Analysis of Decline Curves

Abstract

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