Natural gas hydrates are now considered for storing and transporting natural gas. This is a cheaper alternative than conventional LNG-transport (Liquified Natural Gas). Assuming 4 billion Sm3 gas rate per year and 5500 km transport distance, the capital cost of production, shipping and regasification for NGH-transport (Natural Gas Hydrate) will be 684 million USD less (or 26%) than LNG-transport [1]. If the gas hydrates are stored adiabatically, they will remain stable below the equilibrium area at atmospheric pressure and at temperatures above equilibrium temperature [2]. The gas hydrates are stored in tanks under shipping. The idea for the offloading procedure is to flood the tanks with condensate and then pump the liquidized hydrate onshore where the hydrate will be separated from the condensate [3].
When transporting the crude oil/gas hydrate (oil sorbet) in offloading operations the slurry will behave in sort of a multiphase flow. Accurate data on flow properties (pressure drop, friction factor , viscosity) are needed to investigate if the oil sorbet will flow and how it will flow in pipelines. Earlier work with viscosity measurements of an oil sorbet with a rotational viscosimeter are done by Børge Nerland. His measurements shows that the viscosity of the simulated oil sorbet increases as temperature decreases. It also shows that viscosity increases as ice content of the slurry increases. Bingham plastic model seem to fit his data best. His measurements were done with a slurry consisting of Exxol D80 and ice [4].
The challenge of this study is to measure the viscosity in a slurry consisting of ice and
oil, and then try to do the same experiments with a slurry consisting of oil and gas
hydrates. Then the data should be fitted to rheological models. Then a mathematically
solution for calculation of pressure drop and friction in a horizontal pipe will be
proposed. The slurry will be simulated in the laboratory by using Exxol D80 and ice,
and by using Exxol D80 and gas hydrates. The experimental work will also consist of
formation of hydrates by using a hydrate reactor in the laboratory.
A slurry consisting of ice and Exxol D80 seem to have Bingham plastic rheological
properties.
A slurry consisting of gas hydrates and Exxol D80 seem to have shear thinning
behaviour.
The viscosity of a slurry consisting of ice and Exxol D80 and the viscosity of a
slurry consisting of gas hydrates and Exxol D80 decreases with increasing
temperature.
The pressure loss in a pipeline slurry flow consisting of gas hydrates and oil can be
calculated by using the equation for pseudoplastic fluids given in chapter 7.
[1] Natural Gas Hydrate an Alternative to Liquified Natural Gas,
J.S. Gudmundsson, Norwegian University of Science and Technology,
A. Borrehaug, Aker Engineering, January 1996.
[2] Hydrate Formation and Separation, Diploma Thesis,
Karsten Ofstad, Norwegian University of Science and Technology, Department of
Petroleum Engineering and Applied Geophysics, July 1996.
[3] Hydrate carrier offloading using slurry
Ulf Vegard Jensen, Norwegian University of Science and Technology, Department
of Petroleum Engineering and Applied Geophysics, May 1996.
[4] Rheology of crude oil/gas hydrate-slurry,
Børge Nerland, Norwegian Institute of Technology, Department of Petroleum
Engineering and Applied Geophysics, May 1995.
[5] Dynamics of Fluids in Porous Media, page 32-37,
Jacob Bear.
[6] Applied Drilling Engineering,
Adam T. Bougoyne Jr,
Keith K. Millheim,
Martin E. Chenevert,
F.S. Young Jr, 1991, chapter 4.
[7] Slurry Handling, Design of Solid-Liquid System,
Nigel P. Brown,
Nigel I. Heywood, 1991.
[8] Fann, Model 35 Viscometer Instruction Manual.
[9] Drilling Fluids, borehole hydraulics and pressure controll,
Lecture notes and figures, 1995
Paal Skalle.
i