Effects of Shear and Temperature on Wax Deposition:  Coldfinger Investigation with a Gulf of Mexico Crude Oil

Abstract

The ability to determine the severity of wax deposition is an extremely important issue for the petroleum industry, particularly in the design and development of deepwater fields. Unfortunately, wax deposition is a complex process for which the mechanism is not fully understood. Furthermore, although much progress has been made in the last few decades in better understanding this complex process, the ability to accurately account for all the factors that affect deposition does not currently exist in the wax deposition simulators used in industry today. This paper examines the effects of two factors on the deposition process: shear and temperature. The effects are illustrated through results from coldfinger experiments, which are often used as a simple means to approximate the deposition process in flow lines. Shown are the effects in influencing both the amount of deposition occurring and nature of deposits formed from a medium-gravity Gulf of Mexico crude oil. Although the results do not directly represent flow-line deposition data, the data provide a relatively comprehensive set of illustrative examples on how deposition can vary with changing conditions. In the study, increases in shear have been observed to result in decreases in the amount of total deposition, primarily through a reduction in the amount of entrained crude oil contained in the deposits. The amount of wax in the deposits was determined to be relatively constant, within the range of variation in shear examined. However, the concentration of wax increased as the shear increased, because of the reduction in entrained crude oil. The effect of increasing the temperature differential (between bulk oil and cold surface) led to an expected increase in total deposition. Both the amount of wax and entrained crude oil increased. The concentration of entrained crude oil in the deposits was observed to increase at a greater rate. Hence, higher entrained oil concentrations occurred with larger temperature differentials.

The Influence of Operating Temperatures on Wax Deposition During Cold Flow and Hot Flow of Crude Oil

“…The changes of carbon number distribution in deposit can be seen in Figure 3. From Table 2, the deposition rate and wax content in deposit increase with the coolant temperature decreases from WAT to pour point (PP) when the bulk oil temperature kept constant, that is to say, the deposition rate and wax content in deposit increase with the temperature differential between the bulk oil and coolant increases, which agree with the Jennings's studies (Jennings and Weispfennig, 2005). Figure 3 shows that the low carbon number components increase while high carbon number components decrease, when the coolant temperature decreases from WAT to PP with the bulk oil temperature kept constant.…”

“…When the temperature of the oil near the pipe wall is lower than the wax appearance temperature (WAT), the wax and other substances dissolved in the oil will aggregate and deposit on the pipe wall. It is generally thought by early scholars that wax deposition mechanism can be summarized as molecular diffusion, Brownian diffusion, gravity settling, or shear dispersion, and the molecular diffusion is the predominant contribution in the wax deposition process (Paso and Fogler, 2004;Creek et al, 1999;Jennings and Weispfennig, 2005;Bidmus and Mehrotra, 2009). …”

“…Some showed the deposition rate increased as the temperature differential increased (Burger et al, 1981;Creek et al, 1999;Jennings and Weispfennig, 2005;Bidmus and Mehrotra, 2009;Lashkarbolooki et al, 2010;Zhenyu et al, 2011), and some showed opposite conclusion that the deposition rate decreased as the temperature differential increased (Paso and Fogler, 2004;Bidmus and Mehrotra, 2009;Zhenyu et al, 2011). On the basis of these studies, we selected a series of experimental temperatures (including all the temperature conditions in previously mentioned studies) by using cold finger device and high-temperature gas chromatography (HTGC) to study the effect of operating temperatures on the wax deposition, including the deposit mass, wax content, and carbon number distribution in the deposit.…”

The Study on Calculation Method of Temperature Distribution of Tested Tube for Wax Deposition Experimental Loop

“…The oil temperature and wall temperature of the reference section are consistent, so there is no wax deposition on the reference section. And the test section must ensure that there is a temperature difference between oil and the tube wall, the wall temperature of the test section is lower than the oil temperature in the test section, so that the test section will appear wax deposition (Lashkarbolooki M., 2011;Liu et al, 2012;Ramirez-Jaramillo et al, 2004;Azevedo et al, 2003;Bern et al, 1980;Tian et al, 2014;Singh et al, 1999;Jennings et al, 2005).…”

Targeting High Molecular Weight Wax

“…The wax content and the carbon number distribution of the wax deposit has been shown to have a significant effect on the hardness and yield strength of the deposit (Bai et al 2013) observed in the field. Jennings and Weispfennig (2005) studied the effect of shear and temperature conditions used in CF studies on the amount and nature of the deposits obtained. It was observed that decreasing the thermal gradient between the oil and CF or increasing shear increased the wax content and average carbon number of the deposit.…”