The effect of fracture orientation on permeability

Shale contain tiny pores where oil and gas are trapped, and fracking is undertaken to interconnect them and to release oil and gas. Furthermore, hydraulic fracturing is the process of injecting high-pressure fluids as high as 10 k psi into rocks to fracture them, and permeability is optimised by inducing complicated fractures into the shale. Previous authors have studied fracturing orientation techniques, however, their work considered only simple fractures. In this work, complicated fractures were considered using different shale samples from Mancos and Marcellus. To determine their permeability, these samples were induced with fractures at different orientation angles and tested at an overburden pressure of between 100 and 200 bar using a Nano-Perm machine. Computerised Tomography (CT) was also used to determine the porosity and to characterise the shale samples, with regard to crack width, crack length and to determine if there were any natural fractures. The liquid saturation method was also carried out to find the pore volume, whilst XRF fluorescence was also used to determine the chemical composition of the shale samples.There was a good agreement in the porosity results for the two different methods used, specifically the CT scan and the saturation method. Moreover, it was found that, for the Mancos Shale, the more complex fracture had a greater permeability at ΔP’s of less than 2 bar and an overburden pressure of 100 bar. The greater number of passageways within the shale through which the gas could flow can explain this. However, above a ΔP of 2 bar with overburden pressures of 150 bar on less complex fractures had a lower permeability because the overburden pressures will have reduced the gaps between the passageways due to the greater accumulated passageways, and hence the reduced flow. For the Marcellus Shale, it was found that, at overburden pressures of more than 150 bar and for all ΔP’s, the more complex fractures had a greater permeability. It was also shown that the Marcellus Shale had a greater permeability compared with Mancos. This is because the Marcellus reservoir has a marked ratio of calcium and cementing materials, which comparatively alter the rock structure and render it relatively incompressible