As a reservoir is produced, after an initial peak and plateau period, the production rate starts to decline at certain rate. Analyzing this decline, a Reservoir Engineer can predict the Estimated Ultimate Recovery of the field. This technique is called DCA or Decline Curve Analysis.
While identifying the decline rate or decline trend, one needs to make sure the production is occurring at constant "operating conditions". For example, constant "bean size" in a producing well.
The scope of analysis may be
(a) an individual well or well-string
(b) a given reservoir zone
(c) a field
(d) a bigger area or region
or even
(e) global
However, individual well-string or a given reservoir zone are commonly used or technically valid.
There are other kinds of 'trend analysis', e.g.
(a) Water-Oil-Ratio (WOR) vs. Cum Oil
(b) Gas-Oil-Ratio (GOR) vs. Cum. Oil
These are also used in conjunction with oil decline analysis to determine the economic life of the well or reservoir or the field in question.
For gas reservoir, a different approach, "p/Z-plot" is adopted. It will be discussed separately.
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