Experimental Study on the Strength of Original Samples of Wax Deposits from Pipelines in the Field

The yield stress of the wax deposit, characterizing its mechanical strength, provides critical design basis for pigging. The deposits naturally formed in a pipeline (hereafter, “natural wax deposits”) and those artificially generated from model wax–oil mixtures (hereafter, “model wax deposits”) usually present different yield stress due to structural variations. Investigations on the distinctive yielding characteristics between natural and model deposits are limited in the literature. In this research, we present a comprehensive comparative mechanical and structural analysis of natural and model wax deposits, based on which representative laboratory tests can be designed to guide pigging operations. A rheometer with the vane geometry was enhanced to preserve the microstructure of the deposit sample collected from the field prior to the yielding test. Field wax deposits from different radial positions of the pipe were analyzed. It was discovered that the yield stress of the natural wax deposits increases exponentially with solid paraffin content. Moreover, the deposit layer closer to the center has lower solid paraffin content and lower resulting yield stress than the layer in the vicinity of the inner pipe wall. The original sample of natural wax deposits (called “original sample” for short following) was heated until completely melted and cooled again for the reformed solid sample similar to the model wax deposits in common use. The tested yield stress for the newly formed deposits can be 5–13 times that of the original sample at the same temperature due to the compact microstructure. Consequently, the required pressure to remove the wax deposits in the pipeline could be relatively high estimated based on the yield stress of model wax deposits. On the other hand, the natural wax deposits and model wax deposits formed on the coldfinger or in the flow loop are more alike in structure. So model deposits obtained in these ways should be used in the studies relative to pig motion, rather than the wax–oil gel which is currently very popular.