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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 catonic 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 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 brine on the efficiencies of t he inhibitors were also studied as part of the present work. present work. Several inhibitors have been found to perform with high efficiencies against CaSO4 precipitation at 135 degrees C from solution at a supersaturation of 8. In general, the phosphates based inhibitors have been found to perform better than the inhibitors based on polymers, phosphate esters 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 degrees 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.

Introduction

All geothermal operations will eventually rely on reinjection of the heat-depleted effluent brine and/or injection operations have been discussed previously. In addition, even during the early operations, the mixing of incompatible waters can take place. An example of this situation is the mixing of filtrate of the drilling mud with the formation water. No matter which types of waters enter the formation (whether through injection of drilling fluid invasion), these waters can be rather incompatible with the formation waters. Thus, mixing of incompatible waters takes place at or near the conditions of the reservoir. The mixed waters (now different from the original reservoir water) are subsequently subjected to the various thermodynamic conditions of the geothermal operation.

The problems associated with such incompatible water mixing were discussed previously through computer modelling with previously through computer modelling with examples. The previous publications on the subject briefly described the various ways of treating the imported water to reduce or eliminate the scale problems associated with the mixing of incompatible waters. One of these methods involve the use of chemical inhibitors. The present paper describes the laboratory evaluation of some commercial scale inhibitors. The data presented in this paper can serve as a guideline for the purpose of selection and/or further evaluation of purpose of selection and/or further evaluation of scale inhibitors.