GOVERNING LAW. This Agreement, and the legal relations
among the parties hereto arising from this Agreement, shall be governed by and
construed in accordance with the laws of the State of Texas, without regard to
its conflicts of laws rules.

         Section 11.08 ENTIRE AGREEMENT. This Agreement (including the exhibits
and schedules hereto and the other instruments referred to herein) embodies the
entire agreement and understanding of the parties hereto in respect of the
subject matter contained herein. There are no restrictions, promises,
warranties, covenants or undertakings, other than those expressly set forth or
referred to herein. This Agreement supersedes all prior agreements and
understandings among the parties with respect to such subject matter.

         Section  11.09 HEADINGS. The headings contained in this Agreement are
inserted for convenience only and shall not affect in any way the meaning or
interpretation of this Agreement.

         Section 11.10 COUNTERPARTS. This Agreement may be executed
simultaneously in two or more counterparts, each of which shall be deemed an
original but all of which together shall constitute one and the same instrument.

         Section 11.11 SEVERABILITY. If any provision of this Agreement is held
to be illegal, invalid, or unenforceable under present or future law, such
provision shall be fully severable and this Agreement shall be construed and
enforced as if such illegal, invalid, or unenforceable provision had never
comprised a part hereof, and the remaining provisions hereof shall remain in
full force and effect and shall not be affected by the illegal, invalid, or
unenforceable provision or by its severance herefrom. Furthermore, in lieu of
such illegal, invalid, or unenforceable provision, there shall be added
automatically, as part of this Agreement, a provision as similar in terms and
substance to such illegal, invalid, or unenforceable provision as may be
possible and legal, valid, and enforceable.












                      [THIS SPACE INTENTIONALLY LEFT BLANK]

--------------------------------------------------------------------------------
STOCK PURCHASE AGREEMENT                                                Page 27

<PAGE>   28
         IN WITNESS WHEREOF, the parties hereto have caused this Agreement to be
duly executed and made and entered into as of the date first set forth above.

                                        SELLERS:

                                        /s/ DENNIS G. HARRISON
                                        ------------------------------------
                                        DENNIS G. HARRISON

                                        /s/ SUE ANN HARRISON
                                        ------------------------------------
                                        SUE ANN HARRISON

                                        /s/ WILLIE EDWARDS
                                        ------------------------------------
                                        WILLIE EDWARDS


                                        THE COMPANY:

                                        HARRISON WELL SERVICE, INC., a
                                        Texas corporation


                                        By: /s/ DENNIS G. HARRISON
                                          ----------------------------------
                                          Dennis G. Harrison, President


                                        PURCHASER:

                                        SIERRA WELL SERVICE, INC., a
                                        Delaware corporation


                                        By: /s/ KENNETH V. HUSEMAN
                                          ----------------------------------

                                        Printed Name: KENNETH V. HUSEMAN
                                                    ------------------------
                                        Title: President
                                              ------------------------------



--------------------------------------------------------------------------------
STOCK PURCHASE AGREEMENT                                                Page 28

<PAGE>   29

LIST OF EXHIBITS AND SCHEDULES

EXHIBIT A           THE NOTE
EXHIBIT B           THE SECURITY AGREEMENT
EXHIBIT C           THE BALANCE SHEET
EXHIBIT D           THE NON-COMPETITION AGREEMENT

SCHEDULE 1.02       EXCLUDED ASSETS
SCHEDULE 2.08       EXTRAORDINARY TRANSACTIONS
SCHEDULE 2.10       EMPLOYEE BENEFIT PLANS
SCHEDULE 2.11       OWNERSHIP OF THE STOCK
SCHEDULE 2.12(a)    REAL PROPERTY
SCHEDULE 2.12(b)    PERSONAL PROPERTY
SCHEDULE 2.13       EXISTING LIENS
SCHEDULE 2.17       CONTRACTS
SCHEDULE 2.19       INSURANCE POLICIES
SCHEDULE 2.20       BANK ACCOUNTS
SCHEDULE 2.21       OTHER LIABILITIES



--------------------------------------------------------------------------------
STOCK PURCHASE AGREEMENT                                                Page 29

<PAGE>   30



                                    EXHIBIT A

                             SECURED PROMISSORY NOTE

$625,000.00                     Denver City, Texas           ____________, 2000

         FOR VALUE RECEIVED, the undersigned, SIERRA WELL SERVICE, INC., a
Delaware corporation ("Maker"), promises to pay to the order of DENNIS G.
HARRISON, TRUSTEE ("Payee"), at P. O. Box 1548, Denver City, Yoakum County,
Texas 79316, the principal sum of SIX HUNDRED TWENTY-FIVE THOUSAND AND NO/100
DOLLARS ($625,000.00), together with interest on the unpaid principal balance
from day to day outstanding prior to default or maturity at a floating per annum
rate which shall from quarter to quarter be equal to the lesser of (i) the Prime
Rate (hereinafter defined) in effect hereunder (the "Established Rate")
(calculated on the basis of actual days elapsed in a year consisting of 365 or
366 days, as appropriate) or (ii) the Maximum Rate (hereinafter defined)
(calculated on the basis of actual days elapsed in a year consisting of 365 or
366 days, as appropriate). The interest rate on this Note shall initially be
determined as of the first Friday most immediately preceeding the date of this
Note that is a business day (the "Initial Determination Date") and shall
thereafter be redetermined quarterly until maturity based upon the Prime Rate in
effect as of the first Friday most immediately preceding each ensuing calendar
quarter from the date hereof that is a business day (each such date being
hereinafter referred to as a "Redetermination Date"). Each change in the rate of
interest charged hereunder shall, subject to the terms hereof, become effective,
without notice to Maker, upon the applicable quarterly adjustment date (as
opposed to the Redetermination Date). If at any time and from time to time the
Established Rate exceeds the Maximum Rate, thereby causing the interest payable
to be limited to the Maximum Rate, then any subsequent reduction in the
Established Rate shall not reduce the rate of interest hereunder below the
Maximum Rate until the total amount of interest accrued hereon equals the amount
of interest that would have accrued if the Established Rate had at all times
been in effect. All past due principal of and accrued interest on this Note
shall bear interest at the Maximum Rate.

         As used herein, "Prime Rate" shall mean the floating rate of interest
denominated and published as such by the Wall Street Journal on the Initial
Determination Date and thereafter on each applicable Redetermination Date.

         As used herein, "Maximum Rate" shall mean the maximum non-usurious rate
of interest that at any time, or from time to time, may be contracted for,
taken, reserved, charged, or received under applicable law on the indebtedness
evidenced by this Note, after taking into account, to the extent required by
applicable law, any and all relevant payments, charges, or other amounts under
this Note and all instruments securing payment of this Note. To the extent that
Article 5069-1.04, Texas Revised Civil Statutes Annotated, as amended, is
relevant for purposes of determining the Maximum Rate, Payee hereby notifies
Maker that the applicable rate ceiling shall be the "indicated rate ceiling"
from time to time in effect, as limited by Article 5069-1.04(b); provided,
however, that to the extent permitted by applicable law, Payee reserves the
right to change the applicable rate ceiling from time to time by further notice
to Maker; and, provided further, that the Maximum Rate shall not be limited to
the applicable rate ceiling under Article 5069-1.04 if federal laws or other
state laws now or hereafter in effect and applicable to this Note (and the
interest contracted for, charged and collected hereunder) shall permit a higher
rate of interest.


--------------------------------------------------------------------------------
STOCK PURCHASE AGREEMENT                                                Page 30

<PAGE>   31


         The principal of and accrued interest on this Note are due and payable
one year from the date hereof. This Note may be prepaid in whole or in part
without penalty. Partial prepayments shall be applied first to any accrued and
unpaid interest and the balance remaining, if any, to principal.

         This Note is secured by that certain Security Agreement of even date
herewith, between Harrison Well Service, Inc., as Grantor, and Payee, as Secured
Party, covering all of the well servicing units, vehicles, equipment, and other
tangible personal property therein described.

         It is understood and agreed that in the event of default in the payment
of this Note or any installment hereof, principal or interest, and if such
default shall continue unremedied for more than ten (10) days following written
notice thereof from Payee to Maker, or in the event of any default (other than
non-payment of this Note) in any instrument securing payment of this Note and if
such default shall continue unremedied for more than thirty (30) days following
written notice thereof from Payee to Maker, the entire principal balance of and
all accrued and unpaid interest on this Note shall at once become due and
payable without notice, at the option of Payee. Failure by Payee to exercise
this option on any one or more occasions shall not constitute a waiver of the
right to exercise such option in the event of subsequent default.

         Except as otherwise herein expressly provided, the makers, signers,
sureties, and endorsers of this Note jointly and severally waive demand,
presentment, notice of dishonor, notice of intent to demand or accelerate
payment hereof, diligence in collecting, grace, notice, and protest, and agree
to one or more extensions for any period or periods of time and partial
payments, before or after maturity, without prejudice to Payee; and if this Note
shall be collected by legal proceedings or through probate or bankruptcy court,
or shall be placed in the hands of an attorney for collection after default or
maturity, Maker agrees to pay all costs of collection, including reasonable
attorney's fees.

         All agreements between Maker and Payee, whether now existing or
hereafter arising and whether written or oral, are hereby limited so that under
no contingency, whether by reason or demand for payment or acceleration of the
maturity hereof or otherwise, shall the interest contracted for, charged, or
received by Payee exceed the Maximum Rate. If, for any circumstance whatsoever,
interest would otherwise be payable to Payee in excess of the Maximum Rate, the
interest payable to Payee hereunder shall be reduced to the Maximum Rate; and if
for any circumstance Payee shall ever receive anything of value deemed interest
by applicable law in excess of the Maximum Rate, then an amount equal to any
such excess shall be applied to the reduction of the principal hereof and not
the payment of interest, or if such excessive interest exceeds the unpaid
balance of principal hereof, such excess shall be refunded to Maker. All
interest paid or agreed to be paid to Payee shall, to the extent permitted by
applicable law, be amortized, prorated, allocated, and spread throughout the
full period until payment in full of the principal (including the period of any
renewal or extension hereof) so that the interest hereon for such full period
shall not exceed the Maximum Rate.

         Any provisions of this Agreement to the contrary notwithstanding, if
Maker shall successfully conclude an initial public offering of common stock in
Maker through a nationally recognized stock exchange at any time prior to
maturity or payment in full of this Note, whether stated or by acceleration,
(the "Sierra IPO"), Maker and Payee agree to immediately convert the unpaid
balance of this Note to common stock in Maker at a per share price equivalent to
the per share price at which Maker issued its common stock in the IPO. Maker and
Payee shall effectuate such conversion of indebtedness for stock through the
delivery of this Note by Payee to Maker, marked "PAID," in return for the
delivery by Maker of the requisite number of shares of its common stock to
Payee.

--------------------------------------------------------------------------------
STOCK PURCHASE AGREEMENT                                                Page 31

<PAGE>   32

         This Note is in all respects subject to that certain Stock Purchase
Agreement dated as of December __, 1999, between Payee, as Seller, Harrison Well
Service, Inc., as the Company, and Maker, as Purchaser.


                      [THIS SPACE INTENTIONALLY LEFT BLANK]





















--------------------------------------------------------------------------------
STOCK PURCHASE AGREEMENT                                                Page 32

<PAGE>   33

         This Note shall be governed by and construed in accordance with the
laws of the State of Texas.



                                        SIERRA WELL SERVICE, INC.


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






--------------------------------------------------------------------------------
STOCK PURCHASE AGREEMENT                                                Page 33

<PAGE>   34
                   FIRST AMENDMENT TO STOCK PURCHASE AGREEMENT


         THIS FIRST AMENDMENT TO STOCK PURCHASE AGREEMENT ("First Amendment") is
made and entered into as of the 20th day of March 2000, between DENNIS G.
HARRISON, SUE ANN HARRISON, and WILLIE EDWARDS (collectively, "Sellers"),
HARRISON WELL SERVICE, INC., a Texas corporation (the "Company"), and SIERRA
WELL SERVICE, INC., a Delaware corporation ("Purchaser"), with reference to the
following:

                                    RECITALS

         A. Sellers, the Company, and Purchaser are parties to that certain
Stock Purchase Agreement (the "Agreement") dated as of December 29, 1999,
relating to the sale and purchase of all of the issued and outstanding capital
stock of the Company.

         B. Seller and Purchaser wish to amend the Agreement upon the terms
which follow.

                                    AGREEMENT

         NOW, THEREFORE, for good and valuable consideration, the receipt and
sufficiency of which are hereby acknowledged, the parties agree as follows:

         1. EXTENSION OF CLOSING DATE. Sellers, the Company, and Purchaser
hereby modify and amend existing Section 8.01 of the Agreement to read as
follows:

                  Section 8.01 CLOSING. The closing ("Closing") of the
                  transaction contemplated hereby shall take place at the
                  offices of Purchaser at 406 N. Big Spring, Midland, Texas, on
                  a business date of Purchaser's unilateral selection within
                  thirty (30) days following any successful completion by
                  Purchaser of an initial public offering of common stock in the
                  Company through a recognized national stock exchange (the
                  "Sierra IPO"), but in no event later than June 30, 2000, or at
                  such other place and time as the parties hereto might
                  hereafter mutually agree in writing. Such date or any
                  alternative date so selected by the parties is referred to in
                  this Agreement as the "Closing Date." Purchaser shall afford
                  Sellers and the Company at least ten (10) days prior written
                  notice of its desired Closing Date pursuant to the foregoing.

         2. DEFINITIONS. Except as expressly indicated otherwise herein,
capitalized terms in this First Amendment shall have the same meanings as are
ascribed to them in the Agreement.

         3. CONFIRMATION. The parties hereto hereby ratify, confirm, and adopt
the Agreement, as amended hereby. Except as modified hereby, the Agreement
remains in full force and effect.

<PAGE>   35

         4. FAX; COUNTERPARTS. This First Amendment may be executed by fax and
in multiple counterparts.

         IN WITNESS WHEREOF, the parties hereto have executed and delivered this
First Amendment as of the day and year first above written.


                                        SELLERS:

                                        /s/ DENNIS G. HARRISON
                                        ------------------------------------
                                        DENNIS G. HARRISON

                                        /s/ SUE ANN HARRISON
                                        ------------------------------------
                                        SUE ANN HARRISON

                                        /s/ WILLIE EDWARDS
                                        ------------------------------------
                                        WILLIE EDWARDS


                                        THE COMPANY:

                                        HARRISON WELL SERVICE, INC.,
                                        a Texas corporation


                                        By: /s/ DENNIS G. HARRISON
                                            ---------------------------------
                                             Dennis G. Harrison, President


                                        PURCHASER:

                                        SIERRA WELL SERVICE, INC., a Delaware
                                        corporation


                                        By: /s/ CHARLES SWIFT
                                            ---------------------------------
                                        Printed Name:  CHARLES SWIFT
                                                     ------------------------
                                        Title:         Vice President
                                               ------------------------------



FIRST AMENDMENT TO STOCK PURCHASE AGREEMENT                               PAGE 2