A static mixing device was tested in combination with a dust-energy gamma volume fraction meter and a venture meter in order to determine if this combination could be used to reliably measure oil-water-gas rates under flow conditions typical for offshore oil and gas production especially in the North Sea. A high-rate test facility was used to circulate oil, water and air or nitrogen at rates of up to 60 m3/hr(l 0000 bbl/d) of liquids and total flow of up to 200 m3hr(30000 bbl/d). The tests were carried out primarily in the unsteady slug and bubble flow regimes with gas volume-fraction up to 100 percent and water-oil ratio up to 60 percent. In order to estimate the accuracy of the meter system under test, the single-phase flow rates of the three components were measured independently upstream of the mixing unit which was placed directly upstream of the dual-energy gamma and venture meters.
The results of the tests carried out with and without the mixing unit show that the mixer eliminates the slip between the gas and liquid phases well enough to obtain repeatable estimates of oil and gas volume fractions within ± 10 percent relative error over the entire range of conditions tested for fractions over 30%. Oil flowrates were also measured to within an accuracy of 10 percent compared to the independent single-phase rates over a wide range of conditions up to percent 70 percent gas volume.
No attempt was made in this study to optimize the performance of the combination of the mixer and two meters but the test results and experience indicate scope for improvements to the arrangement between these three main elements and to the dual-energy gamma technique itself. In addition, a follow-up feasibility and pre-engineering study was carried out to establish design criteria for a subsea meter based on the tested technologies. A prototype subsea meter is under design and further testing is expected to begin late in 1991 in the current phase of the project.
The measurement of oil-water-gas production rates by means other than test separators has been a subject of intense research end development because of its potential to eliminate the need for test separators on platforms, to supplement test-separator capacity for satalite developments and for reservoir monitoring. Many methods based on various physical phenomena (such as gamma-ray attenuation, microwave, capacitance)have been tested with varing degrees of success depending on the composition, flow rates of the phases and flow regime or pattern. In all the cases reported where it is attempted to measure the flow without first separating the phases, slip between the phases causes errors which are too large (much greater than 10 percent) to be considered useful for the range of operating conditions that can be expected for typical offshore wells. In those cases where initial separation of the phases was undertaken prior to single or two-phase measurements, additional uncertainty and/or complexity is introduced which limits the usefulness this approach.
