In the extraction of oil and gas, fracturing fluids are used to create and widen a fracture for easier production. The fluid should be compatible with the rocks. Liquid CO2 has been used as a fracturing fluid because it is highly soluble in most oil extracts therefore less damaging to the oil extracted as compared to water-based fluids. The high solubility in oil helps in lowering the viscosity of oil and improves the extraction of oil from the ground.
Liquid CO2 is, however, difficult to control in porous media due to its low viscosity and lack of filter cake formation properties for controlled filtration. This may affect the shape of the desired fracture. Researchers have tried to improve on the viscosity of liquid CO2 by adding thickening agents, however, improvement of viscosity of liquid CO2 by enhancement does not guarantee a change in its filtration performance and liquid CO2 dissolves poorly with thickening agents making the insoluble residues potential pollutants.
Filtration control performance of liquid CO2 was improved without pollution by mixing it with Nitrogen (N2). Gupta et al. stabilized the N2 and CO2 mixture by introducing fluorochemical stabilizers into the liquid CO2 and then bubbling N2 into the liquid to form the N2/liquid CO2 foam. The liquid CO2 was the external phase while the N2 was the inner face and the fluorochemical stabilizers separated the two phases.
Qichao Lv, under the guidance of Professor Zhaomin Li, at China University of Petroleum investigated the dynamic filtration control performance of N2/liquid CO2 foam with a fluorochemical (HFE) as a stabilizer.
The filtration behavior of N2/liquid CO2 foam is uncertain as the external phase of the foam is unstable. Temperature and pressure can affect its density, viscosity and phase and changing the flow properties of the foam. Moreover, the behavior of waterless foam with the unique fluorochemical interface is uncertain. The interface may also be a potential pollutant. The experiment seeks to study the factors that may affect the dynamic filtration performance of N2/liquid CO2 foam including viscosity, foam quality, temperature, pressure, permeability and the damaging effects of the foam on porous media after filtration.
The setup for the experiment was done as shown in their paper where the preparation for the foaming solution was done and the viscosity measurement and dynamic filtration tests were done. The viscosity measurement results showed that the use of foam enhanced the viscosity of liquid CO2. The apparent viscosity is related to temperature, pressure and foam quality. The viscosity increased as the foam quality was increased from 31 % to 71 %. The viscosity of high quality foams were 1 order of magnitude larger than that of liquid CO2 at the same conditions. The apparent viscosity is at a maximum at a foam quality of about 80 % before it starts decreasing as it becomes fragile and sensitive to disturbances such as interactions and pressure fluctuations.
The filtration control performance of the N2/liquid CO2 foam was compared to that of liquid CO2 and a N2/liquid CO2 mixture. The results showed that the filtration control properties of the N2/liquid CO2 foam was better than the others. The leak off coefficient lowered with an increase in foam quality up to 80 % where it increased. Foams of 50 – 80 % quality had a high filtration performance with permeability change. Low initial foam quality foams had better filtration performance at high pressure difference. As the foam enters the porous media, the liquid part would evaporate hence increasing the foam quality with depth. Damage by the foam on the porous media depends upon the pressure difference between the two sides of the porous media. Damage is small under low pressure difference as the CO2 turns to gas under a high pressure media damaging the porous media.
In their study the research team were able to prove that by mixing N2, liquid CO2 and HFE, properties such as viscosity and filtration control performance of the resultant N2/liquid CO2 foam, increased substantially without damage to porous media.
Lv Q, Li Z, Li B, Zhang C, Shi D, Zheng C, Zhou T. Experimental study on the dynamic filtration control performance of N2/liquid CO2 foam in porous media. Fuel. 2017 Aug 15; 202:435-45.Go To Fuel