The role of microporosity on the permeability of volcanic-hosted geothermal reservoirs: A case study from Los Humeros, Mexico

Abstract

Permeability is a key parameter in the analysis of geothermal systems, usually controlled by secondary structures such as fractures, faults or stratigraphic contacts. Contrarily, primary porosity is considered to have a negligible effect on the permeability of those systems, as it is commonly observed that there is low connectivity between macropores (conventionally defined as larger than 1μm). However, we observe that at the micrometric scale this scenario is different, and connectivity is clearly identified. In this study, an updated innovative petrophysical method is developed to calculate microporosity with high precision and simulate permeability on andesite lava flow core-samples from a geothermal production well in Los Humeros Geothermal Field (LHGF), Mexico. The method is based on high-resolution X-ray microCT and numerical post-processing algorithms, producing porosity-permeability estimates, which are in good agreement with other independent petrophysical methods such as Helium porosimetry-permeameter. Our results indicate that macropores account for an average of 3.7 ± 0.9 % of total rock volume fraction, whereas microporosity is generally higher, with a volume fraction in the order of 6.2 ± 0.6 %. Furthermore, when removing microporosity from the pore network, the permeability will account for an average of 3.50 × 10−17 m2, which is only the 17.92 % of the total average permeability for all samples. On the other hand, when microporosity is added, there is an average increment in the permeability of 82.08 %. This means that more than 80 % of the permeability of the samples is due to microporosity, and that microporous space indeed plays a major role in the permeability of these rocks. These results reveal that microporosity plays an important role on the permeability of rocks from volcanic-hosted LHGF. This approach has not been previously fully considered, and in this study, we show that a detailed microporosity analysis is essential for further estimations of permeability of volcanic-hosted geothermal systems.