Evolution of fracture intensity and topology in granitic rocks: insight from Mt. Capanne Pluton, Elba Island, Italy 

Abstract

Field analogue studies of fractured crystalline rocks are important for the clean energy transition and or better understanding the subsurface geothermal systems. In this contribution we present a workflow for multiscale quantitative analysis of fracture network and their connectivity in the monzogranitic pluton of Monte Capanne (Elba Island, Italy). Field structural analysis was integrated with Digital Outcrop Model (DOM) of a 1.5km-long outcrop and with microfracture analysis performed in thin section. The DOM was obtained from images acquired with UAV flights. Field analysis indicate the presence of three main fracture sets with different attributes and showing systematic crusscutting relationships. The quantitative analysis of the DOM was performed with QGIS software and allowed us to characterize the fracture length distributions, density (P20), intensity (P21), and topology (and their parameters). Data derived from field survey and DOM and analysis has been used to create a three-dimensional Discrete Fracture Network (DFN) using a DICE® (https:// github.com/nicmenegoni/DICE) algorithm in MatLab® to calculate the 3-dimensional fracture intensity (P32). In addition, we extended the two-dimensional topology concept in the third dimension. Thus, assuming circular fractures, new topology parameters have been calculated such as number of fracture intersection in volume and intersection fracture length in a volume, i.e., I30 and I31 respectively. Finally, based on the relative fracture chronology, we simulated the step-by-step evolution of 2- and 3-three-dimensional fracture density, fracture intensity and topology, describing the relationship between different fracture sets over time. The preliminary results show how fractures connectivity evolve over time. The ultimate goal of this work is to constrain the evolution of fracture porosity to enhance our ability for modelling fluid flow in crystalline rocks.