Mannon, Robert W.,* Member AIME, Montana College of Mineral Science and Technology, Butte, Mont.

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This paper was to be presented at the 40th Annual Fall Meeting of the Society of Petroleum Engineers of AIME, to be held in Denver, Colorado, October 3–6, 1965, and is considered to an abstract of not more than 300 words, with no illustrations, unless the paper is specifically released to the press by the Editor of the Journal of Petroleum Technology or the Executive Secretary. Such abstract else where after publication in the Journal of Petroleum Technology or Society of Petroleum Engineers Journal is granted on request, providing proper credit is given that publication and the original presentation of the paper.

Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussion may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines.

Abstract

The subject of analyzing oilwell production-decline curves is examined in detail. The problem is approached from the standpoints of raw data validity, techniques of data handling, modes of graphic representation, and final interpretation procedures. Forms of analysis based on mathematical treatments, factors of analogy and experience, and predicted reservoir behavior efforts are discussed. It is evident that all of the methods of decline curve analysis are subject to gross forecasting error in specific situations. Examples are given to illustrate the over-all problem of reliability. The accepted method for classifying decline curves is reviewed. The mathematical definition for hyperbolic decline is found to be too restrictive and a broader definition is suggested.

Gradual or abrupt changes in the producing rate of a well due to reservoir depletion, fluctuation in bottom-hole producing pressure, and changes in conditions in or immediately adjacent to the wellbore., are examined. A method is presented utilizing comparative theoretical and actual productivity index behavior that can be useful in predicting future rates of production.

Introduction

Of the myriad tasks that face the petroleum engineer in his professional duties, the forecasting of future production from a well or group of wells is one of the most formidable. Webster's Dictionary defines the word forecast as meaning: to predict; to foretell; to prophesy. Many a petroleum engineer has felt the need to be possessed of prophetic powers when called upon to predict the future performance of his company's oil wells.

The job of making reliable production forecasts can be laborious and time-consuming. Notwithstanding the effort required, moreover, industry insists that the petroleum engineer'sforecasts be trustworthy. We might ask, "Inview of the rigors of preparing dependable predictions, are oil industry people justified in demanding such exactness?" The answer to this question is, of course, "Yes", and the reasons are quite evident. Oil companies and related organizations rely heavily on production forecasts as an integral part of profitability studies, financing and loan work, capital budgeting, and the trading of oil and gas properties. Much of their capacity for future growth lies in these potential areas.

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