Abstract
We develop a microfluidic experimental platform to study solute transport in multi-scale fracture networks with a disparity of spatial scales ranging between two and five orders of magnitude. Using the experimental scaling relationship observed in Marcellus shales between fracture aperture and frequency, the microfluidic design of the fracture network spans all length scales from the micron (1 μ) to the dm (10 dm). This intentional `tyranny of scales’ in the design, a determining feature of shale fabric, introduces unique complexities during microchip fabrication, microfluidic flow-through experiments, imaging, data acquisition and interpretation. Here, we establish best practices to achieve a reliable experimental protocol, critical for reproducible studies involving multi-scale physical micromodels spanning from the Darcy- to the pore-scale (dm to μm). With this protocol, two fracture networks are created: a macrofracture network with fracture apertures between 5 and 500 μm and a microfracture network with fracture apertures between 1 and 500 μm. The latter includes the addition of 1 μm ‘microfractures’, at a bearing of 55°, to the backbone of the former. Comparative analysis of the breakthrough curves measured at corresponding locations along primary, secondary and tertiary fractures in both models allows one to assess the scale and the conditions at which microfractures may impact passive transport.
Highlights
In the United States and increasingly around the world, a significant volume of oil and natural gas is being produced from unconventional reservoirs that are commonly dominated by shale lithology [1]
The objective of the present paper is to demonstrate that a physical micromodel can be constructed such that the power-law relationship describing shale fracture size and distribution is preserved over multiple orders of magnitude in length, and that the solute tracers can be reliably and reproducibly tracked through this system in real time
We explore the capacity to generate reproducible steady-state flow fields in shale fracture networks, demonstrate successful recovery of solute breakthrough and highlight the difficulties associated with micromodel fabrication and experimental flooding design
Summary
In the United States and increasingly around the world, a significant volume of oil and natural gas is being produced from unconventional reservoirs that are commonly dominated by shale lithology [1]. Shales are ultrafine-grained sedimentary rocks that exhibit variability in structural and chemical features across a broad range of characteristic length scales [2,3,4] They have multi-modal pore size distributions varying over multiple length scales (10−9 to 10−1 m [5,6,7,8,9,10,11]), where nano-scale pores are most common and can be interparticle, intraparticle or even inside the organic matter itself [12]. The rock fabric is very tight and compact with nanometer-scale pathways and nano-Darcy permeability [3,13], but it is inundated with innumerable natural fractures This fracture network follows a characteristic power-law relationship between the fracture aperture and frequency [14,15], which dictates its transport properties [16].
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