Some geologic formations will incur flat or constant injection rates with variable injection pressure, while other formations will have decreasing injection rate with constant injection pressure.
This is primarily due to surface operations and the outer boundary condition of the reservoir (closed or open).
A decreasing injection rate trend can be analyzed and extrapolated to an uneconomic injection rate to infer the ultimate storage capacity of an active injection well.
Mass or volumetric balance can be used to estimate subsurface injected volumes using cumulative injection volume as a function of pore pressure.
Based on the geologic unit, combinations of parameters are used to allow straight line projections of the observed trends to make forecasts of the ultimate storage capacity.