Abstract
The process of hydraulic fracture is well known in both natural (e.g. veining and mineralisation) and engineered environments (e.g. stimulating tight mudrocks and sandstones to boost their hydraulic properties). Here, we report a method and preliminary data that simulates both tensile fracture and fluid flow at elevated pressures. To achieve this we developed a sample assembly consisting of a cylindrical core drilled with an axial borehole encapsulated in a 3D printed jacket permitting fluid from the borehole to move through the freshly generated tensile fracture to a voluometer. The permeability of Nash Point Shale increases from a pre-fracture value of 10−18 to 10−20 m2 (1 microDarcy, μD to 0.01 μD) to 2 × 10−15 m2 (2 milliDarcy, mD) immediately after fracture (at 2.1 MPa confining pressure). Permeability is strongly dependent on confining pressure, decreasing to 0.25 × 10−15 m2 (0.25 mD) at 19 MPa confining pressure (approximately 800 m depth), and does not recover when confinement is removed. Using concomitant measurements of the radial strain as a proxy for fracture aperture, we conclude that the effective permeability is governed solely by the width of the developed cracks, revealed by post-test X-Ray Computed Tomography to be planar, extending radially from the central conduit.
Highlights
Hydrofracturing is a common process in many areas of pure and applied geosciences, such as magma and dyke intrusions[1,2], the development of mineral veins e.g.3 and the intentional hydraulic fracturing of impermeable rock formations in the hydrocarbon and geothermal energy industries e.g
A pore fluid is pumped into a wellbore at rates higher than the radial fluid flow into the surrounding rock, which is a function of the permeability of the rock mass
Each pressure drop and Acoustic Emission (AE) swarm coincides with spikes in the radial deformation time-curve, which increase with every successive step up to a maximum of just under 180μm at the final pressure decrease
Summary
Hydrofracturing is a common process in many areas of pure and applied geosciences, such as magma and dyke intrusions[1,2], the development of mineral veins e.g.3 and the intentional hydraulic fracturing of impermeable rock formations in the hydrocarbon and geothermal energy industries e.g.4–6. Similar work and including Acoustic Emission (AE), the laboratory proxy to tectonic seismic activity, has further expanded on the links between new fracture development and the energy budget using mechanical borehole pressurisation via a rubber liner[5] and via direct water injection[22] Studies such as these have linked fracture energy to the stress and other conditions, measuring the permeability of a newly generated fracture in the same experiment (and minimising sample preparation effects such as sawcuts/split samples) are not common. This is important information, as the permeability of an aligned fracture (without fracture offset) increases by approximately 3 orders of magnitude compared to the virgin shale (matrix) permeability[21]. Whilst the use of a proppant is common in the engineered environment, any measure that leads to an increase in fracture offset will enhance permeability[19]
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