Testing Procedure
The objective of a demulsifier “bottle test” is to investigate the performance of demulsifier in a series of tests that is designed to duplicate the conditions found in the actual production system as closely as possible.
Emulsions can be stabilised by various attributes of the fluids or process. Wax and mineral scale both of which could stabilise emulsions are some of the possible causes.
Three case histories that have been experienced by the Production Chemical are described to illustrate the problems that may occur. Please note there other factors – demulsifier overtreatment, chemical additions upstream, GOR of wells and shearing down hole or at the choke etc.
Wax stabilised emulsions are quite a common occurrence throughout the world. In Figure 1 examples from several West African Oilfield where wax is both precipitating as a wax layer and also producing a wax-stabilised emulsion can be seen.
Examples of Wax-Stabilised Emulsions from Several West African Oilfields
In several fields in Brunei and Eastern Malaysia the produced water is very low in salinity but does contain high concentrations of calcium and bicarbonate ions in solution. As the fluids are produced calcium carbonate scale is precipitated as very small crystals that are less than 5 microns in size. The fine scale particles “coat” the water-oil interface of the droplets of suspended water and effectively stabilise an emulsion. Before this mechanism was isolated, high concentrations of “conventional” demulsifiers were required to “break” the emulsion. In a quest for a more cost-effective treatment it was discovered that a demulsifier containing organic acids would chemically react with the calcium carbonate and de-stabilise the emulsion.
In the Netherlands a film-forming Imidazoline-based corrosion inhibitor was used on an annulus-batch treatment of the “sour” gas wells. The wells produced significant quantities of formation water and hydrocarbon condensate. It was found that the slugs of corrosion inhibitor produced an extremely stable water-in-condensate emulsion that was unresponsive to any known or blended demulsifier or high temperature (80°C). The solution to the problem was to change the corrosion inhibitor from an oil soluble, water dispersible product to a water-soluble, oil insoluble product. The emulsion problem “disappeared” with the change in corrosion inhibitor type.
These three emulsion stabilisation cases have been used to illustrate that emulsions can be produced by “man-made” events but that they can be treated by the optimum selection of demulsifier or a process change. It is necessary to understand all the production system to design an effective bottle test of demulsifiers.
The procedure developed for screening the demulsifiers contained numerous stages that were designed to provide pieces in the treatment jigsaw puzzle.
The first stage is to obtain a demulsifier-free representative crude oil sample. Sometimes this is very difficult to obtain due to the practicalities. This is often the case in demulsification studies and necessitates that a representative “blend” is made from the crude oil from different wells. Producing a two-phase sample of crude oil that can be poured into demulsifier testing settling tubes to ensure a reproducible sample is difficult to the point of impossible when there is “free water” present.
“Free Water” is the formation water that is not emulsified or dispersed in the oil phase and rapidly settles out under gravity to give a water layer. If a bulk oil sample is poured into several tubes different volumes of free water will be transferred and this is no good for comparative testing purposes.
The procedure adopted to obtain a reproducible Crude Oil when we have to collect the sample from the wellheads is as follows:
One litre or two and half litre samples of crude oils to be obtained from each well if possible or several wells. The wells that were selected provided a range of water-oil ratios, were from different production zones, variable temperatures and production rates.
The samples were heated to 40°C to provide a liquid crude oil and the free water from each sample was separated and retained. The “free water”-oil ratio, known as the “water-cut”, of each of the crude oil samples was determined.
The separated oil was blended together to produce a Composite Synthetic Blend.
The various water phases from the samples were combined to produce a composite water blend.
The free water was added to several demulsifier tubes in the proportion that was determined in the “water-cut” test and then the blended oil sample was added to the 100% mark of the tubes.
The samples to be heated if this happens in the process or the bottle test is to be run at the incoming temperature of the crude where the crude enters into the first separation vessel.
The “bottle test” is designed to duplicate, as far a possible, the actual process conditions. However it is not possible to simulate everything and it has been found over many years of worldwide testing that the concentration of demulsifier in the process can be different to that in the laboratory bottle test. The fist stage of testing is to “bench mark” the demulsifier that is currently used . Use the bench marking test this can range from 5ppm to 200ppm depending on the associated problems. . The objective of the bottle testing is not to make the treatment “easy” but rather to work at a concentration where the treatment just occurs at the higher dosage and is possibly not fully treated at the lower rate. It is then possible to see differences in performance and rank each chemical accordingly.
The next stage of the “bottle-test” is to evaluate all the chemicals. The test is comparative so all the chemicals need to be evaluated at the same time to ensure the testing conditions are identical,
Settling tests show the most effective intermediates. The points that are looked for are as follows:-:
Separated Water Visual Quality
Treated Oil Visual Quality
Interface Quality
Water Content of the “top oil”
Water Content of the “interface oil”
Salt Content of Top Oil
The objective of a demulsifier “bottle test” is to investigate the performance of demulsifier in a series of tests that is designed to duplicate the conditions found in the actual production system as closely as possible.
Emulsions can be stabilised by various attributes of the fluids or process. Wax and mineral scale both of which could stabilise emulsions are some of the possible causes.
Three case histories that have been experienced by the Production Chemical are described to illustrate the problems that may occur. Please note there other factors – demulsifier overtreatment, chemical additions upstream, GOR of wells and shearing down hole or at the choke etc.
Wax stabilised emulsions are quite a common occurrence throughout the world. In Figure 1 examples from several West African Oilfield where wax is both precipitating as a wax layer and also producing a wax-stabilised emulsion can be seen.
Examples of Wax-Stabilised Emulsions from Several West African Oilfields
In several fields in Brunei and Eastern Malaysia the produced water is very low in salinity but does contain high concentrations of calcium and bicarbonate ions in solution. As the fluids are produced calcium carbonate scale is precipitated as very small crystals that are less than 5 microns in size. The fine scale particles “coat” the water-oil interface of the droplets of suspended water and effectively stabilise an emulsion. Before this mechanism was isolated, high concentrations of “conventional” demulsifiers were required to “break” the emulsion. In a quest for a more cost-effective treatment it was discovered that a demulsifier containing organic acids would chemically react with the calcium carbonate and de-stabilise the emulsion.
In the Netherlands a film-forming Imidazoline-based corrosion inhibitor was used on an annulus-batch treatment of the “sour” gas wells. The wells produced significant quantities of formation water and hydrocarbon condensate. It was found that the slugs of corrosion inhibitor produced an extremely stable water-in-condensate emulsion that was unresponsive to any known or blended demulsifier or high temperature (80°C). The solution to the problem was to change the corrosion inhibitor from an oil soluble, water dispersible product to a water-soluble, oil insoluble product. The emulsion problem “disappeared” with the change in corrosion inhibitor type.
These three emulsion stabilisation cases have been used to illustrate that emulsions can be produced by “man-made” events but that they can be treated by the optimum selection of demulsifier or a process change. It is necessary to understand all the production system to design an effective bottle test of demulsifiers.
The procedure developed for screening the demulsifiers contained numerous stages that were designed to provide pieces in the treatment jigsaw puzzle.
The first stage is to obtain a demulsifier-free representative crude oil sample. Sometimes this is very difficult to obtain due to the practicalities. This is often the case in demulsification studies and necessitates that a representative “blend” is made from the crude oil from different wells. Producing a two-phase sample of crude oil that can be poured into demulsifier testing settling tubes to ensure a reproducible sample is difficult to the point of impossible when there is “free water” present.
“Free Water” is the formation water that is not emulsified or dispersed in the oil phase and rapidly settles out under gravity to give a water layer. If a bulk oil sample is poured into several tubes different volumes of free water will be transferred and this is no good for comparative testing purposes.
The procedure adopted to obtain a reproducible Crude Oil when we have to collect the sample from the wellheads is as follows:
One litre or two and half litre samples of crude oils to be obtained from each well if possible or several wells. The wells that were selected provided a range of water-oil ratios, were from different production zones, variable temperatures and production rates.
The samples were heated to 40°C to provide a liquid crude oil and the free water from each sample was separated and retained. The “free water”-oil ratio, known as the “water-cut”, of each of the crude oil samples was determined.
The separated oil was blended together to produce a Composite Synthetic Blend.
The various water phases from the samples were combined to produce a composite water blend.
The free water was added to several demulsifier tubes in the proportion that was determined in the “water-cut” test and then the blended oil sample was added to the 100% mark of the tubes.
The samples to be heated if this happens in the process or the bottle test is to be run at the incoming temperature of the crude where the crude enters into the first separation vessel.
The “bottle test” is designed to duplicate, as far a possible, the actual process conditions. However it is not possible to simulate everything and it has been found over many years of worldwide testing that the concentration of demulsifier in the process can be different to that in the laboratory bottle test. The fist stage of testing is to “bench mark” the demulsifier that is currently used . Use the bench marking test this can range from 5ppm to 200ppm depending on the associated problems. . The objective of the bottle testing is not to make the treatment “easy” but rather to work at a concentration where the treatment just occurs at the higher dosage and is possibly not fully treated at the lower rate. It is then possible to see differences in performance and rank each chemical accordingly.
The next stage of the “bottle-test” is to evaluate all the chemicals. The test is comparative so all the chemicals need to be evaluated at the same time to ensure the testing conditions are identical,
Settling tests show the most effective intermediates. The points that are looked for are as follows:-:
Separated Water Visual Quality
Treated Oil Visual Quality
Interface Quality
Water Content of the “top oil”
Water Content of the “interface oil”
Salt Content of Top Oil
Demulsifier bottle testing has a terminology all of its own that is used to describe the appearance of the water, oil and interface and various aspects of the testing.
Different engineers often use different terminology. The terms used during these bottle tests are as follows
Each of settling tube are placed in a tube rack and the visual appearance of the top oil, interface oil and interface are observed. The quality of each parameter for each treatment was subjectively ranked.
Using a special spring-loaded syringe fitted with a 15cm long canula, the oil is removed from the top oil of each settling tube. The canula has a sliding stop fitted that rests on the top rim of the settling tube and is locked in position to allow samples to be extracted from the same point in each settling tube and then placed in a centrifuge tube. The tube and contents are usually heated to 50°C and then placed in a heated centrifuge. The tubes are centrifuged at 3,500rpm for 5 minutes.
The volume of free water emulsion and solids are recorded. Two drops of a special demulsifier knock out drops are then added to each centrifuge, heated, shaken and re-centrifuged as before. The knock out drops can be the incumbent demulsifier cut back or a special product to break virtually any emulsion and will not “over-treat” the oil.
After the knock out treatment the separated water volume may have increased corresponding to the concentration of water in the emulsion.
The centrifuge test should be repeated for the interface-oil
Finally the salt-in-crude of the top-oil is sometimes determined. The equipment that is commonly used is a conductometric salt-in-crude analyser defined in the ASTM D-3230-97 procedure.
This is a quick and accurate technique, taking about 5 minutes per determination. The technique measures the conductivity of a solvent solution that has been mixed with a crude oil sample. This is should be related to a calibration curve corresponding to the ionic components in the formation water. The units used are pptb (pounds per thousand barrels of oil).
source: http://www.productionchemical.com/emulsions.htm
