Prediction of Scaling in Geothermal Systems PDF Download

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Book Description
One of the main objectives of the DOE Geothermal Program is to improve the efficiency and reliability of geothermal operations so that this renewable form of energy can be integrated into the nation's energy system. Scale formation and other chemical problems associated with energy extraction from high temperature brines frequently inhibit the economical utilization of geothermal resources. In some cases, these chemical problems can be so severe that development of a site must be abandoned after considerable capital investment. The goal of our research efforts is to construct an accurate computer model for describing the chemical behavior of geothermal brines under a wide range of operating conditions. This technology will provide industry a cost-effective means of identifying scaling problems in production and reinjection wells as well as in surface equipment, and also devising and testing methods for well as other uses described in table (1) can contribute significantly to meeting the objectives of the Geothermal Program. The chemical model we have developed to date can simulate calcium carbonate scale formation and gas solubilities in concentrated brines containing sodium, potassium, calcium, chloride and sulfate ions as a function of temperature to 250 C and for variable partial pressure of CO2. It can predict the solubility of other scale-forming minerals, such as amorphous silica, gypsum-anhydrite, halite and glasserite, as a function of brine composition to 250 C. The only required input for the model is the temperature, pressure and composition of the brine. Our modeling approach is based on semi-empirical thermodynamic descriptions of aqueous solutions. The model equations are parameterized by careful comparison to a variety of laboratory data. The ability of the resulting models to accurately predict the chemical behavior of even very concentrated high temperature brines is well demonstrated. This ability is an unusual feature of our models which is vital for applications to many important geothermal systems, such as those found in the Imperial Valley of California. In this report, the use of the present version of our model will be illustrated by an application to the prediction of the onset of two phase flow (breakout) in a brine confined by an external pressure. Calculations of this kind are important in assessing the production potential of a geothermal resource because the initiation of breakout in a well bore or power plant is usually simultaneous with the appearance of massive scale deposition. It is therefore necessary to predict breakout and also to assess the consequences of breakout in designing more efficient energy extraction processes. For the geothermal brine for which we have reliable composition and breakout data (East Mesa in California), the model gives results which are essentially identical to the measured values. Calculations also illustrate the importance of contributions of dissolved gases to the total pressure of the brines. Applications to other scale formation problems in Dixie Valley geothermal brines will also be discussed.