Experimental study of fracture deformation, flow and transport using a novel pressure-controlled Hele-Shaw cell

Abstract:

The development of natural resources and engineered reservoirs may lead to the alteration of the equilibrium of the fracture environment, causing changes in the flow- and transport properties. The complexity here lies not only in the number of physical and chemical processes that simultaneously occur within the fracture, but also how they affect each other.

In order to investigate the above, we have developed a novel experimental apparatus and protocol to observe and quantify relevant physical processes involved in fracture flow, and to study the interplay between fracture geometry, flow regimes, and the emergence of non-Fickian transport. The setup is a modified version of the classical Hele-Shaw cell where one can simulate underground conditions by controlling the normal stress on the fracture, and the injected flows and pressures.

Our study uses transparent rock analogs to conduct extensive mili-fluidic experiments. We use different digital fabrication technologies (e.g. 3D-printing) to produce analogue specimens of digitally-generated self-affine fractures (Fig. 1). Digital fabrication provides not only precise control of the geometry but also of material properties, allowing one to control which physical process will dominate in the experiments (e.g. elastic vs. plastic deformation, creep, cracking, etc).

Preliminary tests were conducted on a fracture analog consisting of an array of elliptical asperities (Fig. 2a). This specimen was fabricated using a combination of CNC machining and molding/casting techniques with PDMS. With our setup it is possible to conduct in-situ aperture measurements under different normal loads using light transmission techniques (Fig. 2b). The experiments show how increasing confining pressure results in increasing contact area and decreasing pore throat.

These changes in fracture geometry can be linked to the changes in fracture permeability, expressed, for instance, in terms of its hydraulic aperture (Fig. 3a). Moreover, the transport of non-reactive tracers was studied under different normal stress levels. The measurement of the concentration fields (Fig. 3b) produces direct observation of channeling and boundary layers around individual asperities.

We are currently investigating the onset of non-linear flow and anomalous transport in self-affine fractures.