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The propagation of multiple fractures plays a significant role in the effectiveness of hydraulic stimulation in shale oil reservoirs. Previous studies reported that the laminations and bedding interface in shale oil rocks could influence the propagation of single hydraulic fracture. However, the propagation mechanism of multiple fractures in such rocks is still unclear. Here we use a true-triaxial experimental system, together with the acoustic emission (AE) monitoring system, to investigate the propagation of multiple fractures in shale oil reservoir rocks. The results show that the fracture interference started at the initiation stage seriously affects the propagation of multiple fractures. More clusters per stage could aggravate the fracture interference near the wellhead. The laminations and bedding interfaces are the main causes of fracture interference and could hinder the height of hydraulic fractures. Shear-type AE event signals the generation of fracture interference caused by the slip of bedding interface and the deflection of fractures induced by the laminations. The mechanism of fracture interference not only lies in stress shadow but also in the changes in fluid pressure caused by the high permeability of laminations and bedding interface. The experimental results provide basic and detailed data for studying hydraulic fracturing in shale oil reservoirs. Highlights: True-triaxial experiment is conducted to investigate the propagation of multiple hydraulic fractures. Both the lamination and bedding interface influence multiple hydraulic fracture propagation. The AE monitoring indicates many shear-type fractures when fracture interference appears. [ABSTRACT FROM AUTHOR]

The matrix permeability of shale reservoirs is extremely low. Therefore, massive volume fracturing is needed to form a complex crack network and get adequate sufficient capacity during the well completion. After fracturing, the effective stimulated reservoir volume (ESRV) is vital for developing shale reservoirs, mainly determined by stimulated reservoir volume (SRV) and the increase in permeability. Microseismic monitoring is widely used in the field to describe the crack shape and determine the SRV, to evaluate the stimulation effect. However, no studies have been conducted on the relationship between microseismic parameters and permeability. Thereby, we conducted uniaxial compression tests on Longmaxi shale samples and measured their changes in porosity and permeability before and after loading combining the microseismic monitoring under a laboratory scale (acoustic emission (AE)). Results show that porosity has little influence on the permeability before and after loading, while the propagation and connection of cracks are the most critical factors. As the loading stress increases, the crack volume and sample connectivity both grow. Besides, for the Longmaxi shale, when the stress is loaded to 30~50% of uniaxial compressive strength (UCS), the cracks start to propagate steadily (dilation), the permeability begins to increase rapidly, and percolation occurs, which indicates that the dilation point is closely related to the percolation threshold. The AE rate and accumulative ringing number both increase when it is larger than the percolation threshold value. The variation of AE characteristics can be used to identify the percolation threshold. Finally, the graphic model including AE parameters, crack, and permeability evolution is established based on the experimental results, which could help us understand the relationship between microseismic parameters and permeability and provide a methodological basis for the ESRV evaluation in the field. [ABSTRACT FROM AUTHOR]

The study of the shale rocks' fracturing characteristics reveal general guidelines for hydraulic fracturing processes in shale reservoirs, and the microseismic information provides a direct reflection of the fracturing process. Acoustic emission (AE) testing of rocks in the laboratory can be used for the application of microseismic information acquired in the field. Therefore, uniaxial compression experiments were performed to study AE characteristics during the failure process in shale rocks. In this study, the number of AE hits, along with the energy, a-values and b-values, were used to characterise the AE data and to describe the rock's rupture behaviour. The testing results indicated that the AE rates and energy values increased markedly when slippage occurred and these parameters reflect the generation of micro-cracks in the rock. The b and a-value curves can be divided into four stages corresponding with the fracture processes. The changes in the b and a -values can indicate the distribution characteristics of cracks within the rock. Based on this theory, the AE characteristics from dry and water-saturated shale samples under uniaxial compression were compared to demonstrate that water diffuses pressure in the rock and promotes fracture network generation. The b-values in the elastic-yield stage increase along with the brittleness index, and the b-values in the post-peak stage can offset the limitations of brittleness indexes used in evaluations of reservoir fracability. Exploring the law of rupture for shale under pressure provides a foundation for microseismic-based quantitative evaluation of the hydraulic fracturing stimulation efforts in shale reservoirs; therefore, this knowledge has both scientific and engineering significance. [ABSTRACT FROM AUTHOR]

To understand the influence of fluid saturation on the failure mode of gas shale, a series of triaxial compression tests under the quasi-static loading conditions were conducted using the GCTS (Geotechnical Consulting and Testing System) under different conditions (dry, water saturation, and slickwater saturation). The results showed that the rupture of dry shale samples was accompanied by a transient release of energy, which formed into the main splitting crack. Simultaneously, high-energy tension-type acoustic emission (AE) events were detected around the peak stress. Shale saturated with fluid decreased the strength and increased deformation. The presence of a “step” disturbance was clear in the expansion stage before rupture, and low-energy shear rupture events were detected repeatedly. This indicates that there were internal microslips and distributed micro-cracks to form, which maintained the overall integrity of the cores. Based on the analysis above, two failure modes were discovered: transient main crack failure mode for dry shale and distributed micro-failure mode for shale saturated with fluid. The effect of fluid saturation on the failure mode of shale was clear. This effect can increase the pore pressure; reduce fracture-surface energy; enhance the softening effect caused by water-absorbing clay, which reduced the effective stress on crack surfaces; promoted rock shear slip inside the rock; and changed the energy release characteristic and fracture mode of the shale under a stress condition, which were conducive to the formation of a crack network. The experimental results showed that saturated slickwater had a stronger effect on the failure mode of brittleness gas shale than that of saturated water. [ABSTRACT FROM AUTHOR]

In laboratory rock mechanics experiments, it is common practice to employ single-component acoustic emission (AE) sensors to monitor the rock fracturing process, where the fracture location and fracturing mechanism can be inverted from the single-component AE waveforms. However, since such applications are often based on simplified isotropic hypotheses, accurately capturing the influence of strong anisotropy in rock on particle vibrations at rock surfaces using single-component AE sensors remains challenging. This limitation biases the actual particle vibration amplitude and direction, potentially leading to erroneous estimations in AE waveform inversion mof rock fracturing. Here, we combined three piezoelectric discs to create a three-component AE sensor, allowing us to capture particle vibration along three sensing directions on rock surfaces. Through two calibration experiments and polarization analysis of the three-component waveforms, we verify the new sensor's capability to reliably capture the vibration features of hypocenters that are not easily obtainable using a single-component sensor. We apply this new sensor in uniaxial compression tests on shale, a material known for its strong anisotropy. The results indicate that the AE behaviors monitored by the three-component sensor are consistent with those obtained from the widely used single-component sensor. Moreover, the polarization analysis of the three-component waveforms reveals the presence of coupled hypocenters associated with multiple fractures. This is the first time a three-component AE sensor is used in laboratory rock mechanics experiments, which offers a more comprehensive understanding of the dynamic evolution of fractures in rock formations. • A three-component AE sensor is applied in rock mechanics experiments for the first time. • Polarization analysis of three-component AE waveforms can distinguish P - and S -wave. • A single three-component AE sensor can pinpoint fracture locations in a rock sample. • Three-component AE waveforms can reveal complex rock fracturing processes. [ABSTRACT FROM AUTHOR]

An effective evaluation on fracture network forming capacity is key to the whole process of shale gas exploration. At present, neither a clear standard nor a generally accepted evaluation method exist. In this study, a novel “Soundless Cracking Agent (SCA) fracturing evaluation method” was developed. The fractures were characterized quantitatively using fractal dimension of the trace on the core surface and areal density. Acoustic emission (AE) location was used for dynamic monitoring and analysis. The results show that the fractal dimension can be used for quantitative evaluation of complexity of fracture network. The higher the rock hardness, the smaller the fracture density after fracturing is; the higher the brittleness, the larger the fracture density after fracturing is. The development degree of natural fracture systems and sedimentary bedding is a key factor to control the propagating morphology of fractures. The number of AE events for sandstones with low clay content (<25%) is huge, and there are obvious take-off spots for cumulative curves and frequency distribution curves. The AE events for sandstones are distributed along the main fractures, with simple planar fractures clearly present after fracturing. But for shale, the number of AE events is less, with no obvious take-off spots, and AE events are scatteredly distributed. The higher the clay content and the lower the quartz content, the smaller the number of AE events is, and the smaller the frequency and the sound source amplitude are. For sandstone, the number of AE events decreased by about 75% due to the increase of clay content by 20%. The new method enables a comprehensive reflection of the characteristics of rock hardness, brittleness and natural fractures system. This work is valuable for the evaluation of hydraulic fracturing effects in unconventional oil and gas reservoirs in the future. [ABSTRACT FROM AUTHOR]

In recent years, supercritical carbon dioxide (SC–CO 2) has been attempted in hydraulic fracturing in shale reservoirs as a new type of fracturing tool to overcome the drawback of traditional water-based fluids. Because of the unique physical and chemical properties of SC-CO 2 , its fracturing mechanism is more complicated than traditional fluids and still unclear at present. In this paper, we hydraulically fracture a shale reservoir rock using SC-CO 2 and monitor the fracturing process using acoustic emission (AE) data. The results show the fractures stimulated by SC-CO 2 composite of both shear and tensile fractures. In the initiation stage, SC-CO 2 activates the natural fractures around the wellbore and induces shear fractures. In the propagation stage, SC-CO 2 permeates the fracture tips quickly, results in a dynamic propagation process, and generates plenty of tensile fractures. The phase change of CO 2 could be observed during the fracture propagation process, which is accompanied by a rapid pressure change and local stress shock formations in the fractures. Additionally, the experiments also demonstrate that the existence of bedding structures in shale could constrain the propagation of fractures, thus leading to a smaller volume of fracture network and limiting the complexity of the generated fractures. This research may help understand the fracturing mechanism of SC-CO 2 and shed light on the development of hydraulic fracturing technology in shale reservoirs. [ABSTRACT FROM AUTHOR]

Understanding hydraulic fractures generation and geometric properties are significant for optimizing hydraulic fracturing treatment design which improves the ultimate production from shale reservoirs. The propagation of the hydraulic fractures is influenced by bedding planes. Simple opening or tensile crack model is not enough to meet the hydraulic fractures generation process, and the fault slipping model which is regarding as the cause of the natural earthquake is also not compatible with the actual physical meaning of fracking induced fractures. Hydraulic fractures consist of not only the opening but also slippage. Acoustic emission monitoring combined with the true triaxial hydraulic experiment provides direct information about the propagation and the geometric parameters of fractures. Moment tensor of acoustic emission is related to the mechanical mechanics of fractures. In this paper, the moment tensor interpretation is based on the shear-tensile crack model, then the orientation and the opening width of the hydraulic fractures are calculated. Computerized tomography scanning figures of the samples give support to the interpretation process. The distributions of hydraulic fractures orientations are different in the two samples with different normal stress applied on the bedding plane. Higher normal stress on the bedding plane led to a more complex fracture orientation. And the opening widths of the fractures in the far area from the wellbore are smaller than the fractures nearby the wellbore. These two parameters quantify the fractures geometric properties and provide information for adjusting the next step of the fracturing process. The findings of this study can help for better understanding of hydraulic fractures generation in the bedding shale. Shear-tensile crack model is supported by actual physical models has good adaptability in explaining the hydraulic fracture propagation process. • This paper used the acoustic emission to invert the fracture mechanism. The findings of this study can help for better understanding of hydraulic fractures generation in the bedding shale. • Shear-tensile crack model is supported by actual physical models has good adaptability in explaining the hydraulic fracture propagation process. • New parameters based on shear-tensile fractures model which extracted in the fracturing monitoring can be used for adjusting the next step of the fracturing process. [ABSTRACT FROM AUTHOR]