Use of a Chemical Equilibrium Computer Code To Analyze Scale Formation and Corrosion in Geothermal Brines

Abstract

American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. Introduction The probable size of U.S. geothermal energy reserves indicate we are in the beginning phase of the birth of a new industry. It is easy to visualize 50 to 100 geothermal power plants scattered all over the western United States involving all the electric utilities serving these regions. The development of geothermal energy on such a scale in the next 20 years will require considerable investment. Geothermal power plants will cost $25 to $100 million each. plants will cost $25 to $100 million each. Before such investment in geothermal energy can be justified, a number of questions must be answered including, "Can we assure reliable plant operation over the 20- or 30-year amortization period?" An important part of answering this question hinges on the effects of mineral scaling and corrosion on the performance of geothermal wells and power performance of geothermal wells and power plant components which come into contact with plant components which come into contact with geothermal fluids, and the subsequent plant degradation in electric output and maintenance expense. Under sponsorship of the Electric Power Research Institute (EPRI), Battelle-Northwest Laboratories (BNW) has undertaken research programs which are aimed at answering some programs which are aimed at answering some of the very complex questions relating to scaling corrosion and power plant performance. The major part of the material for this paper is drawn from our EPRI work which is paper is drawn from our EPRI work which is about 50% complete. Thus, this paper will concentrate on the problems we see, approaches we are taking to solve them, and some of our initial results. A prerequisite for the analysis of scaling and corrosion rates in geothermal systems is a comprehensive data base on geothermal brines, and an understanding of which brine components are important. Knowledge of the kinetics of scaling and corrosion processes must be accompanied by an processes must be accompanied by an understanding of the equilibrium conditions of complex, multicomponent brines and associated minerals. Numerous references exist in the geochemical literature for the calculation of equilibrium conditions and the alterations of equilibrium states due to temperature changes. Unfortunately, these studies are inadequate when one considers the extremely rapid changes of state that occur as a geothermal brine passes through a power plant. Supersaturation effects and slow nucleation and growth of crystals an change scaling rates by orders of magnitude during the short residence time of brines in geothermal plant components.