Carbon dioxide is one of the most important gases naturally released from geothermal systems. Establishing the processes and pathways that regulate the CO2 diffuse degassing can provide valuable information for exploration and exploitation purposes of geothermal reservoirs. Areas with high CO2 emissions are indeed able to reveal major upflow zones from deep reservoirs through deep-reaching permeable fault zones. In this work, a high-resolution CO2 flux (with records up to 2927 g m-2 d-1) and soil temperature (with records up to 98.8°C) survey was carried out along with detailed fracture parameters measurements in a selected area of the Monterotondo Marittimo-Sasso Pisano transfer fault (Larderello geothermal system, Tuscany, Italy). The main aim is to define the behavior of diffuse CO2 through the fault system and investigate how the soil CO2 flux and steam change with respect to the architecture of the fault damage zone (i.e., volumetric fracture intensity, permeability, and persistence of the fractures). The presence of multiple populations of CO2 flux suggested that soil degassing is controlled by three different transport mechanisms which can be defined as: i) purely diffusive, ii) mixed diffusive-advective, and iii) purely advective. These three mechanisms are characterized by efflux values of <20, between 20 and 300 and >300 g m-2 d-1, respectively. The spatial distribution of these fluxes well agrees with the fracture distribution and characters of the Jurassic radiolarite (Diaspri Fm) dissected by NNE-striking faults. The interaction between pre-existing fractures and fracture-related fault-zone locally enhances the secondary rock permeability as highlighted by the correlation between Discrete Fracture Network (DFN) modeling and advective flux. The pivotal role of transfer fault zones in controlling fluid emissions is also confirmed by the normalized CO2 output to the fault strip (1350 m2) that resulted in ~155 t d-1 km-2, although when the area characterized by advective flux (460 m2) was considered the efflux rose to 326 t d-1 km-2.
CO2 and steam emissions controlled by enhanced fracture permeability in the Monterotondo Marittimo-Sasso Pisano crustal transfer fault system (Larderello Geothermal Field, Italy)
Taussi M.
;Nisi B.;
2022
Abstract
Carbon dioxide is one of the most important gases naturally released from geothermal systems. Establishing the processes and pathways that regulate the CO2 diffuse degassing can provide valuable information for exploration and exploitation purposes of geothermal reservoirs. Areas with high CO2 emissions are indeed able to reveal major upflow zones from deep reservoirs through deep-reaching permeable fault zones. In this work, a high-resolution CO2 flux (with records up to 2927 g m-2 d-1) and soil temperature (with records up to 98.8°C) survey was carried out along with detailed fracture parameters measurements in a selected area of the Monterotondo Marittimo-Sasso Pisano transfer fault (Larderello geothermal system, Tuscany, Italy). The main aim is to define the behavior of diffuse CO2 through the fault system and investigate how the soil CO2 flux and steam change with respect to the architecture of the fault damage zone (i.e., volumetric fracture intensity, permeability, and persistence of the fractures). The presence of multiple populations of CO2 flux suggested that soil degassing is controlled by three different transport mechanisms which can be defined as: i) purely diffusive, ii) mixed diffusive-advective, and iii) purely advective. These three mechanisms are characterized by efflux values of <20, between 20 and 300 and >300 g m-2 d-1, respectively. The spatial distribution of these fluxes well agrees with the fracture distribution and characters of the Jurassic radiolarite (Diaspri Fm) dissected by NNE-striking faults. The interaction between pre-existing fractures and fracture-related fault-zone locally enhances the secondary rock permeability as highlighted by the correlation between Discrete Fracture Network (DFN) modeling and advective flux. The pivotal role of transfer fault zones in controlling fluid emissions is also confirmed by the normalized CO2 output to the fault strip (1350 m2) that resulted in ~155 t d-1 km-2, although when the area characterized by advective flux (460 m2) was considered the efflux rose to 326 t d-1 km-2.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
