Morphology of paraffin crystals in waxy crude oils cooled in quiescent conditions and under flow

The gel formation in waxy crude oils is a major concern for oil production and transportation. When oil is extracted, the decrease in temperature induces a partial crystallisation of paraffins by a supersaturation mechanism, involving in the first place the longest alkane molecules. Gel formation appears for very low amounts of crystals (around 1–2%) when the oil is cooled in quiescent conditions. However, during normal extraction conditions (pumping or well pressure gradient) the oil remains fluid, although crystallisation still takes place. This investigation deals with the morphology of paraffin crystals in crude oils, cooled either in quiescent conditions or under extensive shearing, controlled by different shear rates. The technique of observation is transmission electron microscopy after cryofixation, cryofracture and replica preparation. The images of gels made under quiescent conditions, distinctly show a lamellar structure: the lamella thickness of the order of the size of a paraffin molecule, the inter-lamella distance decreases with the total amount of crystals. Individual disc-like particles also appear occasionally, which probably constitute the nuclei of crystallisation. Under flowing conditions, only disc-like particles are observed, either isolated or assembled into clusters whose size depends on the shear rate. The discussion deals with the crystallisation mechanism in quiescent conditions and under shear and with the consequences upon rheology of the suspensions.