Abstract
Two-phase oil and gas flow were simulated in an entirely eccentric annulus and compared with experimental data at horizontal, 4, 10, and 90° inclination. The gas-phase was sulphur hexafluoride and the liquid phase a mixture of Exxsol D60 and Marcol 82 for the inclined cases (5–16), and pure Exxsol D60 for the horizontal cases (1–4). The diameter of the outer and inner cylinders was 0.1 and 0.04 m, respectively, for the inclined domains and 0.1 and 0.05 m for the horizontal domain. The cases studied consist of liquid phase fractions between 0.3 and 0.65 and mixture velocities from 1.2 to 4.25 m/s. The mean pressure gradient is within 33% of the expected experimental behavior for all inclined cases. In contrast, the low-velocity horizontal domains exhibit significant deviation, with a drastic over-prediction of the mean pressure gradient by as much as 200–335% for cases 1 and 2. The two remaining horizontal cases (3 and 4) are within 22% of the expected mean pressure gradient. Cases 13–16 are a replication of cases 5–8 at an increased inclination; the mean pressure gradient is within 6.5% of the expected increase due to the increase in hydrostatic pressure. By comparing cases 1–4 to previous published simulations at a lower eccentricity, we found a decrease of the mean pressure gradient by 30–40%, which is in line with existing literature, although for single-phase flows. The simulated and experimental liquid holdup profiles are in good agreement when comparing the fractional data; wave and slug frequencies match to within 0.5 Hz; however, at closer inspection, it is apparent that there is a decrease in the amount of phase-mixing of the simulations compared to the experiments. When increasing the mesh density from 115 k cells/m to 2 million cells/m, the simulations exhibit significantly more phase mixing, but are still unable to produce conventional slugs. In a simplified case, conventional slugs are observed at grid sizing of 1 × 1 × 1 mm, whereas the cells of the 2 million cells/m mesh are roughly 1.5 × 1.5 × 1.5 mm.
Highlights
The purpose of this thesis is to explore, study, and analyze two-phase flow in an annulus by comparing computational fluid dynamics simulations using OpenFOAM with experimental data gathered at Institute for Energy Technology
The literature covers topics such as slug frequency (Perez et al, 2007; Hout et al, 2003; Schulkes, 2011), holdup profile (Beggs and Brill, 1973; Bonnecaze et al, 1971), pressure drop (Strazza et al, 2011; Salem, 2008; Ilic, 1986; Ghajar and Bhagwat, 2014), mechanical losses (Liu et al, 2015) and flow regime (ArchibongEso et al, 2016; Oddie et al, 2003). We frequently find these topics in the body of work related to two-phase flow, the subjects are rare when studying the annulus
If we look past the average holdup increase, which is stipulated by the initial conditions, we can notice that there are some fundamental changes to the holdup pattern
Summary
The purpose of this thesis is to explore, study, and analyze two-phase flow in an annulus by comparing computational fluid dynamics simulations using OpenFOAM with experimental data gathered at Institute for Energy Technology. The oil and gas industry has, for many years, relied upon experimental data and correlations built on empirical methods to predict flow behavior within wells, pipelines, and risers. Through vast data-sets, models are developed to predict flow regime, pressure behavior and flow rates. An empirical model may incorrectly predict flow behavior, and falsely predict the flow regime, predicting for example stratified or annular flow rather than slug flow, which is identified by intermittent large pressure spikes and liquid bodies which cover the cross-section. The modeling, simulation and validation of two-phase pipe-flow and flow regimes are fairly well understood.
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