Using micro-computed tomography and scanning electron microscopy to assess the morphological evolution and fractal dimension of a salt-gypsum rock subjected to a coupled thermal-hydrological-chemical environment

It is well known that salt-gypsum evaporites play a significant role in controlling the physio-mechanical performance of the earth's upper mantle as well as forming extensive “halokineticË® structures that are directly linked to lithologic hydrocarbon reservoirs and petroleum accumulation. Under many geological conditions, hot saline groundwater frequently intrudes into salt-gypsum evaporite sequences. However, relatively little is known regarding the micromechanical behaviour of salt-gypsum in these saline environments, and this behaviour governs the macro-mechanical behaviour and the overall deformation. In this study, we examine the microstructural evolution of salt-gypsum and its weakening mechanisms under varied brine conditions (or coupled thermal-hydrological-chemical environments). A series of laboratory tests, including scanning electron microscopy and micro-computed tomography (MCT), were conducted to study how the petrographic characteristics, including porosity, pore size distribution and fractal dimension, evolved. In total, 81 specimens were prepared and then soaked in brines of 3 different concentrations and at 3 different temperatures. The results showed the following: 1) after brine treatment, the MCT slices of the specimens generally contained four areas: a residual porous skeleton area, an undissolved area, a cracked area and an interface area. 2) For a given concentration, the porosity and fractal dimensions of the specimens gradually increased with temperature, while for a given temperature, the porosity and fractal dimension tended to decrease as the brine concentration increased. 3) Because the nucleation or initiation rate of new voids was slower than the growth and coalescence rate of the original voids, the proportion of 0-1 μm pores gradually decreased over time. However, the proportion of 5 to +∞μm pores gradually increased over time. To study the effects of each factor and the interactions between them on the response variables (porosity and fractal dimension), 2 × 2 and 2 × 3 factorial designs were employed to assess the brine concentrations and temperatures. The results verify that the water temperature significantly weakened the salt-gypsum, while the chlorine ions had a much weaker effect.