Novel Laboratory Cell for Fundamental Studies of the Effect of Polymer Additives on Wax Deposition from Model Crude Oils

Abstract

An in-depth understanding of the effects of polymer additives upon the rate, composition, and structure of paraffin deposition is required in the development of deep, off-shore oil fields to predict treatment strategies. To this end, we have developed a new laboratory-scale deposition cell that enables the measurement of wax deposition under controlled shear stresses and thermal gradients. A model oil with 3 wt % of a multicomponent wax was tested in a parallel-plate laboratory-scale deposition cell under laminar flow at low and high wall shear stress conditions (5−7 and 60−90 Pa, respectively). The addition of 0.1 wt % of poly(ethylene butene) (PEB), which has been shown to reduce the yield stress of the gelled solution 10-fold, actually increased the initial deposition rate. However, the deposit eroded from the surface under low shear stress conditions, while the deposit remained intact under high shear stress conditions. The addition of poly(maleic anhydride octadecene) modified with octadecyl amine and poly(maleic anhydride ethyl vinyl ether) modified with docosanyl amine each prevented deposition under similar conditions. Results provide a consistent framework for understanding the role of polymer additives on deposition in terms of the temperature field above the deposition surface and the cloud point of the solution. Polymers that suppress wax nucleation and suppress the cloud point will prevent deposition if the surface temperature is above the cloud point. This is the case with poly(maleic anhydride octadecene) modified with octadecyl amine polymers. Polymers that prevent wax crystal aggregation by a colloidal stabilization mechanism but that do not suppress nucleation do not prevent deposition. PEB polymers fall in this class. However, these polymers can produce deposited layers with sufficiently low mechanical strength that layer thickness can be controlled by erosion. The mechanism of erosion is demonstrated for the maleic anhydride copolymer. Results from gas chromatography and optical microscopy examine the composition and structure of the deposits.