ABSTRACT The nucleation of (Ba, Sr)S04 scale has been studied on different surfaces representative of those likely to be encountered in oil production. The amount of scale formed and the efficiencies ofthree typical scale inhibitors depended on the surface used. Similar results were obtained from mixtures of seawater with either synthetic or real formation water. INTRODUCTION Mineral scales are frequently formed in oil production, usually as the result of changes in the temperature or pressure of produced fluids, or the mixing of incompatible formation and injection waters. The scale may form a precipitate in thebulk fluid, and be carried along with it in suspension, or it may nucleate on the walls of containing vessels and grow to form an adherent scale. This adherent layer can reduce the flow of fluids in tubulars, prevent the proper functioning of chokes and safety valves, and concentrate some radioactive species from produced fluids to an extent sufficient to create a local radiation hazard. Some of the suspended scale may betransported to the walls to form a particulate deposit that is more porous, mechanically weaker, and easier to remove than those formed by direct nucleation on the surfaces. Despite the widespread occurrence of scale in oil production and other industrial and domestic situations, relatively little attention has been paid to the distinction between adherent and non-adherent scale. This paper describes some of the work being undertaken on scale nucleation on surfaces, and the effects of scale inhibitors on its inhibition and growth, in an on-going research programme at Harwell, supported by a number of oil producing companies. In view of the preponderance of mixed barium and strontium sulphate scaling in North Sea oil production as a result of mixing formation waters and seawater1, work has concentrated on (Ba, Sr)S04 scale formation on a number of surfaces representative of those likely to be encountered both downhole and topside. THE NUCLEATION PROCESS Nucleation of solid deposits is likely to be affected by the nature of the surface involved. The free energy change for the formation of a solid particle in a liquid can be written as(Available In Full Paper) where ø the driving force for the phase change, is given by(Available In Full Paper) in which S is the saturation index or saturation ratio, n is the number of molecular units in the particle,? the surface energy of the particle and A its surface area2. The change in G as the part1cle grows is given by(Available In Full Paper) where ? is the molecular volume and r is a measure of the radius of the particle that also allows for its shape. This change in free energy is thus affected by the saturation ratio, which isdetermined by the chemical composition of its surroundings and their temperature and pressure, and the energy required to create the new surface.