Predicting and Preventing Oilfield Scales

Various types of scales have long been a harassing problem throughout the oil field. Their buildup in the formation as well as along the wellbore decreases production by blocking the flow of oil, either by skin damage in the formation or by plugging off the perforations and lowering pump efficiency. If a tendency for the formation of scale can be predicted, it may save time as well as money in the treatment of that well. The basic types of scale encountered are calcium sulfate (CaSOd), barium sulfate (BaSOd), and calcium carbonate (CaCOq). Their precipitation is mainly due either to the mixing of two different formation waters or the mixing of injected water with formation water, the latter being the more frequent case. Scale is not always found downhole; frequently, it may be found in flowlines, separators and other surface equipment. Scale deposits are usually deposits of inorganic salts that result from the combination of a few parameters. Changes in pressure, temperature and mineral compositions of the waters are the parameters that affect the rate at which scale precipitates. Time is another factor often overlooked; the chemical reactions that produce scale are not spontaneous.