In petroleum production, it is very important to get the gas holdup of oil-gas-water multiphase flows in producing wells accurately. However, it's very difficult due to the complexity and randomness of the multiphase flows.
Because of various flow types of the two-phase flows, existing methods all have limited accuracies for obtaining the flow velocity or holdup of gas phase or have other shortcomings. In order to solve the problem, a research group of State Key Laboratory of Acoustics, Institute of Acoustics, Chinese Academy of Sciences, led by Prof. WANG Xiaomin and Prof. WANG Xiuming, carried out a series of studies and primarily proposed a transient method based on the acoustic resonant technique to measure gas volume fraction of gas-fluid flows.
In the study, the researchers investigate the feasibility of measuring the gas volume fraction in a mixed gas-liquid flow by using ARS method in a transient way theoretically and experimentally. They first investigate the effects of sizes and locations of a single air bubble in a cylindrical cavity with two open ends on resonant frequencies numerically. Then, they establish a transient measurement system for acoustic resonant Spectroscopy (ARS) and study the trends of the resonant frequencies (RFs) and resonant amplitudes (RAs) in the cylindrical cavity with gas flux inside experimentally.
After the experiments, the researchers compare the measurement result got by the transient method with those by steady-state ones and numerical ones. The numerical results show that the RFs of the cavity are highly sensitive to the volume of the single air bubble. A tiny bubble volume perturbation may cause a prominent RF shift even though the volume of the air bubble is smaller than 0.1% of that of the cavity. When the small air bubble moves, the RF shift will change and reach its maximum value as it is located at the middle of the cavity. As the gas volume fraction of the two-phase flow is low, both the RFs and RAs from the measurement results decrease dramatically with the increasing gas volume, and this decreasing trend gradually becomes even as the gas volume fraction increases further. In addition, the transient method for ARS is more suitable for measuring the gas volume fraction with randomness and instantaneity than the steady-state one, because the latter could not reflect the random and instant characteristics of the mixed fluid due to the time consumption for frequency sweeping. The result shows that this study will play a very important role in the quantitative measurement of the gas volume fraction of multiphase flows.
This research result was published on the recently issued journal of SCIENCE CHINA-PHYSICS MECHANICS & ASTRONOMY (Vol.53, No.8, UG 2010, P. 1412-1418).