The deposition and rheology of organic gels.

Abstract

Paraffins can precipitate out of oil solutions and deposit on cold surfaces if the surface temperature is lower than the cloud point temperature. Paraffin deposition during crude oil production and transportation operations is a billion dollar problem. Molecular diffusion, enhanced by convective mass flux, was found to be the dominant driving force for paraffin deposition. The paraffin deposit was found to be in the form of a paraffin-oil gel that is comprised of a network of paraffin (wax) crystals with the solvent (oil) occluded. Laboratory flowloop experiments showed that the solid wax content of the deposit increases with time due to a counter-diffusion process within the gel deposit. Efficient remediation techniques require prediction of the deposition process and the rheological properties of the deposit. A theoretical model was developed based on heat and mass transfer fundamentals, and found to predict the deposition and aging rates under laminar flow very well. Studies of wax deposition under turbulent flow conditions showed that the deposition rate is reduced due to the high shear stress exerted at the deposit-oil interface. Incorporation of this <italic>shear reduction</italic> effect into the mathematical model, along with the development of an approach based on solubility to calculate the mass transfer rate, resulted in good prediction of the deposition process under turbulent flow. The rheological properties of the wax-oil gels, such as the gelation temperature and the yield stress, were found to be strong functions of the wax content, the shear and thermal histories of the gels and the relative amounts and properties of other components such as asphaltenes. Whereas the yield stress of gels formed under static conditions decreased with increasing cooling rate, the yield stress of gels formed under high shear conditions increased with increasing cooling rate. Further, the yield stress reached a maximum with respect to the gelation shear stress. Microscope observations showed that the cooling rate and the shear affected the paraffin crystal morphology, thus influencing the yield stress. The effect of asphaltenes on the gelation and strength of paraffin-oil gels was explored by rheometry, NMR spectroscopy and microscopy, showing that asphaltenes act as natural inhibitors.