A Two-Layer Paraffin Deposition Structure Used To Improve The Assessment of Laboratory and Field Observations
Authors: Toma Peter , Ivory John , Korpany Gerard , deRocco Mario , Goss Chris , Holloway Larry , Ibrahim Jamal , Omar Ismail 
Published: October 2008

The paraffin fouling of pipelines and wells is a dynamic process involving deposition, removal and aging/hardening of deposited wax. The effect of turbulent flow dissipation and wall shear stress on deposition and selective removal of nparaffin components was investigated first using a laboratory loop designed to allow direct observations and detailed sampling of deposit and, then, quantified using a turbulent burst mechanism to explain the removal from the wax deposit of n-alkanes having a lower carbon number.

Direct observations of the deposit, which were made possible with a novel deposition apparatus, and high temperature gas chromatography (HTGC) analysis that was performed on deposited wax offered essential information on the nature and composition of the deposit resulting from different flow regimes. Observations regarding the formation of two distinct layers, particularly for turbulent flow regimes are discussed and quantified. The turbulent burst action in the outer layer was used to explain experimental and field observations indicating the selective removal of n-paraffin fractions and the observed aging tendency.

Experimental results obtained during this study, particularly on the effect of aging and shear removal, are explained using direct observations made on the deposited wax and analytical investigations on the nature (carbon number) of the deposit. A simplified physical model was developed and used to explain the mechanism of paraffin aging during turbulent flow is discussed. The model offers a framework for improved interpretations of a complex and expensive aspect of oil production.

Introduction Deposit Aspect and Composition

The deposition of paraffin wax (consiting of paraffin and naphtenic hydrocarbons (Cn18), also indicated as n-alkanes, takes place in a great number of pipelines and wellbores when the pipe temperature reaches values lower than the cloud point corresponding to the local oil composition and pressure. The high concentration of paraffinic compounds (identified through HTGC) is found in many off-shore reservoirs, which are exploited today by PETRONAS, was the main incentive for initiating a study to better understand deposition and removal mechanisms. It is expected that results and tools presented in this study will lead to an improvement in the effectiveness of pigging and additive selection/injection strategies; with the ultimate goal of increasing oil production and reducing the associated costs and risks.

Figure 1. Conceptual description of deposition-removal - the feedback interaction between key controlling factors (Available in full paper)

It is generally recognized that for a majority of practical flow situations, waxing of pipelines is a dynamic process involving both deposition and removal. Aging, which can be identified through hardening of the deposit, is presented in this paper as an effect of selective removal of lower n-paraffin components from a semi-fluid zone covering the wax deposit.

A conceptual description of the paraffin deposit using a feedback interaction between essential factors responsible for deposition and removal is summarized in Figure 1.

At a certain moment, for any "local" flow situation, heat and mass transfer from the bulk flow to the cold pipe control the deposition process. Similar types of equations that estimate the turbulent heat transfer coefficients (h) through the Nusselt nimber, Nu, are used to calculate the amount of n-paraffin migrating to the cold wall using instead the Sherwood number(Sh); (eqs. 1,a,b,c).