| Abstract |
The Las Tres Vírgenes geothermal field is a liquid-dominated resource located in the Baja California Peninsula (México) that currently has an installed power capacity of 10 MWe. The exploitation of this field has been limited because of a large, mostly calcite scaling potential of produced fluids which makes necessary the use of commercial inhibitors that suppress calcite and anhydrite deposition, in most of the wells. In this work the chemical composition of fluids from wells of the Las Tres Vírgenes field was used to perform equilibrium modeling for a wide range of temperatures to investigate the saturation indices of hydrothermal minerals and to identify desirable production conditions for the wells at which calcite scaling could be avoided. The wells studied were LV-1, LV-3, LV-4, LV-5, LV-11 and LV-13 while input data corresponded to an early production stage of the wells. Chemical speciation was calculated by using the WATCH program, the temperature ranged from 150 to 300C with calculations every 2C. The behavior of the saturation indices of minerals vs temperature for the wells LV-1, LV-3 and LV-5 suggested more than one fluid-mineral equilibrium temperature, (at about 180, 220 and 240C for well LV-1; 220, 250 and 270C for well LV-3 and 180, 200 and 240C for well LV-5) indicating the occurrence of multiple fluid entries. This is consistent with the temperature differences obtained when using different (Na/K, Na-K-Ca and quartz) geothermometers. We conclude that observed scaling phenomena in these wells is probably due to mixing of different fluids in the well. In well LV-4 the mineral equilibrium temperatures were found within the relatively narrow interval 257-268C, indicating no significant multiple fluid entries and reservoir fluid temperature in the feeding area of this well in its early production stage. Based on fluid-mineral equilibria, the reservoir temperature for wells LV-11 and LV-13 could be at least 290C, quartz equilibrium occurs at 290 and 300C respectively, and as was stated for well LV-4, no multiple entries were identified at early production conditions in both wells. Calcite was found to be oversaturated in the wells LV-1 and LV-3 for almost all the studied temperatures; for the wells LV-4 and LV-11 calcite showed equilibrium at 190C; for the well LV-5 at 180C while for the well LV-13 such equilibrium occurs at 225C. Anhydrite showed equilibrium at 200C in well LV-5; 220C in LV-1; 230C in LV-13; 250C in LV-3 and LV-4 and 290C in LV-11. Our results show oversaturation of calcite and anhydrite at higher temperatures, consistently with the reverse solubility of these minerals. Based on these results, separation pressures for the fluids were recommended although it is well known that the separation process itself could promote calcite precipitation, because of fluid degassing. |