粘性の異なる破砕流体による水圧破砕の粒状体個別要素法解析

Field experiences suggest significant interaction between natural fractures and hydraulic fractures in unconventional reservoir and some tight gas sandstone. Microseismic monitoring is a widely used methodology to understand the real-time evolution of fracture geometry during hydraulic fracturing, which is essential for evaluating the SRV under certain treatment, especially in unconventional reservoir. Howerver, the correlation between the real fracture geometry and microseismic monitoring results is still open to discuss. Therefore, understanding the relationships between the induced fractures and acoustic emission in different laboratory conditions is of high importance for studying the mechanism of hydraulic fracturing and ultimately optimizing fracture complexity. A large polyaxial testing system was built for investigating the factors of inducing complex fractures in sandstones, shale and coal for hydraulic fracturing treatment. This paper presents the experimental results of a study conducted to understand the mechanism of fracture initiation and propagation based on acoustic emission monitoring. Large-scale test was conducted in dry sandstone outcrop of 762×762×914mm to simulate the hydraulic fracturing. True triaxial stresses were applied to the specimen. The acoustic emission events associated with fracture propagation or failure process during injection were monitored to provide further data to characterize the fracture. The hydraulic fractures and oil penetration surface were observed after injection. Results show that the acoustic emission monitoring is in good accordance with the split block surface dyed red. AE localizations clearly demonstrate fracture initiation and propagation dynamics in the sample which can be used to control the fracture. The AE rates increased with opening hydraulically induced fracture and its closure. The real-time fracture geometry can be calculated by integrating the borehole pressure, AE localizations, injection rate, fluid leakage and rock mechanical properties. We are currently investigating various ways for building the fracture propagation model. The work presented here a new hydraulic fracturing testing system integrated with geophysical monitoring and processing technologies that will improve laboratory study of hydraulic fracturing mechanism and provide a trial on new ideas and technology

Response characteristics of coal subjected to hydraulic fracturing: An evaluation based on real-time monitoring of borehole strain and acoustic emission

A probabilistic analysis of acoustic emission events and associated energy release during formation of shear and tensile cracks in cementitious materials under uniaxial compression

This paper describes the localization of deformation acceleration in the period prior to dynamic failure in hornblende schist rock under triaxial compression using acoustic emission (AE) monitoring. Rather than stabilize the failure process by controlling axial stress to maintain a constant rate of AE (for monitoring AE hypocenters) as in previous works [e.g., Lockner et al, 1991], we have instead developed a rapid multichannel data collection system. This enables us to elucidate the dynamics of fault nucleation under condition of constant stress (creep) loading, which is a better approximation to low strain rate condition in the Earth and allows both quasi‐static and dynamic crack growth to occur. The waveforms of more than 8000 AE events which occurred mainly during a 15 s period were recorded on 32 channels, with a sampling rate of 50 ns and mask time of 200 μs. Hypocentral locations of AE sources revealed that the fault initiated at one end of the core and then propagated into the unfaulted rock with a process zone (fault front) of intense cracking. We found that there were two different processes operating during the quasi‐static nucleation of a shear fault, namely, a process zone in front of the fault tip and a “wake” of damage zone following the process zone. The process zone had the following features: (1) major tensile cracking, (2) low b value and fewer larger events, and (3) strong self‐excitation. The mechanism of crack interaction and fault growth was, therefore, a mutual enhancement on dilatation due to tensile cracking. On the other hand, the damage zone was characterized by (1) major shear cracking, (2) low b value and more larger events, and (3) weak self‐excitation, indicating that in the damage zone, following the development of a shear fault, linkage between cracks became the major mechanism of crack interaction and fault development. The mutual changes of b value and self‐exciting strength observed in our experiments seem to occur as a result of the hierarchy of fault growth, which was not observed under slowed down loading conditions. Therefore our experimental results, under a realistic approximation of the dynamic condition of the Earth, are meaningful for the problems of earthquakes as well as rock bursts.

ABSTRACT: Hydraulic fracture trajectory near the wellbore directly influences the efficiency of fluid flow into and out of the well. Modeling the near-wellbore hydraulic fracture behaviors has been challenging due to the combined impacts of wellbore perforation placement, stress concentration around the wellbore and perforation tunnels, in-situ stress conditions, as well as pre-existing fractures in the subsurface that interact with hydraulic fractures. In this paper, we have built numerical models to take into account the aforementioned factors and investigate near-wellbore fracture growth and fluid pressure characteristics. The numerical models use a distinct element method (DEM) approach that explicitly represents the well casing, cement sheath, and perforation tunnels, with natural fractures represented by Smooth Joint Model. The results show that natural fractures intersecting the perforation tunnels at a favorable angle with respect to in-situ stress directions can facilitate fracture initiation in hard rocks. Multiple initiation points may occur, while one fracture becomes dominant at a later time in the simulated case. In addition, the simulation results show the termination of hydraulic fracture growth in the highly fractured zone, as well as hydraulic fractures crossing natural fractures that have small effective sizes at the interaction location. This study also demonstrates the efficacy of the near-wellbore model to represent detailed wellbore elements, consider complex discrete fracture networks, and capture the fundamental mechanisms that influence near-wellbore hydraulic fracture behaviors. 1. INTRODUCTION The trajectory of hydraulic fractures near the wellbore directly influences the efficiency of fluid flow into and out of the well, making it a crucial factor that can impact several critical aspects in unconventional reservoirs and enhanced geothermal systems, such as reservoir connectivity, fluid circulation, proppant transportation, and energy production efficiency. Modeling near-wellbore fracture initiation and propagation has been challenging due to complex influencing factors that need to be considered, including the casing-cement sheath-perforation tunnel-rock structure, stress concentrations resulting from the wellbore and perforation tunnels, interference, competition, and merging of hydraulic fractures, interactions within the hydraulic discrete fracture network, and in-situ stress conditions. A model must span multiple scales to accurately capture different fracture behaviors, from fracture initiation along perforation tunnels at a millimeter scale to fracture propagation near the wellbore at a meter scale. In this study, a numerical model was established to investigate hydraulic fracture behaviors in the near-wellbore region. Utilizing a distinct element method (DEM) lattice approach, we analyzed hydraulic fracture initiation and propagation behaviors under the combined impacts of wellbore perforation and discrete fracture network (DFN). The model is based on hydraulic fracturing tests conducted at the Utah FORGE Site. Well casing, cement sheath, and perforation tunnels were explicitly represented in the simulation, and fluid pressure characteristics and corresponding fracture behaviors are discussed in detail.

Investigation of fracture modes in pervious asphalt under splitting and compression based on acoustic emission monitoring

ABSTRACT: Passive seismic monitoring can assist in mapping the generated hydraulic fracture network and determining the fracture source mechanisms during fluid stimulation operations. While this is the most applied technique that provides critical information, it has certain limitations which can influence its capability to estimate the efficiency of the stimulation operation. This study involved the HF of true-triaxially loaded low permeability Barre granite cubes with combined real-time active (ultrasonic) and passive (acoustic emission) monitoring using high viscosity gear oil with the injection rate of 0.1 ml/min. After data filtering, acoustic emission (AE) monitoring successfully mapped the fluid induced damage and its evolution in the brittle rocks mass. Different attributes of the continuous ultrasonic signals (velocity and energy) were analyzed in combination with the detected micro-seismic events which highlighted the difference between fluid permeated and non-fluid permeated fracturing along with various fracturing processes induced due to pressurized fluid injection. These results indicate that utilizing different acoustic techniques together has the potential to better characterize the fluid induced damage in brittle rocks. 1. INTRODUCTION While hydraulic fracturing (HF) is a widely employed technique, it is difficult to predict the HF growth and the underlying mechanisms (Zhuang and Zang, 2021). AE or seismic monitoring at the laboratory or field scales, respectively, is a passive acoustic monitoring technique that has been the most effective and robust technique available for monitoring the fluid induced damage in brittle rocks (Stanchits et al., 2015). This method can record real-time micro-seismic data, which can be utilized not only to map the generated HF but can also be processed to determine the underlying source mechanisms in both field and laboratory scales (Hampton et al., 2018). This detailed analysis of the detected micro-seismicity can greatly assist in determining the efficiency of the stimulation operation (Maxwell, 2014). However, detected micro-seismicity only represents one component of the geo-mechanical response to HF. Micro-seismicity can occur both due to the localized fluid pressure and stress perturbations causing inelastic micro-damage surrounding the propagating fracture (Maxwell et al., 2015). Micro-seismicity will be generated surrounding the injection borehole, even if the fluid is prohibited from entering the surrounding rock, for example through sleeve fracturing (Ishida et al, 2004). Therefore, it may be difficult to distinguish between the fluid permeated fracturing, and non-fluid permeated fracturing. This is a crucial piece of information required for an accurate estimation of stimulated reservoir volume and efficient design of the deep underground stimulation operation.

Hydraulic fracturing mechanisms in coal differ in many aspects from those in inorganic rocks. Due to the lower Young's modulus and higher Poisson's ratio, the stress state in coal is higher than in adjacent formations. This leads to wider and shorter fractures than in inorganic rocks. Fractures are affected by the cleat system in coal, so that multiple fractures may follow in tortuous paths the coal cleats. Coal often has a fracture treatment pressure in excess of 22.6 kPa/m (1 psi/ft.). Several hydraulic fracturing mechanisms may account for that, namely fracture blocking by coal chips, fracture tip plugging by coal or the development of a complex multi fracture system. The orientation of the hydraulic fractures is predominantly horizontal at shallow depths ( 600 m. In between these depths, fracture orientation may be either horizontal or vertical. This paper gives a comprehensive overview of the proposed hydraulic fracturing mechanisms in coal. [Received: July 28, 2015; Accepted: January 14, 2016]

Hydraulic fracturing is a crucial technology for resource development, such as shale gas, and its optimization is necessary to enhance development efficiency. However, evaluating fracture shapes involves technical uncertainties. Japan Organization for Metals and Energy Security (JOGMEC) and Kyoto University have conducted laboratory-scale hydraulic fracturing experiments using coagulable fluorescent resin as the injection fluid (resin fracturing test) to visualize hydraulic fractures and investigate their relationship with acoustic emissions (AEs) generated during fracturing. Since lab-scale experiments can only examine the phenomena near the injection hole owing to size limitations, we designed an experiment to apply the visualization method to the outcrop scale. This paper presents the results from an in situ, outcrop-scale hydraulic fracturing experiment conducted at the Kamioka Mine, Gifu Prefecture, Japan, from 2022 to 2023, with goals similar to those of the laboratory experiments. A resin fracturing borehole (RF1) with a diameter of 76 mm was core-drilled to a depth of approximately 10 m for the resin fracturing tests. AEs were observed in five boreholes drilled around RF1 at the same depth. Resin fracturing tests were performed at two different depths, with breakdown confirmed at both. A core of a larger diameter (205 mm) was recovered by coaxial overcoring around RF1, and resin-filled fractures were observed under black light on the core surfaces. After the resin fracturing experiment, two analyses were performed using the acquired core and AE data to predict the fracture extension and the mechanism of AE occurrence. We compared the distribution of AE events and visualized fractures in the core. Additionally, we compared the stress direction estimated from failure mechanism analysis using AE data with the maximum stress direction estimated from hydraulic fracturing. Our analysis provided several insights into fracture extension. The distribution of AE hypocenters was consistent with the direction of the hydraulic fractures confirmed by coring after the resin fracturing test. The failure mechanisms are classified based on the polarity of the first P-wave motion of the AE waveform. However, the actual scale of oil fields is significantly larger than that considered in this study. Discussing visible fractures created by hydraulic fracturing is deemed meaningful. We expect that the results of this study will provide valuable information for the precise estimation of hydraulic fractures.

This research investigates the mechanical behavior and damage evolution in cross-ply basalt fiber composites subjected to different loading modes. A modified Arcan rig for simultaneous acoustic emission (AE) monitoring was designed and manufactured to apply quasi-isotropic shear, combined tensile and shear loading, and pure tensile loading on specimens with a central notch. Digital image correlation (DIC) was applied for high-resolution strain measurements. The measured failure strengths of the bio-composite specimens under different loading angles are presented. The different competing failure mechanisms that contribute to the local reduction in stress concentration are described. Different damage mechanisms trigger elastic waves in the composite, with distinct AE signatures that closely follow the sequence of fracture mechanisms. AE monitoring is employed to capture signals associated with structural damage initiation and progression. The characteristic parameters of AE signals are correlated with crack modes and damage mechanisms. The evolution of AE parameters during the peak load transition is presented, which enables the timely AE detection of the maximum load transition. The combination of DIC and AE monitoring improves understanding of the mechanical response and failure mechanisms in cross-ply basalt fiber composites, offering valuable insights for possible performance monitoring and structural reliability in diverse engineering applications.

The present paper deals with the acoustic emission (AE) monitoring of fracture behavior of repaired marble specimens. Different types of specimens were ultrasonically interrogated. Subsequently, damage was induced to these specimens by three-point bending. The damaged specimens were repaired using a suitable epoxy agent; then they were mechanically loaded again. Apart from the well-known correlation of pulse velocity to strength for building materials, which also holds for the materials used in this study, AE provides a unique insight in the fracture behavior of the specimens. A statistical analysis of the experimental data has been performed to investigate the correlation between AE parameters and the strength of the specimens. This work discusses the passive monitoring of fracture in repaired marble specimens and shows that AE parameters, well-known to successfully characterize cementitious materials, also provide satisfactory results in characterizing monolithic materials such as marble. It is concluded that AE monitoring during a proof loading can provide good insight information of the materials and characterize their restoration.

first of all, this paper intensive studies the hydraulic fracturing mechanism of heterogeneous material resultant from the seepage – damage coupling, based on which, the coupled seepage and damage effects during hydraulic fracturing is investigated, and the difficulties of hydraulic fracturing study are how to determine failure modes and breakdown pressure. By using numerically testing method, the mechanical mechanism of hydraulic fracturing under asymmetric distribution of pore pressure with three holes, including fracture patterns, stress flied distribution, initial fracturing pressure and breakdown pressure, was studied by using theFSDmodel. According to the results of this research, crack extension direction of hydraulic fracturing is influenced not only by the local pore pressure around crack tip, but also by the gradient distribution of macro pore water pressure. The fracturing direction always propagates towards the regions of higher local pore pressure and breakdown pressure decreases with the increasing of local pore pressure. These results are well aggress with the results in the experiments. This research has important theoretical and engineering value. In engineering practice, we can use the porous asymmetric hydraulic fracturing technology to control the direction of crack propagation. Keywords: hydraulic fracturing; numerical simulation; heterogeneous material; asymmetrical

Identification and prediction of fatigue crack growth under different stress ratios using acoustic emission data

Advancements in the technique for early estimation of the hydraulic fracturing potential of fracturing fluid in different shale formations are needed to successfully stimulate reservoir volume and recover the trap hydrocarbons. The determination of fracture initiation and propagation in shale remains unclear, particularly with regard to the choice of type of fracturing fluid during fracturing operation at high confinement. Hydraulic fracturing with supercritical carbon dioxide (SC CO2) and SC CO2 foam are an encouraging technique to overcome significant use of water in shale gas production. Our study was carried out to explore the fracture propagation and fracture initiation pressure under high confinement stress. The hydraulic fracturing experiments are performed to study fracture propagation in the black shale of three different fields to generalize the scope of work using low, medium and high brittle shale: Eagle Ford, Wolfcamp and Mancos. Three different fracturing fluids were selected for fracturing tests: SC CO2, SC CO2 foam and slick water to investigate the impact of low, medium and high viscous fluids. We evaluated the fracture pressure curve, breakdown pressure (fracture initiation pressure), fracture network variation and the impact of bedding angle, perforation length, injection mediums and formation variations on fracture morphology. During hydraulic fracturing with slick water a consistent rise in the injection pressure is recorded whereas sudden fluctuations are recoded with the injection of SC CO2. With the injection of SC CO2 foam, a sudden decrease in injection pressure appears due to imbibition of SC CO2 foam in the pores matrix. Moreover, fracture initiation pressures in shale rocks varied by changing bedding angle and perforation length. Fracture initiation pressure increases with bedding angles. An appreciable difference in fracture initiation pressure with SC CO2, SC CO2 foam and slick water are 5023psi, 6456psi and 6168psi, respectively at high conferment pressure (3500psi). Comparison of hydraulic fracturing of Eagle Ford, Wolfcamp and Mancos shale with different injection medium shows that SC CO2 foam produced complex fracture networks with high aperture and length for parallel, inclined and perpendicular perforations along the bedding. With all types of shale, SC CO2 foam injection has produced dense fracture network. Thus, fracturing with SC CO2 foam can potentially enhance the stimulated reservoir volume.

In the use of metals, due to industrial advances and the application of more dynamic loads, it is necessary to pay more attention to the fatigue issue. Structural health monitoring (SHM) system is a method of evaluating and monitoring structural health. It has been widely applied in various engineering sectors due to its ability to respond to adverse structural changes, improving structural reliability and life cycle management. Non-destructive inspection methods are used to condition and health monitoring structures at the time of production and even during the service life of parts. Among non-destructive methods, the acoustic emission method has become a standard and reliable method in recent years. The stimulated internal energy of the structure is received in this acoustic emission technique as health monitoring features. A dominating attribute of the acoustic emission technique is its application ability in its loading condition. Therefore, it provides instant damage information within a short period of time. Thus, acoustic emission monitoring tests are often performed in the operating conditions of the structure. Acoustic emissions can inform us of the changes that occur before the final failure and prevent much financial and human damage. In this project, the characteristics of acoustic emission in the fatigue crack growth of aluminum alloy 2025 for online structural monitoring have been investigated and determined. Acoustic emission tests have been performed in two parts: bending fatigue test with the aim of initiation of fatigue cracks in aluminum alloy 2025 specimens and following tensile tests with the aim of growth of fatigue cracks. The acoustic emission signals and parameters sent by the acoustic emission sensor during both tests were received and recorded by the acoustic emission software. According to the received acoustic emission information, various diagrams are plotted. Analyzing the results from online acoustic emission monitoring showed the acoustic emission method can be considered a suitable and reliable technique for detecting crack initiation and crack growth in aluminum alloy 2025.