The impact of hydrothermal alteration on the physiochemical characteristics of reservoir rocks: the case of the Los Humeros geothermal field (Mexico)

Abstract

Hydrothermal alteration is a common process in active geothermal systems and can significantly change the physiochemical properties of rocks. To improve reservoir assessment and modeling of high-temperature geothermal resources linked to active volcanic settings, a detailed understanding of the reservoir is needed. The Los Humeros Volcanic Complex, hosting the third largest exploited geothermal field in Mexico, represents a natural laboratory to investigate the impact of hydrothermal processes on the rock properties through andesitic reservoir cores and outcropping analogs. Complementary petrographic and chemical analyses were used to characterize the intensities and facies of hydrothermal alteration. The alteration varies from argillic and propylitic facies characterized by no significant changes of the REE budget indicating an inert behavior to silicic facies and skarn instead showing highly variable REE contents. Unaltered outcrop samples predominantly feature low matrix permeabilities (< 10–17 m2) as well as low to intermediate matrix porosities (< 5–15%), thermal conductivities (0.89–1.49 W m−1 K−1), thermal diffusivities (~ 0.83 10–6 m2 s−1), and sonic wave velocities (VP: ~ 2800–4100 m s−1, VS: ~ 1600–2400 m s−1). Average magnetic susceptibility and specific heat capacity range between 2.4–7.0 10–3 SI and 752–772 J kg−1 K−1, respectively. In contrast, the hydrothermally altered reservoir samples show enhanced porosities (~ 7–23%), permeabilities (10–17–10–14 m2), and thermal properties (> 1.67 W m−1 K−1; > 0.91 10–6 m2 s−1), but a significant loss of magnetic susceptibility (10–3–10–6 SI). In particular, this latter characteristic appears to be a suitable indicator during geophysical survey for the identification of hydrothermalized domains and possible pathways for fluids. The lack of clear trends between alteration facies, alteration intensity, and chemical indices in the studied samples is interpreted as the response to multiple and/or repeated hydrothermal events. Finally, the proposed integrated field-based approach shows the capability to unravel the complexity of geothermal reservoir rocks in active volcanic settings.

Influence of the smectites in the microstructure and the mechanical properties of an altered basalt

“…A decreased in the velocity of P-waves (V p ) and uniaxial compression strength (UCS) of volcanic rocks according to the type and intensity of the alteration was also found by Tuǧrul (2004), Wyering et al (2014), and Ismaeel et al (2019). In agreement with them, Weydt et al (2022) studying hydrothermally altered and argillized andesites, and Pola et al (2017) studying altered lavas with different weathering or hydrothermal alteration also found that these volcanic rocks tend to have lower V p , higher porosity, higher permeability, and lower UCS than their unaltered counterparts. The variations in the mineralogical and chemical composition and the increase of the porosity influence the loss of mechanical properties, but it is challenging to untangle their individual contributions to the reduction in mechanical strength Heap et al (2022).…”

“…In recent years, there is a growing interest about the influence of the alteration on the mechanical properties of volcanic and subvolcanic rocks, and there are some works focused on the study and correlation of those properties and the degree of alteration, mainly argillitic alteration. The rocks more frequently studied are andesites as in Weydt et al (2022), or more felsic rocks such as ignimbrites (Wyering et al, 2014), tuffites (Frolova et al, 2021) hydrothermally altered and argillized, although in most cases the composition of the neoformed clay minerals is not given. Frolova et al (2021) described a neoformation of kaolinite and smectite while Wyering et al (2014) characterize the neoformed minerals as smectites.…”