Main Results of A Hydraulic Stimulation Test In The Acoculco Geothermal Zone In Mexico

In June 2021 a hydraulic stimulation test was carried out in the exploration well EAC-1, drilled 26 years ago at the Acoculco geothermal zone in Mexico. The 2000 m deep well, was drilled by the Comisión Federal de Electricidad (CFE) searching for a conventional, hydrothermal reservoir, but failed to find geothermal fluids in spite of a measured temperature of 300°C. Thus, the zone was chosen as Enhanced Geothermal System (EGS) demonstration site in the joint EU-Mexico research project GEMex. The test’s objective was to define the main geological-structural and petrophysical parameters of the well and its immediate periphery for the CFE to use them in the future development of an enhanced reservoir in the zone. The test succeeded in stimulating a fractured and permeable zone in the EAC-1 well, located between 800 and 1000 meters deep, which could be further enhanced taking into account the parameters determined in this same test. The injectivity index of the well is around 1.8 l/s/MPa, confirming that the permeability of the target rocks is very low, in the order of 10-17 to 10-18 m2 (10-2 to 10-3 mD). The transmissivity was 10-15 to 10-16 m3 (1 to 0.1 mD/m), and the ratio of the minimum horizontal stress and the vertical stress (Shmin/Sv) is around 1.1, which indicates that the current stress field is dominantly compressive. Thus, this local stress field gives rise to left-lateral strike-slip faults with an inverse component, or to reverse faults with strike-slip component, as confirmed by focal mechanisms based on the micro seismic data obtained during and after the test. The main conclusion is that the Acoculco geothermal zone is suitable for developing a future enhanced geothermal system, for which the results of this test will be very useful.