Numerical Exploitation of Three-Dimensional Hydrofractures Mechanical Perturbation during Multistage Hydrofracturing of Vertical Wells in Laminated Shale Reservoirs

The presence of bedding plane in laminated shale reservoir has an indelible impact on reservoir displacement, deformation, continuity of stress field, and mechanical perturbation among multiple fractures. Rational and thorough exploitation of the mechanical disturbance mechanism of the bedding surface in multistage hydrofracturing provides a shortcut for the production and efficiency increase of unconventional hydrocarbon reservoirs in practical engineering. Based on the framework of bidirectional mechanical coupling adaptive finite and discrete element method and referring to different vertical well fracturing schemes, this article evaluates the fracture evolution discipline after mechanical perturbation caused by multiple coin-shaped nonplanar hydrofractures and bedding planes. The numerical model integrates routinely obtained reservoir geomechanical data with physical properties of the bedding plane to quantify the complexity, mechanical behavior, and residual conductivity of the hydrofracture network. The numerical model is proven to be practical and reliable in theory and physical experiments. The local mesh refinement and coarsening technique around the fracture surface and on the bedding plane provide a reliable guarantee for improving the computation efficiency and evolution stability of nonplanar coin-shaped fractures. The mutual perturbation factor based on weight function theory and the statistical fracture surface area and volume are introduced to quantitatively evaluate and characterize the fracturing efficiency. Numerical experiments indicate that the fluctuation of stress value on the fracture surface is very sensitive to perforation spacing. The mutual disturbance factor and the statistical stimulated reservoir area and volume confirm that there is an optimal reorientation timing due to undirectional squeezing effect in sequential fracturing of a single vertical well and the bedding plane possesses a hurdle effect on unidirectional compressive pressure transmission and reservoir displacement, which makes it discontinuous, mitigates the optimal reorientation phenomena, and leads to a worse fracture network in the case of single vertical well fracturing with the bedding plane. Higher statistical fracture volume and higher pore pressure can be obtained by alternative fracturing with a bedding plane in twin vertical well fracturing. There is a conversion relationship between the statistical fracture area and volume in the specific case of multistage fracturing of the twin vertical well. Isosurface domain of differential vertical principal stress is intertwined in the whole fracturing process, resulting in a lower statistical reservoir area and pore pressure. However, the coverage disturbance domain of the stress field is not necessarily related to the pore pressure, and the disturbed global matrix may be more prone to damage and fractures.