Page 208 - Handbook Of Multiphase Flow Assurance

206 7. Flow assurance deliverability issues

expected to transport sand in a near-horizontal pipe is 1 m/s. This value may differ at
various operator companies. In some production systems operating in stratified multi-
phase flow regime, the chemical inhibitors are injected and are carried in the liquid phase.
Sufficient gas flow velocity is required to entrain liquid droplets and wet the perimeter
of the flowline so that all parts of the production system are treated with the injected
chemicals.
The minimum gas flow velocity consideration is important when top of the line cor-
rosion (TOLC) is expected to be an issue. In multiphase flow with stratified flow regime,
as flowing produced fluid cools down, water may condense from the gas stream and ac-
cumulate as droplets on top of the flow line. Freshly condensed water has no inhibitor
chemicals in it. This brings the concerns of corrosion and hydrate formation. Corrosion
risk may be mitigated by maintaining gas velocity above the minimum value. Hydrate risk
may be mitigated by injection of a volatile hydrate inhibitor such as methanol. It should
be noted that methanol may absorb oxygen from air if kept in a storage tank without a gas
blanketing system. Oxygen thus brought into the system with methanol may increase the
rate of corrosion.
Routing of flowlines should avoid significant elevation changes. It may be more profitable
to increase the line length in order to keep the line mostly flat rather than build it straight over
a mountain or through a canyon. Analysis of the relative cost of extending the flowline length
should be performed together with the flowline size and flow rate optimization to find the
relative impact of a hill or canyon crossing on the backpressure and delivery of wells over the
life of field.

Optimization of flowline sizes

Flowline sizes may be optimized to provide target flow rates over the life of field.
Larger diameter pipelines result in lower pressure drop, but also cost more. In multiphase
flow the liquids accumulation as holdup in low and uphill sections of oversized lines also acts
as a hydraulic restriction and increases pressure drop.
Flow assurance can develop a flow performance analysis, correlating both pressure drop
and liquid holdup with the flow rate and flowline size. There is expected to be a flowline size
when pressure drop and holdup are low. This size is optimal for the operation as it would
reduce the back pressure and increase production from wells over the life of field, and reduce
liquid surge into process equipment.

Artificial lifting

Artificial lift design is in the realm of production engineers, but should be done in collabo-
ration with flow assurance specialists (Table 7.1).
When reservoir pressure is or becomes low, there are several methods which allow to add or
to periodically accumulate the energy in order to lift heavier liquids from reservoir to surface.
Some of the artificial lift methods are listed in the table below. Absence of moving parts
should improve equipment reliability.