A new model for predicting oil-wax gel strength as a function of wax crystal precipitation and structural characteristics

Abstract

Wax deposit strength is a determinant parameter for scheduling crude oil pipeline pigging program. However, previous oil-wax gel strength models only take into account wax precipitation amount (WPA) and ignore other influential parameters, like wax crystal structure. In addition, wax deposit strength measurement remains difficult because its structure would be damaged when using conventional rotational rheometer. In response, this paper develops an oil-wax gel strength model with wax crystal precipitation and structural characteristics incorporated, and proposes an indirect approach to measure the yield stress of real wax deposit. The impacts of WPA, wax crystal roundness, boundary fractal dimension, area fraction and aspect ratio are investigated. Grey relational analysis method shows that WPA, wax crystal boundary fractal dimension and area fraction are highly relevant factors. It is also revealed that a typical pigging process on real wax deposit can be divided into four stages, and wax breaking force is the wax removal force in the second phase. Moreover, it is found that high oil temperature increases WPA, wax crystal boundary fractal dimension and area fraction, and decreases wax layer thickness, accordingly, wax breaking force increases. For model validation, an indirect approach involving wax deposition and pipeline pigging is proposed. It overcomes the uncertainty of rotational rheometer in real wax deposit yield stress measurement. The average and maximum relative errors between predicted and inversely calculated yield stresses are 15.89% and 37.32% respectively, which proves the feasibility of this model to real wax deposit.