A Two-Layer Paraffin Deposition Structure Observed and Used to Explain the Removal and Aging of Paraffin Deposits in Wells and Pipelines

Abstract

Paraffin fouling of pipelines and wells is a dynamic process involving deposition and removal. Deposited wax growth and hardening have been observed in the field. They must be properly evaluated at the design stage in order to develop a suitable and economic flow assurance strategy. Laboratory studies and direct observations on the nature and composition of the deposited wax have been made possible with the aid of a novel, axial wax deposition apparatus. This study resulted in an improved understanding and calculation of wax removal and aging rates. A turbulent burst mechanism was used to explain experimental observations made during this study as well as results recently published by other laboratories and field knowledge of wax removal and the aging/hardening process. Direct observations of the wax deposit nature and composition which were performed on samples removed from the deposited wax, provided essential information on the nature and composition of the deposit resulting from different flow regimes. These observations suggested that two distinct flowing zones (layers) and a wall-attached, thin, solid deposit are part of the flow-related deposition-removal process. A turbulent burst action, which is promoted in the near-wall area by the turbulent flow core zone, was used to explain experimental and field observations suggesting a selective removal of n-paraffin fractions. While the proposed removal mechanism acts selectively depending on the size of nonattached, crystallized wax, the transport of liquid n-alkanes components from the bulk, warmer zone to the near-wall colder region is considered as an overall “n-alkane” transport as accepted and used in calculations so far. The proposed two fluid layer model offers a framework for improving the design and operation (pigging and additive) strategies as well as for revisiting the accepted (safe) ranges of design velocities for waxy crude production and transport limes. This work aims to offer a better tool for estimating the pigging frequency, the optimal flow regime and a better additive testing and application strategy with considerable costs-saving to industry.