Field Tests of Organic Additives for Scale Control at the Salton Sea Geothermal Field

Harrar, J.E., Lawrence Livermore Natl. Laboratory Locke, F.E., Lawrence Livermore Natl. Laboratory Otto Jr., C.H., Lawrence Livermore Natl. Laboratory Lorensen, L.E., Lawrence Livermore Natl. Laboratory Monaco, S.B., Lawrence Livermore Natl. Laboratory Frey, W.P., Lawrence Livermore Natl. Laboratory Abstract A pilot-size brine handling system was operated from Magmamax Well 1 in southern California to study the characteristics of siliceous scale deposition and to evaluate the possibility of treating the brine with chemical additives to control scaling. The rates of formation, chemical constitution, and morphology of the scales were examined as functions of temperature, brine salinity, substrate material, and antiscalant additive activity. Potential antiscalant compounds were screened using a silica-precipitation inhibition test at 90 deg. C. The most active classes of compounds were those containing polymeric chains of oxyethylene and polymeric nitrogen compounds that are cationic in character. The best single compound was Corcat P-18 TM (Cordova Chemical Co. polyethylene imine, molecular weight 1,800). This compound had no effect on the scale formed at 220 deg. C but it reduced the rates of scaling at 125 and 90 deg. C by factors of 4 and 18, respectively, and it also functioned as a corrosion inhibitor. The best additive formulation for the brines of the Salton Sea Geothermal field (SSGF) appears to be a mixture of an organic silica-precipitation inhibitor, a small amount of hydrochloric acid, and a phosphonate crystalline deposit inhibitor. Introduction Interest in utilizing the geothermal resources of the Imperial Valley in California for the generation of electricity has accelerated rapidly in recent years. One resource in particular, the SSGF, is attractive because of its high temperature and size. Recent estimates of its potential for electrical power generation range between 1,300 and 8,700 MW per year (over a 20-year period). The fluid of this resource, however, is a highly corrosive, high-salinity brine containing several constituents that form deposits of scale on power plant components as the brine is cooled. Economical utilization of the SSGF will require techniques for limiting scaling and corrosion to acceptable levels. Scale deposition control at SSGF is particularly difficult because the scale that forms in the portions of the brine handling equipment operating at low pressures and temperatures (100 to 150 deg. C) is predominantly silica and it deposits at rates approaching 0.2 in./D. (Energy extraction systems in which the brine is flashed and injected at high temperature mitigate this problem, but considerable energy is discarded.) Chemical treatment scheme to retard the low temperature scale have been considered, but until recently there have been no systematic investigations of this approach. In 1976, Owen and coworkers demonstrated effective control of the siliceous scales by acidification of the brine with hydrochloric acid, and this technique has been verified in New Zealand by Rothbaum et al. However, for SSGF brines, acidification has several disadvantages:because concentrations >300 ppm of HCl are required, chemical costs are high;the pH of the brine must be lowered from 6 to 3 for complete scale control, and this sharply increases corrosion rates, andacidification tends to interfere with effluent brine treatment Processes involving sludge-bed reactor clarification. Other methods of scale control such as seeding with a silica sludge and the use of scale adhesion inhibitors also have been examined briefly. In this paper we present the results of tests of organic chemical agents for silica scale control in hypersaline geothermal brines. Prior to this work, virtually no knowledge existed on the types of compounds that would interact with silica under the severe geothermal conditions of high temperature, high ionic strength, and high fluid shear rates. Accordingly, to screen a large number of substances rather rapidly, we designed a small-scale flash system as a brine treatment test apparatus and operated it from SSGF Magmamax Well 1 and Woolsey Well 1. SPEJ P. 17^