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In recent years, hydraulic fracturing has emerged as an important technique for the development of unconventional tight gas and oil reservoirs, particularly shale reservoirs. Production from a tight reservoir requires hydraulic creation of a highly fractured network for significantly improving the hydraulic conductivity of the reservoir. Field observations have empirically indicated that fracture complexity tends to be higher in the case of higher rock brittleness which is defined as the combination of the Young's modulus and Poisson's ratio of a rock. The brittleness is considered as one of the most important mechanical rock properties of unconventional tight reservoirs. However, the physical relationship between fracture complexity and the brittleness remains unclear. In this study, a series of hydraulic fracturing simulations were performed using a flow-coupled discrete element method to gain insights into the effect of the brittleness on fracture propagation. The brittleness was adjusted under different combinations of Young's modulus and Poisson's ratio for four rock models. Considering mechanical rock properties and pre-existing fractures, the effective stress in the case of lower brittleness reduced to the tension only around the tip of the hydraulic and pre-existing fractures. On the contrary, the tensile region was more widely distributed along the pre-existing fracture in the case of higher brittleness. The effective tensile stress in a wider region should induce more microcracks, thereby enhancing the possibility of the hydraulic fracture branching off from the pre-existing fracture. These considerations indicate a positive correlation between the brittleness and the complexity of a hydraulically induced fracture system. [ABSTRACT FROM AUTHOR]

In-situ stress measurements were carried out under a 290m overburden in the Kamaishi mine in Iwate Prefecture of Japan. The geology of the measurement site consists of granite. The Hydraulic Fracturing (HF) technique and the Compact Conical-ended Borehole Overcoring (CCBO) technique were applied in the same horizontal borehole. The three-dimensional stress state as measured by CCBO is explained below. The directions of the maximum principal stress (σ1) and the intermediate principal stress (σ2) are almost horizontal. The direction of σ1 is perpendicular to the borehole axis, and the direction of σ2 is parallel to the borehole axis. The direction of the minimum principal stress (σ3) is approximately vertical. In this case, σ1/σ3 ratio is about 5 which is considered to be beyond the scope of the application of the HF technique. To observe the behavior of re-opening and shut-in under a large principal stress ratio, we tried to apply the HF test under these conditions. While, the re-opening pressure could not be evaluated, the shut-in pressure was observed. The results confirmed that σ3 estimated by HF is substantially equal to the vertical stress evaluated by CCBO. [ABSTRACT FROM AUTHOR]

Abstract: There are infinite number of small pores at inside of almost rocks, while the pore’s shape and the interconnectivity among the pores are dependent on the type of rocks. The existence of such pores brings special feature in mechanical behavior of rocks, different to cases of other materials. For the case when the pressure in a drilled borehole is raised by fluid injection into the borehole in hydraulic fracturing carried out for rock stress measurements, the fluid penetrates through the interconnected pores into a rock from the borehole wall, and the fluid penetration develops the change in pore pressure, i.e., fluid pressure in the pore. The developed pore pressure distribution causes a circumferential stress in compression around the borehole, where the mechanics to cause the additional stress is analogous to thermal stress. Furthermore, the pore pressure affects to loose strength in failure following the Terzaghi effective stress law. The point stress criterion can be applied to predict tensile failure of rock occurring in such complicated conditions. Then we should assume that the fracture initiation occurs when the maximum effective stress reaches the tensile strength of a rock at a point that is not on the borehole wall but is inside the rock. This approach allows us to interpret quantitatively strange phenomena associated with fluid permeation such that breakdown pressure, i.e., a critical borehole pressure to cause the tensile failure, increases with the rate of borehole pressurization. [Copyright &y& Elsevier]

The shale rock, with relatively low permeability and porosity, is usually challenging to be developed economically. Hydraulic fracturing is widely used as a stimulation technique for shale gas exploitation. Unlike tight gas reservoirs, shale gas reservoirs appear to show different characteristics under hydraulic fracturing, with a low flow-back rate leading to high productivity. This might be linked to micro-cracks generated by the clay swelling when fluid penetrates the rock, as shown in previous fracturing experiments. However, detailed failure patterns and associated permeability changes under different conditions (e.g., in-situ stress, pore pressure, and clay content) remain ambiguous. In this study, numerical simulations were adopted to address these issues. A solid model with cohesive elements based on the Finite element method was used to predict the generation of micro-cracks. Further, a permeability evaluation model considering the micro-cracks was applied to estimate the permeability changes. Our simulation results indicate that clay swelling can produce shear cracks. The number of shear cracks is substantially influenced by the swelling stress and clay contents. Tensile cracks, together with a complex fracture network, can be observed when the pore pressure or stress difference increases. The permeability increases significantly when a complex fracture network is formed. This study reveals that micro-cracks can be induced by the interaction between fracturing fluid and shale rock, which may help explain how low flow-back rate results in high productivity in shale gas reservoirs. • A numerical model including a solid model and permeability evaluation model has been developed. • Influences of different conditions on micro-cracks and permeability changes have been investigated. • A complex fracture network can be formed by fracturing, resulting in a significant increase in permeability. [ABSTRACT FROM AUTHOR]