油田化学(英文)-第7章 防垢剂.pdf

Chapter 7 Scale Inhibitors Some risk of scale deposition occurs in many operations in the petroleum in- dustry. Scale deposition happens particularly in production, stimulation, and transport. Scaling can occur when a solution becomes supersaturated, which occurs mostly if the temperature changes in the course of injection operations. Also, if two chemicals that will form a precipitate are brought together, a scale is formed (e.g., if a hydrogen fluoride solution meets calcium ions). From a thermodynamic perspective, there is a stable region, a metastable region, and an unstable region, separated by the binodale curve and the spinodale curve, respectively. The scale may consist of calcium carbonate, barium sulfate, gypsum, stron- tium sulfate, iron carbonate, iron oxides, iron sulfides, and magnesium salts [943]. There are monographs (e.g., Corrosion and Scale Handbook [159]) and reviews [414] on scale depositions available in the literature. Scale Inhibition The problem is basically similar to preventing scale inhibition in washing machines; therefore similar chemicals are used to prevent scale deposition. Scale inhibition can be achieved either by adding substances that react with potential scale-forming substances so that from a thermodynamics standpoint the stable region is reached or by adding substances that suppress crystal growth. Conventional scale inhibitors are hydrophilic, that is, they dissolve in water. In the case of down-hole squeezing, it is desirable that the scale inhibitor is adsorbed on the rock to avoid washing out the chemical before it can act as desired. However, adsorption on the rock may change the surface tension and the wettability of the system. To overcome these disadvantages, oil-soluble scale inhibitors have been developed. Coated inhibitors are also available. Often, scale inhibitors are not applied as such, but rather in combination with corrosion inhibitors. Thermodynamic Inhib