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

The chemical composition of the steam discharged by wells at the Cerro Prieto IV (CP IV) sector of the Cerro Prieto (Mexico) geothermal field was used to estimate important reservoir parameters such as steam fractions at the entrance of the wells (also called 'excess steam') and volumetric liquid saturations in the reservoir. The results were analyzed together with chemical and wellhead production data over a time interval to investigate physical phenomena in the reservoir fluid induced by exploitation practices. For starting conditions, the reservoir temperatures at CP IV sector were estimated to be between 270 and 320 degrees C; relatively high reservoir liquid saturations (>0.9) were estimated at the northwest and at the center of the sector, while at the southeast moderate liquid saturations (<0.2) were found. The main reservoir physical processes included (i) moderate boiling with enthalpy increases to values consistently lower than that for the steam (2676 kJ kg(-1)) and with preferential flow of steam to the wells in the periphery of CP IV, (except toward the north), (ii) the entry of shallower, cooler waters into the reservoir at the center of the CP IV sector; and (iii) widespread boiling with high-temperature steam condensation in the south and east of the sector. In some CP IV wells, separated entries for pre-existent reservoir steam and reservoir liquid were identified. According to the analysis of 2005 data, relatively high temperatures (>300 degrees C), high reservoir steam fractions (>0.4), and relatively low volumetric liquid saturations (<0.2) were estimated for fluids entering wells located along the southeast of CP IV. However, at the center of the sector the inflow of cooler waters moving down through the Fault H system was inferred to occur and was related to exploitation.