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.

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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.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractPredictive methods for wax deposition within pipelines make extensive use of diffusion models to account for the amount of wax being deposited. Laboratory testing methods commonly rely on deposition results from so-called coldfinger devices to assess deposition tendencies. While these experiments are primarily done for testing chemical inhibitor performance, they serve here as the basis for a cross-reference to the oftenused diffusion models. An empirical heat transfer model is developed to adequately address the heat transfer within this device. Oil analysis data in combination with thermodynamic prediction routines are used to prepare the necessary nparaffin solubility charts. While the heat transfer and solubility data are used for a diffusion-type wax deposition, an empirical deposition model is developed to describe the deposition process. It is illustrated how practical information such as deposition tendencies can be gained from these cold finger devices. An assessment of chemical inhibitor performance is made.

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