Golden rule of decline curve analysis (DCA)
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The basic assumption in this procedure is that whatever causes controlled the trend of a curve in the past will continue to govern its trend in the future in a uniform manner. 
This rule means that any change in the behavior of the reservoir or well (the beginning or change in the injection rate in neighboring injectors, the addition of artificial lifting, stimulation, etc.), with the exception of the pressure drop in the reservoir itself, will violate the DCA equation. When analyzing the decline curve, one should choose a period with constant conditions (both for matching and for forecasting).