Wax Deposition During Production Operations: SOTA
Authors: Venkatesan R. , Creek J.L. 
Published: October 2008

Arterial wax deposition has been problematic for the industry for many years. The development of new technology progresses in fits and starts. This deposition is differentiated from deposition in the geological sense. The particular characteristics governing the magnitude and occurrence of deposition are:the temperature and pressure must be such that wax precipitates in the flowing phase andthere must be heat flow from the mobile phase to the pipe wall.

We also know the deposits formed are not 100% paraffin wax. Yet, some of the relatively new methods proposed for modeling deposition defy some of these well established principles. Experimental challenges also exist. In general, the heat flux in flowing pipelines is far less than that in laboratory experiments. We also know the flow line is not isothermal yet only typically test at one set of temperatures. More serious work needs to be done to develop an understanding of deposition in turbulent flow conditions, deposition over longer time periods, deposition at low heat transfer, and devising methods to obtain credible field data for comparison and guiding research in this area. This presentation will review the current state of the art and propose ways forward.

Introduction

Wax deposition in production pipelines is one of the major flow assurance risks that need to be considered whether developing new fields or maintaining existing operations. The problems associated with wax deposition are reduced productivity, increased pressure drop and the risk of getting a pig stuck during maintenance operations. It is known that the production losses and remediation operations associated with paraffin deposition cost millions of dollars1. Flow assurance risks, including wax deposition and wax gelation, become a bigger concern as the oil industry continues to expand deepwater operations to greater depths and distances in cold environments.

Precipitation of n-paraffins (waxes) is the precursor to wax deposition. The phase stability of crude oil is dependent on a multitude of factors including the composition of the oil, temperature, pressure, and other operating conditions. At reservoir pressures and temperatures, the paraffin molecules are typically fully dissolved in the reservoir fluid. As the temperature drops in the wellbore and production lines, these paraffins start precipitating out of solution due to solubility limits.

It is important to distinguish between the two phenomena - precipitation and deposition. Precipitation is a necessary, but not sufficient condition for deposition. Unfortunately, this issue seems to be a bit confused in common parlance in the oil industry. Precipitation is, sometimes, incorrectly referred to as deposition. It must be noted that deposition during production operations is a more complex phenomenon than precipitation. Wax precipitation is simply a thermodynamic phenomenon that is observed when the fluid temperature is reduced below the so-called Wax Appearance Temperature (TWAT). As the fluid temperature is further reduced, more waxes precipitate out of solution, possibly leading to a point where the fluid loses mobility due to gelation. The industrial practice is to approximately measure the temperature of "no mobility" using the ASTM D97 method; this temperature is called the pour point temperature.