Page 8 - Handbook Of Multiphase Flow Assurance

2 1. Introduction

Definitions of flow assurance are numerous, including this one: Flow Assurance is the
analysis of thermal, hydraulic and fluid-related threats to flow and product quality and their
mitigation using equipment, chemicals and procedure.

Multiphase production problems: Blockages and restrictions

Oil and gas are currently produced through wells and pipelines. The lack of flow in wells
and pipelines may be due to low reservoir pressure or productivity, due to complete block-
ages or due to partial restrictions.
Flow restrictions may happen in a reservoir, in a well production tubing or a tree, in a
jumper between a well and a flowline, in a flowline, in a riser or in an export pipeline. In
some cases, restrictions may happen in several locations simultaneously. The largest number
I have seen is five blockages in the same production flowline at the same time. Restrictions
may also occur in water and gas injection systems as in wells, flowlines or reservoirs.
Restrictions may be hydraulic, such as liquid accumulation also known as a holdup in
flowlines and risers, liquid loading in wells, or mechanical such as a partly closed valve or
a scraper. Restrictions or blockages may also be solid, including organic (e.g., paraffin wax),
inorganic (scale) or particulate (sand). The hydraulic, mechanical or solid restrictions may be
stationary such as the liquid holdup or moving such as the slugging. A flow assurance prac-
titioner should be able to recognize the signs of and potential for any type of restriction and
either economically design it out of a new system or mitigate it in an existing system.
In some cases, restrictions may lead to other restrictions. There is an early 2000s example
from West Africa offshore production where a wax deposit in a flowline got scraped by a
formed hydrate plug into a solid paraffin wax blockage. Similarly, combined hydrate-paraffin
restrictions have also formed in the North Sea in the early 1990s and asphaltene-hydrate in
the Gulf of Mexico in the 2010s. Paraffin-asphaltene-sand restrictions have been common in
Siberian pipelines through the decades.
Modeling of multiphase flow can be done to find optimal conditions for a stable produc-
tion of gas and hydrocarbon liquids with water. When the gas flow rate is not high enough
to sweep the liquid hydrocarbons and liquid water from a well or a pipeline, these liquids
accumulate in low spots because of gravity.
The liquids can accumulate either downhole in a vertical well or at a heel or a toe, which-
ever is lower, in a horizontal well which is known as liquid loading. Both deepwater and
shale horizontal wells are susceptible to liquid loading.
Severe slugging is one of the issues in multiphase flow also related to gravity. Liquids can
accumulate at a subsea riser base and then get periodically produced to a topsides separator
after a sufficient gas pressure has built up behind the accumulated liquids as a large sudden
gush of liquid preceded by a period of no or limited flow, which is known as severe slugging.
Wells keep producing during severe slugging at a steady rate, but backpressure on wells may
change noticeably between slug accumulation and displacement. Severe slugs keep repeat-
ing, and slug size and momentum are substantial as to cause vibration at pipe bends in flow
geometry, overfill the process vessel or both.
Liquids also can accumulate in the low spots of a near-horizontal pipeline and get period-
ically displaced by a steady flow of gas, which leads to terrain slugging. Terrain slugs keep
repeating and are usually smaller in size and don't overfill the process vessel but may cause