Mineral Alteration and Fracture Influence on the Elastic Properties of Volcaniclastic Rocks

Abstract

In geothermal environments, the physical properties of rocks, such as porosity and alteration, are highly variable and control the reservoir's elastic and hydraulic properties. Elastic wave velocity in volcaniclastic rocks and their relation to fractures, pore shapes, and mineral alteration is mostly unknown. We measure ultrasonic P and S wave speeds on volcanic rocks from the Ngatamariki Geothermal Reservoir, New Zealand. Data clustering of wave speed versus porosity and density allow us to classify lithotypes of variable propylitic and phyllic mineral alteration. Wave speeds increase first due to porosity reduction as a result of mineral alteration and welding and, second, due to the high elasticity of alteration minerals (epidote, chlorite, and carbonates). We model the rock porosity as composed of equant pores and oblate-spheroidal microfractures following an elastic effective medium model. For our samples, microfracture porosities range from 4% in the volcaniclastic tuffs to less than 1% in the ignimbrites, which we validate with pycnometer porosity data under effective pressure. We quantify that within one geological volcaniclastic formation, wave speeds vary up to 47% due to rock alteration and welding, while the effect of microfractures and changes in effective stress on wave speeds is secondary (up to 11%) but not negligible. From the experimental and numerical results we show that equant pores remain open at reservoir conditions (2,000 m) and can retain considerable porosity (10%). Our analysis has implications for microfracture and pore network characterization in geothermal reservoirs, in particular, in vapor-dominated systems where matrix porosity and permeability play a role.

Petrophysical properties, mechanical behaviour, and failure modes of impact melt-bearing breccia (suevite) from the Ries impact crater (Germany)

“…Our data show that Young's modulus decreases as a function of increasing porosity (Figure 7), in agreement with previous studies (e.g., Heap et al, 2020b). Chemical alteration is also known to influence the Young's modulus of rocks and can result in increases to Young's modulus when the alteration is pore-filling (e.g., Frolova et al, 2014;Mordensky et al, 2018;Durán et al, 2019) and decreases to Young's modulus in the case of dissolution (e.g., Frolova et al, 2014). The low Young's modulus of samples from Aumühle (Figure 7) can therefore be explained by a combination of their higher porosity and greater degree of alteration.…”

The Role of Tuffs in Sealing Volcanic Conduits

“…The secondary mineralogy observed here is in contrast to the mineralogy of chlorite, epidote, smectite, observed in the geothermal reservoirs of Taupo Volcanic Zone (Durán, Adam, Wallis, & Barnhoorn, 2019; Wyering et al., 2014). This is likely due to the acidic nature of fluids at Whakaari compared to near‐neutral thermal waters observed in other long‐lived geothermal systems (Giggenbach et al., 2003).…”

Assessing Crack‐Induced Compliance in Low Porosity Rocks Damaged by Thermo‐Hydro‐Chemo‐Mechanical Processes

“…Durán et al. (2019) and others have discussed how the conventional crack density parameter, Γ , should be converted to crack porosity in order to be physically meaningful, but here we are questioning the validity of the parameter itself, which is based on elliptical (or usually, circular) cracks. For instance, as Gurevich et al.…”

“…Indeed, extracting true crack densities is somewhat elusive. Durán et al (2019) and others have discussed how the conventional crack density parameter, Γ, should be converted to crack porosity in order to be physically meaningful, but here we are questioning the validity of the parameter itself, which is based on elliptical (or usually, circular) cracks. For instance, as Gurevich et al (2009) have shown, there is often a large discrepancy between crack density parameters (Γ K and Γ G , respectively) calculated from the effective bulk and shear moduli using the equations of O' Connell and Budiansky (1974).…”