Abstract: A differential equation for production decline rate is set up based on the study of variation of production decline curve, by which the dimensionless decline equation for describing the production decline rule is deduced. This equation shows that after oil production decline of any reservoir occurs, the dimensionless cumulative production rate and dimensionless time in rectangular system are all straight lines which pass through origin of coordinates with slope of 1, hence the cumulative production rate and annual oil production rate of the reservoir could be extrapolated and predicted. The equation for variation of production rate with cumulative production rate is then derived. By this equation, the residual recoverable reserves and abandoned annual production rate could be predicted. In addition, the reasonable limit of reserve-production ratio when production decline occurs is researched. Case studies indicate the correctness and practicability of these equations which can be used in reservoir engineering study and prediction.

Key words: decline rate, production rate, recoverable reserve, reserve-production ratio, prediction