Mechanisms And Factors Of The Localized Deformation In Porous Rocks

The localization of deformation is commonly encountered in the Earth's crust,which always accounts for a variety of geological processes,and natural and engineering disasters.Typically,the deformation behavior in rocks is brittle at low confining pressures,and tends to be ductile at high confining pressures.While the localized compaction may arise from the deformation attached to pore spaces,which can seriously affect volcanic activity,oil and gas exploitation,in-situ fluidized mining due to the weakened transport property.The microstructure and stress condition related to porous rocks are so complex that their actual responses are generally hard to be distinguished,and thus,the deformation of porous rocks constitutes the major subject in this work,where the laboratory experiments and numerical simulations are adopted for an enhanced understanding of the micromechanics and influential factors underlying the localized deformation.The cyclic loading and unloading test is firstly conducted on rock samples of various porosities,to illustrate the damage changes and cracking characteristics in both macroscopic and microscopic levels.It is followed by the establishment of numerical models based on the conceptual microstructure of porous rocks,whose rationality is evaluated through some careful comparisons of mechanical behavior,micro-cracking activity and energy budget between this approach and previous work related with the brittle-ductile transition.Then,several intrinsic factors,such as mineralogy and cementation attributes,grain size,pore structure and porosity,are successively adjusted to investigate the micro-cracking activity and energy budget under contractional regimes,which are intended to discuss the influence of rock microstructure on the micromechanics of localization.Accordingly,some external factors,such as the heterogeneous structure,boundary constraints and borehole,are introduced to describe the related changes in micro-cracking events and grain fragmentation under contractional regimes,and to provide some insights into the effect of stress field inhomogeneity on the localization process.The laboratory results primarily emphasize the nonlinear deformation under compressions and the relentless advance of rock damage,whereas the destruction is a progressive process and the deformation parameters are heavily dependent on stress states;the micro-cracking events are generally localized on the macroscopic rupture region,and both pore space and confining pressure appeal to promote the grain fragmentation.Accordingly,the micro-cracking activity and energy budget across the brittle-ductile transition can be accurately characterized by the numerical models comprised of breakable grains and deformable macro-pores,where the localization is generally accompanied with surges in micro-cracking activity and energy release,and the development of compaction localization requires high magnitudes of porosity,confining pressure and contractional displacement.The rise in confining pressure typically promotes the shear and intra-granular failures,whereas the growth in porosity contributes to the relative abundances of tensile and inter-granular failures.In general,the porosity exerts a first-order control on the mechanical response,while the micro-cracking activity and energy release are affected by the intrinsic factors,such as grain sorting,mineralogy and cementation strength limits,and the relative size and abundance of macro-pore.At high confining pressures,a finer-grained rock favors cataclastic flow,and the role of mineralogy and cementation attributes is typically minimal;the macro-pore is an essential prerequisite for the generation of compaction band,and an increase in either macro-pore size or micro-porosity often facilitates the formation of compaction localization.On the other hand,a rigid and frictional boundary does little to the high-confinement performance of porous rocks,in which the macro-pore structure generally dominates;specifically,the edge macro-pores represent the nucleation of localization,and the stress redistribution originating from heterogeneous structure plays a part in the propagation process.The damage under far-field contractions is often concentrated on the regions of high tangential stress in a perforated rock,of which the detailed characteristics are correlated with far-field stresses,borehole size and rock microstructure.When the drilling process is considered,the damage in wall rock largely depends on the far-field stresses,both the scope and intensity of which may be higher than those in a perforated rock;and accordingly,the micro-cracking events are frequently aggravated by the reaming approach,though their outcrops at the periphery of borehole are settled during the initial phase.