Optic Imaging of Two-Phase-Flow Behavior in Nanoscale Fractures
Authors: Carpenter Chris 
Published: February 2015

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 164549, ’Optic Imaging of Two-Phase-Flow Behavior in Nanoscale Fractures,’ by Qihua Wu, Baojun Bai, SPE, and Yinfa Ma, Missouri University of Science and Technology; and Joeng Tai Ok, Keith Neeves, and Xiaolong Yin, SPE, Colorado School of Mines, prepared for the 2013 SPE Unconventional Resources Conference, The Woodlands, Texas, USA, 10-12 April. The paper has not been peer reviewed.
Gas in tight sand and shale exists in underground reservoirs with microdarcy or even nanodarcy permeability ranges; these reservoirs are characterized by small pore throats and crack-like interconnections between pores. The physics of fluid flow in these rocks, with measured permeability in the nanodarcy range, is poorly understood. Knowing the fluid-flow behavior in the nanoscale channels is of major importance for both simulation studies and calculations of the relative permeability of gas in tight shale-gas systems.
Introduction
Shale-gas reservoirs have pore sizes in the range of 1 to 300 nm. Additionally, the matrix permeabilities of those unconventional gas reservoirs are in the microdarcy to nanodarcy range. When fluid (gas or water) flows in a shale pore, the molecule size is quite comparable to the flow diameter. When some gas molecules strike against the pore wall, they retain a certain velocity that causes them to “slip.” Under those conditions, Darcy’s equation may not be accurate or practical for describing the fluid-flow behavior because there may be other flow mechanisms at work such as slip flow and diffusive flow.
Extensive studies have been conducted on single-phase flow in tight-sand and shale gas systems, and equations have been derived to measure the relative permeability of gas and to determine the single-phase gas-slippage effect. However, under real shale-gas-reservoir conditions, two phases (mostly water and gas) typically exist. It has been reported that in some ultralow-permeability shale-gas reservoirs, water saturation could be much greater than in conventional shale-gas plays. The effects of saturated water on gas permeability/relative permeability and gas slippage still have not been investigated sufficiently.
For laboratory experiments, shale samples were used in most cases in order to represent the real pore structure and distribution in the reservoir. However, there are some limitations of using shale samples in laboratory experiments. For example, water saturation and saturation distribution are difficult to measure. Meanwhile, the laboratory-on-chip technique is becoming a suitable approach for investigating fluid flow in ultrasmall pores. Nano fluidics and microfluidics have been used in different subject areas for both fundamental research and application. Some micromodels are being used to study multiphase flow under both saturated and unsaturated conditions. Furthermore, the laboratory-on-chip technique provides a means by which to directly visualize fluid-flow behavior, such as the displacement of one fluid by another, because most micro-/nanofluidic chips are made of transparent materials. Micromodels have been used to simulate numerous porous-media-transport phenomena, including microorganism transport in unsaturated porous media. However, visualization studies of fluid flow in nanochannels are still lacking, especially for shale gas and tight gas.