Some crude oils from eastern Venezuela, contain large amounts of high molecular weight components such as asphaltenes and waxes. Due to both the foaming characteristics of the crude oil and sluggish efficiency of the oil/gas separation process at the flow stations, both hydrocarbon contaminants asphaltene and wax are carried over to the gas gathering and transmission lines. These contaminants may deposit onto the inner wall of the pipelines producing some operational problems such as a decrease on the transmission capacity of the system and an increase of the cleaning frequency of gas pipelines. On the other hand, asphaltene and wax deposits may create a non-uniform barrier between the wall metal surface and the corrosive agents, interacting with corrosion inhibitors and reducing the corrosion rates.
The scope of this paper is to describe the potential benefits of asphalthene and wax deposition as corrosion inhibitors in gas pipelines, as well to present the experimental set up and some preliminary results from an ongoing research project,
Test conducted with a rotating cylinder electrode in gas sweetening conditions (temperature: 42 °C, PpCO2: saturated) and flow velocities up to 8 m/s showed a reduction of 50% on the corrosion rate of API- 5LB without inhibition and with a prefilmed wax treatment. For prefilmed asphaltene coupons at the same tests conditions, the reduction on corrosion rates were even higher with more than 70% of reduction in the range of 0 to 8 m/s.
Asphaltenes and paraffins are high molecular weight compounds of the crude oil that may precipitate, causing problems in underground and surface production facilities. Due to the foaming characteristics of the crude oil and sluggish efficiency of the oil/gas separation process at the flow stations, asphaltenes and paraffins are carried over to the gas gathering and transmission lines.
In an attempt to characterize the asphaltenes behavior, a methodology was developed at Intevep to study the effect of asphaltene deposition on the internal corrosion of the gas pipelines~5 Through both field and laboratory tests it was observed that asphaltene deposits act as a barrier to the corrosive environment, reducing corrosion rates over 70%. On the other hand, corrosion measurements have been carried out in oil facilities (separators, condensate/water collectors and well heads) exhibiting values below 0,05 mm/y (2 mpy) regardless of the operating condition 67.
A similar methodology was used to determine the influence of paraffin precipitation on the internal corrosion of the gas pipelines and to then compare to the behavior of the asphaltene deposits. The paraffinic crude oil used in this study contained 62.5% paraffin, 3.0% water and has an API gravity of 32.5.
Laboratory tests were carried out in a static autoclave tester, at typical conditions of transmission lines (49°C, 538 kPa CO2 and 0.2 kPa H2S, respectively), in order to evaluate the performance of a paraffin film and determining the behavior of this such layer in presence of different corrosion inhibitor concentrations.
In addition, RCE (Rotating Cylinder Electrode) tests were performed to observe the persistency of the paraffin film as a function of flow rate and to compare with the asphaltene layer performance of a previous study 1-7
