Abstract
Reviewed here is the kinetics of crystal growth of sparingly soluble minerals such as calcium carbonate, calcium sulfate, and barium sulfate, which frequently cause scaling problems in oil fields. For all three electrolytes, the crystal growth is surface controlled and follows a second-order rate law with an activation energy for the growth process of 10 to 20 kcal mol(-1). The growth of calcium sulfate seeded crystal above 100 degrees C demonstrates the importance of characterizing polymorphic transformation processes. Phosphonate scale inhibitors show differing modes of Phosphonate scale inhibitors show differing modes of imbibition in systems precipitating CaCO3 and CaSO4.
Introduction
The formation of crystals of scale-forming, sparingly soluble minerals continues to be a very serious problem for the petroleum engineer. Scaling arises from a specific set of geological, physical, and chemical conditions. Geological factors such as ground water circulation and mineral composition may mediate in scale formation as may physical factors such as pumping rate, well pressure, and the extent of fluid addition to the oil-bearing formation. However, the principal factors regulating scale formation in the oil field are chemical and such investigations can answer many of the problems. For example, scale caused by the addition problems. For example, scale caused by the addition of surface water to an oil-bearing formation can often be eliminated by chemical treatment of the injected water. A more important scaling arises from changes in subsurface mineral solubility due to variations in temperature and pressure under down-hole conditions. The difficulties are compounded by the fact that conditions frequently encountered under down-hole conditions, notably high pressure and high temperature, cannot be readily simulated in the laboratory. Sampling of an aqueous solution brought to the surface for analysis can lead to entirely misleading results owing not only to changes in temperature and pressure, but also to the fact that the solution may be actively depositing scale minerals within the well. In addition, the possible deposition of carbonate scale is dependent possible deposition of carbonate scale is dependent upon the carbon dioxide partial pressure in contact with the solution. The minerals that appear to pose the most serious problems in oilwell scaling are the sulfates of calcium and barium, and calcium carbonate. Calcium sulfate and calcium carbonate have solubility values that decrease with increasing temperature. The higher ambient temperature in the down-hole situation will therefore encourage the formation of scale deposits of these minerals. In the case of calcium sulfate the problem is complicated by the transition between the dehydrate, hemihydrate, and anhydrite phases. These calcium sulfate polymorphs may be stable or unstable under different conditions of temperature or of ionic strength. Barium sulfate presents a particularly serious problem, since it is very insoluble and cannot be dispersed once it has deposited as scale. Numerous studies have been made of the spontaneous precipitation of sparingly soluble minerals from solutions containing concentrations of the crystal lattice ions considerably in excess of the solubility values. Attempts are usually made to use controlled methods of mixing the reagent solutions containing the lattice ions, but it is extremely difficult to obtain reproducible results from such experiments. There are probably no systems that are entirely free from foreign substances or particles that can readily act as sites for the formation of nuclei of the precipitating phase. The attainment of so-called "homogeneous" phase. The attainment of so-called "homogeneous" nucleation conditions is therefore very difficult even when extreme precautions are taken to exclude impurities and foreign particles from the solutions. Experiments are frequently conducted to determine scaling thresholds in the laboratory by mixing solutions of salts containing the lattice ions and observing the appearance of the first precipitate. Such experiments are open to the same objections as those given above, however; moreover, they are frequently carried out in such a manner as to ignore important kinetic factors in the rate of precipitation. Thermodynamic interpretations of the results assume the attainment of equilibrium and involve the thermodynamic solubility products of the precipitating minerals. precipitating minerals. SPEJ
P. 117
