Effect of Shear on the Performance of Paraffin Inhibitors:  Coldfinger Investigation with Gulf of Mexico Crude Oils

Abstract

Paraffin inhibitors are chemicals that are used to reduce deposition of paraffin/wax onto surfaces of flow lines. For some crude oil production systems, paraffin inhibitors can provide significant reduction in wax deposition and be an economically attractive means for increasing production and/or decreasing wax remediation control costs. Paraffin inhibitors function by incorporating into wax deposit structures, altering the deposit structure in order to reduce the deposits' ability to adhere. Generally, effective inhibitors create weaker deposits which are more susceptible to removal from shear forces in the flow field. As such, the performance of the inhibitors can be related to the shear forces present in respective flow streams. In this paper, results of coldfinger experiments using four Gulf of Mexico crude oils are presented to illustrate the effect of shear on inhibitor performance. Trends from the data indicate that inhibitor performance improves with increasing shear. From the test results, the percent inhibition in the amount of depositing wax (for inhibitor treated crude oil tests compared to untreated crude oil tests) was consistently observed to increase with increasing shear in the various experiments. The amount of coldfinger surface area bare of deposition was similarly found to consistently increase with increasing shear. The percent inhibition of the amount of total deposit weight (deposited wax with entrained crude oil) did not always increase with increasing shear thoughseven decreasing in many cases. This is not related to a reduction of inhibitor performance but rather due to variation in the untreated deposit reference with shear. As the amount of entrained crude oil in a deposit decreases with increasing shear, the percent change on the total deposit weight (deposited wax plus entrained crude oil) will vary for a paraffin inhibitor's effect on a fixed amount of deposited wax species.

Deposition apparatus to study the effects of polymers and asphaltenes upon wax deposition

“…The increased deposition of C35+ paraffins resulted in a harder deposit. Jennings and Weispfennig (2006) increased the stirring speed in cold finger tests to create higher shear rates and wall shear stresses, but the stress distribution on the cold finger surface was not uniform and their estimate of the change in shear stresses between high and low stirring rates was only 1-2 Pa. Nevertheless, most additives that they tested were shown to be more effective at reducing the amount of C35+ paraffins at higher shear stresses than lower shear stresses.…”

“…First, there are "cold finger" tests in which a cooled cylinder is immersed in circulating bath of test fluid. Deposition occurs on the cold surface (Jorda, 1966;Singh et al, 2001b;Jennings and Weispfennig, 2006). This test has been modified to induce deposition on discs (Hunt, 1962;Wang et al, 2003).…”

“…Deposition studies using a cold disk or a cold finger with wax control additives have been primarily used to screen for efficacy, measured by a reduced deposit weight upon the cold surface (Newberry, 1984;Machado and Lucas, 1999;Dong et al, 2001;Wang et al, 2003;Jennings and Weispfennig, 2006). Only two of these studies examined the composition of the deposit (Wang et al, 2003;Jennings and Weispfennig, 2006), and only one of these examined the effects of shear stress (Jennings and Weispfennig, 2006).…”

“…Only two of these studies examined the composition of the deposit (Wang et al, 2003;Jennings and Weispfennig, 2006), and only one of these examined the effects of shear stress (Jennings and Weispfennig, 2006). Wang et al (2003) found that the wax control additives tested generally did not mitigate the accumulation of more troublesome, long paraffins (C35+), and in some cases increased it.…”

Evaluation of Paraffin Wax Inhibitors: An Experimental Comparison of Bench-Top Test Results and Small-Scale Deposition Rigs for Model Waxy Oils

“…These bench-top tests are relatively quick and easy to perform with a large number of candidate paraffin inhibitors for a particular crude oil and decisions are often made with those results [1,6,11]. However, the results from bench-top tests do not always translate to tests under flowing conditions, e.g., flow recirculation loops [1,4,12], which seek to reproduce temperature gradients, pressure, flow field, shear environment, etc., that the oil experiences in the pipeline [1,10].…”