Abstract

The extraction of geothermal energy, in situ minerals, liquid and gas hydrocarbons, and subsurface water are all constrained by the flow of fluid through fractured media in the earth's crust, as is the viability of projects involving CO2 sequestration, nuclear and hazardous waste storage, hydrocarbon storage, and subsurface cavities. Subsurface fractures are the main fluid pathways as the matrix permeability is negligible in most rocks. In situ recovery (ISR) or in situ leaching (ISL), particularly in hard rock, poses some challenges currently. One of the main problems is the modelling of fluid flow in fractured rock masses, and this was the primary focus of this project. Modelling fluid flow in fractures can be done in many ways. The modelling showed that ISL in hard rock demonstrates potential. However, the modelling also exhibited the need for advancements in the fluid flow in fractures modelling area. In this paper comprehensive review of developed approaches for subsurface fracture mapping, processing and characterisation to build a fractured rock mass geometry and fluid flow simulation and mineral leachability along with examples were illustrated.

1.
Introduction

The extraction of geothermal energy, in situ minerals, liquid and gas hydrocarbons, and subsurface water are all constrained by the flow of fluid through fractured media in the earth's crust, as is the viability of projects involving CO2 sequestration, nuclear and hazardous waste storage, hydrocarbon storage, and subsurface cavities. Also, fluid flow through fractured media affects the health and stability of the subsurface environment, and the populations that live above. Subsurface fractures are the central fluid pathways as the matrix permeability is negligible in most rocks. So, the presence and nature of subsurface fractures play a fundamental role in many human activities.

Mining in future will be more challenging, because of declining ore grades associated with deeper mining and finely disseminated target minerals in heterogeneous ore bodies, as well as complex mineral association with gangue material, often in locations that are difficult or risky to access. In many areas, ore grades declined by almost 50% over the last 30 years, making mineral processing mostly uneconomical for such minerals. Under these circumstances, innovation in in situ recovery could be a suitable alternative to unlocking resources. The idea of in situ recovery first started with solution mining to extract salt, potash or other minerals as shown in Figure 1(a). It was subsequently developed for recovery from porous media such as sedimentary soil and rock or heavily jointed rock masses as illustrated in Figure 1(b). In situ recovery from porous media has become well established during the last few decades, owing to the presence of void spaces and their connectivity, which facilitate fluid flow from injection wells to recovery wells. In situ recovery of target metals from hard rock is challenging, due to a lack of knowledge about fracture conditions, their connectivity and consequently rock mass conductivity and target metal recoverability, as shown in Figure 1(c).

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