Evolution of Fracture Aperture in Quartz Sandstone under Hydrothermal Conditions: Mechanical and Chemical Effects

Abstract

Fractures efficiently affect fluid flow in geological formations, and thereby determine mass and energy transport in reservoirs, which are not least exploited for economic resources. In this context, their response to mechanical and thermal changes, as well as fluid–rock interactions, is of paramount importance. In this study, a two-stage flow-through experiment was conducted on a pure quartz sandstone core of low matrix permeability, containing one single macroscopic tensile fracture. In the first short-term stage, the effects of mechanical and hydraulic aperture on pressure and temperature cycles were investigated. The purpose of the subsequent intermittent-flow long-term (140 days) stage was to constrain the evolution of the geometrical and hydraulic fracture properties resulting from pressure solution. Deionized water was used as the pore fluid, and permeability, as well as the effluent Si concentrations, were systematically measured. Overall, hydraulic aperture was shown to be significantly less affected by pressure, temperature and time, in comparison to mechanical aperture. During the long-term part of the experiment at 140 °C, the effluent Si concentrations likely reached a chemical equilibrium state within less than 8 days of stagnant flow, and exceeded the corresponding hydrostatic quartz solubility at this temperature. This implies that the pressure solution was active at the contacting fracture asperities, both at 140 °C and after cooling to 33 °C. The higher temperature yielded a higher dissolution rate and, consequently, a faster attainment of chemical equilibrium within the contact fluid. X-ray µCT observations evidenced a noticeable increase in fracture contact area ratio, which, in combination with theoretical considerations, implies a significant decrease in mechanical aperture. In contrast, the sample permeability, and thus the hydraulic fracture aperture, virtually did not vary. In conclusion, pressure solution-induced fracture aperture changes are affected by the degree of time-dependent variations in pore fluid composition. In contrast to the present case of a quasi-closed system with mostly stagnant flow, in an open system with continuous once-through fluid flow, the activity of the pressure solution may be amplified due to the persistent fluid-chemical nonequilibrium state, thus possibly enhancing aperture and fracture permeability changes.

Long-Term Evolution of Fracture Permeability in Slate: An Experimental Study with Implications for Enhanced Geothermal Systems (EGS)

“…Such initial fracture permeability decline was widely observed at the first dozens to hundreds of hours of continuous fluid flow through fractured granitic rocks [4,21], shale [22], novaculite [15,17], limestone [16], and dolomitic anhydrite [18]. However, this time-dependent fracture permeability decay did not occur in some fractured sandstones and mudstones [23,24] with the intermittent flow (flow-stop-flow with a certain time interval). To monitor the influence of pressure on fracture permeability evolution in slates, we continuously measured permeability for several to dozens of hours, followed by stopping the flow for dozens of hours and measured fracture permeability again.…”

“…Typically, such permeability reduction occurs at the early stage of an experiment after pressure build-up and progressively towards a steady state with time. The mechanisms governing the fracture permeability evolution are expected to be an interplay of pressure solution, stress corrosion, and mineral dissolution/precipitation [15,17,20,[24][25][26][27][28][29], which are often referred to as sealing/healing for macroscopic fracture strengthening or strength recovery [30]. Pressure solution and stress corrosion cracking are driven by the imposed (effective) stress on the fracture plane.…”

“…By using an injection fluid close to chemical equilibrium with the rock matrix, precipitation-induced permeability reduction can be minimized [39]. In some cases, permeability can persist to hundreds of days at intermittent flow of deionized water (DI), where fluid-rock interactions tend to equilibrium during the stopped flow stages [23,24]. Even for substantial mineral precipitation in fractures, permeability can remain almost unchanged up to two months duration if the deposition is mainly located behind contact asperities with respect to the flow direction [30].…”

What process causes the slowdown of pressure solution creep

“…Non-hydrostatic dissolution features high reactivity, and moreover possesses attenuation characteristics as if free-face dissolution behaves. The attenuation characteristics show in the manner that the dissolution rate decreases over time, c.f., Yasuhara et al (2006), Van Noort et al (2008a, Yasuhara et al (2011), andCheng andMilsch (2020). Given these two kinetic characteristics, the normalized dissolution rate _ m [mol/m 2 =s] (normalized mass removal rate) in this particular situation follows a more general reaction rate law where the activity of the solid is no longer implicit (Palandri and Kharaka 2004; Taron and Elsworth 2010) (see ''Appendix A.2'' for more information)…”

“…1 Schematic representation of early diagenesis in sedimentary basins. The sediments become consolidated after undergoing the weathering of pressure solution which involves non-hydrostatic dissolution over the inter-granular contact surface, diffusive transport into the pore water outside the contacts, and possible secondary mineral precipitation off after a long runtime (Moore et al 1994;Niemeijer et al 2002;Polak et al 2003;Yasuhara et al 2006Yasuhara et al , 2011Okamoto et al 2017;Cheng and Milsch 2020;Feng et al 2020). To remedy this issue, the cessation of pressure solution creep was attributed to a reduction of the non-hydrostatic stress to the point where it is no longer sufficient to supply the required activation energy (Stephenson et al 1992;Revil 1999;Yasuhara et al 2003;Van Noort et al 2008b;Taron and Elsworth 2010;Lu et al 2017Lu et al , 2018.…”