EVALUACIóN PRELIMINAR DEL PROCESO DE REINYECCIóNEN LA ZONA SUR DE LOS AZUFRES, MICH.(1989-2011) CON BASE EN LAS CONCENTRACIONES DE CLEN LOS FLUIDOS PRODUCIDOS Y DE REINYECCIóN

摘要

Geothermal exploitation requires optimal reservoir polices in order to ensure its longevity. For this reason during the exploitation stage multidisciplinary studies are needed to identify the effects on the reservoir caused by the extraction and eventual reinjection of fluids. Among these studies, those focused on the assessment of reinjection processes are found. Waste fluids reinjection has become a common practice in geothermal exploitation to avoid potential pollution to the environment; to mitigate pressure drops in the aquifer and also to allow a better sweep of the heat stored in the rock improving recovery efficiency. Currently, although reinjection is practically a routine practice in geothermal exploitation there are still complex aspects to be solved regarding its assessment. Traditionally this assessment has been done by means of tracer tests which are effective although they may show some inconveniences i. e. (a) they are expensive; (b) the fluid flow paths can change due to pressure drops induced by production; (c) it is difficult to perform exhaust monitoring of all production wells to identify the tracer. Thus it is recommended to better analyze the movement of fluids in the reservoir by monitoring chemical species in production and injection fluids. In this work a study to assess the impact of reinjection on the south zone of the Los Azufres geothermal field was developed which was based on the variations of chloride concentrations of production and reinjection fluids and on the reinjection mass flow rates, over time. By this approach the reinjection returns produced by production wells were estimated. The study was developed for individual wells and also to a “field” level, considering the south zone as the “field”. Qualitatively it was found that both reinjection wells of the south zone (Az-7 which later became Az-7A and Az-8) have impact on the production wells. Some important results showed that in well Az-2 the production of reinjection returns increased from 55% in 1990 to more than 90% in 2001; in well Az-16 a production of almost 90% of reinjection returns in 1993 was identified with a tendency to decrease from 82% in 2007 to 29% in 2011. For the south zone the maximum production of reinjection returns (42.5%) occurred in 1990 which coincides with a peak of injection while minimum values (between 7 and 8.8%) of reinjection returns。.