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In karstic-fractured carbonate reservoirs, most of the oil resources are hosted in vugs. Connecting as many vugs as possible by hydraulic fracturing is the key to achieving effective development. However, the interaction mechanism between vugs and hydraulic fractures is complicated and has not been fully revealed. In this study, both experimental and numerical simulations were implemented to investigate the interaction between vugs and hydraulic fractures. Key factors, such as vug size, horizontal stress difference, and the internal pressure of the vug, were considered. The results showed that the vug played an important role in the propagation of hydraulic fractures. Three interaction modes of vugs and hydraulic fractures were observed: crossing, arresting, and bypassing. Owing to the variation of the stress concentration existing around the vug, the hydraulic fracture could be arrested by a small vug but would bypass a vug with a larger size. Whether the hydraulic fracture could communicate with the vug was mainly controlled by the horizontal stress difference. Under large horizontal stress differences (≥20 MPa), the hydraulic fracture could cross and connect multiple vugs. The difference between the horizontal minimum stress and the internal pressure of the vug was also particularly significant for fracture propagation. The smaller the difference, the easier the fracture communicated with the vug. The above findings would be valuable and constructive for the optimal design of field hydraulic fracturing in karstic-fractured carbonate reservoirs. [ABSTRACT FROM AUTHOR]

• The unique mineral composition and distribution of carbonate-rich shale were revealed. • Full-scale pore structure and fractal features were characterized. • The comprehensive mechanical properties of carbonate-rich shale were analyzed. • The inner link between micro-structural and macro-mechanical properties was established. • Development strategies for carbonate-rich reservoirs were discussed. As a new growth point of shale gas production, carbonate-rich shale has recently attracted great interest. However, gas production from this reservoir was poor since the properties of the carbonate-rich shale have not been systematically studied. This is an urgent need to be addressed. Downhole cores collected from an exploration well were analyzed by micro-structure experiments (X-ray diffraction, mineral quantitative evaluation, low-pressure nitrogen adsorption, and mercury intrusion piezometry test) and macro-mechanical tests (triaxial compression, Brazilian splitting, and fracture toughness test) to obtain multiscale characterization of carbonate-rich shale. The intrinsic relationship between the microstructural and mechanical characteristics of this reservoir has been revealed. The results showed that (1) The content of carbonate minerals exceeds 30%, just slightly lower than that of quartz. Calcite has large particle sizes and is closely cemented with quartz to form a dense rock skeleton. (2) The maximum adsorption volume, specific surface area, total pore volume and average pore diameter values are poor, which is unfavorable for shale gas accumulation. Limited porosity and permeability and great tortuosity can be obstacles to the release of accumulated gas. (3) High peak deviatoric stress, Young's modulus, and Poisson's ratio were contributed by dense rock skeleton. (4) Narrow maximum main fracture width is not favorable to place proppants and further to create effective gas transport channels. (5) High tensile strength and fracture toughness enhance high breakdown and extension pressure. These comprehensive insights are of great significance for both the exploration and development of carbonate-rich shale reservoirs. [ABSTRACT FROM AUTHOR]

In the process of deep shale gas exploitation through horizontal drilling and large-scale multi-section hydraulic fracturing, the cement sheath can easily fail due to the high peak pressure and the frequent and violent fluctuations. In this paper, firstly, the deformation characteristics of the cement was studied based on the uniaxial and triaxial compression experiments at different temperature conditions. Then, mechanical model of cement sheath was set up. Based on Melan's shakedown theorem, shakedown load of the cement sheath under the periodic alternating loads is obtained and then converted to the internal pressure of the casing. Results shows that: (1) Compared with that at 25 °C, the compression resistance and elastic modulus of cement reduced significantly under 130 °C. The cement exhibits obvious plastic flow characteristics and can be regarded as a rational elastic-plastic material. (2) During the cyclic loading and unloading process, new plastic strains are continuously generated, the width of hysteresis loops is continuously reduced, and the hysteresis loop becomes denser, which indicates that the accumulation rate of plastic deformation is continuously decreasing. (3) The shakedown load is the smaller of the loading and unloading yield pressures, and reverse yielding tend to occur in the depths. (4) The shakedown load of the cement sheath has a positive linear and non-linear relationship with the cohesive force and the friction angle, respectively. (5) the shakedown load has a positive linear relationship with the external pressure. The external pressure, as the most influence of the calculation result of the shakedown load, accounts for close to 80%. The research could provide valuable theoretical and engineering guidance for the design and construction of shale gas well cement. • The deformation characteristics of the cement stone are studied at different conditions. • Mechanical model of cement sheath is set up and analyzed based on Melan's shakedown theorem and get the maximum pressure under periodic loading. • The multiple influence factors of shakedown load are on discussion. [ABSTRACT FROM AUTHOR]

The deep shale gas reservoirs are characterized by great burial depth, high formation pressure, high temperature. During the hydraulic fracturing process, the fracturing fluids will inevitably exchange heat with the surrounding rock, causing the shale in the near-well zone to undergo temperature alternation, the physical and mechanical properties of shale reservoirs are significantly affected, which brings technical difficulties to fracturing optimization design. In this paper, the Longmaxi Formation shale is used to pay attention to the rock physical and mechanical characteristics of the shale undergo temperature alternation, the microstructure and damage and fracture characteristics of the shale undergo temperature alternation are analyzed. The results show that: (1)As the heat treatment temperature increases and the number of heating-cooling cycles increases, the permeability of the shale increases, the P-wave velocity decreases, the tensile strength (TS) decreases, and the brittleness index rises. (2) Heat treatment temperature and the number of heating-cooling cycles have little effect on the triaxial compressive strength (TCS) of shale. However, it has a significant effect on the failure mode of shale. The increase of temperature and the number of heating-cooling cycles will enhance the degree of fracture of the shale failure surface. (3) Based on industrial CT scanning and scanning electron microscopy (SEM) observations, the thermal cracks caused by heating-cooling cycles are mostly inter-granular cracks and inter-layer cracks, which unfold on the surface of the sample and extend to the interior. Heating-cooling cycles also lead to an increase in the diameter of organic pores. The study results can provide a certain theoretical basis for the optimization design of staged fracturing parameters for horizontal wells in the process of deep shale gas development. • Damage of high-temperature shale subjected to heating-cooling cycles was investigated. • Damage induced by multiple heat-cooling cycles becomes more significant than a single air cooling. • Heating-cooling cycles can lead to initiation and propagation of shale cracks. • The number of heating-cooling cycles affects the frequency and width of cracks. [ABSTRACT FROM AUTHOR]

Continental shale is abundant with unconventional oil and gas and has considerable potential for exploitation. However, due to differences in the deposition environment, continental shale presents unique characteristics in terms of mineralogy and mechanical properties compared to marine shale. Furthermore, the mechanical influence of fracturing fluid immersion must be considered because of the dual effects on the productive shale system caused by fracturing fluids remaining in the formation. In this study, tri-axial compression tests were performed on Jurassic continental shale specimens collected from an exploratory well (2778–2816 m deep) in the Sichuan Basin of China before and after exposure to fracturing fluids. Meanwhile, two quantitative indicators, the roughness R , which characterizes the widely distributed yield deformation, and the complexity F , which evaluates the complex multi-fractures, were used to distinguish the different brittle-plastic characteristics of specimens before and after fracturing fluid immersion. The results showed that the immersion of fracturing fluids weakened the mechanical properties of continental shale. Two types of fracturing fluid immersion effects were distinguished by the statistics of the failure characteristics of the different layers: lubrication-dominated and softening-dominated. Unlike conventional smooth tri-axial stress-strain curves, many of the post-peak curves were in a 'step-like' shape, corresponding to two sources: one part came from the formation of multi-fractures (brittle characteristics), and the other part came from the softening deformation of main failure surface (plastic characteristics). A new brittleness index, BI new , was proposed considering failure characteristics with different brittleness effects, which could effectively distinguish the brittleness difference before and after exposure to fracturing fluids. • Mechanical variations of continental shale along longitudinal profile were studied. • Different effects of fracturing fluids were distinguished by failure characteristic. • Different sources of additional energy dissipation were recognized. • New brittleness index was proposed incorporating different failure characteristics. [ABSTRACT FROM AUTHOR]