The production and transportation of oil and gas can be significantly affected by deposition of paraffin and asphaltenes in the reservoir rock tubulars, pumps, vessels, and pipelines. A wide range of solutions has been developed for the operating problems caused by these deposits. Field examples of problems and solutions are cited.
Paraffin and asphaltene control can be expensive and difficult in fields where deposition of these materials is prevalent. The paraffin-deposition problem can vary from very minor to extremely severe depending on the paraffin-wax content of the crude oil, the cloud and pour points of the crude, and the operating temperature. In cases where the paraffin content is high, the paraffin- control costs are significant. In locations where the wax content is much lower, the cost is less, but the problem remains an operating nuisance.
Asphaltene problems also vary widely, depending on the crude-oil composition, pressure, and temperature conditions. In the past, asphaltenes have played a significant part in the production history and economics in such instances as the deep horizons (Zone D-7) in the Ventura field, CA.
Paraffin problems were reported in tubing, rod pumps, and surface equipment as early as the 1920's. Significant paraffin problems have occurred outside the U.S. in the Shengli area of China, in India, and in Russia. Many devices, chemicals and techniques have been proposed and used, both successfully and unsuccessfully, to treat these problems.
Examples of the field problems and the techniques chosen by Shell in the U.S. to combat these problems are cited..
Paraffin deposits usually consist of straight- and branched-chain hydrocarbons (usually ranging from C18 H38 to about C40 H82) mixed with other organic and inorganic materials. The paraffin-wax content has been as high as 50% (Altamont, UT) in some crude oils, and as low as 1 % (South Louisiana) in others. The paraffin wax in the Altamont crude has an usually high carbon number (40 to 60).
Paraffin solubility in crude oil depends on the chemical composition of the crude oil, pressure, and temperature. Paraffin will begin to crystallize out of solution as soon as the equilibrium temperature and pressure is reached (cloud point). The paraffin deposits will often begin on surfaces cooler than the liquid. The viscosity of the crude oil is increased by the presence of the paraffin crystals and, if the temperature is reduced sufficiently, the crude will become very viscous (pour point). Viscosity vs. temperature curves are shown in Fig. 1 for the Altamont and Michigan crudes. The crude- oil viscosity behaves in a Newtonian manner until wax crystals begin to form (Fig. 1) and, subsequently, as the temperature is lowered, behaves in a non-Newtonian manner. The second deflection in the curve is a result of the large amount of crystals present in the pour-point region of the crude. High-pour-point crude oils generally have pour points in the 60 to 125 deg. F range and may be solid at room temperature (80 deg. F). The cloud point of the Altamont crude is as high as 170 deg. F.
The cloud point may be determined by the ASTM D-97-57 method and the pour point by the ASTM D-97-66 method. Test procedures to study effects of dispersants, solvents, and coatings on paraffin deposition have also been describe by Hunt and Jorda.
The viscosity of a waxy crude depends on the viscosity of the oil phase and the state of agglomeration of any paraffin-wax crystals present. Fig. 2 is a photomicrograph showing wax-crystal formation in the Altamont crude. The paraffin-crystal agglomeration depends on the quantities of wax crystals and gas present and the mechanical and thermal history of the crude oil. The apparent viscosity of the non-Newtonian oils can be greatly reduced by mechanical shear. The disintegration of large spherical wax agglomerates appears to be the primary cause of the lower apparent viscosity.