As fluids are produced from a sub-surface reservoir, pressure of the remaining fluids decreases. Generally speaking, oil production will tend to enter natural decline once around 50% of oil on the reservoir has been produced. The annual rate in per cent per year at which production declines is known as the decline rate. Natural reservoir decline can be distinguished from managed decline, whereby efforts are made to sustain reservoir pressure and to enhance total oil recovery. Individual fields can therefore have a multiple production peaks.

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