Thursday, October 02, 2014
 
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Demulsifiers

Introduction

Water/Oil Emulsion

Chemical and Physical Properties of Water/Oil Emulsions

Nature and Action Mode of Stabilizers

Breaking Water-Oil Emulsions

Practical Method for Selection of Demulsifiers

Application Mode

Monitoring the Effectiveness of Demulsifier Treatment

 

Introduction

The presence of water in  crude oils is caused by:

  • Natural mixing with oils during their migration
  • Washing of tubing for scale dissolving
  • Injection in secondary recovery operations to maintain pressure
  • Leaking from formation above the producing one
  • Seawater contamination during marine transportation.

Water causes problems downstream of the production. It increases cost of transportation and corrosion problems in pipelines and vessels. Since  water is almost always salted, it may lead to fouling of heat exchanger and corrosion of distillation equipment in refining. Both water and salt content are regulated. Thus, it is economically important to separate brine from crude oil directly at the production site.

Water mixed in the crude oil can be "free" or as an "emulsion". Free water is separated by simple settling. Emulsions form and stabilize at many areas where there is turbulence such as tubing, pump and valves and require adequate treatment.

Water/Oil Emulsion

They are typically emulsions of water-in-oil. The continuous phase being the oil and the dispersed phase being the water. The quantity of emulsion water can vary in large proportions. It is common to produce wells containing 40 to 50% water and some wells continue to be produced with 95% water content.

Chemical and Physical Properties of Water/Oil Emulsions

  • Density - The density of the emulsion is directly function of the ratio of the liquid phases present and of the densities of each phase.

  • Viscosity - The viscosity increases as a function of the percent of emulsion. A uniform distribution leads to an increase in viscosity. The stable the emulsion, the higher the viscosity.
  • Stability - Thermodynamically, these emulsions are unstable and the coalescence with time of the dispersed particles should lead to the separation of the two phases. It is essentially the existence of an adsorption layer which causes the stability of the emulsion of water in oil.

Nature and Action Mode of Stabilizers

Some natural substances contained in the oil, when adsorbed at the interphase water/oil create a physical barrier which mechanical properties prevent the coalescence of water droplets during their collision. The chemical nature of the adsorbed products is often complex. The main components are asphaltenes, resin, naphthenic acids, porphyrine amines, micro-crystals of paraffin, clay and sand. The respective content of these components in the interfacial layer vary in large proportions:

  • Asphaltenes from 3 to 70%
  • Resin from 7 to 55%
  • Paraffin from 20 to 90%

Some of these components are polar. Naphthenic acids, amines, porphyrines stabilize emulsions with their polar properties and their lack of symmetrical structure favor their concentration at the interface. One part of the molecule, the hydrocarbon part for example has affinity for oil while the other polar part of the molecule has an affinity for the aqueous phase.

The action mode of asphaltenes and other solid micro-particles is different. These are non-polar, they concentrate at the interface and play the role of stabilizer of the emulsion.

Breaking Water-Oil Emulsions

Several methods are used to break water-oil emulsions:

  • Demulsifiers – They may decreasing and cancel the electrostatic forces of repulsion reacting between the water droplets. They also modify the wettability of the solid micro-particles absorbed at the interface or form some inverse emulsion oil-in-water, thus destabilizing the water-in-oil emulsion.
  • Agitation – Mixing increases the collision number between particles and their coalescence. In practice, the mixing is natural during the flow of the emulsion in surface equipment.
  • Temperature - An elevated temperature accelerate the water separation by increasing the probability of the water droplets to collide and decreasing the viscosity of the continuous phase.
  • Electrical Tension (voltage) - An electrical voltage of about 20,000 volts is applied between electrodes, thus creating an electrical field in which the water droplets will be polarized when deformed by elongation. This process increases the number and the energy of the collision between the particles and favor coalescence. This process is economical for water content in oil above 5 percent.

Practical Method for Selection of Demulsifiers

It is practically impossible to predict the product or mixture that will give the best results on an emulsion. The selection tests must be made from fresh samples, e.g. a non-broken and non-oxidized emulsion.

Testing of demulsifiers are made in the production field as soon as the samples are collected. A simple method called the bottle-test is typically used.

1.Introduction of 100 ml fresh emulsion in a calibrated bottle of 200 ml approximate.

2.Placement of the bottle in a bath at the field temperature.

Introduction of candidate demulsifier in the form of 2% in a mixture xylene 75% - methanol 25%.

Mixing of the bottle with successive rotation in a reproducible manner.

5.Restoring the bottle in the bath and reading the separated water at several times. Cleanliness of the water and presence of sludge, filament and cloudiness are noted at the end of the test.

6.Analysis of the oil for water and salinity when the volume of settled water stops increasing. Some compounds while separating rapidly water and oil can leave a large amount of water emulsion and salt in the oil.

The tests are repeated at several demulsifier concentrations in order to determine the optimum concentration and evaluate the performance of several compounds. After selection of the best chemical, a field test of 2 to 4 weeks is performed to confirm the results of the bottle test and also to optimize the concentration of the product injected to reduce treatment costs.

The bottle test, despite its inability of analyzing the emulsion characteristics, is the most rapid and reproducible method to select formulations that are the best for the emulsion.

Application Mode

Demulsifiers are usually injected pure. Dilution of the product is needed only when the injection rate is very small and below the capabilities of the pump. The injection point of demulsifiers depends on several factors including the nature of the emulsion and the surface equipment. The demulsifiers can be injected downhole, in the manifold or pipelines and by batch in storage tanks. The first two are most commonly used.

Monitoring the Effectiveness of Demulsifier Treatment

The monitoring methods are related to the specifications required on the producer such as some tenth of percent for BS&W and some ppm’s of chloride for salinity. The most common methods for BS&W are ASTM D1796, API 2548, ASTM D9570, API 2560, and Aquatest Fina. The methods used for salinity are (1) extraction by washing with water and titration of chloride ions with silver nitrate, (2) direct potentiometric titration with silver nitrate, (3) measurement of conductivity by dilution with a polar solvent, and (4) analysis of sodium , calcium and magnesium ions by flame spectroscopy.


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