Scale Inhibition for Injection of Incompatible Waters

Abstract

ABSTRACT The utilization of scale inhibitors is the most common and promising method of combatting the scale problems in oil and gas field operations. The extension of the scale inhibition technique to geothermal operations faces some problems mainly due to the high temperature and high salinity of the brines. In addition, the injection of foreign waters and their associated mixing of incompatible waters give rise to some additional problems in chosing proper inhibitors. The brines resulting from incompatible water mixing contain non-equilibrium concentrations of anionic and cationic species and sometimes at extremely high supersaturations of scale forming compounds. The present paper gives the results of evaluating commercially available inhibitors in preventing CaSO4 and BaSO4 precipitation from supersaturated solutions. Four different types of commercial inhibitors have been chosen for this evaluation. They are phosphonates, polymaleic acids, polymers and phosphate esters. The conditions chosen for the evaluation represent the conditions found in the injection wellbore (for CaSO4 precipitation) and at the surface facilities of the producing well (for BaSO4 precipitation). The effect of the presence of iron in the producing brine on the efficiencies of the inhibitors were also studied as part of the present work. Several inhibitors have been found to perform with high efficiencies againat CaSO4 precipitation at 135°C from solution at a supersaturaion of 8. In general, the phosphonate based inhibitors have been found to perform better than the inhibitors based on polymers, pbosphate eaters and polymaleic acids. All the inhibitors performed poorly against BaSO4 at high supersaturations. Using high concentrations of the inhibitors and/or mixtures of some specific inhibitors, their efficiencies have been brought to values larger than 0.6. However, the presence of iron in the brine solution caused drastic decreases in the efficiencies of even the best inhibitors and inhibitor mixtures. In the presence of iron, Dequest 2010 (phosphonate inhibitor) in combination with a few of the other inhibitors showed efficiencies larger than 0.4 for times greater than 4 hours. At 120°C the time for failure of the inhibitors is 2 hours or less in the presence of iron. The paper gives all the data in the form of tables.