Summary
The litmus test for downhole multiphase flowmeters is to compare the measured phase flow rates with the rates from a test separator or other surface measurement systems. In most cases, the composition of the measurand is required for flowmeters. This is typically obtained from bottomhole fluid samples. Extracting and analyzing fluid samples is an expensive process mostly done at the initial stages of field development. In some cases, the composition may be old or unavailable, leading to subpar flowmeter performance compared to surface systems. In this work, it is shown that when the data from a surface system such as a test separator are used in conjunction with the mixture sound speed measured downhole, it is possible to optimize a downhole multiphase flowmeter system without obtaining new fluid samples. The optimization process is independent of the downhole measurement device because the required flow-velocity and sound-speed measurements may be obtained from separate devices. For example, the fluid bulk velocity and mixture sound speed can be measured by a local measurement device and a distributed acoustic sensing (DAS) system, respectively. The main challenge in a flow-velocity/sound-speed measurement system is determining individual phase sound speeds so that the mixture phase fraction can be correctly determined using Wood’s mixture sound speed model. The phase fraction from the separator tests can be used as the target value to optimize the performance of the system. The system has two operation modes. In optimization mode, the individual phase sound speeds are calculated backward using the predicted phase fractions from surface measurements. Pressure and temperature variations at measurement locations, as well as pipe compliance effects, are accounted for during the process. After the adjustment of individual phase sound speeds, steady-state operation mode takes over, and a forward calculation is implemented using the same model. The final phase fraction agrees well with the actual value and can be improved further with an iterative approach. This novel method is demonstrated in a North Sea case history. A downhole optical flowmeter in a North Sea field measured mixture velocity and sound speed. Well-test results indicated that water cut from the flowmeter was underreported and phase flow rates did not match test-separator rates. Instead of halting production and going through a fluid sample analysis cycle, the test-separator water cut was used as the target value to optimize oil phase sound speed using Wood’s model in the optimization mode. The difference between the initial and optimized oil sound speeds was extrapolated to other pressure and temperature conditions, and steady-state operation mode showed that separator tests and flowmeter measurements closely matched. Subsequent flowmeter and test-separator data confirmed excellent agreement. Using surface measurements and downhole mixture sound speed to optimize phase flow rates is a novel method that has not been previously demonstrated. This method is independent of device type, is broadly applicable, and improves the understanding of multiphase flow measurement.
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