Page 16 - Handbook Of Multiphase Flow Assurance

10 1. Introduction

completion brines must remain hydrate-free at mudline (seabed) conditions in case light
hydrocarbon gases migrate from the bottom of the well up the wellbore to the location
where well completion fluid is exposed to cold (approx. 277K) seawater if a well is drilled
in offshore environment. Thus, accurate prediction of hydrate equilibria in high salinity
brines is important. Similarly, during production of reservoir fluids through a completed
well, hydrates must still be avoided, lest the well gets plugged with solids. If a chemical,
such as pure methanol or a low dosage hydrate inhibitor formulated in methanol, is used
to control hydrate formation, this may cause a salting out effect as scale forms due to colli-
gative properties of water and inability to dissolve both salt and methanol simultaneously
beyond the solubility limit. This has led to past incidents such as blockage of a North Sea
production line with halite scale [ca.2009]. The ability to account for high salinity brines
with respect to high pressure hydrate equilibria and scale formation is also important
during production.
Usually, stability of brines decreases with decreasing temperature. This may lead to pre-
cipitation and deposition of solid scale such as halite or barium sulfate. In case of carbonate
scales, brine stability decreases with increasing temperature, leading, for example to a cal-
cium carbonate scale in hot systems.

How flow assurance and production chemistry work together

Multiphase flow assurance is complemented by production chemistry whose aim is to
prevent the reduction of product value and process safety such as water in oil, oil in water,
salt in oil, oxygen in water, mercury in fluids, H 2 S in gas, corrosivity, and/or bacteria. Sour
crude is less valuable than sweet, so souring of crude oil in reservoirs may be prevented by
using chemistry. Similarly, it is the goal of production chemistry to maintain specified quality
of product oil and produced water so they are fit for export to refinery, reinjection into a well
or discharge overboard. The chemicals are selected and deployed by chemists in multiple lo-
cations including reservoir, wellbore, flowlines, process facilities and export pipelines to help
produce oil and gas most economically.
The flow assurance and production chemistry work side by side to achieve related goals,
and sometimes that work may be performed by the same person if that person has sufficient
experience in both disciplines.
The combined scope of some of the flow assurance and production chemistry items is
shown below in Table 1.1:
Many of the items from the above table get summarized in a Basis of Design document for
flow assurance work.
The allocation between flow assurance and production chemistry is only suggested de-
pending on whether the issue is more flow-transformation or fluid-transformation related,
and may vary from project to project. For example, foaming may be caused either by a fluid
containing high amounts of natural surfactant or by high shear of flow through a choke or
orifice. Foaming may be prevented by the use of production chemistry to improve separation
of oil and gas or it may be sought by flow assurance to help lift fluids from a well with multi-
phase flow. Both flow assurance and production chemistry specialists should be experienced
in all of the above items.