Crack conductivity and the strength of the lithosphere (NERC EAO DTP)

Understanding impact of both natural and man-made fractures in rock masses upon fluid flow is vital for a variety of both economic and safety reasons. For example a significant proportion of the World’s undiscovered hydrocarbons are in fractured reservoirs. Fractures in aquifers in carbonate rock is a very important water source in many countries. Geo-disposal of nuclear waste relies on sealing nuclear waste from ground water over millions of years, whilst allowing controlled venting of gases produced from the stored materials. Hydraulic fracturing of shale is necessary for gas extraction, and hydraulic fracturing also plays a significant role in the development of geothermal energy projects. All of the above rely on a detailed understanding on how fractures change the bulk permeability of rocks. Despite this there are some major gaps in our understanding of how the stress state in fractured rocks affects their permeability. From observations of heat flow from fractures in boreholes it has been inferred that the hydraulic conductivity of fractures is increased with increasing resolved shear stress (Townend & Zoback 2000). Recent experiments in Manchester (Rutter & Mecklenburgh 2017; Rutter & Mecklenburgh 2018) put this generalization into doubt. We found that for a wide range of rock types (shale, sandstone and granite) the hydraulic conductivity of smooth planar cracks decreased with increased shear stress. Whilst the presence of these simulated fractures always increased the bulk conductivity of the rock, the amount by which it was increased varied with rock type. This can be usefully expressed as the fracture spacing required for the cracks to be able to conduct the same amount of fluid as the matrix. For example shale would require a crack spacing of 1 m or less, sandstone 30 m and granite 300 m. Whilst the matrix permeability of the shale was higher than the granite the cracks were 50 times less conductive. Overall the conductivity is dependent on the roughness of the crack surfaces with shale having the smoothest surfaces and sandstone the roughest. This has important implications for the strength of the crust through the effect of pore pressure on strength. It is often assumed that the pore pressure in rocks follows the hydrostat with depth, but this relies on the vertical permeability being large enough to fully communicate the pores from depth to the surface. If this is done through a network of cracks from our results it is feasible for hard rocks in an extensional tectonic environment, but would require unfeasibly small crack spacing for shale, which always seems to be an effective seal.

The aims of this project will be:
1. To extend the experimental approach used in (Rutter & Mecklenburgh 2017; Rutter & Mecklenburgh 2018) to cracks with higher surface roughness and different rock types.
2. Measure natural fracture patterns and crack roughness in outcrops using lidar and photogrammetric approaches, in order to obtain map 3D networks of fractures (Seers & Hodgetts 2016).
3. Combine these two datasets using reservoir modelling techniques (Discrete Fracture Networks or DFN models, and analogue simulations) to predict fluid flow and rock strength.
By combining experimental, fieldwork and reservoir modelling we will be able to use this approach to make better predictions of how fracture permeability can affect bulk fluid flow in the subsurface. This will have importance in a number of areas, fractured carbonate reservoirs for hydrocarbon and water, nuclear waste disposal, carbon capture and storage and crustal strength.

The successful student will join the world renowned Rock Deformation Laboratory and Basin Studies group in Manchester, becoming a member of two vibrant research groups with a large number of academics, post-docs and PhD students with which to interact. The student will gain skill, in experimental rock mechanics, lidar, photogrammetry, image analysis and reservoir modelling. On successfully completing this PhD the student will have an excellent skill set to work in hydrocarbon exploration and production, carbon capture and storage, nuclear waste disposal, hydrology and geo-technical engineering.