Reports: ND552848-ND5: Interfacial Viscoelasticity of Crude Oils: The Role of Asphaltenes
printer friendlyIt is well-documented that the use of low salinity water in flooding operations can markedly improve oil extraction [1]. There have been several explanations for this benefit including the enhancement of wettability at low salinity. Our work has explored the connection between the influence of salt concentration on the viscoelasticity of oil/water interfaces and its correspondence to oil removal efficiency.
Petroleum fluids used in this work were obtained from two different reservoirs, one from the Gulf of Mexico, and the other form the Middle East. DI water, NaCl, MgCl2, dichloromethane, chloroform and toluene were purchased. Interfacial rheology measurements are performed using a “TA instruments ARG2” rotational rheometer adapted with a double wall teflon vessel and a du Noüy ring. Using this apparatus the elastic and viscous interfacial moduli were measured as a function of time until steady state values were obtained. This was performed at room temperature and at various salt concentrations from DI water to 10% salt. To evaluate the sweep efficiency of displacing these oils with water, a microfluidic flow cell was used. The cell incorporated post arrays that simulated porous media with a pore volume of 110 microliters and a porosity of 32%.
In both crude oils the interface reached a maximum elasticity at 0.01 wt% salinity, regardless of the type of salt. The viscoelasticity of the interface reflects the molecular structure formed at the interface. Amphiphilic molecules, such as asphaltenes, affect the interface structure and these structures can lead to a highly viscous or highly elastic interface. The effect of low salinity on the viscoelasticity may be related to electrical double layer expansion, which can enhance the surface pressure of the adsorbed layers.
Low salinity brines (0.01 wt % salt), where the maximum elasticity is observed, and high salinity brines (1.0 wt % salt) were used to displace the two oils in the microfluidics model. Fluctuations in the pressure profiles are more pronounced in high salinity brine injection. In low salinity brine injection the pressure profiles were found to be smooth. Pressure fluctuations were interpreted to be related to oil snap-off during the course of water injection. In addition, higher sweeping efficiencies were found in the low salinity brines. It is evident that the higher interfacial viscoelasticities of the low salinity water led to higher surface tractions against the water and the oil.
Clathrate (or gas) hydrates form at the interface between water and lower molecular weight hydrocarbons to trap the guest species within cages of hydrogen bonded water [1]. These ice-like inclusions are formed under appropriate thermodynamic conditions and are particularly prevalent in deep sea conditions where high pressures and low temperatures have led to methane, ethane and propane being trapped in massive reservoirs. However, there is a pressing need for research into these structures due problems they create regarding flow assurance. These are manifested in deep sea reservoirs where hydrates can develop in production lines and, in extreme cases, completely block the flow of oil and gas [2]. Cyclopentane forms hydrates at ambient pressures and is often used as a model material. At ambient conditions, the critical temperature below which hydrates form is 7.2C. In this phase of the research effort funded by ACS-PRF, we have used this material to explore the utility of using interfacial rheology to follow the development of hydrates at the interface between water and cyclopentane at different thermodynamic conditions. In order to study hydrate formation, a new design of a double wall ring (DWR) interfacial rheometer was developed. It consists of a brass, double wall cell that sits on top of the Peltier plate of a rotational rheometer. Water is placed in the lower portion of the cell and cyclopentane is poured on top of the water. A du Nouy ring is positioned at the water/hydrocarbon interface and is held from above by the highly sensitive torque transducer.
The experimental protocol first has water introduced into the bottom of the interfacial cell to the level of the du Nouy ring. A layer of cyclopentane is then introduced on top of the water and the interfacial viscous and elastic moduli are continuously monitored. Initially, the temperature was kept at 0C and the interface remain mobile and both moduli remained very small. Upon reducing the temperature below the freezing point of water to -5C, the moduli rapidly rose, indicating freezing of the water. Once this occurred, the temperature was increased above 0C, but below 7.2C. Once this occurred, the moduli again dropped signalling a melting of the interfacial ice layer. However, over time, the moduli monotonically rose to large values with the elastic modulus being greater than the viscous component. This growth in the viscoelasticity of the interface reflects hydrate formation. Indeed, hydrate crystals could be observed spreading across the interface. In this manner, we have demonstrated, for the first time, the kinetics of hydrate formation. Repeating these measurements at temperatures above 7.2C revealed that the interfacial moduli remained small, indicating a lack of hydrate formation.
References
1. Tang, G.Q.; Morrow, N.R., SPE Res Eng SPE-36680-PA 12(1997)(4)269.
2. Sjoblom J, et al., J. Dispersion Sci. Tech., 31(2010)1100.
