Impact of Chemical Environment on Compaction Behaviour of Quartz Sands during Stress-Cycling

Abstract

Decarbonisation of the energy system requires new uses of porous subsurface reservoirs, where hot porous reservoirs can be utilised as sustainable sources of heat and electricity, while depleted ones can be employed to temporary store energy or permanently store waste. However, fluid injection induces a poro-elastic response of the reservoir rock, as well as a chemical response that is not well understood. We conducted uniaxial stress-cycling experiments on quartz sand aggregates to investigate the effect of pore fluid chemistry on short-term compaction (stresses of 0.3–35 MPa, five or ten cycles). Two of the tested environments, low-vacuum (dry) and n-decane, were devoid of water, and the other environments included distilled water and five aqueous solutions with dissolved HCl and NaOH in various concentrations, covering pH values in the range 1–14. In the first loading–unloading stress-cycle, 28–64% of the compaction was inelastic, where fluid-saturated samples produced more compaction than dry samples. In addition, compaction was strongly enhanced in alkaline environments and inhibited in acidic ones, compared to distilled water. With prolonged stress-cycling (up to ten cycles), fluid-enhanced compaction effects disappeared. Acoustic emission data and microstructural analyses revealed that microcracking was prevalent in all samples. We inferred that crack growth was aided by Si–O bond hydrolysis and pH-dependent fluid–solid surface interactions. In addition, crack growth rates reduced with prolonged stress-cycling, leading to less fluid-enhanced compaction with increasing cycle number. Our results imply that fluid injection into a clean, quartz-rich, porous reservoir could evoke or inhibit apparent time-independent inelastic deformation depending on the type of fluid injected.