Quantification of Porosity, Permeability and Mineral Structure in Shales Using Nano- to Micron-scale X-ray Tomography

Gas production from shale gas reserves remains technically challenging as they generally exhibit extremely low permeability. Gas production can therefore only be achieved through artificial stimulation (hydraulic fracturing). Unlike more conventional reservoirs a substantial fraction of the porosity within shale is on the submicron to nanometer scale. Understanding how the distribution and geometry of the macro, micro and nanoscale pore networks control natural and stimulated fracture formation, and gas release can only be achieved using a multi-scale characterisation and modelling approach. Given the scale of the pore spaces and fractures, X-ray imaging is demonstrated to be an effective tool to achieve this goal.

An organic-rich Carboniferous shale was studied from a gas-charged sedimentary basin in the UK. The rock contained detrital quartz and clay minerals (illitic and kaolinitic phases), crystalline authigenic dolomite and pyrite, and amorphous organic matter. The samples were analysed using standard petrographic techniques (optical and SEM), and both synchrotron and lab source x-ray tomography across the required spatial resolution (voxel sizes 10s of microns down to 10s of nanometers). From this data a 3D model was constructed and the pore network was extracted and modelled, with quantification of pore size, shape and co-ordination number. The distribution of mineral phases and fractures was included in this model,. Using bespoke principle component analysis software the shale permeability was modeled and correlated to sedimentary and diagenetic textures, and fracture distribution.

The methodology developed allows a quantitative correlation between the nano-scale properties of shale to macroscopic properties, enabling more accurate characterisation of shale gas reservoirs, and permits more realistic assessment of reservoir potential from core and reservoir scale studies in the future.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California