Thermal, Macroscopic, and Microscopic Characteristics of Wax Deposits in Field Pipelines

摘要

Wax deposition is an important issue in crude oil transportation. Understanding the deposit properties assists in making a suitable schedule for removing wax deposits. However, there was little work published on the nature of wax deposits in field pipelines. This study focuses on the thermal, macroscopic, and microscopic characteristics of wax deposits obtained from the field pipelines. First, differential scanning calorimetry (DSC) was used to investigate the thermal properties of wax deposits and crude oils. The results showed that the wax appearance temperature (WAT) and wax content of wax deposits is much higher than that of crude oils, and both of two parameters increase with the increasing radial distances. For all deposits, the precipitated wax concentration increases significantly with the decreasing temperature near the WAT, while it almost increases linearly when the temperature decreases about 2S℃ below the WAT. Based on DSC results, a correlation between the solid wax content and wax content and temperature was developed. Verification experiments showed that this correlation calculates the solid wax content with the absolute deviation within ±4% and the average relative deviation of 3.8%. Second, the macroscopic structure of wax deposits was visually observed. The yield stresses were also determined using the vane method. The results showed that the loose structure of the original deposits could result in small values of yield stresses, and the yield stresses increase as the radial distances increase. Third, the optical microscopy was used to observe the microscopic characteristics of wax crystals. The average size, aspect ratio, and boundary fractal dimension of wax crystals in deposits vary in the ranges 9.1-14.3μm~2, 1.55-1.77, and 1.13-1.20, respectively. The average size and boundary fractal dimension of wax crystals in deposits are higher than that of the crystals in crude oils.