Gas lift is a means of enhancing oil recovery from hydrocarbon reservoirs. Gas
injected at the production riser base reduces the gravity component of the
pressure drop and thereby, increases the supply of oil from the reservoir. Also,
gas injection at the base of a riser helps to mitigate slugging and thus,
improving the performance of the topside facility. In order to improve the
efficiency of the gas lifting technique, a good understanding of the
characteristics of gas-liquid multiphase flow in vertical pipes is very important.
In this study, experiments of gas/liquid (air/water) two-phase flows, liquid/liquid
of oil/water two-phase flows and gas/liquid/liquid (air/oil/water) three-phase
flows were conducted in a 10.5 m high 52 mm ID vertical riser. These
experiments were performed at liquid and gas superficial velocities ranging from
0.25 to 2 m/s and ~0.1 to ~6.30 m/s, respectively. Dielectric oil and tap water
were used as test fluids. Instruments such as Coriolis mass flow meter, single
beam gamma densitometer and wire-mesh sensor (WMS) were employed for
investigating the flow characteristics. For the experiments of gas/liquid
(air/water) two-phase flow, flow patterns of Bubbly, slug, churn flow regimes and
transition regions were identified under the experimental conditions. Also, for
flow pattern identification and void fraction measurements, the capacitance
WMS results are consistent with those obtained simultaneously by the gamma
densitometer. Generally, the total pressure gradient along the vertical riser has
shown a significant decrease as the injected gas superficial velocity increased.
In addition, the rate of decrease in total pressure gradient at the lower injected
gas superficial velocities was found to be higher than that for higher gas
superficial velocities. The frictional pressure gradient was also found to increase
as the injected gas superficial velocity increased.
For oil-water experiments, mixture density and total pressure gradient across
the riser were found to increase with increasing water cut (ranging between 0 -
100%) and/or mixture superficial velocity. Phase slip between the oil and water
was calculated and found to be significant at lower throughputs of 0.25 and 0.5
m/s. The phase inversion point always takes place at a point of input water cut
of 42% when the experiments started from pure oil to water, and at an input
water cut of 45% when the experiment’s route started from water to pure oil.
The phase inversion point was accompanied by a peak increase of pressure
gradient, particularly at higher oil-water mixture superficial velocities of 1, 1.5
and 2 m/s.
The effects of air injection rates on the fluid flow characteristics were studied by
emphasizing the total pressure gradient behaviour and identifying the flow
pattern by analysing the output signals from gamma and WMS in air/oil/water
experiments. Generally, riser base gas injection does not affect the water cut at
the phase inversion point. However, a slight shift forward for the identified
phase inversion point was found at highest flow rates of injected gas where the
flow patterns were indicated as churn to annular flow. In terms of pressure
gradient, the gas lifting efficiency (lowering pressure gradient) shows greater
improvement after the phase inversion point (higher water cuts) than before and
also at the inversion point.
Also, it was found that the measured mean void fraction reaches its lowest
value at the phase inversion point. These void fraction results were found to be
consistent with previously published results.
| Identifer | oai:union.ndltd.org:CRANFIELD1/oai:dspace.lib.cranfield.ac.uk:1826/8507 |
| Date | 01 1900 |
| Creators | Brini Ahmed, Salem Kalifa |
| Contributors | Yeung, Hoi |
| Publisher | Cranfield University |
| Source Sets | CRANFIELD1 |
| Language | English |
| Detected Language | English |
| Type | Thesis or dissertation, Doctoral, PhD |
| Rights | © Cranfield University 2014. All rights reserved. No part of this publication may be reproduced without the written permission of the copyright owner. |
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