A process fluid flow system comprising: an input, an output, and a plurality of fluid circuits disposed between the input and output; at least one valve (120, 122) disposed to selectably cause process fluid to flow through either a first circuit of the plurality of circuits, or a second circuit of the plurality of circuits, wherein process fluid flows through the first circuit during normal production, and through the second circuit during process fluid sequestration; a process fluid flow measurement device operably interposed between the input and output and configured to measure total flow through the system; a separator disposed in the second circuit, the separator being arranged to allow the process fluid to separate gravitationally over time into immiscible components of the process fluid, the separator having a known internal volume; a level measurement device disposed to measure heights of interfaces within the separator; characterized in that the level measurement device is a guided wave radar level measurement transmitter and further a controller is coupled to the at least one valve (120, 122), the process fluid flow measurement device, the level measurement device, and a differential pressure transmitter coupled to a pair of pressure taps (152, 154) on the separator, the differential pressure transmitter being configured to provide an overall bulk density measurement of the process fluid trapped within the separator, the controller being configured to combine total process fluid flow information from the process fluid flow measurement device with measured height information from the level measurement device and the overall bulk density measurement to provide fractional flow rates for at least one immiscible component of the process fluid. an input, an output, and a plurality of fluid circuits disposed between the input and output; at least one valve (120, 122) disposed to selectably cause process fluid to flow through either a first circuit of the plurality of circuits, or a second circuit of the plurality of circuits, wherein process fluid flows through the first circuit during normal production, and through the second circuit during process fluid sequestration; a process fluid flow measurement device operably interposed between the input and output and configured to measure total flow through the system; a separator disposed in the second circuit, the separator being arranged to allow the process fluid to separate gravitationally over time into immiscible components of the process fluid, the separator having a known internal volume; a level measurement device disposed to measure heights of interfaces within the separator; characterized in that the level measurement device is a guided wave radar level measurement transmitter and further a controller is coupled to the at least one valve (120, 122), the process fluid flow measurement device, the level measurement device, and a differential pressure transmitter coupled to a pair of pressure taps (152, 154) on the separator, the differential pressure transmitter being configured to provide an overall bulk density measurement of the process fluid trapped within the separator, the controller being configured to combine total process fluid flow information from the process fluid flow measurement device with measured height information from the level measurement device and the overall bulk density measurement to provide fractional flow rates for at least one immiscible component of the process fluid.