Research On Micromechanisms Of Rocksalt Rheology And Constitutive Equations

The salt tectonic is an important part of the geological structure, which plays an important role for reservoir engineering. Its geological characteristics are closely related to the long-term evolution of salt basin, which has been proved to be effective occurrence of hydrocarbons. The features of the rocksalt determine that it is the most effective layers for oil and gas basin. In addition, the rocksalt has been widely considered to be the ideal underground storage medium of oil, gas and highly radioactive nuclear waste, since it has low permeability, low porosity, stable mechanical properties, etc. Therefore, the research for the long-term rheological mechanics behavior is of special importance for reservoir engineering, underground storage project. The change for microstructure of rocksalt is often accompanied by the nonlinear curve under creep and stress relaxation conditions. However, influenced by various factors, such as temperature, strain rate, stress level, water content, etc, the strain-rate controlled mechanism may be transformed. It's important for the extrapolation of laboratory scale data and the establishment of a reasonable constitutive model that whether or not the data and images gained by experiments can actually describes the geological process under conditions of long-term deformation. Three microstructural processes that can occur during deformation of rocksalt under the condition of 20-200?, namely dislocation creep, pressure solution and dilatancy, and the typical microstructures & deformation mechanisms are studied based on the data collection, laboratory experiments, thin section observation. By introducing the parameters of damage and fractional derivative order, this article establishes a new constitutive model by means of fractional calculus theory and damage theory, also the data fitting of the full creep curve is studied.1. Typical microstructures of three rheology processes and competitions & transformations of rheological mechanisms under specific conditions are studied. The microstructures of dislocation creep, pressure solution and dilatancy are subgrain & wavy slip line, grain boundary indentation/truncation/over-growth and microcracks respectively. Grain boundary water has great contribution to the grain boundary migration and pressure solution, this recrystallization process involves the dissolution-diffusion-precipitation of grain boundary material, and is driven by chemical potential differences. The dilatancy is influenced by the stress level. Dislocation creep is accompanied by the competition between strain hardening and recovery, and it leads to the nonlinear of three stages of creep curve. The steady-state stage reaches when the hardening and recovery are equilibrium. Considering the influence of water, and defining both recovery and grain boundary migration as softening, then the competition between hardening and softening may lead to oscillating behavior of the quasi steady-state curve. Influenced by temperature, stress, strain rate and grain boundary water, the transformation of strain-rate controlled mechanism will occur in the rocksalt rheological process. Dislocation cross-slip will become the dominant mechanism under high stress and strain rate in the steady-state creep stage, while under low stress and strain rate, the rate-limiting mechanism is the climb of dislocation. For the wet fine-grained rocksalt, pressure solution will be more important. Furthermore, with the increase of grain size, dislocation creep will be strain-rate controlled.2. With the help of etching and?-irradiation, The microstructures of two kinds of natural rocksalt taken from the Hengelo and Qum Kuh are observed. The differences between the microstructures are ascribed to the geologic origin. The formation of Hengelo rocksalt can be explained by salt pan environment, which means that the part of rich of fluid-inclusion is the old one, while the clear halite is characterized of subgrain and grain boundary migration. The cores of Qum Kuh rocksalt are porphyroclast, while the mantles are new substructure-free grains. These nature microstructures indicate that during the long-term geological process, dynamic recrystallization and pressure solution are of great important to deformation mechanism, also grain boundary water has an obvious role in promoting the above process. By consuming old grains, the intragranular fluid-inclusion is transferred to a new grain boundary, and indentation, truncation and overgrowth can be observed. Through calculation of two kinds of rocksalts grain(subgrain) size, the Fine Graining Complexity Measure of the Qum Kuh rocksalt is more significant, which means the deviatoric stress is higher during the geological period. By further calculation of the steady-state creep rates, the creep strain rate caused by pressure solution is more important, which show that for the wet fine-grained salt rocks, pressure solution processes is the dominant mechanism.3. Creep curves and microstructures of wet and dry synthetic rocksalts are analyzed, fluid-assisted dynamic recrystallization and pressure solution are emphasis on: Driven by the chemical potential differences, grain boundary of wet synthetic rocksalts are irregular ascribed to the gain boundary migration and pressure solution, which mean the dissolution-precipitation processes, grains dissolve and diffuse through fluid, then crystallize at new grain boundaries. Wet rocksalts contain substructure-free new recrystallization grain, while dry synthetic rocksalts showing a lot of subgrains and wavy slip lines.Under the softening effect, rheological stress of wet synthetic rocksalts often appear several peak points before steady-state stage, it is concluded that grain boundary migration is the main factor by observing microstructures after deformation.The stress-strain curves are relatively gentle for dry synthetic rocksalts and strain hardening will last for a long time, also the rheological stress is higher when quasi steady-state is reached. Grain size shows no obvious change during the creep of dry synthetic rocksalts, while grain size of wet synthetic rocksalts increases with the influence of dynamic recrystallization.4. Uniaxial and triaxial creep experiments have been carried out, also the acoustic emission data of uniaxial creep test is entirely recorded to study the damage evolution. Furthermore, fractional rheological constitutive model is established, and thin sections of rocksalts after creep deformation are observed with the help of reflected microscope.Uniaxial creep and acoustic emission data can be roughly divided into three stages, the first stage is accompanied by the competition of strain hardening and microcracks, in the second stage the parallel mechanisms are equilibrium, the microcracks development is the main reason for the creep failure along with the third stage. For the triaxial creep experiments, larger deviatoric stress will short steady-state creep stage and larger confining pressure will effectively restrain microcracks development. Competition relationship between strain hardening and recovery of dislocation creep is indicated by means of fractional derivative order, it conclude that recovery is more significant while the value of fractional derivation order is bigger. Considering the microcracks development in the process of dilatant deformation, fractional derivative damage constitutive model is established on the basis of Weibull distribution, fractional calculus and damage theory. The observation of microstructures of nature and dilatant rocksalts manifest that, the nature rocksalts under geological conditions undergo various deformation processes such as dislocation creep and pressure solution. The existent of impurities can restrict the extent of microcracks. The microcracks are inclined to initiate where stress concentrations happen, such as the sharp grain boundaries, paragentic minerals. A large quantity of transgranular cracks and these along with fluid inclusion can be observed.