Page 63 - Handbook Of Multiphase Flow Assurance

58 3. PVT and rheology investigation

heating. This effect can cause a hot reservoir fluid become even hotter during production and
has to be taken into account for material selection and well design. In late life reservoir pressure
declines but reservoir temperature remains the same so additional phases may become stable
or unstable.
Several solid phases can be present simultaneously if there are both sufficient fluid and
appropriate conditions present to form those solids.
Solids usually form from liquids by crystallization or by amorphous freezing. Examples
of crystals are hydrate, ice, paraffin wax. Examples of amorphous solids are asphaltenes and
some forms of wax and naphthenates. Scales are also crystals.
In some cases petroleum solids can form from the gas phase such as diamondoids composed
of adamantane, diamantane and heavier molecules. Diamondoids are also crystals and they
photoluminesce (Clay et al., 2011) which may help identify them among other petroleum solids.
Naphthenates are liquid crystals or micelles (Havre, 2002). Naphthenates have complex
and little studied phase diagrams, and also can form amorphous solid films (Magnusson and
Sjöblom, 2008).
Two structures of hydrate commonly occurring in production operations are shown in
the figure above to illustrate that when water is abundant for a hydrate to form, the propane
and heavier components will be depleted first to form structure II hydrate, and if pressure
is still sufficient to form more hydrate, the lean gas can keep forming structure I. This can
happen when the number of moles of water is approximately six or more times greater than
the combined number of moles of light hydrate forming hydrocarbons such as methane, eth-
ane and propane. Similarly, if all gas is consumed into an exothermic hydrate and water is
still present, ice can form if temperature is below freezing. Thermodynamically hydrate is
more stable than ice at higher pressure because pressure helps water molecules in hydrate
stay connected at higher temperatures, whereas in ice pressure distorts the crystal. However,
kinetically ice forms faster than hydrate because it takes several types of molecules to get
organized in order to form a hydrate crystal, while ice crystal forms with just water. Thus in
an LNG process ice can form together with hydrate from an off-spec stream of hydrocarbon
with a sufficient moisture content. Furthermore, in colder arctic environments hydrate can
be dissociated by pressure reduction, while ice cannot if ambient temperature is below 0 °C.
Depressurization is endothermic or consuming heat and should be done with care if ambient
temperature is below freezing as hydrate upon dissociation releases mainly pure water. If
fresh water released from dissociated hydrate converts into an ice blockage one would need
to wait for the summer.
Phase boundaries in the figure above are qualitative and intend to highlight the relative de-
pendence of phase stability on changes in pressure or in temperature. For example, BaSO 4 scale
is less sensitive to changes in pressure than to changes in temperature. As pressure increases,
less barite forms and as temperature increases, less barite forms. CaCO 3 scale is sensitive to
changes in both temperature and pressure. As temperature increases, more calcite would form.
Also as pressure drops more calcite forms, mainly due to CO 2 evolving from water and hydro-
carbon phases. Carbon dioxide, if present in water and hydrocarbons, helps dissolve calcite in
water, not too dissimilar from resins stabilizing asphaltene in oil. CaCO 3 also can form a film
on pipe surface which can reduce corrosion, unless the film gets sheared away by the flow.
There is a continuous interaction between solid and fluid phases. Solids can act as diffu-
sion barriers or as capillary channels to conduct less or more molecules in the liquid or gas