Predicting the breakdown pressure in hard rock material subjected to hydraulic fracturing and quantifying fluid flow within a fracture network

Abstract

This research is part of the Natural Heat Exchange Engineering Technology for Mines (NHEET) project conducted by Mirarco Mining Innovation. The NHEET project consists of developing a system using natural means to provide economically significant thermal regeneration capacity through a volume of rock fragments for ventilating mine workings. This system can provide heating (during winter) and cooling (during summer) of air on seasonal basis, without using artificial refrigeration. Optimizing the system requires creation of a specific volume of rock fragments having, among other criteria, a pre-determined porosity and fragment size distribution to meet the thermal storage and ventilation requirements of the mine site. This research is part of the NHEET project’s scope of work and investigates an alternative system, which consists of a fractured rock mass with sufficient fracture density and connectivity to admit enough airflow for the NHEET system requirements. This alternative system has the potential of reducing the footprint at surface. Firstly, the hydraulic fracturing (HF) method is investigated for preconditioning the rockmass with the objective of strategically creating additional fractures. Increasing the volumetric fracture intensity and fracture network connectivity within the rock mass can optimize airflow within the fracture network. A numerical predictive model for the breakdown pressure in hard rock subjected to hydraulic fracturing is developed using the lattice spring modeling method for HF simulation. The developed numerical model is calibrated based on the results obtained from a HF field experiment conducted in a northern Ontario mine. Secondly, a laboratory experiment is conducted to quantify fluid flow through a fracture network. In this context, a 3D physical model representing a fractured rock mass is generated using 3D printing technology. The 3D printed model is fixed into an experimental setup for fluid flow measurements. This experiment allowed for establishing the behaviour of the changing pressure to fluid transfer through fracture openings. The flow-pressure measurements are compared to a simple model for the volumetric flow rate in a block of naturally fractured rock with a number of fractures. The numerical model developed, and laboratory results obtained in this thesis provide valuable information for the construction of a NHEET system. The numerical predictive model for the breakdown pressure in hard rock subjected to HF is a tool to evaluate the amount of fluid pressure needed to create additional fractures in the rock mass and facilitate the planning of HF operations. The pressure-flow rate laboratory measurements are key data that can be used to calibrate a subsequent numerical simulation at a larger scale, representative of the NHEET system. Additionally, direct fluid flow measurements in fracture networks are useful to assess the influence of various fracture properties (e.g. intensity, connectivity, aperture) on fluid flow.