Prediction of Deposition of Calcium Sulfate Scale Under Down-Hole Conditions

Abstract

The thermodynamic method of calculating calcium sulfate scale has been improved by including the effects of complicated down-hole phase equilibria. Although the modified method has its limitations, it appears to provide more reliable results. Introduction Calcium sulfate (CaSO4) scale, especially gypsum, still causes unpredictable problems in many oil fields. This type of scale can cause severe plugging of equipment and producing formations; it creates the necessity of costly workovers and stimulation jobs. The best method for combating this scale seems to be the application of inhibitors - for example, by squeeze techniques - before the scale is formed. The reason is obvious: it is much more economical to prevent the formation of scale than to repair the damage caused by it. To apply the most suitable inhibition method, we have to know if and to what extent scaling occurs. At least some knowledge about the likely location of scale within the wall is equally important. The present methods of predicting scale in an oil field do not give all the answers, even though the literature contains enough data to develop and prove a more precise and reliable method. It is still common practice to wait for a production decline in the field, then pull the tubing and find - more or less accidentally - scale on the now-exposed metal surface. This scale sample is considered a proof for the occurrence of scale. This procedure can be very costly, and irreparable damage can be done before the scale is detected. Besides, this method fails frequently because scaling can occur even without the deposition of scale in the tubing. A better method for scale prediction is the use of water analyses and solubility data of CaSO4 compounds in the oilfield brines. There are many publications1–9,12,13,15,16 on the solubility of calcium sulfate compounds in water and brines at 1 atm. Most of the data in these publications are in good agreement. Together with the information from the water analyses, they are used to predict potential scale deposition. Diagrams, empirical equations, or thermodynamic relations are used to find the "maximum" solubility of calcium sulfate, mostly gypsum, in a water of similar composition.6–9,12–14 If the Ca++ and SO4-- concentrations in the actual water approximate the calculated "maximum" concentration of CaSO4, the field brine is said to have a "scaling tendency". We believe that this method can give only vague and unreliable information because it gives the solubility of CaSO4 or its hydrates at 1 atm only.