Synthesis and computer simulation of additives for wax inhibition in crude oils

Abstract

The deposition of wax caused by the precipitation of paraffin from waxy crude oil poses a serious flow assurance problem during the production and transportation of crude oil. The waxy crude oil contains high molecular weight alkanes, which are generally soluble in the crude oil at reservoir conditions. However, upon production and temperature drop, the solubility of these alkanes decrease drastically, inducing precipitation and subsequently forming the wax deposits. The extensive cost of wax management can be significantly reduced if wax deposition can be better predicted and prevented. There are various mechanical, thermal and chemical methods being used to inhibit wax deposition. Though the use of chemical inhibition via wax inhibitors is a widespread industrial practice, the compositional variability of the crude oil remains a challenge as it determines the effectiveness of these inhibitors. This challenge is the motivation of the research presented in this thesis. The primary objective of the research is to develop an effective wax inhibitor used to prevent wax formation in crude oil. To achieve this research objective, the study is divided into two interlinked themes based on molecular dynamics simulation and synthesis of wax inhibitors. Further experimentation is used for performance assessment of the developed wax inhibitors and validation of the proposed concepts as well as refinements of the theoretical models.

Coarse-grained molecular dynamics simulations are used to develop a novel method to determine the appearance of solid phases monitored through equilibrium and kinetic order parameter. The coarse-grained model used in this study maps together three consecutive backbone carbon atoms (CH2CH2CH2) with differentiation of the end group (CH2CH2CH2) allowing as a simple but effective representation of different types of n-alkanes. Parameterisation of the model is made by fitting to alkane phase equilibria data employing the Statistical Associating Fluid Theory (SAFT)-γ-Mie. Two model systems are considered representing a saturated alkane mixture and the full crude. Simulation of the diffusion coefficient is identified as an effective parameter to determine the onset of phase transition during the solidification process. The presence of a slowdown in the particle movement upon a decrease in temperature in the two model systems studied suggests the formation of an arrested state. It observed that the average diffusion coefficients predicted from MD simulations follow the trend obtained from DOSY-NMR, which was demonstrated on a system of varying complexity that replicates the crude oil system. A series of poly(maleic anhydride-olefin) are synthesised as wax inhibitors via free radical polymerisation with free radical initiator 2,2’-Azobis(2-methylpropionitrile) in toluene at 95 °C. The copolymers were characterised using Fourier Transform Infrared Spectroscopy (FTIR), Gel Permeation Chromatography (GPC) and Differential Scanning Calorimetry (DSC). Rheological analysis was used to evaluate their performance; the difference in onset temperature for viscosity increase and the overall viscosity reduction are the two main criteria employed to gauge the effectiveness of wax inhibitor preventing/delaying wax formation.