(635d) Use of Drop Rest Technique to Evaluate the Stability of Waxy Crude Oil

Demulsifying waxy crude oils at low temperatures is a challenging task for offshore platforms. The problem becomes more difficult as the proportion of water in crude oil increases due to the depletion of oil reserves in the reservoir. However, due to space constraints, it is not feasible to increase the capacity of the heater-treaters to match the higher proportion of water in the crude oil. At low temperatures, the wax tends to crystalize at the oil-brine interface. The interfacial crystallization is promoted by the presence of surface-active impurities such as asphaltenes, resins, and naphthenic acids. Wax crystals form a viscoelastic network at the oil-water interface, which dampens the thermal fluctuations, thereby drastically reducing the efficiency of coalescence. The effect of demulsifiers is to either soften or eliminate the wax cover at the interface and thereby facilitate coalescence. Coalescence between two brine droplets occurs when the wax barrier between the drops is weak enough to be broken by thermal fluctuations.

In the present study, the coalescence of individual brine drops at the crude oil-brine interface has been studied through a “Drop Rest Setup”. A brine drop is allowed to fall through a crude oil pool to an otherwise quiescent oil-brine interface. Near-infrared videography is performed to view the motion of the drop as it interacts with the interface. The oscillatory motion of the drop as it impacts the interface is studied to evaluate the viscoelasticity of the interface and the drop rest time is measured and correlated with the interfacial viscoelasticity. The model developed by Huh and Scriven (1968) is modified to evaluate the interfacial viscoelasticity from the trajectory of the drop before it comes to rest. The drop rest time, i.e., the time over which the drop remains under stationary contact with the interface before coalescing with the underneath brine phase is also measured. It is a stochastic variable and is found to follow a log-normal distribution. About 150 drops are studied to determine the expected value and the variance of the distribution. The expected value of this distribution is related to the activation energy required to penetrate the wax network. This activation energy is then related to the interface’s elasticity, which serves as a fitted parameter within the model. For this study, crude oil from the Heera reservoir is used. It is diluted with xylene to control the extent of crystallization of wax. The effect of two surfactants viz., Sodium bis(2-ethyl hexyl) sulfosuccinate AOT (low HLB surfactant) and Sodium Dodecyl Sulphate ( high HLB surfactant) and their mixtures on drop rest time are studied. The interface is allowed to age for fifteen minutes in the presence of surfactant to loosen the wax network, before commencing the measurement of drop rest time.

This study provides a simple experimental technique to relate the coalescence phenomenon with the interfacial rheology as well as determine the stability of the oil-brine interface.