what are Emulsions

Removing water from crude oil often requires additional processing beyond gravitational separation. In selecting a treating system, several factors should be considered to determine the most desirable methods of treating the crude oil to contract requirements. Some of these factors are:

  1. Tightness of the emulsion.
  2. Specific gravity of the oil and produced water.
  3. Corrosiveness of the crude oil, produced water, and casing head gas.
  4. Scaling tendencies of the produced water.
  5. Quantity of fluid to be treated and percent water in the fluid.
  6. Paraffin-forming tendencies of the crude oil.
  7. Desirable operating pressures for equipment.
  8. Availability of a sales outlet and value of the casing head gas produced.

Why Remove Water?

  • Wet Oil has Little Value

– Refineries usually specify <0.5% water

  • The presence of water can lead to corrosion

– In shipping lines and process equipment

– In Refinery process systems

  • System Designs

– Most oil production systems are sized for oil

  • Transport Costs

– Transporting a by-product with no value is wasteful

  • Removing water from oil (Deoiling), although desirable, creates potential problems:

– Disposal of oily separated water

  • Standards from as high as 40ppm hydrocarbon (offshore) to as low as 5ppm hydrocarbon (onshore)
  • Standards often exceeded due to operational problems

– Solids build up in production systems

What is an Emulsion?

  • Definition: A mixture of two immiscible liquids, one of which is dispersed as droplets in the other, this dispersion being stabilized by an emulsifying agent.
  • Water in Oil Emulsions (W/O)

– “Normal” Emulsion

– Continuous phase – Oil

– Dispersed phase – Water

Oil in Water Emulsions (O/W)

– “Reverse” Emulsion

– Continuous phase – Water

– Dispersed phase – Oil

  • Two Immiscible Liquids

– Oil and Water

Source of Oil and Water:

Water Coning in a Waterdrive Reservoir

What is Required For Emulsion Formation?

  • Two Immiscible Liquids

– Oil and Water

  • A Source of Mixing Energy / Shear

– Well Bore, Pumps, Choke, Valves, Bends in Pipework, Flow Regime (Turbulent Flow)

What is Required For Emulsion Formation?

  • Two Immiscible Liquids

– Oil and Water

  • A Source of Mixing Energy / Shear

– Well Bore, Pumps, Choke, Valves, Bends in Pipework, Flow Regime (Turbulent Flow)

  • An Emulsifying Agent(s)

– Solids e.g. Formation Fines, Corrosion Products, Scale

– Treatment Chemicals – Production and Drilling

– Natural Surfactants – e.g. Paraffins, Naphthenic Acid Salts

Major Factors Affecting “Normal” Emulsion Stability

  • Type and Amount of Emulsifying Agent(s)
  • Degree of Agitation (mixing)
  • Viscosity of Continuous Phase – Stokes’ Law
  • Density Differential of Produced Fluids – Stokes’ Law
  • Disperse Phase Content (Water)

– as water content increases emulsion stability decreases (generally)

  • Emulsion Age

– emulsion stability generally increases with time

  • Temperature

– affects other variables

– emulsion stability decreases with increase in temperature

Stokes’ Law:

Stock’s law

V  = Velocity of water droplet

r  = Radius of water droplet

dw = Density of water

do = Density of oil

u  = Viscosity of oil

g  = Constant

  • Water droplet radius is a squared function in Stoke’s Law
  • Therefore, water droplet settling velocity is most easily increased by increasing the radius of the droplets
  • Hence, any means of coalescing the water droplets will increase settling velocity and reduce the settling time needed for water separation.

Resolving “Normal” Emulsions

  • Demulsification

Defined as:-
“The resolution of crude oil emulsions and the subsequent removal of the separated water phase (dehydration)”

read more about Crude Oil Stabilization and Sweetening

The Mechanism of Demulsifiers

  • To Break (or destabilise) a “Normal” Emulsion a Demulsifier Must Achieve the Following:-

– Rapid Migration to the oil/water interface

– Flocculation

– Coalescence

– Solids Wetting

Factors In Demulsification

Oil Phase Emulsion
Without Demulsifier Treatment the Pliable Film Around the Water Droplet Remains Intact when a Collision Occurs
With Demulsifier Treatment the Film Becomes Brittle and Ruptures when a Collision Occurs

Solids Removal

  • Removal of solids from the interface may completely resolve some emulsions
  • Types of Solids:-

– Inorganic

  • FeS, mineral scale (e.g. CaCO3), silt, clay, drilling muds

– Organic

  • Paraffin, asphaltenes
  • Solids removal can be achieved by:-

– Oil wetting (dispersion in oil)

– Water wetting (dispersion in water)

 References:
 1. Surface Production Operations – Ken Arnold & Maurice Stewart.
   2. Production Chemistry & Corrosion Control – Baker Petrolite

Tags