A microfluidic framework for studying relative permeability in coal

• Microfluidic chips are fabricated based on micro-CT imaging of coal cleat structure.
• Microfluidic approach is used to measure relative permeability curve of a realistic coal cleat system.
• Fluid transport and displacement efficiency are visualized and quantified in cleat structure.
• Results provide insights into fluid-fluid displacement in coal cleats and provide a means to study coal relative permeability.

A significant unconventional energy resource is methane gas stored in shallow coal beds, known as coal seam gas. The flow and transport of fluid in coal beds occur in a well-developed system of natural fractures, called cleats. In this study, we developed an efficient workflow for the fabrication of microfluidic chips based on X-ray micro-Computed Tomography (micro-CT) images of coal. A dry and wet micro-CT imaging technique is utilized to image coal cleats that would be otherwise non-resolvable. The obtained image of the cleat network is then etched into silicon wafers and used to fabricate poly dimethyl siloxane (PDMS) microfluidic devices. Fluid transport and displacement efficiency are visualized and quantified in real time by injecting water with a flow rate of 1 μl min− 1 into the fabricated cleat structure initially saturated with air. A microfluidic approach is used to measure the relative permeability of a realistic coal cleat system by monitoring the liquid recovery at recorded saturations after the breakthrough. Relative permeability curves show the cross and end point values for the water and gas flow, and predict a maximum relative permeability of 0.15 for the water phase. Understanding the relationship between coal cleat structure and the resulting relative permeability is crucial for the optimization of methane gas extraction and to reduce the environmental concerns of excess surface water production. Also, pore network modelling based on the Maximal Ball (MB) concept is applied to predict relative permeability curves numerically. Our experimental results are in good agreement with pore network modelling outcomes and provide consistent and smooth macro-scale relationships along with direct observation of the pore-scale physics. Therefore not only can the microfluidic approach be used as a validation tool for multiphase flow numerical models but it can also provide direct visualization of transport properties unique to coals. Overall, our developed system provides a better understanding of fluid flow behaviour in coal and presents novel relative permeability data for coal seam gas reservoirs under identical conditions and cleat sizes.