| Abstract |
Fractures are the main pathways for water circulation through rock masses. Thus, to characterize a geothermal reservoir, beyond other thermo-physical and mechanical properties, the knowledge of fractures geometrical features and fractures network connectivity are key aspects. A total of 415 fractures were analyzed in four selected sites (Areas 1 to 4) around Kuujjuaq village, Northern Québec, Canada, in outcrops of variable lithology – Suite de False (migmatitic paragneiss; Areas 1 and 2) and Complex de Kaslac (diorite and granoblastic diorite quarzitic strongly foliated to mylonitic; Areas 3 and 4). The geometrical features were obtained by scanline sampling method. From those sites, one (Area 2) was selected to carry out fracture network connectivity studies by topology method. For the statistical analysis, according to qualitative aspects the fractures were divided in master joints, secondary joints, highly altered joints and faulted fractures. Corrections for orientation, truncation, censoring and size biases were also carried out. The focus of this work was on the master joints as water circulation can be restricted in smaller fractures. Area 1 is located 2 km NE of Kuujjuaq and Area 2 is 6 km SW of the village. In terms of fractures directions, Area 1 has two equally preferential orientations NNW-SSE and ENE-WSW, while Area 2 has as the preferential orientation E-W with scattered fractures in NNE-SSW. The master joints show a systematic behavior in both areas. Lengths from 2.4 to 25 meters minimum, apertures from closed/very tight to wide, and spacing between joints from few centimeters up to 2-3 meters were observed. The joint sets in this area, based on the dihedral angle between joints, are conjugate and orthogonal. Regarding fracture network connectivity in Area 2, the system is composed by abutting fracture terminations with some isolated and cross-cutting fractures, with 3.97 as the ratio between number of branches and lines. A measure of connectivity is given by the average number of connections per line and the number of connections per branch. Values of 3.66 and 1.67 were observed for these two parameters, respectively. Area 3 and 4 are located ~ 2 km from Kuujjuaq, the first is in the E direction and the second to the NW. The preferential fracture orientation is NW-SE in both areas, with scattered fractures oriented NE-SW. Area 3 is characterized by master joints with lengths that vary from 1 to 11 meters and apertures that are closed/very tight. In contrast, Area 4 has mainly systematic master joints of 1 to 10s of meters in length with closed/very tight to moderately wide apertures. The fracture spacing is averaging 1 to 2 meters. Area 4 has mainly orthogonal joint sets, but some conjugate sets also occur. The results presented in this work are part of an on-going project that aims at assessing the shallow and deep geothermal energy potential in the Inuit community of Kuujjuaq, Northern Québec, Canada. The fracture network characterization carried out will be used to estimate mechanical and hydraulic properties of rock masses in order to (1) simulate the performance of an enhanced geothermal system, and (2) evaluate the efficiency of underground thermal energy storage as an alternative to diesel fuel used for space heating in this region. |