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Weathering
Published in Richard J. Chorley, Stanley A. Schumm, David E. Sugden, Geomorphology, 2019
Richard J. Chorley, Stanley A. Schumm, David E. Sugden
Experimental investigations of weathering rates commonly involve simulated attempts to speed up the effects of natural processes by increasing the rates of heating-cooling and wetting-drying cycles, the amounts of heating and wetting, the concentration of chemical solutions, etc. This time compression introduces scale problems which distort the effects of natural lag and long-term fatigue effects and the increase in the intensity of the processes further removes them from their supposed real-world counterparts. Thus experimental work tends to throw more light on relative, rather than absolute, rates of weathering. A coarse granite block was exposed to an accelerated equivalent of 244 years of diurnal temperature ranges of 110°C with little effect, but after a period equivalent to only two and a half years where cooling was effected by a water spray, the surface exhibited significant cracking and loss of polish. Although ignoring the longer-term fatigue factor and lack of rock confinement, this experiment showed the efficacy of wetting and drying in association with thermal changes. The effect of crystal growth, hydration and thermal expansion in association with salt crystallization was investigated, where a number of rocks were immersed in a salt solution at 17°C – 20°C for 1 h, dried at 60°C for 6 h and then dried at 30°C for 17 h. The resulting changes of weight by surface fragmentation are shown in Figure 9.13 against number of daily cycles of wetting and drying, indicating that chalk, sandstone and limestone are more susceptible to salt weathering than are igneous rocks and black shale. The most effective salt used was Na2SO4 and rates of disintegration appeared to be related to water absorption capacity. This weathering mechanism is particularly applicable in deserts and arctic coastal areas. Other controlled experiments have involved the exposure of different rock types to natural weathering processes, although choices of initial particle size have been arbitrary. Figure 9.14 gives the percentage decreases of particles of the initial 10–20 mm diameter range when exposed to seventeen years of weathering in Central Europe, with sandstone and mica schist being more susceptible to weathering than limestone. In a series of observations between 9 December 1962 and 19 March 1964 at Denver, Colorado, the natural weathering of a piece of Entrada Sandstone was measured against that for two sets of other Colorado Plateau cliff-forming sandstones (Figure 9.15). During the total eight periods the rocks were subject to surface granulation and cracking under the influence of 21.53 in precipitation (including 3.87 in of snow in periods 1, 2, 6,7 and 8) and 124 freeze-thaw cycles (period 1–33; period 2–15; period 6–13; period 7–22; period 8–41). These observations indicated the high present susceptibility of all these sandstones to weathering, the especial importance of freeze-thaw effects particularly after periods of wetting, and the marked vulnerability of the Entrada Sandstone which exhibited a rate of weathering more than four times the average of the rest.
Coupled hydro-chemo-mechanical model for fault activation under reactive fluid injection
Published in European Journal of Environmental and Civil Engineering, 2023
H. Tounsi, A. Pouya, J. Rohmer
The geomechanical processes associated with geologic carbon storage, namely, the combined effects of the lithostatic stress and the poromechanical loading induced by CO2 injection and their potential consequences on fault stability and caprock integrity are fairly well documented (Morris et al., 2011; Rutqvist, 2012; Rutqvist et al., 2016; Snell, 2019; Zhang et al., 2015), whereas the influence of the geochemical processes of dissolution/precipitation on the geomechanical behavior of the storage system is often neglected and hence not well understood yet. Historically, the effect of chemically active fluids flow has long been studied to understand long-term evolution of faulted regions of the crust (Pili et al., 1999) and, with the advance in numerical methods in geomechanics, some numerical approaches were presented in literature for modeling dissolution/precipitation phenomena in fractures (Elsworth & Yasuhara, 2010; Yasuhara & Elsworth, 2006) as well as subcritical crack propagation applied, for instance, to wellbore stability (Park et al., 2007). The effect of chemical processes on the mechanical stability has also been studied and modeled numerically in the framework of continuum mechanics, in the context of weathering in underground galleries (Ghabezloo & Pouya, 2006). However, few studies exist on the experimental characterization of interfaces between CO2 or CO2-enriched brine and fault minerals associated with carbon storage, and even fewer focus on the coupling between the above-cited chemical processes, the fault-related flow and the ground mechanical behavior at different time and space scales. In an exhaustive literature review of laboratory experimental studies available in literature in relation with the interaction between chemical and mechanical processes in the context of CO2 storage, Rohmer et al. (2016) noted that carbonate reservoirs are most concerned by dissolution alteration, which impact on their mechanical behavior is inevitable, especially when they are subjected to long-term CO2 exposure. It was also reported that no experimental data on the shear strength weakening of CO2-infiltrated calcite-rich faults is available in literature. Lately, Espinoza et al. (2018) measured the reduction of shear strength and stiffness of Entrada sandstone and Summerville siltstone samples that were exposed to CO2-charged brine resulting in mineral dissolution. It is thus widely agreed, until now, that the dissolution due to reactive flow is likely to increase the rock porosity and, consequently, to decrease the rock stiffness and strength under certain conditions of pressure and stress and other site-specific conditions.