A CLOSER LOOK AT SHALE: REPRESENTATIVE ELEMENTARY VOLUME ANALYSIS WITH LABORATORY 3D X-RAY COMPUTED MICROTOMOGRAPHY AND NANOTOMOGRAPHY

摘要

Though naturally occurring in many regions of the world,shale rock microstructure continues to be much of a mystery.Pore sizes may be very small,typically low 100s of nanometers and even below 10s of nanometers.It is thus very important to determine the volume size that must be examined to understand the oil reserves in a macroscopic shale rock formation,as the small features require a very high resolution imaging system, which usually come with limited field of view.This makes precise quantification of the microstructure a daunting challenge,especially when the analysis needs to be performed in 3D to capture the tortuous paths taken by the pores. The introduction of ultra-high resolution imaging systems is now shedding light on the problem,with the commercialization of precise laboratory x-ray imaging tools.Here,a novel suite of x-ray computed tomography systems is shown to provide unique insight into shale microstructure.Large volumes are measured with as high as sub-1 ?m resolution using laboratory-based x-ray computed microtomography(VersaXRM) to localize regions-of-interest(ROIs) for further higher resolution analysis.A ROI of cubic volume with ~65 ?m on each side is isolated for precise analysis with a novel laboratorybased x-ray computed nanotomography system(UltraXRM) capable of 50 nm resolution for quantification of porosity within the shale sample. Using the multi-length scale resolution imaging systems described here,a representative elementary volume(REV) quantification has been performed,which identifies ~30 ?m as the minimum volume that must be considered in order to quantify pores in shale down to 150 nm linear dimensions.Using a 3D field of view capable of sampling ~4 of these REVs,a precise microstructure analysis is carried out,within which further calculations of pore tortuosity and connectivity are demonstrated.The non-destructive nature of x-ray imaging further opens the door to innovative experimentation,such as time-evolution and studies of microstructure response to varying environmental parameters,such as temperature cycling or surfactant treatment.