TABLE OF CONTENTS Production-decline analysis is the analysis of past trends of declining production performance, that is, rate versus time and rate versus cumulative production plots, for wells and reservoirs. From about 1975 to 2005, various methods have been developed for estimating reserves in tight gas reservoirs. These methods range from the basic material balance equation to decline- and type-curve analysis techniques. There are two kinds of decline-curve analysis techniques, namely,
The classical curve fit of historical production data
The type-curve matching technique
Some graphical solutions use a combination of decline curves and type curves with varying limitations. General principles of both types and methods of combining both approaches to determine gas reserves are briefly presented in this chapter.
Decline curves are one of the most extensively used forms of data analysis employed in evaluating gas reserves and predicting future production. The decline-curve analysis technique is based on the assumption that past production trends and their controlling factors will continue in the future and, therefore, can be extrapolated and described by a mathematical expression.
The method of extrapolating a "trend" for the purpose of estimating future performance must satisfy the condition that the factors that caused changes in past performance, for example, decline in the flow rate, will operate in the same way in the future. These decline curves are characterized by three factors:
Initial production rate, or the rate at some particular time
Curvature of the decline
Rate of decline
These factors are a complex function of numerous parameters...
