Abstract
Laboratory studies designed to provide the strength of rocks are typically performed on either dry- or water-saturated rock samples. To better understand the mechanical behaviour of rocks in a geothermal context, we provide here the short-term strength and static elastic modulus of sandstones from exploration well EPS-1 at the Soultz-sous-Forêts geothermal site (France) saturated in hydrothermal brine sourced from the adjacent production well GPK-2. We performed 59 uniaxial compressive strength experiments on samples of Buntsandstein sandstones sampled from six depths (1008, 1022, 1069, 1107, 1290, and 1414 m). Samples were deformed oven-dry (i.e., unsaturated), water-saturated (deionised water), or brine-saturated. Relative to the dry state, strength was reduced by between 24 and 39% and elastic modulus was reduced by between 9 and 19% when the samples were saturated with water. However, we observed no measurable difference in strength and elastic modulus between samples saturated in water and brine. These reductions in strength and elastic modulus in the presence of water or brine are considered the result of a reduction in specific surface-free energy. Changes to short-term strength and elastic modulus in the presence of hydrothermal brines likely require brines with higher salinities and/or lower values of pH than those typically found within the Upper Rhine Graben (the brine from GPK-2 has a salinity of ~ 10% and a pH of 5.5). These new data can be used to assist reservoir prospection, stimulation, and optimisation strategies at the geothermal sites within the Upper Rhine Graben.
Highlights
Experimental studies designed to understand the mechanical behaviour and failure modes of sandstones are often performed either dry or using deionised or distilled water as the pore fluid (e.g., Menéndez et al 1996; Wong et al 1997; Baud et al 2000; Klein et al 2001; Bésuelle et al 2003; Baud et al 2004; Fortin et al 2005; Heap et al 2009a). The results of these studies may not be directly applicable to the study of geothermal resources, where the rocks in situ are saturated with hydrothermal brines that
These curves show that the uniaxial compressive strength is lower when the sample is saturated with water or brine
It is reasonable to assume that the high salinity (TDS of 97 g/l) (e.g., Shukla et al 2013; Rathnaweera et al 2014) and low pH (5.5) (e.g., Parks 1984; Singh et al 1999; Wang et al 2015) of these brines could influence the strength and elastic modulus of these sandstones
Summary
Experimental studies designed to understand the mechanical behaviour and failure modes of sandstones are often performed either dry or using deionised or distilled water as the pore fluid (e.g., Menéndez et al 1996; Wong et al 1997; Baud et al 2000; Klein et al 2001; Bésuelle et al 2003; Baud et al 2004; Fortin et al 2005; Heap et al 2009a). Nara et al (2014) found that the velocity of a crack growing subcritically decreased and did not change, respectively, as salinity (NaCl) was increased up to 1.0 mol/l for Berea (clay-poor) and Shirahama sandstone (clay-rich). Feucht and Logan (1990) found that the strength of sandstones saturated with low ionic strength solutions (NaCl, CaCl2, and N a2SO4) did not change with respect to the water-saturated case, but that intermediate ionic strengths (1.0 M) increased the strength of the sandstone (by up to 20%) and high ionic strengths (5.0 M) decreased strength (by up to 20%). These authors considered that the presence of clays in the Shirahama sandstone influenced subcritical crack growth by changing the width of the electric double layer. Double-torsion experiments have shown that subcritical crack growth rates increase as the hydroxyl (OH−) concentration is increased (Atkinson and Meredith 1981)
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