Gas geochemistry related to wellhead production data to investigate physical reservoir phenomena in geothermal reservoirs: application at Cerro Prieto IV (Mexico)

摘要

The chemical composition of the steam discharged by wells at the Cerro Prieto IV (CP IV) sector of the Cerro Pri-neto (Mexico) geothermal field was used to estimate important reservoir parameters such as steam fractions at thenentrance of the wells (also called ‘excess steam’) and volumetric liquid saturations in the reservoir. The resultsnwere analyzed together with chemical and wellhead production data over a time interval to investigate physicalnphenomena in the reservoir fluid induced by exploitation practices. For starting conditions, the reservoir tempera-ntures at CP IV sector were estimated to be between 270 and 320u0002C; relatively high reservoir liquid saturationsn(>0.9) were estimated at the northwest and at the center of the sector, while at the southeast moderate liquidnsaturations (<0.2) were found. The main reservoir physical processes included (i) moderate boiling with enthalpynincreases to values consistently lower than that for the steam (2676 kJ kg)1n) and with preferential flow of steamnto the wells in the periphery of CP IV, (except toward the north), (ii) the entry of shallower, cooler waters intonthe reservoir at the center of the CP IV sector; and (iii) widespread boiling with high-temperature steam conden-nsation in the south and east of the sector. In some CP IV wells, separated entries for pre-existent reservoir steamnand reservoir liquid were identified. According to the analysis of 2005 data, relatively high temperaturesn(>300u0002C), high reservoir steam fractions (>0.4), and relatively low volumetric liquid saturations (<0.2) were esti-nmated for fluids entering wells located along the southeast of CP IV. However, at the center of the sector theninflow of cooler waters moving down through the Fault H system was inferred to occur and was related tonexploitation.