Experimental investigation of gas lift technique in viscous fluids

Ibrahim, Abubakr (2017) Experimental investigation of gas lift technique in viscous fluids. PhD thesis, University of Nottingham.

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Oil and gas often naturally flow to the surface driven by the high pressure of the reservoir. Over the time oil fields suffer a decline in production primarily caused by the decrease in the reservoir pressure coupled with the fact that fluids become thicker and more viscous. In addition, there are huge reserves of oil that have not been exploited because of the high drilling and pumping costs owing to the high viscosity and density of the oils within. The feasibility of drilling new wells or continuing production from ’dead’ reservoirs depends to a great extent on the pumping cost. Pumping is achieved via several methods including gas-lift. It is applied by injecting gas to the base of the oil well which in turn reduces the weight of the oil column in the well riser. The decrease in pressure head results in an increased liquid flow.

The aim of this thesis is to study the dynamics of gas liquid flows in vertical large diameter pipes, with particular emphasis on viscous fluids. The fundamental study to understand the underlying physical mechanisms underpinning the gas-liquid interactions when the viscosity is increased will thereafter be employed to investigate the performance of gas lift technique and explore avenues for optimisation. Ultimately resulting in improved modelling of the flow behaviour leading to an optimised design approach and a maximised oil productivity.

The aforementioned aim is achieved experimentally by simulating the flow behaviour in a 127mm vertical pipe. The facility employed is capable of operating as a gas-lift facility and a fixed flow loop that is able to simulate the flow behaviour at controlled gas and liquid turbulence levels. The selection of simulant fluid is key in this work, the selected liquid has physical properties closer the petroleum oil. It is paramount to ensure that when the viscosity is increased, other relevant physical properties such as density and surface tension remain virtually constant. Therefore, silicone oils with four different viscosities were employed ; namely 4.0, 25.4, 51.1, 104.6 cP while varying the liquid superficial velocity from 0.07-0.86 m/s and the gas superficial velocity from 0.01-5.40 m/s, generating a matrix of 720 runs. Void fraction was measured using high spatial and temporal resolution measurement techniques: Electrical Capacitance Tomography (ECT) and the Wire Mesh Sensor (WMS) at 5 different axial stations along the 10.12m length of the test section.

First, a novel parametric study on the effect of viscosity in large diameter vertical pipes is presented; whereby the effect of viscosity on various two phase flow attributes is assessed and analysed both qualitatively and quantitatively. The results suggest that void fraction in general decreases with increasing viscosity. Also, the study reveals the presence of Taylor bubbles in the large diameter pipes at the high viscosities studied. Second, the performance of the state of the art models is assessed against the unique experimental data generated. Most models are found to grossly depart from the experimental data. In addition, new improved global models for various multiphase flow features are proposed. Thirdly, we discuss the issue of flow development and elucidate on how the entrance effect varies with increasing viscosity. That was achieved by employing three different injector nozzles for the four different viscosity fluids, producing 2160 experimental runs. The study suggests that the flow becomes independent of the injection method at 63D axial distance from the injection point. Finally, an investigation of the performance of an actual large scale gas-lift pump is presented. The efficiency is observed to dramatically decrease with increasing liquid viscosity. The discussions extend onto assessing the performance of the state of the art models and proposing improved models based on conclusions drawn from the fundamental experimental study.

Quintessentially, the outcome of this research would help engineers and operators in the oil and gas industry to estimate, with greater accuracy, how much oil will they be getting for any specific gas input. Additionally, it provides improved estimation of pressure gradient and other global parameters that are essential for the design of wells and risers. The high resolution phase distribution information obtained in this work could serve as a benchmark data to test the performance of Computational Fluid Dynamics (CFD) codes developed for similar conditions against.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Hewakandamby, B.
Azzopardi, B.J.
Keywords: Gas-lift, Two-phase flow, Viscosity, Viscous flow, Large diameter flow, vertical pipes, pressure gradient, WMS, Tomography, Taylor bubbles, Multiphase flow
Subjects: T Technology > TN Mining engineering. Metallurgy
Faculties/Schools: UK Campuses > Faculty of Engineering
Item ID: 47560
Depositing User: Ibrahim, Abubakr
Date Deposited: 13 Dec 2017 04:40
Last Modified: 06 May 2020 13:03
URI: https://eprints.nottingham.ac.uk/id/eprint/47560

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