Drilling and multi-stage hydraulic fracturing bring a large amount of water into the formation, and clay-bearing shale reservoirs interact with water, which may lead to reduction of gas production, attenuation of fracturing effects, and even wellbore instability. Because of the complex fabric of shale, a thorough understanding of changes in shale micromechanics and corresponding mechanisms when exposed to water remains unclear. In this work, representative terrestrial and marine shale samples were selected for experiments based on clay enrichment. Then, contact resonance (CR) technique was performed to characterize micromechanics of shale after exposure to water. Visual phenomena provided by environmental scanning electron microscopy (ESEM) assisted to explain the underlying mechanisms. It was found that the hydration effect lowered both the storage modulus and stiffness of samples, but with different contributions from brittle minerals and clay, as well as variations depending on bedding plane orientation. Owing to the difference in composition, terrestrial shale exhibited stronger water sensitivity and anisotropy, with a general 15%-25% decrease in modulus, while marine shale changed relatively little (-5%-15%). Moreover, microscopic observation experiments revealed that complex interaction mechanisms may have existed that produced the mechanical changes. The reduction of capillary force and the interlaminar swelling of clay particles after water adsorption weakened the strength-related behavior of shale. However, the swelling-caused confining effect or void space closure during the water imbibition process might have offset this weakening effect, and even increased mechanical properties. At mesoscale, excessive shrinkage caused the growth of micro-cracks, which significantly attenuated overall mechanical behavior.

Accurate numerical modeling of fracture propagation and deflection in porous media is important in the development of geo-resources. To this end, we propose a novel modeling framework to simulate nonplanar three-dimensional (3-D) fracture growth within poroelastic media, using an iteratively coupled approach based on time-/scale-dependent fracture stiffness. In this approach, the propagating fractures are explicitly tracked and fitted at each growth step using triangular elements that are independent of the matrix discretized by hexahedral grids. The finite volume/finite element method (FVFEM) is employed to solve the hydro-mechanical system, based on the embedded discrete fracture model (EDFM). The calculated pressure in fractures and the stress state of the host grid of the embedded fractures constitute the boundary conditions for the boundary element method (BEM). The BEM module, in turn, renders the evolving fracture stiffness and aperture for the FVFEM module. Finally, the total stresses and the fracture-tip displacements are computed at the end of each time step to estimate the velocity and direction of newly created fractures ahead of the fracture tip. The proposed model is first validated against analytical solutions. Then, in three different examples, results are shown from the fracture’s footprint under layered stress conditions, simultaneous propagation of two nonplanar 3-D fractures, and the mechanical interaction of en échelon arrays. This work presents an efficient framework to simulate propagation of nonplanar fractures, and establishes the foundation to build an integrated simulator for fracture propagation, proppant transport, and production forecasting in unconventional formations.

Strong heterogeneity and anisotropy exist in fractured-vuggy reservoirs, resulting in complex flow and relatively low oil recovery. Therefore, the mechanism of water flooding and gas injection displacement in fracture-vug medium constitutes a key issue in oil production. In this study, based on similarity criteria, physical models of fracture-vug medium are designed and constructed through 3D printing technology. Then, by combining the LED (light-emitting diode) backlight visualization method (BVM) and the particle image velocimetry (PIV) technique, experiments of multiphase flow (i.e., oil-water and gas-oil) through the printed fracture-vug medium are carried out. During the experiments, the morphological changes of the fluid interfaces are captured with BVM, and the velocity field and streamlines of the fluid in the system are determined by the PIV technique. In addition, we also investigate various factors affecting the recovery efficiency of fracture-vug medium, such as injection velocity, gravity, outlet position, and shape factors. Results show that oil recovery in fracture-vug medium varies with injection velocity (or Reynolds number), shape of the vug, and fracture-vug structure. Specifically, the distribution of remaining oil is affected by the shape of the vug. Gravity also exerts a great influence on the morphology of bubbles in the case of gas injection. The present study leads to a better understanding of multiphase flow in fracture-vug medium and a valuable experimental dataset.

Organic matter is an important constituent in organic-rich shale, which influences the hydrocarbon generation, as well as the mechanical behavior, of shale reservoirs. The physical, chemical, and mechanical properties of organic matter depend on the source material and the thermal evolution process. Previous works attempted to investigate the impact of thermal maturation on the mechanical properties of organic matter. However, owing to the lack of maceral classification and the limitation of data volume during the mechanical measurement, no consistent trend has been identified. In this work, vitrinite reflectance test, scanning electron microscope observation, nanoindentation, and micro-Raman analysis were combined for geochemical and mechanical characterization. A total of 114 test areas were selected for testing, enhancing reliability of the test results. The Young’s moduli of organic matter are from 3.57 GPa to 8.32 GPa. With the same thermal maturity, inertinite has the highest Young’s modulus, while the modulus of bitumen is the lowest. The Young’s moduli of different organic types all increase with vitrinite reflectance. When vitrinite reflectance increases from 0.62% to 1.13%, the modulus of inertinite and vitrinite is increased by 57% and 78%, respectively. In addition, with the increase of thermal maturity, the micro-Raman test results show a decrease of intensity ratio of D peak to G peak, indicating an increase of the ordered structure in organic matter. Organic type and thermal maturity reflect the diversity of the source material and chemical structure change during the thermal evolution process, and together they influence the mechanical properties of organic matter.