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Register a new userFailure patterns caused by localized rise in pore-fluid overpressure and effective strength of rocks
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In order to better understand the interaction between pore-fluid overpressure and failure patterns in rocks we consider a porous elasto-plastic medium in which a laterally localized overpressure line source is imposed at depth below the free surface. We solve numerically the fluid filtration equation coupled to the gravitational force balance and poro-elasto-plastic rheology equations. Systematic numerical simulations, varying initial stress, intrinsic material properties and geometry, show the existence of five distinct failure patterns caused by either shear banding or tensile fracturing. The value of the critical pore-fluid overpressure at the onset of failure is derived from an analytical solution that is in excellent agreement with numerical simulations. Finally, we construct a phase-diagram that predicts the domains of the different failure patterns and at the onset of failure.
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Cracking of rocks and rock-like materials exhibits a rich variety of patterns where tensile (mode I) and shear (mode II) fractures are often interwoven. These mixed-mode fractures are usually cohesive (quasi-brittle) and frictional. Although phase-field modeling is increasingly used for rock fracture simulation, no phase-field formulation is available for cohesive and frictional mixed-mode fracture. To address this shortfall, here we develop a double-phase-field formulation that employs two different phase fields to describe cohesive tensile fracture and frictional shear fracture individually. The formulation rigorously combines the two phase fields through three approaches: (i) crack-direction-based decomposition of the strain energy into the tensile, shear, and pure compression parts, (ii) contact-dependent calculation of the potential energy, and (iii) energy-based determination of the dominant fracturing mode in each contact condition. We validate the proposed model, both qualitatively and quantitatively, with experimental data on mixed-mode fracture in rocks. The validation results demonstrate that the double-phase-field model -- a combination of two quasi-brittle phase-field models -- allows one to directly use material strengths measured from experiments, unlike brittle phase-field models for mixed-mode fracture in rocks. Another standout feature of the double-phase-field model is that it can simulate, and naturally distinguish between, tensile and shear fractures without complex algorithms.
To explore the role of the olivine grain size and crystal preferred orientation (CPO) on the evolution of the microstructure and the mechanical behaviour of upper mantle rocks up to large strains, we performed axial extension experiments at 1200C, 300MPa confining pressure, and constant displacement rate on three natural peridotites: a fine-grained mylonitic harzburgite with a weak CPO and two coarse-grained well-equilibrated dunites with CPO of variable intensity. Initial flow stresses show a limited range of variation (115-165MPa), with the fine-grained sample showing the highest initial strength. However, the evolution of both mechanical behavior and microstructure differs between fine and coarse-grained peridotites. In the fine-grained harzburgite, necking resulted in decrease in the apparent differential stress. Focusing of strain and stress produced increase of the olivine recrystallized fraction and decrease of the recrystallized grain size in the neck. Analysis of the final stress and strain in the neck indicates softening due to evolution of the microstructure and CPO. In contrast, necking of the coarse-grained samples produced weak or no decrease in the apparent differential stress. This implies hardening, consistently with the increase in bulk intragranular misorientation and final stresses in the neck similar or higher than initial ones. Coarse-grained dunites deformed heterogeneously. Crystals well oriented to deform by dislocation glide became elongated, whereas those in hard orientations remained almost undeformed. In the neck, stress and strain concentration resulted in formation of kinks in hard crystals and dynamic recrystallization in soft crystals. We interpret the more effective strain-induced softening of the fine-grained peridotite as due to easier dynamic recrystallization, probably due to the higher proportion of grain boundaries acting as nucleation sites.
The growing concentrations of the greenhouse gases CO2, CH4 and N2O (GHG) in the atmosphere are often considered as the dominant cause for the global warming during the past decades. The reported temperature data however do not display a simple correlation with the concentration changes since 1880 so that other reasons are to be considered to contribute notably. An important feature in this context is the shrinking of the polar ice caps observed in recent years. We have studied the direct effect of the loss of global sea ice since 1955 on the mean global temperature estimating the corresponding decrease of the terrestrial albedo. Using a simple 1-dimensional model the global warming of the surface is computed that is generated by the increase of GHG and the albedo change. A modest effect by the GHG of 0.08 K is calculated for the period 1880 to 1955 with a further increase by 0.18K for 1955 to 2015. A larger contribution of 0.55 +/-0.05 K is estimated for the melting of polar sea ice (MSI) in the latter period, i.e. it notably exceeds that of the GHG and may be compared with the observed global temperature rise of 1.0 +/- 0.1 K during the past 60 years. Our data also suggest a delayed response of the mean global temperature to the loss of sea ice with a time constant of approximately 20 years. The validity of the theoretical model and the interrelation between GHG-warming and MSI-effect are discussed.
Acoustic emission (AE) characterization is an effective technique to indirectly capture the progressive failure process of the brittle rock. In previous studies, both the experiment and numerical simulation were adopted to investigate AE characteristics of the brittle rock. However, as the most popular numerical model, the moment tensor model (MTM) did not reproduce the monitoring and analyzing manner of AE signals from the physical experiment. Consequently, its result could not be constrained by the experimental result. It is thus necessary to evaluate the consistency and compatibility between the experiment and MTM. To fulfill this, we developed a particle-velocity-based model (PVBM) which enabled directly monitor and analyze the particle velocity in the numerical model and had good robustness. The PVBM imitated the actual experiment and could fill in gaps between the experiment and MTM. AE experiments of Marine shale under uniaxial compression were carried out, of which results were simulated by MTM. In general, the variation trend of the experimental result could be presented by MTM. Nevertheless, magnitudes of AE parameters by MTM presented notable differences with more than several orders compared with those by the experiment. We sequentially used PVBM as a proxy to analyze these discrepancies quantitatively and make a systematical evaluation on AE characterization of brittle rocks from the experiment to numerical simulation, considering the influence of wave reflection, energy geometrical diffusion, viscous attenuation, particle size as well as progressive deterioration of rock material. It was suggested that only the combination of MTM and PVBM could reasonably and accurately acquire AE characteristics of the actual AE experiment of brittle rocks by making full use of their respective advantages.
The enhanced oil recovery technique of low-salinity (LS) water flooding is a topic of substantial interest in the petroleum industry. Studies have shown that LS brine injection can increase oil production relative to conventional high-salinity (HS) brine injection, but contradictory results have also been reported and an understanding of the underlying mechanisms remains elusive. We have recently developed a steady-state pore network model to simulate oil recovery by LS brine injection in uniformly wetted pore structures (Watson et al., Transp. Porous Med. 118, 201-223, 2017). We extend this approach here to investigate the low-salinity effect (LSE) in heterogeneously wetted media. We couple a model of capillary force-driven fluid displacement to a novel tracer algorithm and track the salinity front in the pore network as oil and HS brine are displaced by injected LS brine. The wettability of the pore structure is modified in regions where water salinity falls below a critical threshold, and simulations show that this can have significant consequences for oil recovery. For networks that contain spanning clusters of both water-wet and oil-wet (OW) pores prior to flooding, our results demonstrate that the OW pores contain the only viable source of incremental oil recovery by LS brine injection. Moreover, we show that a LS-induced increase in microscopic sweep efficiency in the OW pore fraction is a necessary, but not sufficient, condition to guarantee additional oil production. Simulations suggest that the fraction of OW pores in the network, the average network connectivity and the initial HS brine saturation are key factors that can determine the extent of any improvement in oil recovery in heterogeneously wetted networks following LS brine injection. This study highlights that the mechanisms of the LSE can be markedly different in uniformly wetted and non-uniformly wetted porous media.
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