Crude oil from Norte de Monagas field, in Venezuela, contains large amounts of asphaltenes. Some of them are very unstable with a tendency to precipitate. Because liquid is carried over from the separation process in the flow stations, asphaltenes are also present in the gas gathering and transmission lines, precipitating on the inner wail of pipelines. The gas gathering and transmission lines contain gas with high partial pressures of C02, some H2S and are water saturated; therefore, inhibitors are used to control internal corrosion. There is uncertainty on how inhibitors perform in the presence of asphaltene deposition. The purpose of this paper is to describe the causes that enhance asphaltene deposition in gas pipelines and present some results from an ongoing research project carried out by the Venezuelan Oil Companies.
Crude oil from Norte de Monagas field in Venezuela contains large amounts of asphaltenes. Some of them are very unstable and have a tendency to precipitate. They also have foaming characteristics. Asphaltenes are hydrocarbon constituents of crude oil, with high molecular weight. These polycyclic, aromatic substances are colloidally dispersed in the crude oil [1].
Due to the foaming characteristics of the crude oil and a low efficiency of the oil/gas separation process of the separators in the flow stations, liquid earned over (crude oil) is often present in the gas gathering and transmission lines. Once this liquid is inside the gas lines, the asphaltenes present in this liquid may precipitate and deposit onto the inner wail of the pipelines producing the following operational problems[2]:
A decrease in the transmission capacity of the system. Glycol contamination of dehydration systems. An increase of the frequency of cleaning gas pipelines. Obstruction of the path for cleaning tools. Vibration problems on compressors. Shut-downs of gas processing plants. Delay of oil/gas production due to obstructions in flowlines.
Asphaltene deposition in gas pipelines is affected by mechanical, thermodynamic, chemical and electrical factors, Mechanical factors such as friction, shear forces and forces produced by the impact between asphaltene molecules cause pressure and temperature drops that affect the stability of the asphaltene molecule, resulting in precipitation. Thermodynamic factors [3] include changes of pressures and temperatures caused by flow velocities, obstruction of the path of the flow caused by valves, changes in flow direction due to the different geometries of the pipeline. Chemical factors include changes in pH, volubility characteristics of the liquids, the presence of acids [4], (i.e. carbonic acid) the use of organic compounds, i.e. corrosion inhibitors.
Corpoven, the operating company of the Norte de Monagas Field, has established a number of asphaltene control methods [2, 6] including the development and application of a chemical additive, designed as an asphakene dispersant and to remove the asphaltene deposits from gas pipelines. Another control method has been the modification of the internal components of oil/gas separators to increase their efficiency and thus reduce the amount of liquid carried over.
In addition to the above, there are potential corrosion problems in the gas gathering and transmission lines.
