UNCOVERING NANOSCALE ISSUES IN SHALE GAS SYSTEMS

Organic rich shale beds are the origin of conventional hydrocarbon resources and may still contain notable amounts of oil or gas, depending on the kerogen types they contain and their maturity stage. The natural gas confined in the shales is either in form of compressed free gas occupying micro-fractures and the nanopore network, adsorbed gas covering surface of the kerogen bodies within the rock or dissolved gas dispersed within kerogen bodies. Because of their abundance compared to conventional reservoirs, shale gas reserves can be a breakout play in natural gas market.

Nevertheless, shale gas has a complex system that is not yet completely understood and its recent popularity in the energy industry has made it subject of ongoing research. Atomic Force Microscope (AFM) has been commonly used in many high-tech fields such as biotechnology and material science industries for broad spectrum of nano-science investigations. It can provide 3D topographic image and nanoscale structure of sample surface, characterize surface material properties and measure surface interaction forces up to atomic resolution.

Using AFM topographic imaging, we detected various nanopores over the shale gas samples. AFM phase imaging helped us to differentiate the softer regions that possibly reveal kerogen bodies. We then performed contact mode AFM imaging and force curve analysis over those regions and compared the phase images side by side with molecular friction images to characterize those regions more precisely. We performed force curve measurements over various distinguished regions on the shale gas samples. Finally, we used statistical analysis of the force curves to extract some attributes of tip and surface materials and contact geometry such as, van der Waals jump-in force, maximum adhesive force and adhesion energy loss.

In this work we utilized AFM to comprehensively study the detailed structure and behavior of shale gas systems at nanoscale for the first time. Shale gas systems contain nano-features that can play an important role in the resource production. Size, abundance and connectivity of nanopores and relative location of the nanopores and kerogens are significant parameters in determining shale gas production. AFM analysis of gas producing mudrocks is an important supplement to the data obtained by logging, geochemistry, SEM and thin section studies.