Since production curtailment for other than engineering reasons is graduallydisappearing, and more and more wells are now producing at capacity and showingdeclining production rates, it was considered timely to present a brief reviewof the development of decline-curve analysis during the past three or fourdecades. Several of the commoner types of decline curves were discussed in detail andthe mathematical relationships between production rate, time, cumulativeproduction and decline percentage for each case were studied. The well-known loss-ratio method was found to be an extremely valuable toolfor statistical analysis and extrapolation of various types of curves. Atentative classification of decline curves, based on their loss ratios, wassuggested. Some new graphical methods were introduced to facilitate estimationof the future life and the future production of producing properties wherecurves are plotted on semilogarithmic paper. To facilitate graphical extrapolation of hyperbolic-type decline curves, aseries of decline charts was proposed, which will make straight-lineextrapolation of both rate-time and rate-cumulative curves possible. Introduction During the period of severe production curtailment, which is now behind us, production-decline curves lost most of their usefulness and popularity inprorated areas because the production rates of all wells, except those in thestripper class, were constant or almost constant. While production-decline curves were thus losing in importance forestimating reserves, an increasing reservoir consciousness and a betterunderstanding of reservoir performance developed among petroleum engineers.This fact, together with intelligent interpretation and use of electric logs, core-analysis data, bottom-hole pressure behavior and physical characteristicsof reservoir fluids, eliminated a considerable part of the guesswork inprevious volumetric methods and put reserve estimates, based on this method, ona sound scientific basis. At the same time, a number of ingenious substituteswere developed for the regular production-decline curve, which made it possibleto obtain an independent check on volumetric estimates in appraisal work, eventhough the production rates were constant. With the now steadily increasing demand for oil to supply the hugerequirements of this global war, proration for reasons other than prevention ofunderground waste is gradually disappearing. More and more wells are, or willbe, producing at capacity or at their optimum rates, as determined by soundengineering practice. T.P. 1758
Published in Petroleum Transactions, AIME, Volume 207, 1956, pages 182–191. Abstract This paper reviews the methods currently in use for estimating primary oil reserves and discusses the principles on which these methods are based. Particular emphasis is placed on how these methods change with the type of information available during the life cycle of an oil property. This paper contains various novel estimating methods and shortcuts heretofore unpublished. Introduction Estimating oil reserves is one of the most important phases of the work of a petroleum engineer since the solutions to the problems he deals with usually depend on a comparison of the estimated cost in terms of dollars, with the anticipated result in terms of barrels of oil. His recommendations to management regarding the best course of action are therefore normally based on the most favorable balance between these two. Specific engineering problems which require such a knowledge of recoverable oil reserves and a projection of future rates are:the exploitation and development of an oil reservoir;the construction of gasoline plants, pipelines and refineries;the division of ownership in unitized projects;the price to be paid in case of a sale or purchase of an oil property, and the magnitude of the loan which it will support;the proper depreciation rate for the investment in oil properties; andevaluation of the results of an exploration program. This discussion will be confined to the various methods and tools which are currently in use for estimating oil reserves to be obtained during the primary phase of an oil-producing reservoir and for a projection of the future production rates. Reserves which may be obtained by secondary recovery methods or fluid injection programs and gas and gas condensate reserves will not be discussed in this paper.
A ptwcticwl solution to the inherent "blindness" O/ the rotury drii[ing systent has been developed and field tested successfully, [t consists of a novel teleoteiry system which lnukes it possible to of)min at the surface, conct.wretn with the driiiing of u well, continuous logs on one or more pilysic.d parameters of the freshly penetrated fortnation at the bit. The method wili also be made avaiiable for var-. ious warning attd control systems pertaining to the conclitimt or operation of the driliing equipment at the bottotn of fhe hole.A measurement at tile bottom of the /roIe is trattsiated ilito a puise code which periodically triggers a hydrauIic tmui valve in the drill collar. Partiai ciosings of this valve for a short period oj titne create pressure pulses in the ingoing mud sfream which are readiiy detected at the surface as small increases in pretisure. After decoding this pressure-pulse pattern arriving at the surface, the resulting readings are autowetically piotted on a recording paper strip wilich moves as the dril! progresses downward.Examples of continuous logs are shown, and a genera[ description of the equipment used in the teientetry system is given, htrodltction The Need for Continuous Logging in Rotary Drilling of Exp[orwtory WellsThe basic purpose of drilling exploratory wells is to find and evaluate new oil and gas reservoirs, The emphasis is therefore on gaining significant information about possible productive zones, 'and the drilling of the well itself is in most cases merely a means to this end, Whi[e drilling, an exploratory well often encounters unexpected geological conditions and new potential reservoirs. Obviously, the best time to acquire pertinent information about such reservoirs is when they are freshly penetrated and least disturbed by mud filtrate. AIso, optimum drilling speed and minimum drilling cost can I.@y .be obtained if the exact nature of the changing formations being drilled is known at all. times.Beta-!se o! these 'cotisiderations, "the' 'search-for a-reliable-'and 'diagnostic" -method -of gaining such information tvhiie driiling tind IZOt afterward is as old as the drilling industry itself,
Published in Petroleum Transactions, AIME, Volume 204, 1955, pages 120–127. Introduction Since the introduction of the relative permeability concept in the middle thirties various investigators have shown how the basic equations for the flow of oil and gas through porous media can be utilized to compute the recovery from depletion type or solution gas drive type reservoirs under certain conditions, when the necessary physical data pertaining to the reservoir rock and its reservoir fluids are known. In the study by Muskat and Taylor in 1945, the effects of viscosity, gas solubility, shrinkage, and the permeability-saturation characteristics of the producing formation on the production histories and the recovery of gas drive reservoirs were analyzed. Each parameter was varied a limited number of times, while keeping the others constant. The work was not carried to the point where the data could be used directly to estimate the primary recovery in those cases where certain rock and fluid characteristics were known or could be assumed. Since the publication of this classic study, the work by Beal in 1946 and Standing in 1952 has established general correlations of oil viscosity and shrinkage with gas solubility and oil gravity. In addition, a considerable number of relative permeability relationships for different types of reservoir rocks has been published. Also, the burden of the numerical work required in the solutions of the differential equation has, during the last few years, been greatly reduced with the availability of modern electronic computing facilities. The purpose of this paper is therefore to present the results of a large number of such recovery computations covering the normal range of variation of the main parameters such as the type of rock, the API gravity of the oil, and the amount of gas in solution. As is to be expected, the ultimate recovery is found to increase with the oil gravity, except for the higher solution gas-oil ratios.
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