Micro-mechanics based numerical simulation of NaCl brine induced mechanical strength deterioration of sedimentary host-rock formations

Abstract

Artificial fracture stimulation in low-grade sedimentary ore deposits is one method to improve mineral extraction efficiency of In-Situ Leaching (ISL) process. Low to moderate saline environments are found in most sedimentary ore deposits that deteriorate intact rock strength over time. It is important to recognize the host rock strength properties prior to artificial fracture stimulation. Therefore, to identify the effect of salinity on the mechanical properties of brine-saturated sandstone, a series of uniaxial compressive strength (UCS) tests and Brazilian tensile strength tests were performed on specimens saturated with water, and with varying NaCl brine concentrations (5.0%, 7.5%,10% and 12.5%). Scanning electron microscopy and inductively coupled plasma mass spectrometry (ICP-MS) tests reveal that increasing salinity deteriorate the mechanical integrity of sandstone by accelerated clay mineral dissolution. Consequently, 12.5% saturation fluid salinity resulted in a 40% reduction in UCS, 22% reduction in Young's modulus and 33% reduction in tensile strength. The strength deterioration observed in the experimental study was then successfully simulated using Particle Flow Code 3D (PFC3D) in consideration of the bond strength deterioration mechanism for sandstone. The calibrated model was used to accurately replicate the damage mechanism of sandstone under the influence of brine saturation. The model forms an accurate intact rock assembly for numerical modelling of saline sedimentary host-rock formations.