Fluid flow through rock fractures undergoing chemical interactions: A look at pressure solution from an experimental perspective

Fluid transport within the Earth’s crust is predominantly controlled by planar void space like fractures and crack networks. Characterizing the time dependent hydro-mechanical properties of these rock-structural elements therefore is of paramount importance for natural geosystem understanding and geotechnical applications alike.

In this contribution we outline the protocol and results of a long term flow-through experiment of more than 4 months conducted with one single-fractured, pure quartz, and centimeter-sized Fontainebleau sandstone sample displaying very low matrix permeability.

The cylindrical sample was axially split to generate one single and rough tensile fracture and the obtained sample halves were manually offset in axial direction by 200 µm resulting in geometric mismatch of the two fracture faces yielding asperity contacts and high contact stresses upon loading.

The experiment was conducted at constant temperature (333 K) and pore fluid pressure (1 MPa), three different confining pressure levels (2, 18, and 30 MPa), and with two different fluids (deionized water and 0.3 mM SiO₂ solution).

The sample was continuously flown through and the experimental procedure consisted of several successive stages during which confining pressure and fluid type were systematically varied in time intervals of several weeks each.

The experiment yielded results of continuous sample and fracture permeability measurements, the derivation of time dependent changes in hydraulic fracture aperture, a complete ICP-OES chemical analysis of Si concentrations in the effluent in one day time intervals, and a full before/after microstructural investigation of mechanical aperture, contact area ratio, as well as asperity and free fracture face morphology.

Overall, this experiment yields evidenced insights into the low-temperature dynamics of fracture permeability when, concurrently, chemical interactions between fluid and rock are taking place. Moreover, the investigations emphasize the role of pressure solution (creep) in this context as opposed to, e.g., free face dissolution or subcritical crack growth. Finally, conclusions are drawn on the rate-limiting sub-process of pressure solution with possible implications for fluid history matching in quartz-rich fractured rock masses.