A multi-scale framework for digital core analysis of gas shale at millimeter scales

Digital Core Analysis (DCA) is emerging as a complementary technology that adds value to traditional lab-based petrophysical measurements of the macroscopic properties of rock samples. DCA involves: digital characterization of porous rock samples, construction of pore-scale models, numerical simulations of pore-scale physicochemical processes on those models, and determination of macroscopic properties. DCA may have great potential for predicting the sample-scale fluid flow properties of very tight materials, commonly encountered in shale gas extraction. This potential arises because of the challenges of conducting appropriate lab measurements for these rock types: shale and other tight rocks are dominated by very small pores, making it time-consuming or impossible to obtain valid measurements of the full range of macroscopic properties when using conventional laboratory experiments.

A fundamental issue in DCA is to obtain suitable images of the sample. Recent advances in imaging technology have made it possible to obtain high-quality images of samples over a large field of view at a high resolution. However, the ratio between the field of view and the image resolution, though already on the order of 103, still prevents simple DCA from making reliable and robust predictions of the macroscopic properties of gas shale. Gas shale contains sub-micron to sub-nanometre pores that are important to gas flow, but the most advanced imaging techniques can image pores only down to a few nanometers in diameter on a shale sample of 10s of microns in size. Such a small sample is not large enough to be considered as a representative element volume (REV), and this limitation has hindered the use of DCA for these materials.