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To understand the effects of laminar structure on fracture propagation and proppant transportation intuitively, an improved true triaxial fracturing device with a proppant pumping unit was used to carry out sand-laden fracturing on shale oil reservoir samples with multiple lithological-combination and different laminar structures. Based on high-precision CT scanning technology and acoustic emission (AE) monitoring technology, the propagation mechanism of hydraulic fractures (HFs) and proppant transportation characteristics were analyzed, and the critical condition for lamina slip was proposed. The results show that laminas with initial width tend to be activated by fracturing fluid, resulting in diversion or offset. Closed laminas tend to be penetrated by HFs and are hardly activated by fracturing fluid. Rock with dense initial width laminas tends to form "#" shaped fractures interwoven with activated laminas and vertical fractures. In contrast, rock with closed laminas tends to form simple fractures dominated by vertical HFs. The width of HFs varies greatly from the perforation layer to the neighboring layer. As the difference in tensile strength between the interlayer and the perforated layer increases, the degree of decline in HF width significantly increases. Intensive AE activity was monitored at the intersection of vertical HFs and activated laminas, indicating that decreased fracture width causes proppants to bridge and block at the diversion and offset. Therefore, most proppants are filled in wide fractures near perforation, blocking the diversion and offset; there is almost no proppant in activated laminas. Reducing proppant diameter is conducive to placing the proppant in the activated laminas and interlayer HFs. Compared with placing 200 mesh and 120/140 mesh with similar fracture morphology samples, the proppant placement volume ratio of 400 mesh proppant placing samples increased by 7%. The findings significantly improve the scheme decision-making and parameter design of fracturing technology for thin interlayered shale oil reservoirs. [ABSTRACT FROM AUTHOR]

Exploring the adaptability of CO2 and water-based fracturing to shale oil reservoirs is important for efficiently developing shale oil reservoirs. This study conducted fracturing experiments and acoustic emission (AE) monitoring on the Jurassic continental shale. Based on high-precision computed tomography scanning technology, digital reconstruction analysis of fracture morphology was carried out to quantitatively evaluate the complexity of fractures and the stimulation reservoir volume (SRV). The results show that the fracturing ability of a single water-based fracturing fluid is limited. Low-viscosity fracturing fluid tends to activate thin layers and has limited fracture height. High-viscosity fracturing fluid tends to result in a wide and simple fracture. A combination injection of low-viscosity and high-viscosity water-based fracturing fluid can comprehensively utilize the advantages of low-viscosity and high-viscosity fracturing fluids, effectively improving the complexity of fractures. CO2 fracturing is adaptable to Jurassic shale. The breakdown pressure of the supercritical CO2 (SC-CO2) fracturing is low. Branch fractures form, and laminas activate during SC-CO2 fracturing due to its high diffusivity. Under high-temperature and high-pressure conditions, the aqueous solution formed by mixing CO2 with water can promote the formation of complex fractures. Compared with water-based fracturing fluid, the complexity of fractures and effective stimulation reservoir volume (ESRV) increased by 8.7% and 47.6%, respectively. There is a high correlation between SRV and ESRV, and the proportion of AE shear activity is also highly correlated with the complexity of fractures. The results are expected to provide better fracturing schemes and effectiveness for continental shale oil reservoirs. [ABSTRACT FROM AUTHOR]

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]

As a non-aqueous fracturing technology, carbon dioxide (CO 2 ) fracturing is eliciting increasing attention in the exploitation of tight oil/gas reservoirs. This study explored the growth behavior of supercritical CO 2 (SC-CO 2 )-induced fractures in layered tight sandstones through a series of laboratory fracturing experiments under triaxial stress states. Acoustic emission (AE) monitoring and computerized tomography scanning were performed to analyze the AE focal mechanisms and fracture geometry. The features of SC-CO 2 fracturing were discussed through a comparison with the features of x-linked guar and slickwater fracturing. The influences of horizontal differential stress and pumping rate during SC-CO 2 fracturing were also examined. Results showed that SC-CO 2 likely promoted the dilation or shearing of bedding planes and natural fractures, resulting in a complex fracture network even under a high horizontal differential stress. Statistical analysis of the P wave polarity of AE waveforms indicated that the AE events associated with the SC-CO 2 induced factures were dominated by shear failure. Most of the branching fractures induced by SC-CO 2 fracturing were extremely narrow that they impeded proppant transport. For SC-CO 2 fracturing, injection pressure (especially breakdown pressure) was low and likely dissipated as the growth of branching fractures. AE activities, indicating the fracture growth, more likely occurred when the injection pressure rose high (e.g., in tens of seconds before and after breakdown) or fluctuated remarkably. A large pumping rate was necessary to maintain high injection pressure which can open the branching fractures widely and persistently because of the extremely large leak-off rate of SC-CO 2 . [ABSTRACT FROM AUTHOR]

• CO 2 -gel fracturing was introduced to stimulate shale oil reservoirs. • AE monitoring was employed to reveal the failure mechanisms of CO 2 -gel fracturing. • The fracture-height enhancement mechanism of CO 2 -gel fracturing was elucidated. CO 2 -based fracturing was widely introduced to stimulate shale oil reservoirs for its multiple advantages. However, fracture height containment may become noticeable as the low-viscosity CO 2 used in the shale with massive mechanically weak bedding planes (BPs). To address this issue, a novel CO 2 -gel fracturing design is introduced in this study. The design consists of initially pumping gel to break through the BPs at the near-wellbore zone and then injecting supercritical CO 2 (Sc-CO 2) to further extend these gel-induced hydraulic fractures (HFs) and to reactivate far-wellbore BPs. In this paper, the design feasibility was demonstrated through laboratory multi-stage fracturing experiment results, and their comparisons to single-phase Sc-CO 2 , slickwater, and gel fracturing results. Computerized tomography scanning and acoustic emission monitoring were employed to reveal the fracture geometry and the tension, shear and compression failure mechanisms. Fracturing with low-viscosity Sc-CO 2 and slickwater encountered the problem of fracture height containment caused by high infiltration ability of these two fluids, which would lead to opening of wellbore-connected BPs dominated by tensile failure. High-viscosity gel fracturing can create multiple transverse HFs but with a few fractures on BPs. When CO 2 -gel fracturing fluid was used, the fracture height is enhanced and more far-wellbore BPs are activated mostly in slip. The isolation effect of gel on BPs can lower the leak-off amount of Sc-CO 2. Consequently, CO 2 -gel fracturing is relatively far to the leak-off dominated regime, which means more elastic energy of high-compressibility Sc-CO 2 could be attained for fracturing the reservoir rocks. CO 2 -gel fracturing can provide a useful alternative in improving the vertical fracture height during the fracturing treatment. [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]