Page 171 - Handbook Of Multiphase Flow Assurance

Paraffin wax 167

• Vacuum insulation tubing
Vacuum insulation tubing (VIT) is an effective albeit costly method to reduce heat transfer
in a well downhole. As the temperature difference between hot oil and colder ambient con-
ditions is the main driving force for wax deposition, the effective insulation prevents wax
deposition.
Vacuum insulated tubing is relatively costly at over USD 100/ft.
• Electrical heat
Electrical heat has been used to effectively remove wax deposits from well tubing. Several
studies found the downhole electrical heating method to be most cost-effective in onshore
applications if the commercial cost of electric power is low in the region.
Subsea systems have used actively heated pipelines of the EH (electrical heating or pipe
and cable), DEH (direct electrical heating or pipe in pipe) and ETHPIP (electrically trace
heated pipe in pipe) mainly for hydrate control, but wax deposition also benefits from the
active heating.
• Comparative economics of prevention techniques
Scraping is the second most common method of wax management, after pipe insulation.
Pipe may be insulated either by addition of insulating layer, by burial underground or both.
Scraping frequency optimization is one of the most profitable technology studies for an oper-
ator. A justified reduction of downtime for scraping by reduction of the frequency of mainte-
nance scraping brings substantial uptime and revenue to the operator.

Remediation techniques
Wax tends to deposit as fluid flow rate declines and the fluid cools to conditions below the
wax appearance temperature. Once the wax deposits, there are several methods to remove it.
• Mechanical
Mechanical removal of wax is accomplished by scraping, wireline scraping, coiled tubing
jetting or pressure pulsation.
• Thermal
Thermal removal may include hot oiling and SGN exothermic chemical reaction. Hot oil-
ing is not always effective as the fluid has to be hotter than the wax dissolution temperature.
Wax dissolution temperature is a temperature at which a concentrated wax deposit dissolves
in crude oil. It may be 10–40 °C higher than the wax appearance temperature. Hot oil may
cool below the wax dissolution temperature while still flowing to the deposit.
SGN chemical reaction is seldom used as it requires to simultaneously time the arrival of
two chemicals at the wax deposit location. The chemicals have different viscosity, and the
lower viscosity fluid catches up to the higher viscosity fluid. Upon mixing, they react exother-
mically to produce nitrogen and release heat which dissolves the wax deposit.
• Chemical
Chemical removal may be accomplished with a solvent or a dispersant. Solvent efficiency
in wax removal depends on its composition (diesel, xylene, kerosene) and temperature.
Temperature of the solvent is a process-safety related issue. Volatility and ignition tempera-
tures must be closely evaluated before the selection of the chemical solvent.
Water-based dispersant may help remove non-aged wax deposits from wellbores by a sur-
factant mechanism.