Fracture Mechanism And Evaluation Method Of Rock Shale In Shale Reservoir

Shale oil and gas resources is one of the key development of unconventional energy in the country. In order to obtain the desired production, shale formation should be fractured so as to improve the flow capacity of SRV. Whether a shale reservoir has the ability to be effectively fractured or not depends on the geological factors such as Natural Fractures, In-situ Stress Difference Coefficient and Rock Brittleness. Shale rock with higher brittleness is one of the necessary conditions for the formation of the fracture network. Now Jarvier’s or Rickman’s brittleness index has been used to characterize the brittleness of shale rock, but neither of them can reflect the integrated nature of shale formation in the process of fracturing. In this article, we first conducted a series of laboratory tests including Mineral Composition Tests and Triaxial Tests to analyze the irrationality of Jarvier’s or Rickman’s brittleness index. Second, Based on the triaxial tests results, the stress-strain characteristics, crack expansion characteristics, intensity evolution of the energy dissipation characteristics and failure modes were studied in the process of deformation and failure of brittle shale, the laws of deformation and failure of brittle shale were analyzed. The characteristics of the rupture planes and the factors controlling the brittle failure of shale have been analyzed by3D Laser Scanner and Environmental Scanning Electron microscope Quanta450. Third, based on the Microscopic monitoring of the rupture planes, the lithologic features of shale and generalized Hooke’s law, considering large amount of lithologic beddings like siltstone with different mechanical properties, a mathematical model of shale under three-dimensional stress environment has been established to reveal the effects that different mechanical properties among layers have on the complexity of the fractures. Finally, based on the laws of deformation and failure of brittle shale and the factors affect the formation of complicated burst mode as well as the ability that shale getting tension and resist deformation and failure because the mechanic properties itself under stress, we established an new mathematical formulas of brittleness index to predict the brittleness of shale formation,and established an new logging method to acquire the while brittle profile of oil or gas well. The main results are as follows:(1) Jarvie’s mineral brittleness index ignores the effect of stress condition and Calcite on the brittleness, and shale rock with the same mineral constituent may behave different brittle failure characteristic due to the different stress conditions.(2) Rickman’s brittleness index ignores the effect that break strength have on the brittleness. The high brittleness layer may be the blocked layer for the extension of fracture network due to the high break strength.(3) The confine pressure affects the threshold value of the rupture. The higher the confine pressure is, the higher the stress during expansion, the rupture stress and the energy required to be destroyed are, the more difficult for the rock rupture.(4) Shale rupture is a cohesion weakening-friction reinforcing process. The pre-peak strength is mainly controlled by the bonding strength and is collectively controlled by the bonding strength and friction strength after the formation of rupture plane. With the further rupture the bonding strength is becoming lower and the friction strength is becoming higher. Finally, both of them come to a stable level.(5) The suggested formula for the evaluate of the brittle index based on the mineral constituent and the failure mode is:BI=(Quartz+Calcite)/(Quartz+Carbonatite+clay) Tentative explore the effects that the mineral constituent have on the breaking model. The material basis under the circumstance of confining pressure at40MPa and the temperature at70癈for the formation of complex failure mode is:the total content of Quartz and Calcite is more than40%, the brittle index is more than50, the content of Calcite is not less than6.4%.(6) The macroscopic and Microscopic characteristics of rupture plane of shale after triaxial test has been analyzed with the using of3D Laser Scanner and Environmental Scanning Electron Microscope Quanta450. The contour line of rupture planes of Splitting failure sample are Disorganized. This kind of fractures are more likely to form high conductivity fractures after closure. The rupture planes of shear failure are smooth and the width are more narrow after closure. Multi-dimensional rupture planes mainly comes from Brittle shearing. The stronger the shear failure is, the fewer the planes are. The formation of Multi-dimensional rupture planes is mainly caused by tensile fracture controlling mechanics.(7) Based on the experimental results, the lithologic characteristic and the generalized Hooke’s law, considering large amount of lithologic beddings like siltstone with different mechanical properties, we have established a mathematical model of shale under three-dimensional stress environment. Results showed that a segment with lower Young’s modulus or higher Poisson’s ratio would be subject to higher compressive stress, and this segment will be prone to tensile fracture, and vice versa. The difference mechanical properties among different layers is the basic reason the complexity of failure. The compressive stress is controlled by the mechanic properties and has nothing to do with the spatial location of the layer.(8) The main factors affect the formation of complex failure mode are Young’s modulus, Poisson ratio and Confining pressure.(9) An new mathematical formulas of brittleness index was established to predict the brittleness of shale formation, and an new logging method was also established to acquire the while brittle profile of oil or gas well. The higher of Young’s modulus, the lower of the Poisson ratio, tensile strength of rock and confining pressure, the stronger of the brittleness of rock, the better for the extension of the fracture network and the stimulated reservoir volume.