Predicting potential scaling problems can be difficult, and numerous saturation indices and computer algorithms have been developed to determine if, when, and where scaling will occur. The Langelier, Stiff-Davis, and the Oddo-Tomson saturation indices, all widely used in the oil field, are compared and contrasted relative to calcium carbonate scale. New saturation indices for barium, strontium, and calcium sulfate scale formation are introduced and discussed, along with an updated version of the Oddo-Tomson calcium carbonate index. An updated version of the CaC03 saturation index is presented that includes correction terms for fugacity effects and changes in the solubility of CO2 in oil and gas wells as functions of temperature, pressure, water cut, and hydrocarbons present. The CaC03 saturation index does not require a measured pH and can accommodate the presence of weak acids, such as H2S, and weak organic acids in the system. The sulfate scale prediction methods (for gypsum, hemihydrate, and anhydrite) are easy to use, reliable, and designed for field use by an operator who may be untrained in chemistry. The prediction methods can be applied to any production well where calcium carbonate, calcium sulfate, strontium sulfate, or barium sulfate scale occurs.
As the petroleum industry produces more water to recover hydrocarbons more efficiently from existing fields, scale formation and corrosion will become increasingly difficult problems. Calcium carbonate and sulfate scale problems are incipient to some fields but also occur where incompatible waters are introduced to the production scheme during waterfloods or workovers. A general discussion of saturation indices and sulfate and carbonate predictive equations are given later.
The sulfate scales of Ca, Sr, and Ba are well described. However, the chemistry of the individual scale formers is described briefly, along with supporting field application to lend credibility to the predictive equations developed here.