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Hydraulic fracturing as a method for reservoir stimulation depends on the properties of the media that fracture propagates in it. Discontinuities in the media and their mechanical properties greatly affect the geometry and propagation of hydraulic fractures. In this research, the interaction between the hydraulic fractures with the media layers interface, fracture propagation pattern and termination in multi-layered media were investigated. The true tri-axial cell was utilized to conduct experimental tests on cube multi-layered samples with discontinuities. The tests were aimed to investigate propagation of fractures from soft to stiff, stiff to soft media and also the effect of elastic properties of rocks in hydraulic fracturing. Results showed that the condition of discontinuities (healed, open or filled) and elastic properties of the layers influences the geometry and propagation pattern of hydraulic fractures. In the block with the bounded interfaces, the fracture propagates and interacts with the interfaces, then penetrates in the adjacent layers. However, for the block with unbounded interfaces the fracture propagates from the borehole up to the interface, then after filling the interface with the fluid the new fracture will propagate in the adjacent blocks. In sample where the interface was filled, the fracture propagation was terminated and then the fluid started to leak off in the interface. The results also show when the fracture reaches the interface, the pressure increased immediately and more pressure is needed for fracture propagation across the interface. In comparison between the length and width of fractures in soft and stiff layers, the study displays that the fracture width and its penetration length in soft layers are greater than those in stiff layers.

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Abstract: Hydraulic fracturing as a method for reservoir stimulation depends on the properties of the media that fracture propagates in it. Discontinuities in the media and their mechanical properties greatly affect the geometry and propagation of hydraulic fractures. In this research, the interaction between the hydraulic fractures with the media layers interface, fracture propagation pattern and termination in multi-layered media were investigated. The true tri-axial cell was utilized to conduct experimental tests on cube multi-layered samples with discontinuities. The tests were aimed to investigate propagation of fractures from soft to stiff, stiff to soft media and also the effect of elastic properties of rocks in hydraulic fracturing. Results showed that the condition of discontinuities (healed, open or filled) and elastic properties of the layers influences the geometry and propagation pattern of hydraulic fractures. In the block with the bonded interfaces, the fracture propagates and interacts with the interfaces, then penetrates in the adjacent layers. However, for the block with unbonded interfaces the fracture propagates from the borehole up to the interface, then after filling the interface with the fluid the new fracture will propagate in the adjacent blocks. In sample where the interface was filled, the fracture propagation was terminated and then the fluid started to leak off in the interface. The results also show when the fracture reaches the interface, the pressure increased immediately and more pressure is needed for fracture propagation across the interface. In comparison between the length and width of fractures in soft and stiff layers, the study displays that the fracture width and its penetration length in soft layers are greater than those in stiff layers.

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A two-dimensional (2D) eXtended Finite Element Method (XFEM) simulation is presented for propagation of hydraulic fractures from wellbore that the minimum principal in-situ stress is in horizontal plane. One primary role of hydraulic fracturing is to provide a high conductivity pathway along which reservoir fluids can flow to the well. In this study, enriched element has been applied with a maximum principal stress damage criterion, for initiation and propagation of crack in Abaqus 6.12. The properties and input data for rock models were extracted experimentally from Ahwaz reservior- Bangestan wellbore specimens. The specifications of crack were studied by analyzing the rock model without any crack or flaw and under different condition, such as in-situ stresses, pore pressure and elastic modulus. The results show that the critical position of crack initiation is perpendicular to minimum principal in-situ stress and stress condition of borehole and by increasing the pore pressure to the rock models, the pressure of injection fluid decreases. The results show that the pressure of injection fluid decreases at initiation step to constant amplitude after crack propagation. These results are in close agreement with the theoretical data, so that our simple procedure is efficiency and flexible.

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From a statistical perspective, internal erosion and piping are from the main causes of failure in earth-rockfill dams. If these dams were located in a narrow valley, the steep slopes of the valley walls can cause increasing in stress transfer in the core. Therefore, the occurrence of hydraulic fracturing phenomenon in this kind of dams is more probable. Bidvaz dam is an earth-rockfill dam with a thin inclined clay core. The dam is located in the Northeast of Iran with a distance of twenty kilometers from the Esfrayen city. It has a height of 66 meters from the foundation and constructed in a narrow valley with a width of 40 meters on river bed and a wall slope of more than 60 degrees relative to horizontal direction. After about seven and a half years of starting first filling, a subsidence was observed at the upstream slope surface of this dam. The initial assessments, based on the data recorded in instruments which were installed inside the body and dam foundation, show at the lower level of the core and adjacent to left abutment, pore water pressure gradually has been increasing and finally reached to the reservoir water pressure, and at the same time effective stress with abnormal rate reduced to zero. These observations confirm the occurrence of internal erosion in the lower levels of the core adjacent to the left abutment. Due to the steep valley walls and noticeable difference of compressibility properties between the core and shell materials, it is expected occurring significant stress transfer in the core especially adjacent to the valley walls. Therefore, the hydraulic fracturing can be considered as a main cause initiating the process of internal erosion in this dam. The main objective of this paper is to assess the validity of this hypothesis. To achieve this purpose, this paper used a three-dimensional numerical model to simulate the behavior of the dam during construction and reservoir filling. This model has improved in the environment of a finite difference software, called FLAC3D. In the formulation of numerical model, the flow and mechanical equations have been solved simultaneously. The 3D model has been calibrated based on the recorded data from the instruments. With using a number valid suggested theoretical and empirical relationships, hydraulic fracturing potential have been calculated and the contour distribution of fracturing pressure at upstream side of the core has been presented. Also, the contour distributions of factor of safety against occurrence hydraulic fracturing phenomenon were determined for all of the suggested relationships at the upstream side of the core. The findings show that, as expected, the steep slopes of valley walls and the difference of the compressibility properties of the core and the shell materials caused significant stress transfer at lower parts of the core and adjacent to the valley walls. Moreover, the values of factors of safety against occurrence hydraulic fracturing phenomenon in upstream side of the core are less than unity near the walls. So, the hydraulic fracturing phenomenon is the one of the main causes initiating the process of internal erosion in the core.

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From oil and gas engineering point of view, one of the challenges in low permeable or damaged wells is improving the productivity. There are different methods to increase the productivity of low permeable wells and one of the most efficient one is hydraulic fracturing. In this study, two-dimensional modeling of hydraulic fracturing using finite element method and cohesive element approach through traction-separation law has been performed. This approach avoids the singularity in the crack tip and the cohesive zone fits naturally into the conventional finite element method. Hydraulic fracture is assumed to propagate in a poroelastic and permeable medium with a constant injection rate and under quasi-static conditions and the criterion for fracture initiation is quadratic nominal stress criterion. Also as a propagation criterion, Benzeggagh Kenane (BK) approach has been considered. Two types of elements have been implemented in the model which are 4-node bilinear displacement and pore pressure reduced integration and 6-node displacement and pore pressure two- dimensional cohesive element. Cohesive elements have three degrees of freedom that two of them are in X and Y directions and one of them is pore pressure. Mesh size in the near fracture region is small enough to consider the stress and pressure distribution efficiently and avoid any problem in convergence. Meantime, to decrease the computation cost the mesh size gradually increases from fracture area to the boundaries. Also, to increase the accuracy of the model, the time steps for fracture propagation is 0.01 second. In addition, the effect of fracturing fluid has been directly included in the model which means that the fluid pressure would be applied along the fracture without any simplifying assumption. To validate the model, the results have been compared with KGD approach. The results indicate that in the initial steps the pressure at the wellbore wall is high which decreases with time significantly and eventually it gets a steady and uniform trend. In other words, in the initial steps, the fluid pressure should be high enough to overcome the hoop stress around the wellbore and after some injection periods, the fracturing fluid pressure would reach the breakdown pressure and the fracture starts to initiate and propagate. It is clearly observed that increasing the injection rate would lead to faster propagation of hydraulic fracture and in the models with higher injection rate the fracture tends to grow in the propagation direction. This indirectly means that increasing the injection rate would affect both opening and length of the hydraulic fracture which can result in increasing the productivity. The results reveal that the peak of the normal effective stress profiles corresponds to the fracture tip position, where the fracture opening is zero,and the peak value equals the cohesive strength of the material,as expected.Moreover,with increasing thedistance from the fracture tip,the stress decreases rapidly and approaches the initial stress value. The way that Young’s modulus affects the overall characteristics of hydraulic fracture implies that higher Young’s modulus would lead to longer fractures. In other words, formations with higher Young’s modulus can be fractured easily but the opening of the hydraulic fracture would reduce at the same time. This also indirectly means that Young’s modulus would play an important role in the productivity.