Foam Flooding is an important technique to improve the enhanced oil recovery (EOR) through increasing the efficiency of the injection fluid to cover the volume of the reservoir and at the same time increasing the displacement of oil. The foam can be generated either at the surface before injection or in the reservoir pore space. Under challenging reservoir conditions common foam agents are often chemically not stable making it necessary to have the means to screen different foaming agents for their suitability under high temperature (up to 130 °C) and high salinity (220 g/L).
In order to understand the foam behaviour under these conditions researchers need to be able to run their tests in an environment that resembles the reservoir as closely as possible. Amongst the key parameters to study and optimize foams for foam flooding applications is the interfacial tension and the viscoelastic modulus E*. Both parameters can be measured with a spinning drop video tensiometers of the SVT series that can be used with special capillaries withstanding the high pressure of superheated water at up to 130 °C and can vary the rotational velocity in an oscillating manner in order to determine rheological values like the viscoelastic modulus E*.
Capillary for superheated aqueous solutions at up to 130 °C
Spinning Drop Tensiometer Oscillation Experiment: Viscoelastic Modulus E*The viscoelastic modulus E* characterises how fast surface active molecules can respond to a changing interfacial area. For the measurement, a droplet surrounded by an outer liquid phase is placed into a capillary which is rotated with up to 20000 rpm. When the rotational velocity is varied in an oscillating manner, the centrifugal force on the drop changes periodically and hence the interfacial area changes periodically. Surface active agents from the bulk migrate to additionally created interfacial area and leave it when it decreases. This migration is not instantaneously and hence during oscillation the surfactant concentration at the interface changes and with it the interfacial tension. This results in a phase shift between the interfacial area and interfacial tension and can be used to calculate the viscoelastic modulus E*.
To illustrate the interesting results that can be achieved with this kind of equipment we want to highlight the latest work from Wei and Pu and their teams from the State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation in Chengdu, China . They studied different non-ionic surfactants, amphoteric surfactants and anionic surfactants regarding their foaming characteristics at harsh reservoir conditions.
Measuring cell for oscillation experiments and temperatures up to 180 °C
Many surfactants disqualified during the study due to their low thermal stability or precipitation under high salinity. A mixture of Glycoside Surfactant (A) and Hydroxy Sulfobetaine (B) in a ratio of 3 to 1 showed a lower surface tension and a larger viscoelastic modulus compared to pure A or B which leads to better foam properties at high temperature. These findings indicate that the absorption at the gas-liquid interface is improved for the 3 to 1 mixture and the foam is strengthened to a certain extent which can be illustrated by the fact that the mixture will lead to a closer packed molecular film which lowers the gas permeability and thus increases the foam stability.
They furthermore studied the surface tension and viscoelastic modulus E* at different temperatures (90 °C, 100 °C, 110 °C) showing that both parameters decrease with increasing temperature which is consistent with the fact that the molecular motion is increasing which leads to a higher film permeability and lower foam elasticity and stability as well as an increase in adsorption at the gas-liquid interface improving the foaming ability.
The key to successfully understand and quantify the behaviour of foaming agents under harsh conditions is a specially designed capillary that allows for a safe superheating of aqueous solutions to temperatures of up to 130 °C without boiling as well as the possibility to investigate interfacial rheology by spinning drop tensiometry. If you want to know more about the content of the article, you can directly refer to the literature information below:
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