New Insight into the Wax Precipitation Process: In Situ NMR Imaging Study in a Cold Finger Cell

Abstract

A cold finger cell intended for the wax deposition measurements was fabricated and integrated into an NMR imaging probe for the noninvasive study of wax precipitation processes in situ. The cell was first tested with a model system; then, a series of experiments with different thermal gradients applied to the cell were performed for a waxy crude oil. NMR imaging of the operating cell revealed the formation of a deposit with the morphology and dynamics strongly correlating with the temperature regime. At higher temperatures of cold finger, the incipient wax gel ages uniformly, giving rise to the hard and thin inner layer of deposit accompanied by a branched loosely consolidated outer layer. Conversely, the lower temperatures facilitate formation of a thick deposit which no longer ages uniformly and slow down the diffusion-controlled growth of the branched deposit structure. The results obtained are consistent with the majority of the data previously reported. Thus, gelation of the wax at temperatures below the cloud point and subsequent thermal-driven diffusion processes are considered to be the dominant mechanisms of the deposit formation. The counter diffusion and Ostwald ripening aging concepts were found to be relevant in the case of the cold finger study and account for the phenomena observed in this work. The information obtained via NMR imaging is highly complementary to the results obtained by other techniques that can aid in understanding the essential processes behind the wax precipitation phenomena. The approach developed can be effectively extended to study any thermal-driven phase separation process.

Experimental Investigation of Time-Dependent Thickness and Composition of Multicomponent Wax Deposits on Cold Surfaces

“…This apparatus allows measurement of wax deposition onto a cylindrical surface where the temperature of both the surface and the waxy oil are held constant. In previous cold finger studies, where the cold finger could not be visualized in situ in real time, each data point of deposit thickness versus time was obtained through repeatedly stopping the experiment and extracting the cold finger for thickness measurement, , or through invasive but more difficult and expensive measurements such as MRI and NIR . Mahir et al, on the other hand, measured the full-time history of the wax deposition by using a transparent model oil containing a single wax component, which allowed real-time visualization and video recording of deposit thickness.…”

Fluid-solid phase transition of n-alkane mixtures: Coarse-grained molecular dynamics simulations and diffusion-ordered spectroscopy nuclear magnetic resonance

“…Morozov et al . 65 observed the formation of gel when the temperature of crude oils is reduced below the WAT, and showed that the deposition of wax can be accelerated with significant temperature decrease. Similar findings were reported by Dimitriou et al .…”

Wax Deposition Characterization under Flowing Conditions Using an Oscillatory Flow Setup

“…To study wax deposition mimicking conditions encountered in the subsea flowlines, a variety of lab-scale devices have been developed, with the most prevalent being the coldfinger and flow loop. ,− Coldfinger is a small cylindrical heat exchanger made of metal, which is submerged into a bulk oil phase . By separately controlling the temperature of the coldfinger and the bulk oil phase, a specified thermal gradient can be achieved for wax deposition.…”