Advancements in Paraffin Testing Methodology

Abstract

Abstract Predictive methods for wax deposition within pipelines make extensive use of diffusion models to account for the amount of wax being deposited. Laboratory testing methods commonly rely on deposition results from so-called coldfinger devices to assess deposition tendencies. While these experiments are primarily done for testing chemical inhibitor performance, they serve here as the basis for a cross-reference to the often-used diffusion models. An empirical heat transfer model is developed to adequately address the heat transfer within this device. Oil analysis data in combination with thermodynamic prediction routines are used to prepare the necessary n-paraffin solubility charts. While the heat transfer and solubility data are used for a diffusion-type wax deposition, an empirical deposition model is developed to describe the deposition process. It is illustrated how practical information such as deposition tendencies can be gained from these cold finger devices. An assessment of chemical inhibitor performance is made. Introduction One of the most serious problems in the transportation of crude oil is the flow obstruction within pipelines due to paraffin deposition1,2 (commonly referred to as wax deposition). This deposition in pipelines is a constant concern for operators inasmuch as it may partially or totally block production lines and thus cause production to decrease or halt. Since pipeline blockage remediation becomes more and more expensive as the production of oil moves into deeper water, it is crucial to have a good understanding of wax precipitation and deposition processes. This understanding enables the operator to foresee potential problems and to deploy suitable prevention and/or remediation means. This paper focuses on the development of an empirical model to describe the wax deposition process within coldfinger devices as illustrated in Fig. 1. A rigorous mathematical derivation illustrates the method to correlate the coldfinger deposition process to those in pipelines. The basic form of this model can be set forth asEquation (1) in which the temperature difference ?T is expressed asEquation (2) provided that the applicable temperature difference remains positive. Eq. 1 relates the mass deposition rate to the temperature difference between the bulk oil and the wall. This temperature difference is schematically illustrated in Fig. 2 for an exemplary pipeline profile as a function of distance. The temperature range for which wax deposition may become problematic is indicated in Fig. 2 by a thick line. The far left corner represents the point at which the oil temperature at the wall reaches the wax appearance temperature (cloudpoint). This location within the pipeline is the first point at which wax deposition may occur. The range for which coldfinger devices are used to obtain wax deposition data according to Eq. 1, is indicated by the 2 vertical dashed lines in Fig. 2. Most of the experiments are carried out with the bulk oil temperature set to the wax appearance temperature (cloudpoint) and the coldfinger wall temperature at some lower temperature. Typical values for ?T vary from 0°C to about 20°C.