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UBC Theses and Dissertations

DFN analysis to quantify the reliability of borehole derived volumetric intensity Ojeda Selame, Pedro Pablo


Volumetric fracture intensity (P₃₂) is a parameter that plays a major role in the mechanical and hydraulic behaviour of rock masses. Although it is not possible to measure P₃₂ directly using current technologies, P₃₂ can be estimated from borehole and surface data using either simulation or analytical solutions. In this thesis we use Discrete Fracture Network (DFN) models to addresses the problem of estimating P₃₂ using information from boreholes (1D data), and we also investigate the problem of quantify the uncertainty range of the calculated P₃₂. Based on the comparison between actual P₃₂ and the intensity sampled using synthetic boreholes, we propose a new methodology to estimate P₃₂ variability from linear intensity. This methodology can be useful to quantify and understand the expected variability of P₃₂ values of a project when linear intensity is the only information available. It is common practice, when calculating P₃₂ based on Terzaghi Weight (1965), to use drill run lengths, and limit the minimum angle between the borehole and the intersected fractures. The analysis presented in this thesis indicated that limiting the minimum angle of intersection would result in an underestimation of the calculated P₃₂. Additionally, the size of the interval has a great impact on the variability of the calculated P₃₂. To account for that we propose a methodology to calculate P₃₂ using variable lengths, depending on the angle between the fractures and the borehole. This methodology allows to capture the spatial variation in intensity and at the same time it avoids the artificial increasing or decreasing of the intensity sampled along borehole intervals. This can be useful when the interval intensity is used as input for interpolating P₃₂ values in block models. Finally, the research has addressed another fundamental issue, that is the impact of boundary effects in DFN models. The results confirmed that DFN models do present boundary effects with respect to the modelled fracture intensity and that these boundary effects are dependent on the size of the generation box in relation to the volume of interest and the size of the fractures.

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