Experimental Investigation on the Mechanical Behavior of Victorian Brown Coal under Brine Saturation

Successful hydraulic fracturing depends on the characteristics of the induced fracture network, which controls the enhancement of permeability and ultimate gas production. Among the various factors, the brine concentration in the pore fluid can significantly affect the process, through alterations in microstructure and the impacts of this on rock mechanical behavior. This paper investigates the mechanical behavior of coal in terms of UCS, brittleness, and volumetric-strain deformation and characterizes the induced fracture network caused by monocyclic uniaxial compression on coal samples saturated with varying brine concentration (i.e., 0%, 10%, and 20% NaCl by weight). The mechanical analysis suggests that the softening effect due to water saturation and the chemical interactions between ions in the solution and rock mass cause a significant strength reduction. At a higher order of NaCl concentrations that are near to the solubility limit of NaCl in water, a crystal accumulation in near-surface pores during air drying imparts a high strength and brittleness, reversing the reaction-based strength reduction. Analysis of the resulting fracture network by acoustic emission and micro-CT images shows that the fracture characteristics of a brine saturated coal mass vary due to (1) fracture reopening due to coal softening effect during saturation, (2) micro crack initiation and extension/widening of natural cracks due to chemical interactions between coal mass and ions in saturation fluid, and (3) expansion or initiation of cracks during mechanical loading. The strain contour maps and the volumetric deformation analyses infer that the brine saturated coal specimens subjected to mechanical loading exhibit a dilatancy behavior, confirming the higher plastic deformation undergone by the samples due to softer nature and the existence of excessive fractures in the samples. Although the fracture network becomes dense and wide during saturation, the coal mass remains soft and highly deformable, so that mechanically induced fractures may be extensive. This leads to the potential for damaging the rock formation and of contaminating adjacent aquifers. Thus, the design parameters of a stimulation method such as hydraulic fracturing should be carefully determined by considering the pore fluid characteristics, in order to optimize the process and to minimize reservoir damage.