Abstract
The potential of shale reservoirs for gas extraction is largely determined by the permeability of the rock. Typical pore diameters in shales range from the μm down to the nm scale. The permeability of shale reservoirs is a function of the interconnectivity between the pore space and the natural fracture network present. We have measured the permeability of the Whitby Mudstone, the exposed counterpart of the Posidonia Shales buried in the Dutch subsurface and a possible target for unconventional gas, using different methods and established a correlation with the microstructures and pore networks present down to the nanometer scale.Whitby Mudstone is a clay rich rock with a low porosity. The permeability of the Whitby Mudstone is in the range of 10−18m2–10−21m2. 2D microstructures of the Whitby Mudstone show no connected pore networks, but isolated pore bodies mainly situated in the clay matrix, whereas 3D data shows that connected pore networks are present in less compacted parts of the rock. A closely spaced interconnected fracture network is often required to speed up transport of fluids from the matrix into a producing well. For fluids within the matrix the nearest natural fracture is on average at a distance of approximately 10cm in the Whitby Mudstone. The combination of the permeability data and the porosity data with natural fracture spacing of the fractures present in outcrops along the Yorkshire coast (UK) resulted in new insights into possible fluid pathways from reservoir to well.
Highlights
Gas in shales is trapped in poorly connected micro pores by adsorption on and in particles of organic material and clay minerals in the matrix of the host rock
The high resolution Precision Ion beam Polishing (PIPS)-Scanning Electron Microscope (SEM) mosaics covered areas larger than 300 × 300 μm2 and are representative for the microstructure present in one layer of the sample, where the Representative Elementary Area for the microstructure is in the order of 200 × 200 μm2 (Houben et al, 2016b)
The Whitby Mudstone is a matrix rich rock with silt-sized grains floating within the matrix
Summary
Gas in shales is trapped in poorly connected micro pores by adsorption on and in particles of organic material and clay minerals in the matrix of the host rock. At the nm-cm scales, transport pathways exist of often poorly connected pore networks present in the clay matrix with pores and pore throats often below the μm, whereas at the cm-m scales, an open network of natural fractures control the permeability. A closely spaced interconnected network of open fractures is often required to form high permeable pathways for transport of fluids from the tight shale matrix into a producing well (Curtis, 2002; Gale et al, 2014). It has been suggested that the initial peak is related to a fracture network permeability reaching the easy accessible fluids (either stored in the fractures or close to the fractures), whereas the sustained low production level is related to the matrix permeability of the shale (Hyman et al, 2016), where closure of the fractures and pores with time due to decreasing fluid pressures plays a role
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