Page 133 - Handbook Of Multiphase Flow Assurance

Hydrate of natural gas 129

Cold flow
Cold flow had been proposed as a way to achieve chemical-free production in subsea sys-
tems. The concept involves forming a suspension of hydrate particles and recirculation of
a fraction of the stream with hydrate particles into the uninhibited produced fluid stream.
Hydrate particles from the recycle stream will act as nucleation sites for the conversion of
water in the main stream to hydrate. Hydrate will grow on the suspended particles without
adhesion to pipe walls. The recycle stream needs to be added at a position in a flowline where
produced fluid stream has cooled down to the hydrate formation condition. The cold flow
method had been evaluated in laboratory setting in 2003 and showed good results for both
hydrate control and promising results for wax deposit control. The results were published
(US7261810 patent, 2003; Wolden et al., 2005). However, economics of the cold flow approach
are constrained by the need for a recirculation pump located close to the wellhead, a recycle
stream pipeline and the increased viscosity of the hydrate suspension. The combination of
these requirements makes cold flow applicable only in a narrow range of flowline distances.
This method has not been tested in the field. Another implementation of cold flow with a field
pilot test was reported by Turner and Talley (2008).
Partial gas separation
Partial gas separation may be used to avoid hydrate if supported by appropriate labo-
ratory verification. With low GOR oils, the amount of gas available to form hydrate is low.
Partial gas separation may remove enough gas from the hydrocarbon mixture to shift hydrate
equilibrium outside the operating envelope. The method can be modeled with PVT software
but the true shift of hydrate stability conditions should be validated in a laboratory.
Static mixer
Static mixer concept for hydrate control has been tested in the field with promising re-
sults. The summary of the method and the field test had been presented at the 2008 ICGH in
Edmonton, Canada (ICGH, 2008).

Modeling of gas hydrates
Currently several rigorous models are available for calculation of gas hydrate stability.
These models have been developed based on the van der Waals and Platteau 1959 work
which has been discussed in Sloan (1990).
Early practical examples of software implementation for modeling of hydrate stability
were developed by Robinson (1988) and Erickson (1983) with the source code presented in
the dissertation.
The working version of the software for use on PC computers is also available at the
Colorado School of Mines, as described in Chapter 11. This software should be able to help
achieve safe operation of production facilities and to raise awareness of operators about gas
hydrates.
A number of empirical correlations have been presented over the years.
Katz had developed a graphical method to find hydrate formation conditions for natural
gases with various gravities (Katz, 1945; Katz et al., 1959). A detailed example of this method
allowing one to calculate the gas gravity and to find the appropriate hydrate conditions is
also presented in Sloan (1990).