To study the influence of cyclic stress on the nonlinear behavior of saturated sandstone, the residual strain properties and energy dissipation characteristics of the sandstone under tiered cyclic loading were experimentally investigated. The axial/radial residual deformation and energy dissipation characteristics of sandstone at different cyclic stress stages were analyzed in detail. By combining the mathematical statistics, fluctuation coefficients of the residual strain and energy dissipation, and correlation coefficients of axial/radial residual strain and energy dissipation were defined to describe the process. It was determined that these newly defined physical variables were closely related to the elastic-plastic state (or instability failure state) of the rock.
Based on a self-developed triaxial seepage device, a new loading and unloading experimental method is proposed in this paper to eliminate sample variations. The results show that the strength of sandstone sample and the axial strain at failure increased with the increasing initial hydrostatic stress, but decreased with the increasing loading-unloading rate. Following the alternating loading-unloading test, the stress-strain curves of specimens advanced in wave form, the waves' volatility decreased and then increased. It is found that near the ultimate strength, volatility is the biggest, and the stability of waves' volatility increased along with the increasing initial hydrostatic stress. The similarity of stress-strain curves between the conventional loading-unloading tests and the alternating loading-unloading tests increased along with the increasing initial hydrostatic stress and the increasing initial velocity for the alternating loading-unloading method. Along with the increasing initial hydrostatic stress, the failure behaviour of the sandstone samples tested under loading-unloading methods changed from a tensile state to a tensile-shear coexisting state, and finally to a fully shear failure state. The degree of failure modes increased with the increasing loading-unloading rate.
Summary Foam has proved to be effective and economical in underbalanced operations (UBO) and is gaining wider applications in many areas. It provides the desired flexibility in controlling pressure profile and equivalent circulating density (ECD). However, the knowledge of rheology and hydraulics of polymer-thickened foams is still limited. This paper summarizes the significant effects of polymer on foam rheology and presents a hydraulic model that simulates aqueous and polymer-based foam flow in directional and horizontal wellbores. Experimental studies on the rheology of polymer-enhanced foam were conducted using a specially designed flow-through rotational viscometer and pipe viscometers with different concentrations of hydroxyethylcellulose (HEC) polymer. Correlations have been developed for rheological parameters of aqueous- and polymer-based drilling foams. On the basis of the experimental results of foam rheology and a steady-state momentum balance equation, a foam-flow hydraulics model was developed to predict pressure profile, ECD, foam velocity, and foam quality along a vertical/inclined/horizontal wellbore. For practical applications, a simulator has been developed and validated by experimental flow-loop data obtained from the Advanced Cuttings Transport Facility of Tulsa University Drilling Research Project. The effects of polymer concentration, backpressure, and wellbore trajectory on foam hydraulics were studied extensively using the simulator. Results show significant impact of polymer on foam hydraulics. When 0.5% volume to volume (v/v) HEC polymer is added to aqueous foam, bottomhole pressure (BHP) and foam density are significantly increased, while foam quality and velocity are greatly decreased. The polymer effects are more pronounced in vertical wells than in horizontal wells. Simulation results also indicate that it is possible to use foam to create a pressure profile within the narrow window between continuously changing pore-pressure and facture pressure gradients, which is not possible with conventional fluids. Those responsible for hydraulic optimization and well control in managed-pressure drilling/UBO where foam is used will find this paper useful for practical design applications.
An underground coal gasification (UCG) process is strongly exothermic, which will cause thermal damage on rock cap. We proposed a new thermal damage numerical model based on a two dimension particle flow code (PFC2D) to analyze the inception and extension of cracks on pre-cracked red sandstone, which were thermally treated at a temperature of 25~1000 °C. The results indicated that: (1) a thermal damage value DT obtained by extracting the thermal crack area of scanning electron microscope (SEM), which can be used as an indicator of the degree of thermal damage of the sandstone; (2) a thermal damage numerical model established by replacing the flat-joint model with the smooth-joint model based on the thermal damage value DT, this approach can properly simulate the mechanical behavior and failure patterns of sandstone; (3) the critical temperature for strength reduction was 750 °C. The peak strength increased as pre-treatment temperature increased from 25 to 750 °C and then decreased. The elastic modulus E1 decreased with the increasing thermal treatment temperature; (4) micro-scale cracks initiate from the tip of the prefabricated fissure, and expand along the direction of prefabricated fissure, finally developing into macroscopic fracture. This approach has the potential to enhance the predictive capability of modeling and presents a reliable model to simulate the mechanical behavior of thermally damaged sandstones, thereby offering a sound scientific basis for the utilization of space after UCG.
The deformation and fracture characteristics of shale in the Changning-Xingwen region were experimentally studied under triaxial cyclic loading with a controlled pore-water pressure. An RLW-2000M microcomputer-controlled coal-rock rheometer was used in the State key Laboratory of coal mine disaster dynamics and control in Chongqing University. These experimental results have indicated the following. (i) The shale softened after being saturated with water, while its failure strength decreased with the increase of axial strain. (ii) A complete cyclic loading–unloading process can be divided into four stages under the coupling action of axial cyclic loading and pore-water pressure; namely the slow or accelerated increasing of strain in the loading stage, and the slow or accelerated decreasing of strain in the unloading stage. (iii) The axial plastic deformation characteristics were similar when pore-water pressures were set to 2, 6 and 10 MPa. Nevertheless, the shale softened ostensibly and fatigue damage occurred during the circulation process when the pore-water pressure was set to 14 MPa. (iv) It has been observed that the mean strain and strain amplitude under axial cyclic are positively correlated with pore-water pressure, while the elastic modulus is negatively correlated with pore-water pressure. As the cycle progresses, the trends in these parameters vary, which indicates that the deformation and elastic characteristics of shale are controlled by pore-water pressure and cyclic loading conditions. (v) Evidenced via triaxial compression tests, it was predominantly shear failure that occurred in the shale specimens. In addition, axial cyclic loading caused the shale to generate complex secondary fractures, resulting in the specimens cracking along the bedding plane due to the effect of pore-water pressure. This study provides valuable insight into the understanding of the deformation and failure mechanisms of shale under complicated stress conditions.
Elucidating and understanding the dynamic fracture characteristics of rocks play an essential role in the application of rock engineering and geophysics. In this study, based on a self-developed dynamic damage model, a rock notched semi-circle bend test with the Split Hopkinson Pressure Bar technique is numerically simulated. The study focuses on three aspects including damage evolution, energy evolution, and failure mode of rock under different loading velocities. From the simulated results, the following conclusions can be conducted: 1) the damage range increases gradually with the increase of loading velocity; 2) the crack propagates to the loading point along the symmetry axis of the samples under different loading velocities; 3) the loading velocity has an important influence on the failure mode of straight notch semi-circular marble, whose mechanism can be explained by that the local high strain rate leads to the obvious randomness and uncertainty of crack activation in rock; and 4) the energy evolution of notched semi-circle bend is vitally affected by loading velocity, and the deformation and the failure process of straight notch semicircular marble under dynamic loading can be divided into five stages according to the ratio of internal energy to total energy. The beneficial findings may provide some references in practice design from engineering problems.
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