Springer Science and Business Media LLC

The renewable energy is the best energy potential to exploit, because they are economic, not pollutant and permanent. As a kind of renewable energy, geothermic which becomes more and more widely used in this field. In a geological setting regional on the effectiveness of the process solar thermal, offering a greater supply of geothermal energy, the study of Ghardaia’s case are based on data of soil temperature and especially using local meteorological data, Accurate estimates of mean daily soil temperature (MDST) are needed. In this study, we will use the capability of Gaussian process regression (GPR) for modeling MDST using 3 years of measurement (2005–2008), in a semi-arid climate. It was found that GPR-model based on mean air temperature as input, give accurate results in term of mean absolute bias error, root mean square error, relative square error, and correlation coefficient. The obtained values of these indicators are 0.0021, 0.5036, 0.0029 and 100 %, respectively, which shows that GPR is highly qualified for MDST estimation in semi-arid climate.

Dynamic failure widely exists in rock engineering, such as excavation, blasting, and rockburst. However, the quantitative measurement of the dynamic damage process using experimental methods remains a challenge. In this study, a SHPB modeling technique is established based on Voronoi-based DDA to study the damage evolution of Fangshan granite under dynamic loading. The assessment of cracking along the artificial joints among Voronoi sub-blocks is conducted using the modified contact constitutive law. A calibration procedure has been implemented to investigate the rock dynamic properties quantitatively. The dispersion and damping effect can be effectively eliminated by regular discretization in SHPB bars, based on which the dynamic stress equilibrium can be satisfied. To reproduce the loading rate effect of the dynamic compressive strength, which has been observed in the experiment, a modification strategy considering the influence of the rate effect on the strength meso-parameters is proposed. Using this strategy, the peak stresses of the transmitted waves predicted by DDA match well with those obtained from experiments conducted at different loading rates. The simulation results show that more microcracks are generated and the proportion of tensile cracks decreases as the loading rate increases. Furthermore, the dynamic mechanical behavior and fracturing process have also been discussed and compared with the experiments. The results show that the established SHPB system is a powerful tool for quantitative analysis of rock dynamics problems and can handle more complex problems in the future.

Underground chambers or tunnels often contain inclusions, the interface between the inclusion and the surrounding rock is not always perfect, which influences stress wave propagation. In this study, the imperfect interface and transient seismic wave were represented using the spring model and Ricker wavelet. Based on the wave function expansion method and Fourier transform, an analytical formula for the dynamic stress concentration factor (DSCF) for an elliptical inclusion with imperfect interfaces subjected to a plane SH-wave was determined. The theoretical solution was verified via numerical simulations using the LS-DYNA software, and the results were analyzed. The effects of the wave number (k), radial coordinate (ξ), stiffness parameter (β), and differences in material properties on the dynamic response were evaluated. The numerical results revealed that the maximum DSCF always occurred at both ends of the elliptical minor axis, and the transient DSCF was generally a factor of 2–3 greater than the steady-state DSCF. Changes in k and ξ led to variations in the DSCF value and spatial distribution, changes in β resulted only in variations in the DSCF value, and lower values of ωp and β led to a greater DSCF under the same parameter conditions. In addition, the differences in material properties between the medium and inclusion significantly affected the variation characteristics of the DSCF with k and ξ.

Conventional hydraulic fracturing techniques are often found problematic for extracting geothermal energy in hot dry rock (HDR). As an alternative, employing the less viscous gas to replace water as the fracturing fluid showed great potential for more effective fracturing of HDR. In this work, the failure behavior and mechanism of granite during gas fracturing under different confining pressures and gas injection rates are comprehensively examined. It is shown that the breakdown pressure increases with the increase of confining pressure, whereas higher gas injection rate can result in evident decrease of the breakdown pressure. As the confining pressure grows, the acoustic emission (AE) event increases rapidly, with much higher AE counts observed at high gas injection rates than at low injection rates. Comparatively, the AE energy decreases under high confining pressure, due probably to granite transitioning from brittle to ductile. It is interesting that the b-value of AE varies dramatically as the gas injection rate becomes higher with significant fluctuations, indicating the ratio of large fracture and small fracture changes drastically during gas fracturing. In addition, the length of the induced fractures decreases with the increase of confining pressure during gas fracturing, and the length and width of vertical fractures are evidently larger when at high gas injection rate. Last, a novel theoretical predictive model is proposed for estimating breakdown pressure during gas fracturing based on the average tensile stress criteria, which is featured by considering the effect of confining pressure and gas flow behaviors. The theoretical prediction agrees with the experimental results. The present study can provide valuable results for theoretical analysis and engineering applications of gas fracturing in stimulating the HDR reservoirs.

Hydraulic fracturing and waterflooding are both widely applied methods for improving the recovery of oil and gas resources. These methods have increasing commonality because many waterfloods are being carried out at high enough pressure to generate hydraulic fractures. Even so, it is challenging for engineers to make an optimal wellbore pressure design for layered and otherwise complex underground formations. An overly aggressive injection pressure may lead to uncontrollable fracture height growth into non-producing layers adjacent to the reservoir. In contrast, when using classical but highly simplified height growth models, the pressure limits can be far too conservative which may lead to lower recovery rates and inefficient use of resources invested in developing producing reservoirs. Therefore, it is necessary to investigate the mechanism of fracture height growth while considering the coupling effect from multiple dominated factors. This research contributes an experimental approach to evaluating the role of stresses, weak interfaces, and mechanical properties of a three-layer system in promoting or containing hydraulic fracture height growth from a central reservoir into neighboring barrier layers. In all cases, the experiments agree that the pressure required to induce substantial height growth exceeds the stress applied to the barrier layers and is far above classical predictions. Additionally, when the reservoir layer is softer than the barriers, the containment is sustained to even higher pressures than for layers with similar material properties. Finally, the experiments show that permeability of the barrier layer can induce a more sudden transition to uncontrolled height growth when fracture reaches the bedding interfaces. Hydraulic fracture height growth is mitigated by weak interfaces between layers. Unstable height growth typically requires fluid pressure to exceed the in-situ stress in the bounding layer(s). Contrasting layer stiffness and permeability often leads to further mitigation of height growth.

It is of great importance to investigate the effect of multiple loading rates on the crack propagation of brittle material such as concrete using acoustic emission because engineering structures are subjected to multiple loading conditions. Although material behaviour under single loading mode has been extensively studied, very limited research has been conducted to investigate the performance of brittle materials subjected to varying loading conditions. This paper presents an experimental study of the effects of single and multiple strain rates on cement mortar samples using acoustic emission. A total number of 81 concrete specimens were tested, in this 28 samples were tested in constant strain rate, whereas rest 53 samples were tested in multiple loading rates. Axial strain, lateral strain and acoustic emission counts were recorded continuously until the specimens had failed. The results showed that concrete behaves differently under multiple loading rates. Acoustic emission was in close agreement with the crack propagation and damage of concrete samples.