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.