Phase segregation, boiling, and reinjection at the Los Azufres Geothermal Field, Mexico, monitored by water stable isotopes, chloride, and enthalpy

Abstract

Monitoring the impact of brine reinjection is crucial to the sustainable exploitation of geothermal resources. Reinjection monitoring at the Los Azufres Geothermal Field (Mexico) has historically been carried out by investigating the temporal and spatial evolution of chloride and the stable isotopic composition of water. Here, we review the evolution of the stable isotopic composition of the total fluid discharge (TD) since the earliest measurements in 1980, integrating new data obtained in 2014. Based on δDTD and δ18OTD values measured in 2014, two groups of fluids were identified. The first group is characterized by δDTD and δ18OTD similar to those measured in previous survey years, 1980–1982, 1983–1986, and 2000. The second group shows a large positive shift in the hydrogen isotopic composition (ΔDTD of +10‰) compared to the first group. The evolution of reinjected brines and production wells over time between 1980 and 2014 shows δ18OTD and δDTD enrichment until 2000. This is followed, between 2000 and 2014, by a slight decrease in δ18OTD values and a net increase in δDTD, explained by extensive phase segregation and boiling, at temperatures of over 280 °C. The observed trend suggests that a steam phase, caused by boiling, is invading the reservoir, and the phenomenon is observed in both production and reinjected fluids. A comparison of the spatially kriged 2000 and 2014 δ18OTD and δDTD datasets reveals that reinjected brine is spreading progressively to the eastern part of the field, mainly in the southern production zone (SPZ), and through E-W corridors delimited by the E-W fault systems, which underlie the secondary permeability of the reservoir. However, reinjection is still not able to suppress boiling in the main production zone. This situation needs to be taken into account in developing future reinjection strategies to counterbalance the substantial fluid extraction from the field.