Study on Fracture Conductivity of Mahu Sandy Conglomerate Tight Oil Reservoir

Objectives/Scope: Effective development of tight oil and gas depends on the generation of artificial fractures, and continuous and efficient development of tight oil and gas requires the use of proppants to maintain the diversion effect of artificial fractures. At present, the microscopic mechanism of damage to fracture conductivity of sand conglomerate reservoir is not clear. Methods, Procedures, Process: Taking the sandstone conglomerate in Mahu sag as the research object, the experimental study on the fracture conductivity of the sandstone conglomerate in Mahu sag is carried out. First, the stress sensitivity analysis of the sandstone conglomerate is performed on a pore scale using a self-made permeability measurement method, then, the fracture width, pressure and flow rate are measured under the condition of fracture scale to analyze the change law of conductivity during fracturing fluid injection. Results, Observations, Conclusions: The results show that the permeability of gravel decreases with the increase of confining pressure, and the stress sensitive damage is strong. After a cyclic loading condition, permeability will not recover to the initial value, causing irreversible damage to the pore and roar. As the fracturing fluid continues to be injected, a large amount of proppant becomes embedded in the fracture and leads to a decrease in conductivity. The whole diversion curve can be divided into two stages. In the first stage, the diversion damage is great, and in the second stage, the diversion damage decreases somewhat. The damage of conductivity is closely related to the content of clay minerals.With the increase of clay mineral content, the conductivity damage rate increases rapidly, especially the existence of illite and Aimonite mixed beds can significantly improve the conductivity damage rate. Novel/Additive Information:The results provide a solution for the optimization of proppant concentration, the improvement of tight oil production and the study of gravel diversion damage mechanism in the Mahu area.

Multiscale Geomechanical Behavior of Fiber-Reinforced Cementitious Composites Under Cyclic Loading Conditions—A Review

As construction materials, cementitious composites such as cemented paste backfill (CPB), cemented soil, and concrete may be subjected to extreme dynamic loadings including impact, blast, and/or seismic loads during their service life. To improve mechanical performance under dynamic loadings, fiber reinforcement technique has been considered a promising approach and extensively used in practice. In this manuscript, a new perspective on the multiscale geomechanical behavior of fiber-reinforced cementitious composites (FRCC) is provided through a comprehensive review on the macroscale constitutive behavior and the associated mechanical properties, and microscale failure processes under cyclic tensile, shear, and compressive loading conditions. For the macroscale mechanical response, this review includes a detailed analysis of the state-of-the-art research in stress-strain behaviors including pre- and post-peak response and hysteretic behaviors. Moreover, the effects of pore water pressure on the dynamic response of soft FRCCs such as CPB are discussed. Furthermore, the link between microscale crack propagation (including the formation of the interfacial transition zone and fracture process zone) and damage accumulation is established for each type of cyclic loading condition. In addition, a critical discussion on the future development of fiber reinforcement is conducted as well. Therefore, this review not only offers guidance and references to the experimental investigation on the multiscale behavior of FRCCs under cyclic loadings, but also promotes the further development of fiber reinforcement techniques.

Effects of the Replacement Length of Concrete with ECC on the Cyclic Behavior of Reinforced Concrete Columns

This paper presents an experimental investigation on the effects of the replacement length of concrete with engineered cementitious composites (ECC) on the cyclic behavior of a reinforced concrete (RC) column. A conventional RC column specimen and two RC composite columns designed with ECC were fabricated. To investigate the cyclic behavior of each specimen, a series of cyclic loading tests was performed under a reversed cyclic loading condition with a constant axial load. Test results showed that ECC columns exhibited higher cyclic behavior in terms of load carrying capacity, ductility, and energy dissipation capacity compared to the RC column. It was also found that when applying ECC to the column specimen with a length of 3.6d or more, the energy dissipation capacity was greatly increased.

Cyclic deformation behavior of rare-earth containing magnesium alloys

Lightweighting in ground vehicles is considered as one of the most effective strategies to improve fuel economy and reduce anthropogenic environment-damaging, climate-changing and costly emissions. Magnesium (Mg) alloy, as a strategic ultra-lightweight metallic material, has recently drawn a considerable interest in the transportation industry to reduce the weight of vehicles due to their high strength-to-weight ratio, dimensional stability, and good machinability and recyclability. However, the hexagonal close-packed crystal (HCP) structure of Mg alloys gives only limited slip systems and develops sharp crystallographic textures associated with strong mechanical anisotropy and tension-compression yield asymmetry. For the vehicle components subjected to dynamic loading, such asymmetry could exert an unfavorable influence on the material performance. These problems could be tackled through texture modification via addition of rare-earth (RE) elements. These RE-Mg alloys possess relatively weak initial textures, which lead to improved ductility and strength, and a reduction of the tension-compression asymmetry present in the conventional wrought Mg alloys. Despite the fact that the addition of RE elements sheds some light on the alterations in the mechanical anisotropy and the tension-compression yield asymmetry, the potential advantage of such RE-Mg alloys as structural components under cyclic loading condition has not been well appreciated. Thus, the main objective of this dissertation was to explore the cyclic deformation behavior of RE-Mg alloys under varying strain amplitudes and strain ratios, and correlate the behavior to the microstructural change and crystallographic texture weakening in the RE-Mg alloys in different states (extruded and heat-treated). Unlike the RE-free Mg alloys, these alloys exhibited essentially cyclic stabilization and fairly symmetrical hysteresis loops due to the weaker texture and reduced twinning-detwinning activities. While these alloys had a lower cyclic strain hardening exponent than the RE-free extruded Mg alloys, it had a longer fatigue life which can also be described by the Coffin-Manson law and Basquin’s equation. Fatigue crack was observed to initiate from the specimen surface with some cleavage-like facets near the initiation site. Crack propagation was basically characterized by fatigue striations in conjunction with secondary cracks. A detailed analysis for understanding the obstructive role of the precipitate to twinning has been also presented.

Cyclic deformation behavior of rare-earth containing magnesium alloys

Lightweighting in ground vehicles is considered as one of the most effective strategies to improve fuel economy and reduce anthropogenic environment-damaging, climate-changing and costly emissions. Magnesium (Mg) alloy, as a strategic ultra-lightweight metallic material, has recently drawn a considerable interest in the transportation industry to reduce the weight of vehicles due to their high strength-to-weight ratio, dimensional stability, and good machinability and recyclability. However, the hexagonal close-packed crystal (HCP) structure of Mg alloys gives only limited slip systems and develops sharp crystallographic textures associated with strong mechanical anisotropy and tension-compression yield asymmetry. For the vehicle components subjected to dynamic loading, such asymmetry could exert an unfavorable influence on the material performance. These problems could be tackled through texture modification via addition of rare-earth (RE) elements. These RE-Mg alloys possess relatively weak initial textures, which lead to improved ductility and strength, and a reduction of the tension-compression asymmetry present in the conventional wrought Mg alloys. Despite the fact that the addition of RE elements sheds some light on the alterations in the mechanical anisotropy and the tension-compression yield asymmetry, the potential advantage of such RE-Mg alloys as structural components under cyclic loading condition has not been well appreciated. Thus, the main objective of this dissertation was to explore the cyclic deformation behavior of RE-Mg alloys under varying strain amplitudes and strain ratios, and correlate the behavior to the microstructural change and crystallographic texture weakening in the RE-Mg alloys in different states (extruded and heat-treated). Unlike the RE-free Mg alloys, these alloys exhibited essentially cyclic stabilization and fairly symmetrical hysteresis loops due to the weaker texture and reduced twinning-detwinning activities. While these alloys had a lower cyclic strain hardening exponent than the RE-free extruded Mg alloys, it had a longer fatigue life which can also be described by the Coffin-Manson law and Basquin’s equation. Fatigue crack was observed to initiate from the specimen surface with some cleavage-like facets near the initiation site. Crack propagation was basically characterized by fatigue striations in conjunction with secondary cracks. A detailed analysis for understanding the obstructive role of the precipitate to twinning has been also presented.

The influence of stress level on the ratcheting magnitude and progress rate in steel alloys under uniaxial loading conditions

Engineering materials in their service life undergo symmetric or asymmetric fatigue loading, which leads to fatigue damage in the material. Ratcheting damage is due to the application of mean stress under cyclic loading condition. From deformation behavior perspective, application of mean stress under stress-controlled fatigue loading gives rise to accumulation of plastic strain in the material. Ratcheting strain increases with an increase in applied mean stress and stress amplitude. In addition, ratcheting behavior will increase in cyclic damage with the rise in strain accumulation and it can be illustrated by a shift in the hysteresis loop towards large plastic strain amplitudes. This study focuses on the ratcheting behavior of different steel materials under uniaxial cyclic loading condition and suggests a suitable method to arrest ratcheting by loading the materials at zero ratcheting strain rate condition with specified mean stress and stress amplitudes. The three dimensional surface is created with stress amplitude, mean stress and ratcheting strain rate for different steel materials. This represents a graphical surface zone to study the ratcheting strain rates for various mean stress and stress amplitude combinations.

The influence of stress level on the ratcheting magnitude and progress rate in steel alloys under uniaxial loading conditions

Engineering materials in their service life undergo symmetric or asymmetric fatigue loading, which leads to fatigue damage in the material. Ratcheting damage is due to the application of mean stress under cyclic loading condition. From deformation behavior perspective, application of mean stress under stress-controlled fatigue loading gives rise to accumulation of plastic strain in the material. Ratcheting strain increases with an increase in applied mean stress and stress amplitude. In addition, ratcheting behavior will increase in cyclic damage with the rise in strain accumulation and it can be illustrated by a shift in the hysteresis loop towards large plastic strain amplitudes. This study focuses on the ratcheting behavior of different steel materials under uniaxial cyclic loading condition and suggests a suitable method to arrest ratcheting by loading the materials at zero ratcheting strain rate condition with specified mean stress and stress amplitudes. The three dimensional surface is created with stress amplitude, mean stress and ratcheting strain rate for different steel materials. This represents a graphical surface zone to study the ratcheting strain rates for various mean stress and stress amplitude combinations.

Research on Fracture and Energy Evolution of Rock Containing Natural Fractures under Cyclic Loading Condition

For rock engineering in cold regions, the naturally fractured rock is susceptible to repeated freeze-thaw (F-T) weathering, coupled fatigue conditions of freeze-thaw (F-T), and stress disturbance act on rock mass, which can lead to the instability of rock engineering and even occurrence of geological hazards. Knowledge of how natural fracture affects the overall fracture evolution of freeze-thawed rock is crucial to rock mass stability. Laboratory multilevel cyclic loading tests are conducted to reveal the fatigue behavior and energy evolution for naturally fractured marble, as well as the influence of natural fracture volume on fracture evolution. The test results show that the preexisting natural fracture impacts fatigue strength, lifetime, and energy dissipation. The dissipated energy is correlated to all kinds of natural fracture (i.e., opening-mode, closing-mode, and filling-mode), and it decreases with the increase of the total natural fracture volume. The dissipated energy presents a first slow and then faster pattern as the cycle number grows. Compared with newly formed cracks, the proportion of energy consumed by stimulating natural cracks is smaller.

Mechanical Behavior and Morphological Study of Polytetrafluoroethylene (PTFE) Composites under Static and Cyclic Loading Condition

The key goal of this study was to characterize polytetrafluoroethylene (PTFE) based composites with the addition of bronze particles and mineral fibers/particles. The addition of individual fillers was as follows: bronze—30–60 wt.%, glass fibers—15–25 wt.%, coke flakes—25 wt.% and graphite particles—5 wt.%. Both static and dynamic tests were performed and the obtained results were compared with the microscopic structure of the obtained fractures. The research showed that the addition of 60 wt.% bronze and other mineral fillers improved the values obtained in the static compression test and in the case of composites with 25 wt.% glass fibers the increase was about 60%. Fatigue tests have been performed for the compression-compression load up to 100,000 cycles. All tested composites show a significant increase in the modulus as compared to the values obtained in the static compression test. The highest increase in the modulus in the dynamic test was obtained for composites with 25 wt.% of glass fibers (increase by 85%) and 25 wt.% of coke flakes (increase by 77%), while the lowest result was obtained for the lowest content of bronze particles (decrease by 8%). Dynamic tests have shown that composites with “semi-spherical” particles are characterized by the longest service life and a slower fatigue crack propagation rate than in the case of the long glass fibers. In addition, studies have shown that particles with smaller sizes and more spherical shape have a higher ability to dissipate mechanical energy, which allows their use in friction nodes. On the other hand, composites with glass fiber and graphite particles can be successfully used in applications requiring high stiffness with low amplitude vibrations.