Effect of oil temperature on paraffin waxing of oil field equipment

Abstract

Analysis of paraffin-waxing of oil field equipment at West Siberian oil fields has shown that the number of wells troubled by wax deposits has been going up every year. Wax deposition causes fall in oil recovery and entails additional cost for deposit removing. Many researchers have studied the distinctive features of the process of wax deposit formation from model solutions [1‐3]. Analysis of the obtained data has revealed that they are highly inconsistent even for such simplified systems as artifi cial mixtures of wax and solvent. That is why, to prevent resin-wax deposits (RWD), it is essential to study their formation process in real oils. Until now, this process has been poorly studied because of wide diversity of properties and compositions of oils. In view of development of new oil fields characterized by heightened bed temperature and substantially increased wax content as well as of oil fields located in regions having permafrost soils, there is a pressing need for investigating the composition and properties of deposits formed in a wide range of temperature gradients. The reagents under development generally act selectively on the oil or a group of oils distinguished by a definite ratio of waxes, resins, and asphaltenes. It was therefore necessary to conduct further research into the composition and properties of the formed resin-wax deposits as a function of the difference in the temperatures of the oil and the metal rod being cooled ⌉t that reached 60°C. It covers the temperature variation range in which wax deposition on well equipment occurs. In the experiments, we used in-situ oils with diverse component compositions, which were collected from four clusters of wells located in various corrosion-active areas of the Samotlor field (Table 1). An aliquot of the oil was heated to 7 0°C and held at this temperature for 30 min. Next, the vessel containing the oil was placed in a water container and a cylindric steel rod filled with ice was dipped into the oil. The oil temperature was monitored by a thermocouple. The RWD formation process continued for 1.5‐2 h, after which the deposits formed were extracted and submitted to thorough analysis. The deposit composition was studied by using well-known procedures [4], which made possible identification of waxes, resins, and asphaltenes. The composition of the identified waxes was determined by gas-liquid chromatography, the surface activity of the resins separated from the deposits was determined from the acid number (GOST 11362‐76), and the carbon dioxide content in their pyrolysis products was determined by a procedure described in [1]. The properties of the RWD were determined by using conventional procedures described in [5, 6]. As evident from Fig. 1a‐c, the mass fraction of the deposits from the well clusters III and IV is 2.0‐6.2 mass %. The variation in the mass fraction of the deposits with temperature gradient has a maximum in the temperature region where the oil is saturated with wax. This maximum is assignable to crystallization of the wax at a temperature lower than the saturation temperature [7]. For the oils from the well clusters I and II (these oils are relatively high in viscosity), a 10‐15%