Effects of fluid composition and salinity on subcritical crack growth in granite

Abstract

Using the double-torsion technique, the fracture mechanical response of granite specimens was investigated under the influence of fluid with different electrolyte types and salinities. The fracture toughness (KIC) and critical mechanical energy release rate (G0) of the granite in electrolyte solution are weakened compared to those in distilled water. The KIC in all electrolyte solutions is 2.08–2.28 MPa·m1/2, which is 8.4 % − 16.5 % lower than that in distilled water, and it is insensitive to ion type and salinity. In contrast, the subcritical crack growth index (n) shows different salinity dependence in three salt (AlCl3, CaCl2, and NaCl) solutions. The slight increase in n with rising salinity of the AlCl3 solution is attributed to crack tip blunting caused by over-dissolution of minerals. The n and G0 decrease and then increase with rising salinity of CaCl2 solution, which may be related to the salinity-dependent dissolution rate of quartz and the weakening of repulsive force between crack surfaces. The n and G0 shows a negative correlation with salinity and decrease by 26.4 % and 15.4 %, respectively as the NaCl salinity increases from 0.1 M to 1.0 M. Limitations in the dissolution reactions of the main minerals (albite, biotite and diopside) is accountable for enhancement of n in silica solutions. Experimental results emphasize the critical and subcritical fracture behavior of the granite in response to chemically reactive fluids with implications for both hydraulic fracture stimulations in geothermal fields and caprock integrity for CO2 sequestration. In these applications, the fracture properties of the reservoir rock can be enhanced or weakened by changing the electrolyte type and salinity of the fluid, depending on the actual requirements.