A Decrease of Calcium Carbonate Scaling Potential Due to an Increase of CO2, a Case Study: PGM-29, Miravalles, Costa Rica

Abstract

Well PGM-29 is located in the southeastern sector of the Miravalles Geothermal Field; in this area all of the wells present high calcium carbonate scaling tendencies. From February 1997 to April 1998 a 5 MWe backpressure unit rented from the Comisión Federal de Electricidad of Mexico (CFE) was installed in this well. During that time, the mass production of PGM-29 was 200 kg/s and the chemical composition of the produce fluid was sodium- chloride-bicarbonate type waters with a content of non-condensable gases in the steam of 5% w/w at 7.0 bar absolute separation. Due to the high scaling potential (1296 kg of CaCO3 per day) dosages of calcium carbonate inhibitor were from 4 to 8 ppm. After several years of being out of commercial production, in January 2007 another 5 MWe backpressure unit owned by ICE (Instituto Costarricense de Electricidad) was installed in PGM-29.

During this second production period the chemical composition of PGM-29 fluids remained the same type but the TDS increased around 700 ppm and the non-condensable gases in the steam went up to 88% w/w. According to the chemical inhibition monitoring system, there were no calcium carbonate deposits although this well has been characterized by a high scaling potential. This behavior was related to an increase of CO2 concentration has an effect in the bicarbonate-carbonate equilibrium that increases calcite solubility.

PGM-29 has been producing continuously since January 2007 and the non-condensable gases content has decreased to a value around 22% w/w. Calcite saturation indexes calculated utilizing WATCH software (v. 2.3) indicate that PGM-29 fluids are only slightly oversaturated in this mineral and experience has shown that calcite is not forming. Under these circumstances, in PGM-29 a dose of 2.5 ppm of inhibitor is injected as a precaution in case the CO2 content decreases and a very strict monitoring plan is maintained in order to detect any change in chemical composition of its fluids that would modify the calcite saturation conditions.