| Title | A preliminary interpretation of the gas composition of the CP IV sector wells of the Cerro Prieto field, Mexico |
|---|---|
| Authors | Rosa María Barragán Reyes, Víctor M. Arellano Gómez, Enrique Portugal Marín, Alfredo Pérez Hernández, Marco Helio Rodríguez Rodríguez y Jesús de León Vivar |
| Year | 2006 |
| Conference | Mexican Geothermal Congress |
| Keywords | Cerro Prieto, geothermal fluids, gas composition, chemical equilibrium |
| Abstract | In order to increase the electric generation capacity of the Cerro Prieto geothermal field from 620 MW to 720 MW, the Cerro Prieto IV (CP IV) sector of the Cerro Prieto geothermal field was developed. This is located at the NE portion of the exploited field, where fourteen new wells were drilled since 2000 to date. The wells in CP IV zone produce two-phase fluids at wellhead with heterogeneous characteristics regarding steam fraction: at the central zone and towards the NW, the wells are liquid-dominated while those towards the E and S produce a relatively high steam fraction. In this work a study of the gas composition of the produced fluids was developed to obtain reservoir parameters such as temperature and steam fraction to identify the presence of different type of fluids at reservoir. A method based on the Fischer Tropsch reaction and H2S equilibria with pyrite-pyrrhotite as mineral buffer (FT-HSH3) was used. The results for the “natural state” showed the presence of fluids with reservoir temperature from 275 to 310° C and excess steam values from -1 to 50%. Data are aligned in a FT-HSH3 trend suggesting that the well discharges are constituted of a mixture in different proportions of two end members. One of them seems to be a liquid with a temperature of more than 300° C and a negative or negligible excess steam while the other seems to be a two-phase fluid with a temperature of about 275° C and an excess steam fraction of about 0.5. According to the data for single wells, depending on the production conditions of the wells, mixtures in different proportions of liquid and steam reservoir fluids could occur. Data for 2005 that included wells drilled after 2000, suggested the presence of steam phase at reservoir. This steam could be generated by boiling of deep reservoir fluid due to pressure drop. The mixing trend obtained for the natural state was also seen for 2005 data but lower temperatures (from 265 to 295° C) compared to those for natural conditions were obtained. The entry of lower temperature fluids descending through the H Fault to the reservoir was also inferred, since results for the wells affected by this process (located at the center of CP IV area), showed small or negative excess steam values and slightly lower temperatures (265-270° C). |