Carbonate Scale Inhibition in Republic's East Mesa Geothermal Operations

Abstract

Republic Geothermal, Inc., Lawrence Berkeley Laboratory and Monsanto Chemical Company have jointly funded a study by Vetter Research to evaluate and control carbonate scale in geothermal surface production and injection equipment. Republic's East Mesa geothermal field was chosen for the field experiments because of the utilization of downhole pumps in this field. Pumping prevents carbonate scale formation within the producing wellbore and shifts the entire scale problem into the surface equipment. Field experiments designed to control the scale problem consisted of pumping the wells for extended periods of time and adding a phosphonate scale inhibitor (Monsanto's Dequest 2060) at the wellhead. Fluid flow was directed through a test loop which simulated a flash cycle power plant. it was found that all carbonate scales can be avoided at a cost of less that 0.3 mills/KWhr in a planned 64 MW power plant. The critical range of scale inhibitor concentration is very small. Field studies were backed up by laboratory studies using the same inhibitor labeled with radioactive carbon. the pseudoscale collected in the field was analyzed and attempts were made to duplicate the field results in the laboratory.