Salinity-dependent strength and stress–strain characteristics of reservoir rocks in deep saline aquifers: An experimental study

• Salinity effects on mechanical properties of deep saline aquifers are investigated.
• AE counts, SEM and ARAMIS analyses on varying brine saturated sandstone are performed.
• Crystals develop in the rock’s pore structure, depending on the bine concentration.
• Increased salinity enhances rock brittleness and reduces crack initiation stress.

This paper presents an experimental study of the effects of salinity on the mechanical properties of reservoir rocks in deep saline aquifers. Nineteen sandstone specimens saturated in NaCl brines of varying salinity concentrations (0%, 10%, 20%, and 30% NaCl by weight) were tested in a uniaxial compression testing machine and the corresponding fracture propagation patterns were recorded using an advanced acoustic emission (AE) system. The stress–strain curves were analysed, with the simultaneous recording of the acoustic signals and the failure mode. In addition, a digital image correlation system, ARAMIS, was used to measure the lateral and axial strains during the loading period. Scanning electron microscopy (SEM) analysis was performed to understand the changes observed in the uniaxial compressive strength (UCS) and stress–strain behaviour of the rock specimens. According to the experimental results, the UCS and stress–strain behaviour of the rock specimens change with the increasing NaCl concentration of the host fluid. The SEM results show only minor changes in mineral structure during immersion. However, some depositions of NaCl crystals in the rock’s pore space were observed. Interestingly, the growth of the NaCl crystals depends on the brine concentration, the amount of growth increasing with increasing brine concentration. The observed changes in AE analysis are also explained by the crystallisation of NaCl in the pore space, which transfers more acoustic energy from the cracks to the AE sensors due to the crushing of NaCl crystals during the compression process.