Page 89 - Handbook Of Multiphase Flow Assurance

84 4. Hydraulic and thermal analysis

Joule-Thomson effect
Minimum temperature in the production system is expected to occur in one of the two
scenarios:
- During restart due to JT cooling across the choke (so-called chilly choke conditions).
- During system depressurization due to JT cooling across the flare system valve.

In the first case a reservoir is below the bubble point pressure, reservoir fluid is saturated
with gas, and the well is produced in multiphase natural depletion mode without artificial
lift. Fluid settles downhole in such a well during a shut-in, and gas remains near the wellhead
and cools down to the ambient temperature. During a restart, the gas is produced initially be-
fore the well unloads the liquids and multiphase production resumes. The initially produced
gas is at a high wellhead shut-in pressure and at low ambient temperature. Flow of such gas
across a partly opened wellhead tree choke causes it to expand and cool.
When pressure drop across the valve is less than approximately 50%, gas expands and its
temperature drops as described by a thermodynamic equation of state in proportion to the
change in pressure and compressibility, which in turn depends on pressure and temperature.
Reservoir may also be above the bubble point pressure. Then the JT effect will cause heating if
pressure downstream of the choke is still above the bubble point pressure. JT effect will cause
a combination of heating and then cooling if the pressure downstream of the choke is below
the bubble point pressure.
In the second case of flow to flare an operator may be requested to depressurize the sys-
tem, for example to remedy or to prevent a hydrate blockage. Flare system is normally rated
to pressures much lower than the production system, so a greater pressure differential may
be expected. In case of depressurization there will always be JT cooling.
When the pressure drop is greater than 50%, for example from 50 bar upstream of the
valve to 10 bar downstream, critical flow or choked flow occurs as exiting gas velocity
approaches the speed of sound in gas at the outlet pressure and temperature, which limits
the cooling of the expanding gas. Typical coldest temperatures for hydrocarbon gas in crit-
ical flow downstream of a restriction is of the −50 °C order of magnitude, not much colder.
Flow velocity across a critical flow choke can then only be increased if pressure upstream
of the choke increases. The choked flow discharge velocity may limit the depressurization
flow rate.
In both cases flow from high to low pressure causes the fluid to change temperature which
may affect the integrity of materials in the production or the depressurization systems. The
materials are exposed to ambient temperature, which supplies heat and offsets the JT cooling.
A smaller valve opening may be used to limit the flow of cold gas mass in order to maintain
temperature of the system above the material minimum temperature limit.

Flow modeling

Correlations

A number of correlations have been developed over the years using various data sets to
more accurately predict pressure drop and liquid content in a flowing fluid system. Most of